Suitable locations for salt marshes as natural coastal defence systems in the Ems-Dollard estuary

Initiator: dr. ir. M.J. Baptist Supervisors: drs. P. Bron, Ing. P. Smit Author: E. Lindemans Date: August the 22nd 2011

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A report on research into suitable locations for salt marshes as natural coastal defence systems in the Ems-Dollard estuary Picture Salt Marsh (Vastenhouw, 2010) Final thesis Coastal Zone Management Projectnumber: 594000 Student: Eric Lindemans student nr: 790129001 Tel: 0654324555 eric.lindemans@wur.nl Principal: University of applied science Van Hall Larenstein Initiator: Dr. Ir. Martin Baptist Tel: 0317-487068 Martin.Baptist@wur.nl Graduation Coordinator: Ing. Peter Smit Tel: 058-2846100 peter2.smit@wur.nl Supervisors: Drs. Patrick Bron Tel: 058-284406 patrick.bron@wur.nl Ing. Peter Smit Tel: 058-2846100 peter2.smit@wur.nl Leeuwarden August the 22nd 2011

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Index

Introduction ................................................................................................................................ 7 Chapter 1: Climate change and land subsidence ...................................................................... 12

1.1: Sea level rise ................................................................................................................. 12 1.1.1 The sea level rise in the estuary .............................................................................. 12 1.1.2 The accelerated sea level rise .................................................................................. 12 1.1.3 The effect of accelerated sea level rise on the salt marshes .................................... 13

1.2: Storm surges .................................................................................................................. 15 1.2.1 The effect of climate change on storm surges ......................................................... 15 1.2.2 Storm surges and salt marshes ................................................................................ 15

1.3: Rain and drought ........................................................................................................... 16 1.4 Tidal changes .................................................................................................................. 17 1.5: Land subsidence ............................................................................................................ 17 1.6 Conclusion: The effects of climate change and land subsidence on the salt marshes of the estuary ............................................................................................................................ 18

Chapter 2: The salt marshes of the Ems-Dollard estuary ......................................................... 20 2.1: The history of the present salt marshes ......................................................................... 20 2.2: The acreage ................................................................................................................... 20 2.3: The different types of salt marshes ............................................................................... 21 2.4: Zonation and succession ............................................................................................... 22 2.5: The vegetation ............................................................................................................... 24 2.6: The Referential acreage ................................................................................................. 26 2.7: Conclusion: ................................................................................................................... 27

Chapter 3: Known methods of developing salt marshes .......................................................... 27 3.1: Natural development ..................................................................................................... 27

3.1.1 The transportation of sediment ................................................................................ 28 3.1.2 The deposition of silt ............................................................................................... 31

3.2: The Sleeswijk-Holstein system and its variations ......................................................... 33 3.3: How salt marshes are currently designed to function as coastal defence systems ........ 35 3.4: Other possibilities .......................................................................................................... 35

3.4.1 Other ways to develop salt marshes ........................................................................ 35 3.4.2: Building with nature ............................................................................................... 37

3.5 Conclusion: ..................................................................................................................... 39 Chapter 4: Direct human influences on the development of salt marshes in the Ems-Dollard estuary ...................................................................................................................................... 40

4.1 dredging: ......................................................................................................................... 40 4.2 Coastal squeezing and the straightening of the coast: .................................................... 41 4.3 Sand suppletion: ............................................................................................................. 42 4.4 Conclusion: ..................................................................................................................... 42

Chapter 5: Analysis .................................................................................................................. 44 5.1 Criteria to divide the area and compare the locations .................................................... 45 5.2: Dividing the area ........................................................................................................... 45 5.3: Suitable locations for salt marshes as a natural coastal defence system in the Ems-Dollard estuary ..................................................................................................................... 48 5.4: The most promising methods for the development of salt marshes as a natural coastal defence system on the suitable locations .............................................................................. 50 5.5: Imaginable combinations of salt marsh development methods in the Ems-Dollard estuary .................................................................................................................................. 51

Chapter 6: Conclusion .............................................................................................................. 53

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Chapter 7: Discussion and recommendations .......................................................................... 58 Sources ..................................................................................................................................... 60 Appendix 1 ............................................................................................................................... 65 Appendix 2 ............................................................................................................................... 66 Appendix 3 ............................................................................................................................... 67

Sedimentation and erosion ................................................................................................... 67 Tidal maps ............................................................................................................................ 68 Current velocity .................................................................................................................... 75 Wave velocity ....................................................................................................................... 75 Buildings .............................................................................................................................. 76 De-embanked Breebaartpolder ............................................................................................. 76 Primairy coastal defence system .......................................................................................... 77 Most recent vegetation map of salt marshes in the estuary that has been found during the study for this report .............................................................................................................. 77 Depth .................................................................................................................................... 78 Duration of being above sea level ........................................................................................ 78 Intertidal areas ...................................................................................................................... 79 Sediment size ........................................................................................................................ 79 Silt concentration in the soil ................................................................................................. 80 Silt concentration in the water .............................................................................................. 80 Waterways ............................................................................................................................ 81 Oyster banks ......................................................................................................................... 81 Mussel banks ........................................................................................................................ 82

Appendix 4 ............................................................................................................................... 83 Appendix 5 ............................................................................................................................... 90 Appendix 6 ............................................................................................................................... 92

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Summary The global sea level is expected to rise at an accelerated speed due to global warming. The coast of the Ems-Dollard estuary will therefore need more protection against flooding. Salt marshes are a nature-friendly alternative for enlarging the sea dikes. This report is intended to provide the answer for the following main question: Which aspects should be taken into account when designing salt marshes that will have to function as natural coastal defence systems in the Ems-Dollard estuary and which locations in this region are suitable to develop a salt marsh that meets these demands? On the main land, the coast of the estuary stretches from the Ems harbour in the west to Krommhörn in the east. This line has been studied to find suitable locations for the development of salt marshes. The German part of this coast line has been studied in less detail and has not been taken into consideration in the final stage of pointing out suitable locations because a certain amount of essential information could not be retrieved. Salt marshes are pieces of land which have developed over time by the deposition of sediment that border shallow tidal areas, like the Wadden Sea. They form in places where the water movements (currents and waves) are calm enough for a net deposition of sediment instead of erosion (especially the deposition of the small silt particles is important because they make the land more fertile). Their development starts when a mud flat has gained enough height for the settlement of salt tolerant vegetation (halophytes). These halophytes slow down the water movement even further which leads to more sedimentation. The first settling halophytes are called the pioneer vegetation and their presence marks a zone which is called the pioneer zone. After enough sediment has accumulated to reduce tidal flooding to a point where less salt tolerant species were able to settle, the next zone has evolved which is called the low salt marsh zone. This zonation continues till a point where the fresh water species dominate the area. Therefore a salt marsh is defined as the zone where the halophytes are dominant. Along the main land of the estuary there is a total acreage of 741 ha. of salt marshes (aside from the unknown acreage of salt marshes in the German part of the estuary). These salt marshes are the result of land reclamation works. Claiming land from the estuary has stopped around 1950 and currently the salt marshes are eroding. A total acreage of 700 ha is considered to be enough for the salt marshes to stay in a good ecological condition. Due to the current erosion of approximately 2 ha per year the area is expected to degrade towards a mediocre condition however. The increased risk of the flooding of the land due to the accelerated sea level rise is what initiated this study into how to develop salt marshes as a coastal defence system. According to the most likely scenario, the sea level is expected to rise between 35 and 85 cm this century. According to a more extreme scenario, which was created to ensure enough protection against flooding, the sea level could also become 120 cm higher this century. Whether the salt marshes are able to keep up is very uncertain and being researched. The estimations for this tipping point extend from 30 cm to 2 m per year. Besides the sea level rise, the salt marshes also have to withstand the effects of land subsidence (partially caused by gas extraction), climate changes, dredging, coastal squeezing and the straightening of the coast. These events cause changes in current velocities, wave velocities, tidal patterns and the sediment transportation for instance. From these events dredging seems to be the most destructive for the salt marshes in the estuary. It has been said that 40% of the acreage that was present in

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1900 has disappeared due to dredging activities. Dredging has increased the tidal range, caused the tide to come in faster and brings more sediment into suspension which is damaging to the aquatic ecosystem. Within this ecosystem there are bio engineers which create favorable conditions for the deposition of silt. To point out the most suitable locations for the development of salt marshes a multi criteria analysis has been carried out. Then the most promising methods to develop a salt marsh that has to function as a natural coastal defence system on these locations have been selected. In the figure that is shown below, it is explained which methods have been selected for the most suitable locations (a larger example of this map is shown on page 55). The methods that were considered as an option are: 1: Scooping a large amount of sediment onto the mudflat 2: Stimulating sediment deposition by using sedimentation fields 3: Building a structure in the sea, for instance a dam 4: Protecting the salt marsh from erosion with groynes 5: De-embanking summer polders 6: Relocating the sea dike further inland and (partially) excavating the foreland 7: Building a new sea dike further inland and allowing sea water to flow through pipes (for instance) that are built in the former sea dike. 8: Creating oyster or mussel banks 9: Stimulating the settlement of halophytes

Suitable locations and the most promising methods for the development of a salt marsh that has to function as a natural coastal defence system

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Introduction

As a result of global warming the sea level is expected to rise between 35 and 85 centimetres this century (KNMI, 2011). Winds will also increase in strength and the land will descend in many locations (Rijksoverheid, 2011) due to geological processes and human activities like gas extraction. Therefore it is necessary to adjust the coastal defence structures. This can be done by raising the dikes but using salt marshes for coastal protection would be a more nature-friendly solution because instead of possibly claiming nature by heightening and widening the dikes, nature will be added (Ministeries van V&W et al., 2010). The salt marshes of the Wadden area are marked with dark green in figure 1. The total acreage is approximately 38.000 hectares of which 9.500 hectares is located in The Netherlands and 22.290 hectares in Germany. All of it is protected by law (Esselink et al., 2009).

Figure 1: Salt marshes in the Wadden area (Common Wadden Sea Secretariat, 2011) The boundary of the Ems-Dollard region is laid down in the Ems-Dollard treaty. In figure 2 this boundary is highlighted in red. As shown in figures 2 and 3 the coast line of the area

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extends from the Ems harbour in the west to Krommhörn in the east. Along this line suitable locations for the development of salt marshes as coastal defence systems may be found.

Figure 2: Boundary of the Ems-Dollard estuary Figure 3: Map of the estuary (Google Maps, 2011) (Altenhofen et al., 2010) The Ems-Dollard estuary is about 10.000 hectares in size (Wadden Sea World Heritage, 2011). The Dollard was created in the year 1277 after the sea broke through. Soon after that event salt marshes started to develop on the edges of the flooded land. The 741 hectares of salt marsh currently present in the Ems-Dollard estuary is a result of land reclamation in the midst of the previous century. This land reclamation has ceased and now the salt marshes are slightly eroding on the sea side. The percentage of salt marshes in the Ems-Dollard is still relatively high and for the total acreage of salt marshes, the Ems-Dollard is deserving of the predicate “good ecological condition”. Due to the erosion there soon will be a deficiency however (Dijkema et al., 2005) and without human intervention this unique landscape will

eventually disappear (Kwelderherstel Groningen, 2011a). Salt marshes are muddy pieces of land that border shallow tidal areas like the Wadden Sea. At high water levels salt marshes become submerged with sea water. Sand and silt particles which are carried in with the sea water have the opportunity to sink between the vegetation where they will not be washed back to sea. Due to this accumulation, salt marshes steadily become higher (Natuurinformatie, 2011) and have the ability to keep up with the pace of an increased rising of the sea level and decreasing height of the land (Dijkema et al., 2005). It is still uncertain at which degree of sea level rise salt marshes are unable to keep up anymore. At the moment this is an important subject for monitoring (Ministeries van V&W et al., 2010). One of the reasons for this type of land to be so unique is the accompanying vegetation. Because the surface is regularly flooded with salt water it is only possible for salt tolerant plants to survive here. A few examples of these characteristic salt marsh species are: samphire (Salicornia dolichostachya), sea lavender (Limonium vulgare), seablite (Suaeda maritima), sea purslane (Halimione portulacoides), sea aster (Aster tripolium) and sea wormwood (Artemisia maritima) (Natuurinformatie, 2011).

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Limonium vulgare during extreme high tide (Ecomare, 2011b) Apart from being internationally highly valued nature, salt marshes are also a natural foreland for our sea dikes. Therefore, in the German Wadden Sea, salt marshes are considered part of the coastal defence system. Measurements of wave heights during storm floods on German coasts with and without these forelands underline this point of view (Dijkema et al., 2005). In The Netherlands this function to reduce wave energy has not yet been adopted in the examination of high water safety (Baptist, 2011). One reason for conducting the research for this report was to find out if it is an option to increase the quantity of salt marshes that serve as natural defence systems in the estuary. I’d like to thank everyone who has assisted me by providing data as well as advise. The following people, presented in alphabetical order, have been of great help to me: Martin Baptist Anton Bartelds Patrick Bron Willem van Duin Herman Mulder Peter Smit Kerstin de Vries Problem description Van Hall Larenstein University of Applied Sciences is involved in the Climate(Ex)Change Dollard Ecology project to determine how water engineers can develop safe coastal defence systems while respecting the natural environment at the same time. The project focuses on nature, management and designing dikes and salt marshes in the Dollard region. This report is meant to provide an answer to a part of that question by looking at which natural aspects should be incorporated in a design for salt marshes which are meant as a natural coastal defence system in the Ems-Dollard estuary and by pointing out where these salt marshes can be constructed (if possible). In some places in the estuary salt marshes are already present but the total acreage is decreasing. This indicates that there are natural or human causes that prevent the formation of salt marshes in the estuary. Looking at why that is happening and how this can be turned around plays a role in determining at which locations salt marshes can be formed. Therefore various aspects that influence the formation of salt marshes in the estuary have been studied to explain how and where they can be created.

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However, the focus has been placed on the natural aspects that can influence the formation of salt marshes because of the available time for the assignment. For these natural aspects one can think of sedimentation, vegetation, currents, wave action, tides, depth and channels. Regarding human aspects, the influences of climate change, land subsidence, dredging and more have been studied. The purpose, within this research, of creating more salt marshes, is protection from flooding. Therefore it is important to find out how salt marshes are currently designed to be able to do that as well. To solve this problem the following questions need to be answered. Main question: Which aspects should be taken into account when designing salt marshes that will have to function as natural coastal defence systems in the Ems-Dollard estuary and which locations in this region are suitable to develop a salt marsh that meets these demands? Sub-questions: How do nature and men create salt marshes? In which ways are salt marshes currently designed as a coastal defence system? What are the differences between German and Dutch methods for salt marsh development? Which influences of nature play a role in the development of a salt marsh? To which extent can mussel and oyster banks contribute to the development of a salt marsh? Where are currents, wave movements and sediment transports located in the estuary and what are the threshold values for developing, maintaining and eroding a salt marsh? Is the natural formation of salt marshes being held back in any locations in the Ems-Dollard region? Which extra measures are necessary to develop a salt marsh on a location where it would not be created by nature itself? Which locations in the estuary are suitable to develop a salt marsh that has to serve as a coastal defence system? Methods The information in this report is mainly derived from literature and the consultation of experts. Starting the research no theories have been formulated that need to be tested. The aim for this research was to gain information on how salt marshes can be designed. Therefore this research had a descriptive and explorative character (Baarda et al., 2001). During the research a set of maps that provide field data for various subjects that are discussed in this report has been used (appendix 3) to analyse which locations in the Ems-Dollard region are suitable for the development of a salt marsh. The outcomes are mainly discussed in Chapter 5. The results of the research will be presented in three products: this report which answers the main question and explains how the answer was extracted, a poster in A3 format to show the outcomes of the report in a more accessible way and a presentation to briefly discuss this information.

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The chapters in this report are arranged in a way that information which is necessary to understand what is being discussed has been explained in the previous paragraph(s). A logical order is maintained as well by outlining the cause of the problem in chapter 1, followed by describing the research’s key subject in chapter 2, we will discuss which other factors may effect the outcome in chapters 3 and 4. Possible solutions will be presented in chapter 5. In chapter 6 a clear answer to the main question is formulated. This is followed by a discussion of what may have affected the results or could be done in the future to further increase the chances for success in chapter 7.

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Chapter 1: Climate change and land subsidence The purpose of this chapter is to give an idea of the changes the area will undergo in the future as a result of climate change and land subsidence. A lot about this is still uncertain but what we do know shows to which extent salt marshes might be able to defend the land surrounding the Ems-Dollard estuary against flooding. The predicted climate change will influence the salt marshes of the estuary in many ways. Some of the more significant alterations will be discussed in the following paragraphs. Land subsidence is as well a collection of several processes but with only one outcome that is relevant for this research and will therefore be discussed in total in paragraph 2.5.

1.1: Sea level rise Salt marshes are able to gain height because they gather sediment during high tides. Therefore, to some extent, while the sea level rises, the salt marshes rise with it. Due to global warming the sea level is expected to rise at an accelerated pace. This poses the question if salt marshes are able to accumulate enough sediment to keep up with the accelerated sea level rise as well. If this is not the case then it rules out the possibility of using salt marshes that are only maintained by nature to defend the coast.

1.1.1 The sea level rise in the estuary The rate of sea level rise in the Wadden region has been constant at about 1.8 mm/year for the past century (RIKZ, 2004). According to Talke et al. (2006) the global sea level has risen 1.8 mm/year over the past 100 years but the mean sea level rise in the Ems estuary has remained constant at about 1-1.2 mm/year since 1901. This difference might be due to the difficulty to compare the global mean tidal level (MTL) with the MTL along the North Sea coast because of a subsidence of its ground (in between 2 and 8 cm/century) and distortion of the tide by intertidal mudflats (Talke et al., 2006) (land masses, narrow passages and shallow depth areas distort and impede tide waves for instance because the friction absorbs a lot of energy (Schwartz, 2005)).

1.1.2 The accelerated sea level rise The Royal Netherlands Meteorological institute (KNMI) ’06 scenario’s give an idea of the climate in The Netherlands for the year 2050 and 2100 (in comparison with 1976-2005) and predict the most likely amount of sea level rise at the Dutch coast (see figure 4). These scenario’s do not differentiate at a regional level (Klein Tank et al., 2009) so they are equally applicable to the Ems-Dollard estuary as any other part of the Dutch coast. The climate change could be more extreme within a few decennia, for instance because green house gas emissions could increase faster than the most extreme emission scenario of the Intergovernmental Panel on Climate Change (IPCC) predicts. Climate scenarios reflect the scientific knowledge at a certain moment. They do not present absolute upper or lower limits for, for instance, temperature or sea level rise. The rule of thumb is: the more extreme the scenario, the smaller the chances of the scenario becoming

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reality. Nevertheless extreme climate changes with a small chance of occurrence but with big consequences can be important (Klein Tank et al., 2009).

Figure 4: Scenario’s for the expected sea level rise along the Dutch coast in 2050, 2100 and 2200. (The year of reference is 1990 and the effects of land subsidence are not taken into account) (Klein Tank et al., 2009). Since the publication of the KNMI ’06 scenarios, several more extreme climate scenarios have turned up, each for its own purpose (Klein Tank et al., 2009). One of those is made by the Delta commission. This commission, installed on behalf of cabinet Balkenende 4, was assigned in 2007 to give advise about how The Netherlands should deal with the climate change or more precisely how the land can remain safe in 2100 and beyond from the expected sea level rise and an increasing precipitation resulting in higher river drainage

(Deltacommissie, 2008a). The commission presented a climate scenario in 2008 to determine a plausible upper limit for the sea level rise. The KNMI ’06 scenarios were maintained to determine this for the year 2050 but there are differences from there on upward (see figure 4). The Delta commission departs from a global temperature rise of 2 to 6°C in 2100 which would cause the ocean water to expand more and therefore the sea level to rise further. The commission indicates an upper limit of 55 to 120 cm in 2100 (land subsidence not included). The KNMI ’06 scenario’s, which are meant to describe the most likely scenario, extent from 35 to 85 cm (land subsidence not included) (Klein Tank et al., 2009). This comparison is meant to point out that the sea level rise estimated by the KNMI might be most likely to become reality but that it is especially important with coastal defence to be aware of the more extreme scenarios because if those are to become reality the people in the vicinity have to be safe as well. In general a trend seems to be developing in which new insights result in an upward adjustment of the expected sea level rise (Deltacommissie,

2008b).

1.1.3 The effect of accelerated sea level rise on the salt marshes Salt marshes situated along the mainland are capable of keeping up with an accelerating sea level rise or land subsidence of 1 to 2 cm/year according to the WOK (2010) due to the combination of silt accumulation and plant growth (Van Duin et al., 2008). However, it is still uncertain at which tempo of sea level rise salt marshes will not keep up anymore (Ministeries van V&W et al., 2010). This confusion is emphasized by the statement of the Delta

commission (2008b) that the salt marshes in the Wadden Sea are probably unable to keep up with a sea level rise of 0.3 to 0.6 cm/year (which is expected in 2050 and onwards) without

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artificial sedimentation. At the moment this tipping point is an important subject for monitoring (Ministeries van V&W et al., 2010). The pioneer zone (the area with the plants that are first to settle) might have more trouble to keep up with the sea level than the rest of the salt marsh. This is also the case without an accelerated sea level rise and land subsidence. Because there is less vegetation, which predominantly exists out of one year living plants, there is less protection of the deposited sediment and therefore not as much accumulation of silt. Eventually this deficit of silt, compared to the higher part of the salt marsh, could lead to cliff erosion. This means that the salt marsh will keep growing vertically but the acreage crumbles away towards the land due to lateral erosion (Van Duin et al., 2008). The difficulty to predict if the salt marshes located in the estuary will remain to exist under the various circumstances is easier to comprehend when looking at their morphology. Under the current conditions salt marshes expand and retreat as well. Some locations become more vulnerable to erosion, others to deposition. During monitoring of the “natural sedimentation” (in contrast to artificial forms of sedimentation) between 1985 and 1990, the average silt deposition on the Dutch salt marshes of the Ems-Dollard estuary was estimated to be 0.558 cm/year. For the salt marshes on the German half of the estuary this was 0.466 cm/year (RIKZ, 2004). During another monitoring of the Dutch salt marshes of the estuary, which were artificially protected against waves and currents, the deposition was measured as 1.2 cm/year for 1960 till 1997 and 1.1 cm/year nowadays (WOK, 2010). This indicates that salt marshes are indeed capable of gaining height at a rate of 1 to 2 meters per century, which was mentioned earlier, but as stated at the beginning of this subparagraph, the precise upper limit is still uncertain (and as the measurements indicate, dependable on the extent of artificial influences). Possibly because all the salt marshes in the estuary are men made which means that the sediment depositions are influenced. It is well known that more sediment will be deposited under certain conditions. Creating embankments and draining ditches for instance has a positive effect on the amount of material that will be left behind on a salt marsh by the flood (Kamps, 1956). Monitoring has shown that a mud flat located South-East of the Ems harbour collected sediment in the period of 1985-1990 at a rate of 5.320 cm/year (influenced by dredging activities). In the Ems harbour it was even more than twice as much (natural sedimentation) (RIKZ, 2004). As these numbers indicate the deposition rate has a close relation to the shape of the environment. Vegetation for instance makes the water less turbulent which gives more sediment particles the chance to settle down (Sips, 2009). Therefore it seems that deposition rates above 1.2 cm/year can be achieved on salt marshes in the estuary. Maybe 2 cm/year or more is possible as well under the right circumstances (for instance behind an embankment, between the vegetation or close to a draining ditch). Salt marsh development and morphology will be discussed in more detail in chapter 3. Which locations in the estuary currently gain or lose sediment and in which quantities is, for as far as this information could be retrieved, shown in the sedimentation and erosion map in appendix 3. The total acreage of salt marshes in the estuary decreases for various reasons (many will be discussed later in this report) but that doesn’t mean that there are no places alongside the estuary where salt marshes are forming or where this could be achieved by altering the environment. At this moment there is only a limited number of spots within the Wadden Sea where thorough observations on sediment composition, transport and the force of tidal currents are being carried out. It is not clear if locally observed changes in the sediment composition occur in general as well. The climate change influences the tidal currents and with it the

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sedimentation through sea level rise and wind dynamics. The precise results are however hard to predict. There are too many uncertainties to do so for the sedimentation processes in the Wadden Sea (Van Leeuwe, 2010). A sea level rise ranging from 35 cm/century to 85 cm/century does not seem to exceed the elevation capacity of the salt marshes in the estuary presuming they are located in favourable conditions (in contrast to conditions that cause them to erode such as high water turbulence or a lack of vegetation). The more extreme scenario of a sea level rise of 120 cm/century makes the salt marshes’ survival more uncertain but creating an environment to accumulate more sediment could be the way to solve this problem. Obviously more knowledge is required to gain more certainty.

1.2: Storm surges During a storm surge the risk of flooding increases because the sea water is forced towards the land. This increases the depth of the water, the height of the waves and the height of the dike that is reached by the sea water. Salt marshes can contribute to the protection against flooding due to storm surges. Therefore salt marshes might become more important if storm surges will become more severe in the future.

1.2.1 The effect of climate change on storm surges Small changes in the atmosphere can have a large impact on the storm climate. A small alteration in the position and strength of the jet current above the Atlantic Ocean for instance (where strong western winds develop at an altitude of about 10 km and get carried away elsewhere) can be of big importance locally (Klein Tank et al., 2009). The storm climate is very variable by nature. Even from one decennium to the next, substantial alterations take place. Besides that it is also hard to predict trends for heavy storms because of their rarity. The KNMI ’06 scenarios predict however that the climate change will have a small influence on the Dutch storm climate. That is because various models show that the natural variations in the storm climate are larger than the changes which are caused by the green house effect (Klein Tank et al., 2009). For estimating the risk of flooding one should not only focus on the strength of the storm but also the direction the wind is coming from. Wind coming from a northerly direction drives the most water up the Dutch coast. There are however no indications for more or stronger winds coming from the north. This means that in the future the water will not be pushed up higher by storms than nowadays. Southwest winds dó seem more likely (Klein Tank et al., 2009).

1.2.2 Storm surges and salt marshes If storm surges would get stronger, the water in the estuary will be driven up the shores more fiercely as well (Ministeries van V&W et al., 2010). When the land seaward of the sea dike is higher, wave height and wave stowage are reduced. In the German and Danish parts of the Wadden Sea salt marshes are therefore considered part of the coastal protection (Dijkema et al., 2007).

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After a storm surge one can tell by the position of the high water line till which height the waves have hit the dike. The difference in altitude points out that dikes behind a salt marsh are better protected against storm surges (Van Duin, 2011). The reason is that a salt marsh makes the water in front of the dike shallower which makes waves tumble over sooner. This is beneficiary for the safety of the dike because water getting over it erodes the dike on the land side which may cause it to breach. Dikes are generally weaker on this side than on the sea side where it is covered with a harder substrate (Van Duin, 2011). After a severe storm on the 1st of November 2006, which caused the highest recorded water level in Delfzijl ever, (KNMI, 2006) the regional water authority of Noordzijlvest measured the height of the dike that was reached by the waves between Lauwersmeer and Delfzijl. These results indicated that salt marshes reduce the height of the waves even during severe storms (WOK, 2010). Numerous models have been used by others to generate estimations for the estuary about storm surges, the tidal range, sediment transport and mixing and dispersion processes. Reasonable results for short term processes (in the order of days) have been found this way but long term morphological changes can not yet be predicted according to Talke et al. (2006). The climate scenario suggests that climate change will not have a big influence on the storm surges in the estuary. It could however happen (for instance due to natural variations in the storm climate) that storm surges become stronger and therefore it could be preferable to increase the protection of the land by the development of salt marshes. Even under the current circumstances it might be beneficial to create additional protection with salt marshes to decrease the risk of flooding.

1.3: Rain and drought Global warming and changing precipitation patterns may have an effect on the salt marshes of the estuary. Till which extent is very uncertain. The information below gives an idea of what can be expected. According to the KNMI ’06 climate scenarios, the winters in The Netherlands will become 7 to 28% wetter on average and the amount of rainy days in the summer will decrease (with a loss of 3 to 38 wet summer days on average). The rain will fall down more often in the form of heavy showers. If the precipitation patterns change a lot (for instance: more rainfall in winter and less in summer) it could lead to changes in the sedimentation patterns within the estuary. What these changes will be is being studied by several governmental organisations at this moment (Ministeries van V&W et al., 2010). The temperature in The Netherlands and the surrounding countries has increased twice as much in the past years as the global temperature. This rise seems to be systematic and not based on a natural fluctuation. The average rise in temperature in The Netherlands in the years between 1999 and 2008 is 0.8° C higher than in the years between 1976 and 2005. Due to the rise in temperature (assuming there is enough moisture present in the soil and the net radiation will stay as it is) the evaporation will increase (Klein Tank et al., 2009). The KNMI ‘06 scenarios predict an increase of 3 to 15% for the summers in the years around 2050. This will lead to dehydration of the soil if it is not compensated by increased rainfall or by human intervention.

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The North Sea’s temperature has a demonstrable effect on the distribution of precipitation in The Netherlands. Along the coast there can be up to 15% more precipitation per degree Celsius of rising temperature of the North Sea when coinciding with certain air current patterns. This effect is strongest to an extent of about 30 kilometres inland. Long climate simulations with regional models that take into account the influence of the North Sea’s water are however unavailable at this moment. Therefore the statements about future precipitation in the coastal region are merely indicative (Klein Tank et al., 2009). Obviously, dehydration or more water absorption of the soil, changes in salinity and alterations in the flow of sediments may have consequences for the salt marshes and its vegetation in the estuary. Yet the effects of climate change are diverse and uncertain and the monitoring nowadays seems insufficient to give a good idea of what is going to happen. Analyses point out that more knowledge is needed about the possible effect of climate change on morphology, the hydrological regime and the ecology of the Wadden area as well as the stability and resilience of present systems. To assess the effects of climate change a more integral and continuous monitoring of basic data of the waddensystem is needed. Which kind of long term monitoring is necessary (aside of the regular obligated monitoring) is under investigation, according to the ministry of Infrastructure and the environment et al. (Ministeries van V&W et al., 2010).

1.4 Tidal changes Deepening the estuary for shipping has increased the tidal difference in the estuary and caused the acreage of salt marshes to decrease with 40% compared to 1900 (ZOETZOUT, 2011). The unnatural, deep and straight channels make the floods come in much faster and stronger (Natuur en Milieufederatie Groningen, 2010). Enlarging the channels and dredging and dumping activities since circa 1970 cause the estuary to become more cloudy as well (Raad voor de Wadden, 2010) (more about dredging and clouding in chapter 4, paragraph 1). The tidal difference is not caused by the dredging activities in the estuary alone. The mean tidal range along the entire German coast has increased with 13 cm/century between 1855 and 1990. Between 1965 and 2001 it increased at a much faster average pace however: 51 cm/century. Scientists have suggested that this accelerated increase may be a derivative of the accelerated sea level rise. The mean high water (MHW) and mean low water (MLW) values for the estuary have changed as well. Due to the increasing MHW the estuary became more vulnerable to storm surges. The decreasing MLW lowers the groundwater table which negatively impacts the salt marshes (Talke et al. 2006). The tidal range in the Dollard averaged 3.3 meters in the year 2000 according to Esselink (2000) (a more recent figure has not been found during the study for this report). During incoming tide the water in Delfzijl raises up to 3 meters nowadays (Natuur en Milieufederatie Groningen, 2010). This is more than anywhere else in the Wadden Sea according to WaddenZee.nl (2008).

1.5: Land subsidence The land of the Ems-Dollard estuary is subsiding due to gas extraction and natural processes. One of these processes is isostasy (movement of the land as a result of a shifting load that pushes down the earth’s crust). Till about 12.000 years ago, large ice caps covered Scandinavia and Great Britain. This caused the earth’s crust to descend and push aside a part of the material of the earth’s mantle beneath it. Since the ice has melted, the earth’s crust has been returning to its former position and still does so today. The material that was once pushed aside and moved certain parts upward is returning as well, causing the earth’s crust to

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descend at these places now as is happening in The Netherlands. Geological data and modelling calculations indicate that the Wadden area is subsiding because of isostasy within 0.1 and 0.5 mm/year. The earth’s tectonics (movement of the earth’s crust along the earth’s faults) causes The Netherlands to descend with 7 mm/century in Noord-Holland and rise with 3 mm/century along the eastern border. Over the past 2 million years this movement has caused the Wadden Sea to subside with an average speed of 0.1 to 0.2 mm/year. The remaining natural cause of the Netherlands to subside is compaction (the settling down of material under its own weight). In the case of the Wadden area water is being pressed out of layers of clay and peat. The current speed at which this is happening below the Wadden Sea

probably does not exceed 0.1 mm/year (Waddenvereniging, 2011a). The Groningen-gasveld (a gas extraction site in the province of Groningen) is in the top 20 of the biggest gas extraction sites in the world (NOM, 2010). Gas production from this field will be continued till 2070 (recently it was expected to seize in 2050) (NAM, 2005). As a result the overlying land is going to descend, probably with 40 to 54 cm at the deepest point till 2070. A value of 47 cm is thought to be the most likely (NAM, 2005). In appendix 1 is shown how the expected subsidence is divided over the estuary in 2070. According to the RIKZ (2004) monitoring salt marshes and several other locations of the estuary and the bordering influenced areas has led to believe that land subsidence caused by gas extraction is minor compared to the natural dynamics and that the sedimentation of the salt marshes is not dependent on this kind of land subsidence at all (RIKZ, 2004). The data from the previous paragraphs will be compared in the next paragraph to draw our own conclusion.

1.6 Conclusion: The effects of climate change and land subsidence on the salt marshes of the estuary The map in appendix 1 shows that a large part of the estuary is expected to descend due to gas extraction with a maximum of 34 cm in 2070 at its most influenced part. This value is in comparison to the height of the land in 1964 when the extraction of gas from the Groningen-gasveld started (binnenland.nieuws.nl, 2010). This comes down to a maximum descent of 3.2 mm/year. If we compare this figure to the expected sea level rise (between of 3.5 mm/year and 8.8 mm/year) it is clear that gas extraction has a significant effect on the salt marshes located in those areas if their maximum “growth rate” would indeed be 10 to 20 mm/year. If we ad up the lowest estimations for the sea level rise and causes of land subsidence we find that the salt marshes would need to grow in height with nearly 3.8 mm/year to 7 mm/year to compensate for these effects depending on how much they are influenced by the gas extraction. In the worst case (with the Deltacommissie estimates) a salt marsh would need to get higher with 15.5 mm/year to keep up with the sea level. Since it is uncertain if they are able to do so it might be a good idea to give nature a helping hand. It is important to keep in mind that the climate scenarios have a big margin of uncertainty. They do not present absolute upper or lower limits for, for instance, temperature or sea level rise.

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Salt marshes contribute to the protection of the coast from the sea even during severe storm surges. At the moment it is too hard to predict if storm surges will occur more frequently in the future. However, the expectations are that the climate changes will not have a large impact on the Dutch storm climate. Winters are expected to become wetter and summers dryer. When it rains it is expected to fall down more often in the form of heavy showers. These changes may have consequences for the salt marshes but more knowledge is needed to predict the effects. Along with the sea live rise, the MHW and the tidal range in the estuary are increasing at an accelerated pace as well. Partially this has been caused by dredging activities and partially by changes in the climate. The increasing tidal range has an eroding effect on the salt marshes and makes the estuary more vulnerable to storm surges. Based on the scenarios and the salt marshes’ ability to cope with them we can conclude that it is uncertain if the salt marshes in the Ems-Dollard estuary will remain to exist. Their chances for “survival” are growing smaller. They increase coastal protection but defending the land against flooding is of such high importance that it would not be wise to rely on salt marshes to provide for this in the future without filling up the deficits in knowledge mentioned in this chapter. However, they could be used as an extra safety measure till there is more certainty or have a more important role where there is no alternative. In those cases it seems a good idea to monitor the condition of the salt marsh (for instance deposition/erosion rate, acreage or characteristics of the vegetation) frequently and to intervene where it is necessary. For the development of a new salt marsh it is advisable to take preliminary actions and to select a location and design that is less likely to be effected by the influences mentioned in this chapter (locations that promise higher deposition rates are for instance preferable). The following chapter will give a description of the salt marshes in the Ems-Dollard estuary to explain which characteristics are important to incorporate into a design.

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Chapter 2: The salt marshes of the Ems-Dollard estuary This chapter describes the salt marshes by briefly explaining how they have developed over time in paragraph 1, then pointing out where they are located in paragraph 2. Paragraph 3 addresses their characteristic differences in comparison with other salt marshes in the Wadden Sea. The 4th paragraph explains the relevance of zonation and succession. The 5th paragraph describes the vegetation and its contribution to the ascend of salt marshes. The 6th paragraph goes into their ecological condition and is followed up by a conclusion. This information should give a better understanding of which locations within the estuary are suitable for developing new salt marshes and which characteristics are important to carry over.

2.1: The history of the present salt marshes In 1509 a storm surge flooded an area of 200 square kilometres in the Dollard region. At the time there were many small settlements in the area from people who lived from the fertile soil

(Waddenvereniging, 2011b). After the storm surge, the inhabitants started with the reclamation of the land. They did so continuously (except during WW2) till 1950. In that year the Dollard commission declared that there was no need for more agricultural land. Nowadays, only a small strip of salt marshes is left which is kept in place. There is hardly any dynamic and without human intervention (such as grazing or digging trenches) the ecological diversity decreases. The marshes could also grow in size and diversity by claiming more land from the sea but this is forbidden by the habitat directive according to de Ranitz et al. (2009). To make matters worse, the 741 hectares of salt marshes in the estuary (all created by land reclamation) is eroding from the sea side (RIKZ, 2005). However there seem to be certain ways to legally claim more land from the sea but since this report primarily focuses on the natural aspects concerning salt marsh development it is advised to consult page 23 of the following source if one wants to find more information about it (Atsma et al., 2011).

2.2: The acreage The estuary measures 10.000 hectares (Wadden Sea World Heritage, 2011) and the total acreage of salt marshes along its coast is currently 741 hectares. A large part of the estuary is brackish because the fresh water of the rivers the Ems and Westerwoldse Aa flow into the estuary (Wadden Sea World Heritage, 2011). The salt marshes extend inland from where the first salt tolerant plants (halophytes) are found to where the freshwater vegetation dominates over the halophytes. In other words: it is an area of muddy soil, located next to the sea, where halophytes dominate the vegetation (Van Duin, 2011). Under natural circumstances the halophytic vegetation is able to settle from 20 cm below the mean high water line and upwards. In sedimentation fields (areas that are fenced with brushwood groynes for land reclamation) the halophytic plants can be found at 40 cm below the mean high water line and upwards (Dijkema et al., 2001). In the appendix 3, the exact location of the salt marshes in the estuary is shown on the map called “Most recent vegetation map of salt marshes in the estuary that has been found during the study for this report”. We will restrict ourselves in this paragraph to a brief description of the separate salt marshes. Moving down along the coast from the North-Eastern part of the estuary, the first salt marsh that crosses our path is located on the Punt van Reide. The Punt van Reide is a piece of land

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of 46 hectares situated between the Dollard and the Ems (upper half of the estuary). A large portion of it is salt marsh of which some is grazed, mowed and hayed. A stone wall protects

this salt marsh from erosion (Kwelderherstel Groningen, 2011c). Moving further down the coast we find the salt marsh of the Dollard. The Dollard distinguishes itself from the rest of the estuary because of the size of the terrain that is uncovered by the sea during low tide which is 85% of the total area (Coumans, 2006). The salt marsh in The Dollard is partially privately owned (de Ranitz et al., 2009) and from that part it is recorded that it erodes with approximately 2 hectares/year. It extends from the southern edge of the Breebaartpolder (appendix 3 “De-embanked Breebaartpolder”), which is located immediately south of the Punt van Reide, till the border between The Netherlands and Germany. This salt marsh covers 694 hectares, is at some points over a kilometre wide

(Kwelderherstel Groningen, 2011c) and is by far the biggest in the estuary. Approximately 500 meters to the east of the border between The Netherlands and Germany it is intersected by the river The Westerwoldse Aa. According to Coumans (2006) there have been salt marshes in the German part of the estuary in the year 1902 but these have all been impoldered and in 2006 there were no salt marshes in the entire German half of the estuary. Dijkema et al. (2007) and Van ‘t Hof (2006) mention a total acreage of 741 ha of salt marshes for the estuary. This indicates that the previously mentioned salt marsh in the Dollard and Punt van Reide are indeed the only salt marshes in the estuary. An examination of the area with Google Maps however, raises the question if this is correct because some land formations show a lot of resemblance with the appearance of the two previously mentioned salt marshes. A vegetation map provided by Van ‘t Hof (2006) shows that there were typical salt marsh vegetation types present in the German part of the Dollard in 1976 (see appendix 2). Esselink et al. (2009) mention 225 ha of salt marsh in the German part of the Dollard and 280 ha of salt marsh in Krommhörn. This however is all the data that could be retrieved during the study for this report and since it is not enough to know where there currently are salt marshes present in the German part of the estuary we will keep to the 741 ha mentioned in the majority of the literature that has been found on the subject. An explanation for this deficit of information has been given by Van Duin (2011) by stating that certain aspects of the salt marshes in the estuary have been monitored more intensively in The Netherlands compared to Germany.

2.3: The different types of salt marshes According to Dijkema et al. (2005) The Dutch salt marshes and schorren (name used for salt marshes in the Delta region (Natuurinformatie, 2011)) can be divided into 6 types: the “North sea type”, “sandy, island salt marshes”, “muddy mainland salt marshes”, “salt salt marshes”, “brackish salt marshes” and “brackish schorren”. The types of salt marshes differ because of the kind of water body they are situated next to. A distinction is made between: transitional waters, sheltered coastal waters and North Sea coastal waters. The types also differ with respect to the sediment (sandy-muddy), the salinity (salt-brackish) and their location (Southerly-Northerly). All the salt marshes in the estuary are categorised as being brackish salt marshes. These brackish kinds of salt marshes have a different composition of the vegetation. Species that have a lower tolerance for salt are able to grow closer to the shore because they have less

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difficulty with brackish than salt water. The term brackish salt marsh also plays a role within zonation. How it is used there will be explained in the next paragraph.

2.4: Zonation and succession As is shown in figure 5, the height of a salt marsh increases proportionally with its distance to the sea. This difference in elevation has the consequence that the frequency in which parts of the salt marsh get flooded with sea water decreases in the same direction. The part nearest to the sea does not only get flooded with more water but more often and for a longer period as well. This results in differences in the composition of for instance sand, silt and salt in the soil of the salt marsh. These different conditions have the effect that there is also a difference in the vegetation based on the ecological preferences of these organisms. Therefore, in general, it is possible to distinguish various zones on a salt marsh. These zones also reflect several stages of succession of the vegetation. Succession is an ecological process that ranges from a stage where only the first settling organisms are present (the pioneer stage) to the climax stage where the last settling organisms dominate the ecosystem.

Figure 5: zonation of a salt marsh (text in this figure has been translated and is therefore no exact copy of the source) (Dijkema et al. 2005) Mud flat zone: Seaward of the salt marsh is where the mud flat zone is located. This zone is no part of the salt marsh but it is very important in relation to sediment deposition and erosion. It is also of importance for defending the coast from incoming sea water because it dampens the wave action due to the impact it has on the depth of the water (Van Duin, 2011). Without a mildly sloping talud on the mud flat the formation of the pioneer zone is impossible (Van Loon-Steensma, 2011). Generally no vegetation is found here. Pioneer zone: According to Dijkema et al. (2001) the lower part of this zone is located at 40 to 20 cm below the mean high water level. In the estuary it is flooded twice a day. Here some species are found that play an important role in the formation of a salt marsh for example: algae, Saltwort (Salicornia procumbens) and Common Glasswort (Salicornia europaea). These plants

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increase the amount of sediment that is being deposited (Rijkswaterstaat, 2011a). Most of the plants in this zone are annual species and therefore there are periods when there is hardly any vegetation present. At these times less sediment is deposited in the pioneer zone causing cliff formation and cliff erosion. This effect gets amplified by sea level rise and land subsidence. This makes the pioneer zone more vulnerable to these changes (Dijkema et al., 2007). The natural process of cliff erosion and deposition makes a salt marsh more dynamic. The low salt marsh: The vegetation of the pioneer zone is capable of collecting enough silt for plants that live longer than 1 year to settle within 5 years. These plants require a height that is a little above the mean high water level. This zone gets flooded between 300 and 150 times per year. Generally 12 different plant species are found here on salt marshes in The Netherlands.

Grazing influences which species are dominant (Rijkswaterstaat, 2011a). When the salt marsh is not grazed and a layer of 15 to 20 cm has been deposited, it can occur that the vegetation will develop into a climax stage. Yet, this increases the risk of erosion after which the cycle will start over (Dijkema et al., 2005). The middle salt marsh: This zone starts from a little more than 30 to 40 cm above the mean high water level. Here the land gets flooded 100 to 70 times every year. Which plants dominate here depends a lot on if the land is being grazed. The high salt marsh: This zone and the next two are not visible in figure 5 but the information here is sufficient to understand where they fit in. The high salt marsh is situated circa 70 cm above the mean high water level and gets submerged 30 to 20 times each year. Only during spring tide or storms this part is flooded

(Rijkswaterstaat, 2011a). The climax zone: The climax zone rarely gets flooded. The vegetation here is generally taller and the species diversity lower (Dijkema et al., 2005). The climax zone is the final stage of succession. According to Esselink, P. (de Ranitz et al., 2009) experiments on grazing have shown that salt marshes where grazing was halted for 20 to 27 years had developed into a climax stage with a lower species diversity. According to Dijkema et al. (2007) grazing can indeed delay or prevent the development into a climax stage. The brackish salt marsh (zone): Locations where rain water accumulates present an opportunity for species that have a lower

salt tolerance (Rijkswaterstaat, 2011a). This creates zones in the vegetation as well which are more clearly distinguishable on salt marshes that are not situated besides transitional waters.

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Figure 6: Acreage and zonation in hectares in the year 1981, 2000 and 2006 (Dijkema et al., 2007) Figure 6 shows how these zones are divided across the salt marshes of the Ems-Dollard region. As can be seen most of the area is taken up by low salt marshes but other zones are present as well. Almost a quarter has reached its climax stage. The figure also shows how this distribution has developed from 1981 to 2006. As can be seen, the acreage of pioneer vegetation has increased. Note also the decline of the total area of salt marches.

2.5: The vegetation There are more or less 25 plant species that survive the conditions of the salt marshes in the

estuary (Kwelderherstel Groningen, 2011b). Some of them are rare (Wadden Sea World Heritage, 2011). Only the ones of which evidence has been found, during the research for this report, that they play an important role in the development of the salt marshes are discussed below. The geographic distribution of the zones in which they can be found is shown in the map called “Most recent vegetation map of salt marshes in the estuary that has been found during the study for this report” in appendix 3 (with an exception for diatoms). According to Kamps (1956) there are only 4 plant species that play an important role in the sedimentation process. The contribution of the others does not even come close in comparison. These 4 species will be discussed first. After that, three species will be discussed that play another important role in the development processes of salt marshes in the Ems-Dollard estuary. Common glasswort (Salicornia europaea): This species only lives 1 year and can reach a height of 25 cm. After it dies in October the seeds that were embedded in the tissue are released. The ligneous part of the plant often stays behind on the salt marsh as a bare bush but since the weather gets rougher during the autumn and winter its capacity to hold the sediment in place then decreases. Common glasswort is not a fast growing plant either and its direct value for land reclamation is often overestimated. A denser vegetation is more effective in capturing silt especially when it covers the ground. The large influence this species has on sediment deposition is its contribution to the settlement of common saltmarsh-grass (Puccinellia maritima). This grass is for a large part dependent on the presence of Common glasswort because common saltmarsh-grass multiplies mainly through resettlement of viable plant parts that have been cut free from the rest of the

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plant. These plant parts often become entangled with the dead or living glasswort bushes when they are washed ashore and then get the chance to form roots that enable the plant to hold on to the ground. Common saltmarsh-grass can only survive winters at locations that are 20 cm above the minimum height required by common glasswort so not all these plants get the chance to fully develop. Artificially planting common glassworth is very well possible (Kamps, 1956) but how this is done exactly will not be discussed in this report because no indications have been found during the research that nature is not capable of supplying enough seed to do this but if one would like more information about this topic then it is recommended to follow the source in this subsection where it is described in great detail. Common saltmarsh-grass (Puccinellia maritima): This species can cover large areas in a relatively short amount of time and once it has settled on its preferred height it is unlikely to disappear again. (Kamps, 1956). In the estuary it is able to live there where the vegetation is flooded twice a day which means it lives in the pioneer zone (Vreeken-Buijs, 2002). It can only survive winters at locations that are 20 cm above the minimum height required by common glasswort. Cattle likes to graze on it which contributes to its dispersal. Flowering plants are not often found (Kamps, 1956). It grows in sunny spots on wet, nutritious and salty soils, mostly on mud but on sand as well. It can reach a height of 70 cm (Dijkstra, 2011). Measurements have shown that this species is a lot more effective in trapping sediment then common glasswort. Experiments with artificially increasing the population by spreading plant parts and pressing them into the ground did not lead to clear results (Kamps, 1956). Information on more recent experiments has not been found during the research for this report. Common cordgrass (Spartina townsendii): This species was imported from England in 1924 and planted on salt marshes in the province of Zeeland two years later. In 1927 it was planted in Germany and 4 years after that in Denmark. From these locations it has spread further through the Wadden region (K. Dijkstra, 2011). It has been planted extensively to catch silt but its rapid growing rate poses a threat to

other species. Therefore it is also considered a pest (Ecomare, 2011a). In the estuary it can be found on the low salt marsh where grazing is moderate and the soil is silty. It has also been found there where the vegetation is flooded twice a day which means it lives in the pioneer zone (Vreeken-Buijs, 2002). It is able to become 1.4 meters high (Dijkstra, 2011). Cows and sheep are unable to digest this plant (schorrenwerkgroep, 2011). Experiments to artificially increase the population in Groningen have been successful (Kamps, 1956). Creeping red fescue (Festuca rubra): This type of grass lives in the high salt marsh zone. It does not survive a regular flood of salt water like common saltmarsh-grass. Therefore it plays a less important role in the sediment deposition. Saltwort (Salicornia procumbens): This species only lives 1 year. It needs sunny, open areas on a wet, nutritious and silty soil either sandy or muddy. Its maximum length is 40 cm. It is mainly located beneath the mean high water level (Dijkstra, 2011). In the estuary it is able to live there where the vegetation is flooded twice a day which means it lives in the pioneer zone (Vreeken-Buijs, 2002). Saltwort enables the first creeks to be formed on the salt marsh (K. Dijkema et al., 2007) (this will be discussed in detail in subparagraph 3.2.1).

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Common reed (Phragmitis australis): Because the salt marshes in the Ems-Dollard estuary are brackish, the vegetation differs from the vegetation on most of the other salt marshes in the Wadden Sea that border the main land. Especially common reed is a plant that is remarkably more dominant on these brackish salt marshes (Van ’t Hof, 2006). According to Bakker et al. (1997) common reed is capable of overgrowing most of the salt marshes in the Dollard and thus leading to a climax vegetation with very little species diversity. Diatoms: These small plants live in the mud flat zone in a capsule of mucus which probably enables them to continuously rise above the deposited silt and catch sun light. The mucus forms a protective lens over the silt which therefore does not erode during calm weather. The sheets of mucus can not withstand turmoil water movements (Kamps, 1956). Of course other plant species slow down the water and hold down the soil as well and therefore contribute to the sedimentation rate and prevention of erosion. According to Dijkema et al. (2001) the sedimentation rate in the low salt marsh zone can increase to 1 or a few cm/year thanks to the vegetation. Without it there would be less sediment deposition due to the lower number of floods.

2.6: The Referential acreage Small salt marshes are more vulnerable. A certain size is needed to protect the biodiversity and rejuvenation by cyclic changes. Therefore, as part of the Water Framework Directive, scientists have estimated which size the acreage of salt marshes in the estuary should have to ensure a good ecological condition. They did so by investigating what size they had in a time when there was an equilibrium between impoldering and new development of salt marshes. This figure is called the potential reference (P-REF). The original idea was to estimate a referential acreage (REF) based on what the size had been before human intervention but this could not be reconstructed. The minimum acreage that is necessary to ensure a good ecological condition is called (GET). GET however would be based on REF and since this was not possible GET was based on P-REF and called P-GET. According to the research, the minimum acreage of salt marsh in the Ems-Dollard estuary that is necessary to ensure a good ecological condition is 700 ha (Dijkema et al., 2005). In figure 7 is shown which size is required for the salt marshes in the estuary to fall into the different categories. Since the acreage is currently 741 ha the conclusion is that it is in a good ecological condition (Dijkema et al., 2005). P-REF P-GET Mediocre

(<25% below P-GET)

Insufficient (25-50% below P-

GET)

Poor (>50% below

P-GET)

Ems-Dollard estuary

1000 700 700-525 525-350 <350

Figure 7: required size for the total acreage of salt marshes in ha to determine their ecological condition (Dijkema et al., 2005). The total acreage of the Salt marshes in the Ems-Dollard estuary is declining, as shown in figure 6. As mentioned in paragraph 2.2 this erosion takes away approximately 2 ha/year at a

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certain location. The total acreage of salt marshes in the estuary has 41 hectares to lose before it falls into a mediocre condition which means that the biodiversity and rejuvenation by cyclic changes are threatened.

2.7: Conclusion: There are too many uncertainties about the German salt marshes in the Ems-Dollard estuary to completely incorporate them into this report. The 741 ha of salt marsh that is known to be present in the Ems-Dollard estuary is located in The Netherlands. This acreage is eroding and is headed towards becoming too small to ensure a good ecological condition. Growing towards a climax stage poses a threat for the plant diversity. Smaller acreages of salt marsh are more at risk of a decline in species diversity and are less capable to rejuvenate. Allowing cattle to graze on salt marshes is a method used to rejuvenate salt marshes and counteract on the disappearance of species. The known location of the Dutch salt marshes in the estuary gives a better idea of where new salt marshes are desirable. The suitable locations will be pointed out chapter 5. Many characteristics of salt marshes that are important to understand salt marsh development, such as zonation, different types of salt marshes and vegetation have been described in this chapter. By doing so, a part of the influences of nature on the development of salt marshes has been explained because the vegetation plays an important role in the development of salt marshes. It for instance influences the soil composition, where more sediment is being deposited and where the salt marsh is better protected against erosion. The other natural influences will be discussed in the following chapter, mainly in paragraph 3.1.

Chapter 3: Known methods of developing salt marshes From chapter 1 and 2 we gained an understanding of the cause and scale of the problem (the increased risk of flooding of the land for the given reasons) and how well salt marshes are able to cope with it. We also got familiar with the area for which we want to know if developing salt marshes as a coastal defense system can be (part of) the solution. Before we can answer that question however, we first need to understand how salt marshes can be developed because this gives us understanding of how these tools can be used to deal with the problem. In this chapter a description of how salt marshes are created by nature is given first. Then the same will be done for how salt marshes have been developed by men. After that we will focus on which adjustments have been made to use salt marshes as coastal defense systems and finally we will look at which other alterations might help our cause. An extensive report on natural and artificial development of salt marshes, written by Dr. L.F. Kamps, was published in 1956. Since almost all the information we need for understanding how salt marshes are created by nature and have been created by men till that time is given in his report, a lot of the information in the first two paragraphs came from this source.

3.1: Natural development At high water levels salt marshes become submerged with sea water. Sand and silt particles which are suspended in the sea water have the opportunity to sink between the vegetation where it is less probable that they will be washed back to sea. Due to this accumulation, salt marshes steadily become higher (Natuurinformatie, 2011). So much we already knew but where the silt is coming from and where it will be deposited is important to know as well for

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when one wants to understand where salt marshes can be created in the Ems-Dollard estuary. Therefore this is what we will look at in the following subparagraphs.

3.1.1 The transportation of sediment Sediment is an umbrella term for solid material that is or has been transported by water, air, ice or gravity to the ground. It is deposited when the weight of the sediment is greater then the carrying capacity of the force that moves it (USDA, 1996). The word silt is generally being used in different ways. Some people use it as a name for all the material that is being deposited, others think of it as particles of a certain size and composition (Kamps, 1956). In this report we will use the following definition given by Kamps (1956): “Silt exists out of small particles that have clustered into groups of the same or different types of material which behave as separate units during transport in the water.” This means that silt is a type of sediment. Only units smaller than 63 micrometers are regarded as silt (WaddenZee.nl). It is this silt that makes salt marshes exceptionally fertile (Waddenvereniging, 2007). The silt forms salt marshes together with other material that is deposited on the coast (Kamps, 1956). Understanding the sediments composition gives a better idea of where it is deposited. The material that is deposited can be divided into four groups from which particles can cluster to form silt or which can move through the water as separate units (Kamps, 1956). These 4 groups are: Sand: Most of the mass of what is being deposited along the coast of Groningen exists out of sand. Sand often gets transported through the water as separate units, especially the larger grains (Kamps, 1956). Sand is much less fertile then silt (USDA, 1996) which explains why less vegetation is found on beaches then on salt marshes. Clay minerals: These are deposited in a much smaller quantity compared to sand but they are of great importance to the salt marsh because they fertilize the land. If these particles would not cluster but be transported as separate units then the amount of clay would not be deposited in such quantities along the coast of Groningen that it would make land reclamation worth while (Kamps, 1956) meaning that the land would not be fertile enough for agricultural purposes. It is the formation into groups that makes these particles sink faster then separate clay minerals. Often these particles also form groups with other materials (Kamps, 1956). Calcium carbonate (CaCO3):

The size of this material ranges from entire sea shells to less than a micrometer. Along the coast of Groningen 5 to 25% of the dry matter exists out of these particles. As happens with sand, larger particles are often transported separately while the smaller ones group with other small particles of sediment (Kamps, 1956). Organic material: The organic material probably varies most in form. Examples are: proteins, carbohydrates, silicic acid skeletons, Common cordgrass and entire Saltwort bushes. Another important material is slime which is excreted by many different organisms and which plays a role in the sedimentation by binding particles. Under certain conditions silt also contains many living organisms. These too are sometimes capable of binding the material, once it is deposited.

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The larger parts of organic matter are transported separately and the smaller ones group with other materials to form larger units as well (Kamps, 1956). In general it can be said that larger particles are transported separately and smaller ones are transported in clusters. However, there is no clear division between these larger and smaller particles. Particles measuring 50 micrometres can still regularly be found in complexes but are also transported as separate units. Even smaller particles, measuring 20 micrometers, are frequently transported separately. Below 20 micrometres however, in comparison, only a few particles are transported as separate units (Kamps, 1956). The previous information gives a good idea of what the sediment exists out of. In the following text we will focus on where the sediment (with an emphasis on silt due to its importance for the salt marshes) is coming from and how it is transported to the Ems-Dollard estuary. During the Holocene (past 10.000 years, roughly) circa 64 billion cubic metres of sand and 19 billion cubic metres of silt has been deposited in the Dutch part of the Wadden Sea. Approximately 20% of this silt is currently located outside of the sea dikes. This has created a layer of about 4.5 meters of sand and 1.5 meters of silt, averaged over the sea floor. Much of this silt is mixed with the sand and unevenly distributed. In general, the amount of silt increases proportionally to the distance from a passage to the sea or channel. At the moment, a large quantity of silt is located in the Ems-Dollard estuary and other, more sheltered, areas. For instance along the coastal zone or on the end of a channel system, in salt marshes, former channels and mussel banks. This deposited silt frequently resuspends but these quantities are minute compared to what is stored in the sea floor (Waddenacademie, 2009). The estimated gross amount of silt that is internally transported in the Wadden Sea is more or less the same as the amount that is carried in and out through the sea passages (108 to 109

m3/year). The concentration of silt that is suspended in the water varies greatly, even from one year to the next due to changing current velocities, tidal changes, the amount of silt in the sediment, the wave climate and the balance between bio-builders and bio-breakers (more information about them is presented further on in this subparagraph). It ranges from circa 25 to 150 mg/l with an average of 80 mg/l. This translates to an average of approximately 1 to 2 million kilos of material that is suspended in the Wadden Sea. On the seafloor of the Wadden Sea, this amount would create a layer of 0.5 to 1 mm thick (Waddenacademie, 2009). In this report we focus primarily on the part of the estuary south of the line between the Ems Harbour and Krommhörn. As can be seen in figure 2, the estuary extends further to the north. In this northern part 32 x 106 tonne/year of silt flows through the sea passage between Borkum and Rottumeroog. The total silt flux of the estuary is at least 130 x 106 tonne/year (Waddenacademie, 2009). These figures indicate that most of the silt that is suspended in the water of the Ems-Dollard estuary (northern part included) originates from the Wadden Sea and the estuary (northern part included) itself because only 1.5 x 106 tonne/year (circa 1% of the total internal flux) is estimated to be (net) imported from the Ems (Sips et al., 2009) and Westerwoldse Aa (Waddenacademie, 2009). These figures also show that the amount of silt that goes into the estuary (northern part excluded) mainly comes from the Wadden Sea instead of being carried in by the Ems and Westerwoldse Aa. Mulder (2011) emphasizes this by estimating that 90 to 99% of the silt that is transported to the estuary (northern part excluded) originates from the Wadden Sea (and partially the North Sea). The North Sea contributes a relatively small amount of silt to the Wadden Sea’s sedimentation process. The majority of the suspended silt has been stirred up in the Wadden Sea itself (Waddenacademie, 2009). This, in comparison, small contribution from the North Sea might be explained by the measured value for the concentration of depositable silt

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particles. Measurements indicated that a relatively high number of particles is too small to sink fast enough (Kamps, 1956). We have now discussed where the sediment is coming from and will focus more on how it is being transported in the following text. According to the Wadden Academy (2009) measurements in the field led to believe that the concentration of suspended sediment in the estuary is mainly dominated by the wind. The concentration differences are largely in accordance with variations in wind velocity and the resulting wave action (Waddenacademie, 2009). Figures 8 and 9 show how the wind driven, internal silt transport takes place. These pictures illustrate that the silt in the Wadden Sea does not simply come from óne source and subsequently flows to its final destination. Instead, it is a dynamic process in which particles from various locations are taken up into the water column and are deposited in places from where they might as well be picked up later to enter the water column again and get carried back to where they came from or to wherever else the currents take them.

Figure 8: silt transport from a channel to an Figure 9: silt transport during low tide intertidal flat during high tide (Waddenacademie, 2009). (Waddenacademie, 2009). During tides or when waves are created by the wind, silt is being suspended when the current velocity exceeds the sea bed’s shear strength. The resistance of the sea floor against erosion of silt is partially affected by organisms living in or on the soil which have a (de)stabilizing influence. Uninterrupted surfaces containing animals or plants are able to trap more sediment for instance. Examples of such bio-engineers are: mussel- and oyster banks, banks of sand mason worms (Lanice conchilega), salt marsh vegetation, cyan bacteria and diatoms capable of creating thin films and seagrass fields (Waddenacademie, 2009). Among the animals a group of molluscs called the lamellibranchiata have the largest influence on the silt deposition. These animals are equipped with gills for their respiration but they also filter food out of the water with them. Indigestible sand and silt particles are covered in slime and extruded (pseudo faeces). The smaller ones however will go through the intestinal tract from where they are discharged together with other indigested particles in the form of a solid, clingy mass (real feaces). Because the particles are attached they are able to sink faster. They are also able to stay together for long enough to be transported over long distances until they are deposited (Kamps, 1956). In subparagraph 3.4.2 we will discuss these organisms further. Besides bio-builders there is also another group of organisms called bio-breakers. These have a destabilizing effect on the seabed. Examples are: shrimps (for instance Corophium volutator), cockles and lugworms (Arenicola marina). These kinds of organisms can diminish populations of bio-builders which causes silt trapped by them to become suspended (Waddenacademie, 2009).

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Throughout history there have been many human activities that influenced the sedimentation processes in the Wadden Sea as well. Some of them have also been carried out in the estuary. Examples are: the construction of the Afsluitdijk, reducing the size of the Wadden Sea due to land reclamation, dredging in harbours and channels, bottom trawling fishery, sand suppletion, shipping and recreation (Rijkswaterstaat, 2010). In chapter 4 we will focus more on reducing the size of the Wadden Sea, dredging and sand suppletion and the influence it has on the sedimentation patterns in the estuary. However, how much sediment this adds to the water column is difficult to say. Knowledge about silt quantities in he Wadden Sea is merely based on rough estimations of what is present in the sea bottom, the water column and the amount of silt that flows through a certain area per second (flux). These estimations all have a large margin of uncertainty. Recent measurements of net fluxes in the Marsdiep have shown that 10 times more silt is coming in from the North Sea then what was generally expected (Rijkswaterstaat, 2010). The numbers previously mentioned in this paragraph about sediment import from the North Sea came from a report that was published 1 year earlier. This indicates that there is indeed a lot of uncertainty about the sedimentation processes in the Wadden Sea. The aim of this paragraph is to describe how silt is transported to the salt marshes so therefore the debate about how much of the silt is carried in from the North Sea, how much of it is depositable and how much is ending up in the estuary has not been studied any further in the conviction that the collected data provides for a sufficient understanding of how this system behaves. Now that we know where the silt is coming from and how it is transported (the most significant human influences will be discussed in chapter 4) we will discuss the deposition of these particles in the next subparagraph.

3.1.2 The deposition of silt When the silt has become suspended it gets carried away by the water currents. In places where the current velocities are low and where there is not much wave action the silt has a chance to sink and will stay (at least temporary) on the bottom. Such places are between marine vegetation, mussel banks and salt marsh vegetation for instance but also in places where two tidal currents meet or human structures such as harbors (Kamps, 1956). Depositions of silt in other locations then salt marshes can become beneficiary for the growth of nearby salt marshes. An example of this is excreted silt from molluscs as was mentioned in the previous subparagraph. We will discuss that in more detail in subparagraph 3.4.2. For now we will focus on the salt marshes’ ability of capturing sediment. The salt marsh vegetation and the role of individual plant species in the sedimentation process has already been discussed in chapter 2. In general, denser vegetation is more effective in capturing silt especially when it covers the ground. This is partly a result of the vegetation slowing down the water currents but also because the roots stabilize the soil (Kamps, 1956). In the pioneer zone this effect is the smallest because the vegetation here is not dense at all but the higher we move up the salt marsh, the denser the vegetation gets. However, because the vegetation here gets flooded less often, the deposition decreases according to the height of the salt marsh (Dijkema, 2007). For the depositable particles Kamps (1956) mentions a minimum size of 16 micrometers. According to Van der Lee (2000) the velocity with which silt sinks depends on both the size and the density of these units. Therefore we can conclude that the size of depositable units is variable and that the sinking velocity is dependent on the water movement, size and density of the silt. Therefore it is not

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surprising that the salt marsh vegetation stimulates the deposition of smaller sediment particles because it makes the water less turbulent and decelerates the waves (Sips, 2009). In general it is said that pebbles of 1 cm in diameter erode when the current speed exceeds 150 cm/s. They are transported by currents that exceed 90 cm/s and are deposited when the current speed is less than 90 cm/s. Fine sand particles of 100 micrometers in diameter erode when the current speed exceeds 30 cm/s and are deposited when the current speed is below 15 cm/s. The smaller silt and clay particles are deposited at lower current velocities (Packham et al., 1997). An exact value for the current speeds that lead to erosion or deposition of these particles has not been retrieved during the study for this report. The values given in this subsection are only meant to give an indication of which current velocities lead to accretion or erosion of a salt marsh. As we have learned in the previous subparagraph, the resistance of the sea floor against erosion of silt is partially affected by organisms living in or on the soil which have a (de)stabilizing influence. The current velocities in the estuary are shown in appendix 3, in the set of tidal maps which covers a time distance of one low sea water level to the next in 1976 and on the map called “Current velocities” that probably shows the mean current velocity of a longer, however unknown time period. Appendix 3 also contains a map on which the wave velocities in the estuary are shown. For this map no evidence was found either, during the study for this report, about the time frame it represents. Clearly these maps are not ideal but, in spite of numerous attempts, no better ones have been retrieved during the research for this report. Looking at how the velocities are divided over the several maps shows where the circumstances are more and less favorable for salt marsh development. In a natural salt marsh the vegetation has a big influence on the formation of creeks in the soil. In spots that are covered with plants more sediment is being deposited while in open areas the flow of water is more concentrated which results in less sedimentation and even erosion (Dijkema, 2007). Creeks improve the drainage of the salt marshes. This gives the deposited sediment the chance to dry faster which makes it less vulnerable to future water movements (Kamps, 1956). Some scientists have mentioned that the larger creeks originate from before the pioneer stage and were formed in the mud flat zone (Dijkema, 2007). Figure 10 illustrates this. Not only water but also nutrients and sediments are brought into the salt marsh through these creeks. Therefore the formation of a salt marsh is not exclusively determined by the sediment transport from the sea but also by the transport through a creek system. When the acreage of a salt marsh increases, the main creek becomes elongated and new creeks and more branches will appear. These new creeks are formed mostly due to erosion when the water retreats. The sides of the creeks are vulnerable to erosion as well because while the creek deepens, the capability of the plants roots to hold the sediment in place decreases. This causes the sides to collapse and gives creeks their meandering shape. In inner corners relatively more sediment will be deposited while it erodes in the outer corners. In the pioneer zone creeks are usually wide and shallow. Where the salt marsh is more developed, especially the smaller creeks have become deeper and smaller (Dijkema, 2007). In figure 10 these different stages are shown.

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Figure 10: Formation of creeks in a salt marsh (Dijkema, 2007) altered by removing some information that is not relevant for this report. The salt pans shown in figure 10 create a more saline environment which leads to a difference in distribution of the vegetation based on the salt tolerance of the species. The figure also shows the development towards a climax stage. Now that the natural formation of salt marshes due to the deposition of silt has been discussed, the artificial ways to create a salt marsh will be explained in the following 3 paragraphs. This is done in a chronological order in the sense that the first paragraph discusses the traditional methods, the second one the currently practiced methods to add to the protection of the coast against flooding and the final one discusses the current methods that are not meant for coastal defence and more innovative possibilities for creating salt marshes in the Ems-Dollard estuary in the future.

3.2: The Sleeswijk-Holstein system and its variations Kamps stated in his report of 1956 that before the Sleeswijk-Holstein system was introduced there has been a time in Groningen when all the salt marshes were formed by nature and that humans only constructed dikes around the areas that became high enough. Later, they build the dikes further away from the sea because this had the advantage that the lower part of the dike did not need to be protected with stones. Instead, covering it with clay and sods was sufficient. The steadily increasing height of the terrain seaward of the dike and the part of the salt marsh that contained plants, located in between, protected it against storms (Kamps, 1956). Obviously back then the people had already noticed the advantages of placing salt marshes in front of a dike. After the dike was finished they started digging trenches seaward of the dike. This was done to stimulate the drainage and sedimentation and consequently speeding up the process of the development of salt marshes.

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In 1935 the Dutch government started carrying out land reclamation activities based on the German Sleeswijk-Holstein method. With this method dams made from clay and brushwood groynes were used (Kamps, 1956). Brushwood groynes exist out of a double row of poles that are driven into the ground with birch or willow twigs placed in between. In figure 11 an example is shown which is almost entirely covered in sediment. These dams were made to calm down the water and give sand and silt a better chance to sink. With these dams, fields of 400 meters long and 400 meters wide were created. When enough sediment had accumulated to expect the settlement of plants it became time to dig trenches and create another deposition field further towards the sea. In Sleeswijk-Holstein this newly formed land was for the greater part perceived as a coastal defence while in Groningen it was solely created to obtain more land (Kamps, 1956).

Figure 11: Brushwood groyne (Ministerie van onderwijs et al., 2011) There was a difference however between the Sleeswijk-Holstein method and the way it was used in Groningen. In Groningen a second sedimentation field, located further towards the sea, was created at the same time. In both of these fields channels were dug out immediately. These channels were dug out wider to collect more sediment. Shifting the land reclamation activities further towards the sea also had its disadvantages. The quality of the soil decreased, the risk of erosion increased and there was not as much time to carry out the activities (Kamps, 1956). During the study for this report, the most detailed information that had been found was about how the salt marshes in the Dutch part of the Ems-Dollard estuary were formed. However, it is known that the majority of salt marshes that border the mainland of Lower Saxony originate from salt marsh works (Esselink et al., 2009). The salt marsh at The Punt van Reide was not created by land reclamation and is from a geomorphologic point of view the only natural salt marsh of a substantial size that borders the mainland in The Netherlands (Arcadis, 2006). However, this salt marsh was not developed by nature but, as described in paragraph 3.1, it was created when storm surges eroded the surrounding land in the 17th century. The Punt van Reide was better protected, by an old dam. The salt marsh in The Dollard was created by using clay dams and sods (Arcadis, 2006). This proves that letting nature create salt marshes (in the Dutch part of) the Ems Dollard estuary under the current circumstances is not an option because no natural development of salt marshes has taken place since the present ones were artificially created.

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3.3: How salt marshes are currently designed to function as coastal defence systems In the German Wadden Sea, salt marshes are considered part of the coastal defence system (Dijkema et al., 2005). In The Netherlands this function to reduce wave energy has not yet been adopted in the examination of high water safety (Baptist, 2011). Till which extent a salt marsh and the bordering mud flat add to the protection against flooding is currently being researched by the Dutch government (Atsma, 2011). According to Esselink a width of 100 meters would be sufficient to protect the coast in the estuary against flooding (de Ranitz et al., 2009) but no information has been found during the study for this report that explains if a 100 meters wide salt marsh is able to protect the coast sufficiently on its own or if a combination with a dike will still be required. German nature-conservation agencies protect the salt marshes of Lower Saxony from erosion to provide for a better coastal defense. Land claim and former coastal protection works have narrowed the salt marshes located in front of the seawall but nowadays the present extent of the salt marshes is preserved as much as possible. This is done by trying to restore development under natural conditions and also by constructing sedimentation fields to create a buffer against wave energy to protect the seawall. This is done (for instance) on locations where the foreland is exceptionally small or non existent. The intention is to stimulate sedimentation until a dense vegetation has developed. Former creeks are restored to improve drainage (however this is kept to a minimum to increase the naturalness) and summer banks have been reopened to increase the acreage. In some areas strengthening of the seawall is necessary to protect the land against flooding (Esselink et al., 2009).

3.4: Other possibilities In this paragraph a collection of other techniques to develop salt marshes is discussed. The purpose for this is to complete the set of options for salt marsh development which makes it possible to point out the preferable combination for the suitable locations mentioned in chapter 5. Ecological engineering is described in a separate subparagraph because of its unique approach.

3.4.1 Other ways to develop salt marshes In addition to what has been said in earlier paragraphs there are some examples of modern techniques that have not been developed primarily to defend the coast but might also be useful for this cause. A few theories about how to create salt marshes will be discussed in this paragraph as well. No conclusions on which methods are preferable to use for salt marsh development in the estuary are given here. That part of the analysis will be presented in chapter 5. An easy but not very realistic way to create salt marshes would be to scoop a large amount of sediment onto the mudflat zone and spread it out. According to Dijkema et al, (2005) this however would not work because various biological, geomorphological and physical processes that stabilize the sediment, create an ecosystem and give a salt marsh its characteristic shape would not be executed. It would also conflict with the way salt marshes are managed nowadays. Everywhere in the Wadden Sea the maintenance of channels, creeks and clay dams has stopped to make the salt marshes more “natural” (WOK, 2010). Intertidal

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mud flats are also internationally valued areas that are protected by the habitat directive (Ministerie van V&W, 2011). This method has been practiced in Texel and in a few other areas and the results have led to the same conclusions (Dijkema et al., 2001). Land reclamation by using sedimentation fields has led to further enhancements of the Sleeswijk-Holstein system. More durable wood for the brushwood groynes has been found, the most efficient size of the fields is now believed to be 200 x 200 meters (WOK, 2010) (constructing additional groynes will reduce the fetch and increase the sedimentation rate (Esselink, 2000)) and the construction height of the brushwood groynes has been elevated to 30 cm above the mean high water level for example (Dijkema et al., 2001). In order to enhance the opportunity for the natural formation of creeks the creation of ditches in salt marshes is nowadays minimalised (Van Loon-Steensma, 2011). If this is also advised for the reclamation of land by using sedimentation fields has not been discovered during the study for this report. The general approach for this type of salt marsh development is still the same however. According to Esselink the habitat directive prohibits any further seaward reclamation of land in the Dollard (Ranitz et al., 2006). The Common Wadden Sea Secretariat however states that it might be the best way to protect the seawall in the future (Esselink et al., 2009). Something else to keep in mind is that the creation of a salt marsh in this way will replace another type nature, the mudflat (Dijkema et al., 2005). Building a structure in the sea, for instance a dam, to calm the water can lead to higher sediment depositions as well. This would go at the expense of the mud flat in return for a more artificial terrain than when one would use sedimentation fields to claim land. No reasons to presume that sedimentation fields would not be effective enough to claim more land have been found during the study for this report. According to Van Loon-Steensma (2011) it has been proven throughout history that the placement of hard objects, for instance dams, often not only leads to the desired formation of salt marshes but also to unforeseen and undesirable effects such as erosion in unpredicted areas. This is why Van Loon-Steensma recommends using various models for different circumstances to gain insight into the possible effects of the creation of such a structure in the concerned area. Dams also have the disadvantage that they can not be relocated as easily, when necessary, as is possible with brushwood groynes. In many locations in the estuary the salt marshes are protected from erosion by groynes. They also stimulate the deposition of sediments. However, the prevention against erosion by groynes does make salt marshes less dynamic (Esselink et al., 2009). The purpose of this technique is to prevent erosion and not to expand salt marshes. In The Netherlands and Lower Saxony plans to de-embank 15 impoldered salt marshes have been implemented since 1973. The goal is to reintroduce the tidal influences by removing banks from the marsh bed, and (partially) removing sluices, culverts or dams. The success rate varies. Where there is an unrestricted tidal access and a moderate grazing regime the chances for success are optimized (Esselink et al., 2009). The rate at which the salt marsh develops and the quality of the newly formed salt marsh have been promising. The de-embankment of these polders is a legal objective as well (Dijkema et al., 2005). No evidence has been found during the study for this report that de-embanking these polders leads to more protection of the coast against flooding. Another option to create a salt marsh is to relocate the sea dike further inland and (partially) excavate the foreland. This would probably lead to some similar problems as caused by scooping sediment onto the mudflat. However this is all very speculative because during the

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study for this report no evidence has been found that this method has been put into practice yet. The report of Dijkema et al. (2005) suggests it has not been implemented before 2005 but that it might be the only way to achieve or maintain P-GET in the western part of the Wadden Sea. This method is not expected to work when there is a low sediment import. It would probably be an expensive method and support for this idea among the population is scant (Dijkema et al., 2005). According to Dijkema et al. (2001) this method has been proposed as a way to adapt to the accelerated rise of the sea level by the WWF, Rijkswaterstaat and in the design for the 3rd PKB-Waddenzee (environmental plan of the Dutch government). He also states that artificial protection against erosion has to be incorporated in such a design because the natural formation of salt marshes along the main land will not take place unless the coast is irregular enough to provide for sheltered areas. Creating salt marsh vegetations inland of the sea dike and building a new dike further inland as well is a method that is often perceived as an alternative for relocating the dike. To do this, sea water is allowed to flow inland through pipes for instance. The halophytic vegetations that arise in this way are no alternative for salt marshes however because the complex geomorphologic, physical and biological processes that are an essential part of a salt marsh, do not occur this way (Dijkema et al., 2005).

3.4.2: Building with nature An especially nature-friendly way to expand the acreage of salt marshes in the estuary could be to let nature create them for us. As seen in subparagraph 3.1.1 molluscs contribute to the total amount of suspended, depositable silt. Around 1956 it was estimated that the Groninger part of the Wadden Sea contained 7.5 x 109 mussels which would be able to produce 1.3125 x

106 tonne of depositable silt annually (Kamps, 1956). This is almost as much as the amount of silt that is estimated to be (net) imported from the coastal water into the Ems estuary. Mussel banks also reduce wave action as was mentioned in subparagraph 3.2.1. Mussels are not the only molluscs capable of excreting depositable silt. Cockles (Cerastoderma edule) and sand gapers (Mya arenaria) behave in the same way. These species were however much less common in Groningen back then (Kamps, 1956). No exact numbers were found for the amount of molluscs in the Ems-Dollard estuary. In the Wadden Sea the amount of mussels and cockles has been decreasing for decades however. This is partly due to the shell fish fishery, which is now forbidden. The climate change also has a negative effect on several mollusc species. Because the winters became less fierce, predation on molluscs starts sooner. The juvenile molluscs therefore have less time to reach a size that prevents them from being eaten (Waddenvereniging 2009). The amount of silt excreted by the mussels in the Groninger part of the Wadden Sea was estimated to be close to 1% of the total amount of suspended silt in the estuary in 1956. Therefore it seems unlikely that the molluscs which are present in the estuary nowadays or in the near future will be capable of producing a significant quantity of silt either. The number of Japanese oysters (Crassostrea gigas) in the Wadden Sea is increasing fast (Deltamagazine, 2011). In theory this offers an opportunity to stimulate salt marsh growth by enlarging the population in the estuary. This exotic species however has such a large impact on the ecosystem, for instance by suppressing other mollusc species (Deltamagazine, 2011), that increasing their number to build salt marshes may not be considered a nature-friendly solution. Besides their damaging effect to the ecosystem they are able to form a reef-like environment which is beneficial to the surrounding nature as well however. At this moment experiments are being carried out in the Oosterschelde to see if creating oysterbanks is a

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successful way to reduce wave action and trap enough sediment to prevent the erosion of sandbanks. These experiments take place in two locations where baskets of 200 meters long and 10 meters wide are placed near the mean low water line and filled with oyster shells. The idea is that this will provide a stable basis for oyster larvae to settle and build a reef. An earlier study on a smaller scale showed that the oyster shells indeed provided a suitable environment for the oyster larvae (Van Loon-Steensma, 2011). Instead of letting the currents carry the silt towards the salt marsh it might be a better idea to place the mussels in sedimentation fields. Such an experiment has taken place. The animals were placed in rows of 4 meters wide, between small dams and with 10 meters of room between each row. The experiment showed that in this way “more then enough” silt was being deposited. 80 mussels are capable of producing 4.2 kg of depositable silt (dry matter) in an environment with a lot of suspended material. The problem was however that, although they stayed in place during the summer, the mussels and the silt they had produced were washed away during autumn storms. If it would be possible to keep the mussels and enough of the silt in place for 1 or 2 years then it would also be possible to gain land at a fast pace in parts where the mudflats’ height is equal to or lower then NAP (Kamps, 1956). On the map called “Depth” in appendix 3, the range of depth in the estuary is shown. During the study for this report it has not become clear if these are the heights in comparison with NAP. It also has not been discovered when this map has been created. Unfortunately no map that is more useful has been found during the study for this report. The same uncertainty about the date is true for the maps in appendix 3 that show where mussel and oyster banks are present in the estuary. Another bio engineer is seagrass (Zostera spec.). Fields of seagrass slow down the water currents which leads to a larger quantity of deposited silt. Seagrass is rare in The Netherlands

and nowadays there is only 150 ha left (Rijkswaterstaat, 2011b). It is not exactly known what caused seagrass to disappear from the Wadden Sea. It may be caused by an increase of suspended sediment but that is speculative (Rijkswaterstaat, 2010). One of the few places where one can find a field of seagrass is in the Ems-Dollard estuary close to Delfzijl

(Rijkswaterstaat, 2011b). Projects to maintain and reintroduce seagrass have been going on for years but clearly did not lead to a rapidly increasing population. The final possibility we will discuss is increasing the deposition by stimulating plant growth on the salt marsh. As was mentioned in paragraph 2.5, common cordgrass is very effective in capturing sediment but since it is an exotic plague species it probably would not be nature-friendly to increase their population. Therefore common saltmarsh-grass could be a better choice. It depends on the number of present plants if it is sensible to enlarge their population but as mentioned in paragraph 2.5, no indication has been found that nature is not capable of supplying enough halophytes to populate the salt marshes. One could however use cattle for instance to increase the population of a certain species (Natuurkennis, 2010). However, no evidence has been found that salt marsh vegetation does more then heightening the salt marsh and preventing erosion instead of expanding the acreage.

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3.5 Conclusion: The sediment that forms the salt marshes exists out of sand, clay minerals, calcium carbonate and organic material. Only grouped particles smaller than 63 micrometers, are regarded as silt. These are the most important parts for salt marsh development because they make them exceptionally fertile. The silt is exported to the salt marshes in the Ems-Dollard estuary by (predominantly wind driven) currents, mainly from the sea floor of the Wadden Sea but also from the North Sea and for a small part from the bordering rivers. This transportation does not simply begin in one place until it reaches the salt marshes. Instead, it is a dynamic process in which particles from various locations are taken up over and over until some reach the salt marshes or are carried away from them through erosion (the most significant human influences on this transportation process will be discussed in the next chapter). Where sediment is deposited and where it erodes depends on the current velocity. Calm waters are favourable for the development of salt marshes and therefore more sheltered areas and salt marsh vegetation are beneficial to the accretion of salt marshes. Together with the geological influences discussed in chapter 1 and the biological influences discussed in chapter 2, these biological and physical influences complete the description of the main natural influences on the development of the salt marshes in the estuary and how nature creates salt marshes in the estuary. Apart from the human influences that will be discussed in the next chapter, the artificial methods with which salt marshes have been created in The Netherlands and Germany have been discussed in this chapter as well. No evidence has been found during the study for this report that salt marshes in The Netherlands or Germany are currently designed or created to protect the coast against flooding with techniques that differ from more traditional techniques such as the Sleeswijk-Holstein system. The main difference is the approach to keep the salt marshes more natural but doing so does not increase the protection against flooding. No significant differences between the German and Dutch methods for salt marsh development have been discovered during the study of this report. From the traditional methods to claim land from the sea up to the current methods to protect the salt marshes against erosion and a decline in “naturalness”, they are all very similar. The largest difference here is that the creation of sedimentation fields has continued in Germany while it has stopped in The Netherlands since 1950. Exact threshold values for the accretion or erosion of a salt marsh have not been found during the study for this report. The current velocities that lead to deposition or erosion of fine sands particles that have been mentioned in this chapter, give an idea however of which velocities prevent the natural formation of salt marshes and where the currents will have to be slowed down (for instance with brushwood groynes) to make the location more suitable for the development of salt marshes. Although the transportation of sediment in the estuary has been discussed, no maps have been found during the study for this report to show where these transports start and end up exactly. As explained this would however be difficult to show on a map because of the very dynamic and unstable nature of this system. Besides that, it has also been mentioned that knowledge about silt quantities in he Wadden Sea is merely based on rough estimations of what is present in the sea bottom, the water column and the amount of silt that flows through a certain area per second. In the next chapter we will focus at human activities that have a large influence on this system.

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Chapter 4: Direct human influences on the development of salt marshes in the Ems-Dollard estuary In this chapter we are going to look at relevant influences on salt marshes that are not as unavoidable as climate change or land subsidence. These influences have not been discussed in chapter 1 because they are easier to understand with the knowledge about salt marsh development discussed in the previous chapter. We will begin with what is the most significant human influence in the sedimentation processes of the Eems-Dollard estuary which is dredging. In the second paragraph we discuss coastal squeezing and in the final paragraph the influence which has the smallest impact of these three, which is sand suppletion, will be discussed.

4.1 dredging: The biggest changes to the characteristics of the Eems-Dollard estuary are caused by physical changes to the natural morphology to accommodate shipping. Since approximately 1970, channels have been deepened to facilitate the Ems harbor and the harbors of Emden and Delfzijl. Since 1984 there have also been intensive dredging activities in the River Ems and activities to straighten this river. All these interventions have led to a change in the tidal dynamics of estuary. The flood is coming in faster now (Provincie Groningen, 2010) and the tidal range has increased with circa 25 to 45 cm per century (partially caused by sea level rise) (Sips, 2009). 40% of the salt marshes in the estuary is said to have disappeared since 1900 due to these changes (ZOETZOUT, 2011). The hydrodynamic changes influence the silt transport as well as the sedimentation and erosion processes (Provincie Groningen, 2010). This fact is emphasized by the higher concentrations of suspended silt that are measured in the estuary. Enlarging channels by dredging and dumping activities are the main causes for the increased clouding in the estuary (Provincie Groningen, 2010). This large increase of silt (clouding) has a large negative impact on the aquatic ecosystem. The higher concentration has as a result that more sunlight is being blocked which leads to a smaller primary production (Raad voor de Wadden, 2010) (of for instance algae and seagrass (Stichting Het Groninger Landschap et al., 2011)) and a decrease of the available food for organisms higher in the food chain. Along with the increased concentration of silt, the amount of suspended organic material in the water increases. Through biodegradation this leads to a decreasing oxygen concentration which causes large problems for organisms that are oxygen dependent (Raad voor de Wadden, 2010). One could think that the increase of suspended organic material is good news for the filter feeders in the estuary but according to Stichting Het Groninger Landschap et al. (2011) molluscs, like mussels, have trouble finding enough food due to the increase of indigestible particles. Therefore, dredging has a wide range of influences on the salt marshes in the estuary: the population of silt trapping bio-engineers decreases while the tidal range, the current velocity and the amount of suspended sediment increases. Since the total acreage of salt marshes in the estuary is decreasing we can conclude that the increased amount of suspended sediment does not lead to the expansion of the salt marshes. A reason for this seems to be the increased water movement. It may however lead to more sedimentation in combination with brushwood groynes but no monitoring results have been found that prove this assumption. Future adaptations to dredging activities could again change the sedimentation processes in the estuary and the aquatic ecosystem. It is likely that the dredging activities will continue for years because of the economic activities which depend on it but considering the damaging

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effects it has on the ecosystem one can conclude that doing so is not a sustainable way to treat the surrounding nature which is why the Waddenvereniging (a non governmental organization) is taking action to counteract the dredging activities (Waddenvereniging, 2007). It was beyond the scope of the study for this report to look for ways to change the dredging policy because, in this study, we focus on natural aspects that influence salt marsh development. Therefore we will not differentiate between silt concentrations, current velocities or tidal forces which are caused by dredging or nature. This chapter is meant to explain which influences there are on the sediment transport and the salt marshes that had not been discussed in the previous chapters. Because these influences are man made they need a more political approach to change them and it is also hard to predict which way they will go. Therefore in the following paragraphs only a brief description about the human activities will be given as well, without discussing methods to alter them.

4.2 Coastal squeezing and the straightening of the coast: Salt marshes will develop (under natural circumstances) on tidal flats that have a sufficient height, protection against waves and currents and supply of sediments, plant parts and seeds. This natural development of salt marshes has become rare in The Netherlands. One of the causes could be that the Wadden Sea became too narrow. The natural reaction of the Wadden Sea to sea level rise would be to expand inland and to “push” the salt marshes further inland as well (Dijkema et al., 2005). Dikes and impoldering prevent this from happening however and have probably made the Wadden Sea too narrow for the natural formation of salt marshes (Dijkema et al., 2007). This coastal squeezing has changed the current patterns and increased the wave action on the coast which has decreased the room for calm waters and deposition of silt. This could also have led to an increase of clouding (Waddenacademie, 2009). Another possible cause for the lack of naturally developing salt marshes along the main land of the Wadden Sea is the straightening of the coast line with sea dikes. An irregular coast line offers more protection against waves and currents and therefore provides better circumstances for the natural development of a salt marsh. Nowadays, areas with a low wave and current energy in the Ems-Dollard estuary can still be found behind the shelter of land reclamation works and a few German and Dutch dams. Natural salt marsh development in the Dollard has declined however, likely due to dredging activities (Dijkema et al., 2005). We can conclude that coastal squeezing and the straightening of the coast both reduced the area of suitable locations for the development of salt marshes due to its increasing effect on the currents and wave action along the coast. It has even been mentioned that the Wadden Sea has probably become too narrow for the natural formation of salt marshes. In chapter 1 we concluded that it is uncertain if the salt marshes in the estuary will be able to keep up with the increasing sea level and subsiding land but that their chances for survival are growing smaller. Together with the information in this paragraph we can assess that the chance for natural formation of salt marshes in the estuary is very slim indeed. We can not rule out the possibility that nature will develop new salt marshes in the estuary but we can hereby rule out the idea of letting nature do this for us under the current circumstances without our help.

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4.3 Sand suppletion: Apart from previously given reasons, this subject will only be discussed briefly because the concentration of suspended sediment in the Ems-Dollard estuary is already so high, for instance due to dredging, that it seems very unlikely that adding more sand to develop salt marshes is a good idea. Besides that, importing sand into the sedimentation system of the Wadden Sea by sand suppletion is carried out in such a way that the amount changes

whenever more or less sand is deemed necessary (Deltacommissie, 2008b). This amount

varies year by year (Rijkswaterstaat, 2011c). As was mentioned in subparagraph 3.1.1, how much sediment is added to the water column by dredging, reducing the size of the Wadden Sea and sand suppletion is difficult to say. Knowledge about silt quantities in he Wadden Sea is merely based on rough estimations of what is present in the sea bottom, the water column and the amount of silt that flows through a certain area per second. These estimations all have a large margin of uncertainty (Rijkswaterstaat, 2010). According to Sips (2009) it is also unknown which long term consequences sand suppletion has for the concentration of suspended silt and clouding in the Wadden Sea.

According to the Delta commissie (2008b) the Wadden Sea will require more sand to maintain large tidal areas due to the sea level rise. Currently almost no sand (net) that is supplied to the Dutch coast comes from the North Sea and the rivers. Therefore the deficit of sand in the Wadden Sea is growing. Sand suppletion is carried out to fulfil this constant “need”. To elevate the Dutch coast, including the Wadden region, in accordance with the sea

level rise, 7 million m3 sand per mm sea level rise has to be added (Delta commissie 2008b). Theoretically, sand suppletion would add sediment to the salt marshes and therefore have an increasing effect on the total acreage of salt marshes in the estuary. The previously discussed information however shows that scooping sand onto the salt marshes or adding it to the water column has a destructive influence on ecosystems that play an important role in the development of salt marshes. Finding out if one could add sediment without negatively influencing these systems was beyond the scope of this report and is therefore not considered to be an option.

4.4 Conclusion: The human influences on the Ems-Dollard estuary that have been discussed in the first two paragraphs have a large impact on the natural development of salt marshes in the estuary (or lack thereof). It has even been mentioned that they cause the salt marshes to disappear. Sand suppletion does not seem to be the solution for this problem in the Ems-Dollard estuary because adding more sand into suspension would probably only make the situation worse. For the future development of a salt marsh in the estuary it is common sense that it is preferable to do this far away instead of nearby these human influences. Therefore the most suitable locations which are presented in the next chapter have been selected by determining where conditions such as current velocity and wave action are favourable for the development of a salt marsh. These values are not solely dependent on the human influences but there does not seem to be a need to differentiate because, as stated before, it is beyond the scope of this study to look for ways to alter the human activities which have been discussed in this chapter. Now that the natural and artificial influences on salt marsh development in the estuary are explained and the methods to reduce the influences that lead to a decline in salt marsh acreage have been described in chapter 3, it has been made clear which of the measures that have been

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discussed in this report can be taken to develop a salt marsh where it would not be created by nature itself. Overall we can conclude that in the Ems-Dollard estuary the main reason for the lack of salt marsh formation is that there is too much water movement. Based on previously discussed information (for instance about dredging, the straightening of the coast the lack of salt marsh development in the estuary, etc), it seems that this is true for the entire Dutch coast of the estuary. This impact can be reduced by creating more sheltered environments to calm the water with the methods described in chapter 3. In the next chapter an analysis will be presented that points out which locations in the estuary are more and which locations are less suitable to develop a salt marsh that has to serve as a coastal defence system with the methods described in chapter 3.

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Chapter 5: Analysis In this chapter we will analyse the information that has been presented in the previous chapters to point out the most suitable locations in the Dutch part of the estuary for the development of salt marshes that serve as natural coastal defence systems and the most promising ways to develop them on these locations. Due to the various uncertainties mentioned in this report (for instance about the threshold values for erosion and accretion or the date of the maps in appendix 3) the outcomes merely indicate which locations and methods are more suitable or less suitable. In other words: the outcomes do not guarantee success or failure. Another reason for why it can not be predicted with certainty if the development of a salt marsh on a certain location with a certain method will lead to success is that the effects of climate change and the salt marshes’ ability to cope with them are not clear yet. The comparisment of the locations, to point out which locations are more suitable and which locations are less suitable for the development of a salt marsh that has to function as a natural coastal defence system, is made with a multi criteria analysis (MCA). In paragraph 1 the criteria that are used for this analysis and for dividing the area into separate locations will be discussed. In the second paragraph it is explained how the area has been divided according to the criteria. The MCA is presented in the third paragraph. In the fourth paragraph the most promising method to develop a salt marsh has been pointed out for the individual locations. An explanation for these choices is given in the paragraphs as well. In the fifth paragraph a few imaginable combinations of salt marsh development are discussed. The flowchart in figure 12 shows how analysing the data has led to the selection of certain of the salt marsh development methods from chapter 3 for the most suitable locations in the estuary. It also shows in which order the information will be presented in this chapter.

Figure 12: Flowchart

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5.1 Criteria to divide the area and compare the locations To divide the Dutch part of the Ems-Dollard estuary into separate locations and to point out which locations are more suitable and which locations are less suitable to develop salt marshes as a natural coastal defence system, the most significant influences on the development of these salt marshes have been selected from the information in the previous chapters and appendix 3 to ensemble the list of criteria below. All the information about the maps that could be found during the study for this report is shown in the appendix 3 (this is relevant because for some of the maps it is unknown when they have been created or which time frame they represent for instance). The criteria have been arranged according to their influence on salt marsh development. The first mentioned criterion is estimated to be the most influential and criterion 11 is estimated to be the least influential. In appendix 6, the explanation for why these criteria have been selected, and why one criterion is estimated to be more important then the next is given behind the hyphen. Between the brackets, the title(s) of which map(s) from appendix 3 contain(s) the values for the criteria in the Ems Dollard estuary is/are given. 1: Sedimentation and erosion 2: Current velocity 3: Wave velocity 4: The presence of valuable buildings or other man made structures 5: The presence of salt marshes in the foreland 6: The depth of the terrain 7: Sediment size 8: Silt concentration in the water 9: The distance to nearby waterways 10: Nearby presence of oyster or mussel banks 11: Contribution to the total acreage of a bordering salt marsh Which criteria will be used to divide the Dutch part of the estuary in separate locations and to compare these locations in a MCA, why these criteria have been chosen and according to which reasons they are arranged from most to least important has now been explained. In the next paragraph we can therefore discuss the division of the area into separate locations.

5.2: Dividing the area Based on the criteria from the previous paragraph, the Dutch part of the Ems-Dollard estuary has been divided as is shown in figure 13. These locations all extend 100 meters seaward or 100 meters inland depending on whether they are an option for the protection against flooding more seaward or by retreating inland. The width of 100 meters was based on the earlier mentioned estimation for what is expected to be enough to defend the coast against flooding. The drawn width of the locations in figure 13 is not in proportion to the rest of the map. The width has been exaggerated to make the locations more visible. The blue line representing the primary coastal defence system is not an exact fit to the rest of the map. This is most evident in the lower-right corner of figure 13, above the salt marsh of the Dollard. The relation between the height and width of the layers has been left unchanged in comparison to the source maps which are included in appendix 3.

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What has caused this deformation remains a mystery, however the line still gives a good indication of where the sea dikes are located.

Figure 13: the possible locations for salt marsh development in order to defend the coast against flooding. The map has been composed by using elements from the “Intertidal areas”, “Primary coastal defence system” and “Most recent vegetation...” maps which can be found in appendix 3. The legend shows which elements of these maps have been used. The green line has been used as a basis, the blue line and multi coloured area have been reduced in size without altering the proportion between their height and width. These layers have then been fitted together (by the author of this report) in an attempt to resemble their relative position in the estuary. The list on the next page explains according to which criterion each location has been distinguished from the others (apart from the division between land and sea). Consulting the maps (in appendix 3) that have been linked to these criteria in paragraph 5.1 therefore demonstrates that each location distinguishes itself from its surrounding locations by having noticeably different values for the criteria that have been summed up behind the locations number in the list (which value that is, is mentioned behind the criterion). The numbers correspond to the numbers in figure 13.

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1A: Criterion 2 high current velocities, criterion 4 Ems harbour, criterion 7 relatively large sediment size 1B: Criterion 2 high current velocities, criterion 7 relatively large sediment size 2A: Criterion 2 high current velocities, criterion 4 Buildings, criterion 6 relatively large shallow area, criterion 8 relatively low silt concentration, criterion 9 relatively large distance to nearest waterway, criterion 10 mussel banks 2B: Criterion 2 high current velocities, criterion 6 relatively large shallow area, criterion 8 relatively low silt concentration, criterion 9 relatively large distance to nearest waterway, criterion 10 mussel banks 3A: Criterion 1 high sedimentation rate, criterion 6 relatively large shallow area, criterion 8 relatively high silt concentration, criterion 9 relatively large distance to nearest waterway, criterion 10 oyster and mussel banks 3B: Criterion 1 high sedimentation rate, criterion 6 relatively large shallow area, criterion 8 relatively high silt concentration, criterion 9 relatively large distance to nearest waterway, criterion 10 oyster and mussel banks 4A: Divided by the relatively high silt concentration of area 5B (criterion 8) 4B: Divided by the relatively high silt concentration of area 5B (criterion 8) 5A: Criterion 8 relatively high silt concentration 5B: Criterion 8 relatively high silt concentration 6A1: Divided by the relatively high silt concentration of area 5B (criterion 8) and Delfzijl (criterion 4) 6A2: Criterion 4 Delfzijl and the harbour of Delfzijl 6B1: Divided by the relatively high silt concentration of area 5B (criterion 8) and the harbour of Delfzijl (criterion 4) 6B2: Criterion 4 Delfzijl and the harbour of Delfzijl 7A1: Divided by the build up area of location 7A2 (criterion 4) 7A2: Criterion 4 build up area 7A3: Divided by the build up area of location 7A2 and the salt marsh of location 8A 7B: Criterion 1 relatively high sedimentation rate, criterion 3 relatively high wave velocity, criterion 6 relatively large shallow area, criterion 9 relatively long distance to the nearest waterway, criterion 11 potential to contribute to bordering salt marsh 8A: Criterion 5 salt marsh 8B: Criterion 5 salt marsh 9A: Criterion 4 Breebaartpolder 9B: Criterion 4 Breebaartpolder 10A: Criterion 5 salt marsh and divided by the erosion in location 11A (criterion 1) 10B: Criterion 5 salt marsh and divided by the erosion in location 11A (criterion 1) 11A: Criterion 1 erosion 11B: Criterion 1 erosion Now that the Dutch half of the estuary has been divided into separate locations we can compare them in the next paragraph.

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5.3: Suitable locations for salt marshes as a natural coastal defence system in the Ems-Dollard estuary To get an indication of which locations are more and which locations are less suitable for the development of a salt marsh as a coastal defence system, we will compare the locations in a multi criteria analysis with the 11 criteria which have been mentioned in paragraph 5.1. This MCA is shown in figure 14. A weighting factor has been assigned to the criteria to let their importance influence the outcome. The first 5 criteria have a weighting factor of 3 because they are estimated as being capable of making the location unsuitable for the development of a salt marsh as a coastal defence system. Criteria 6, 7, 8 and 9 have a weighting factor of 2 because they are estimated as being very influential on the development of a salt marsh but less capable of preventing their formation. The final two criteria have a weighting factor of 1 because they are estimated as being hardly capable to prevent the formation of a salt marsh. The weighting factors are also shown between the brackets in the upper row of the multi criteria analysis. Without the weighting factors each location can score a maximum amount of 3 points, 2 points or a minimum amount of 1 point on each criterion. This ordinal scale has been chosen to make a clear separation between less, moderate and more favourable conditions. It has also been chosen as a way to create uniformity in the various values of the maps in appendix 3. The scores for the locations on a certain criterion are relative to one another. No calculations on how the values on several maps for a criterion could be translated to a figure of 1 to 3 have been made because there are too many uncertainties to define threshold values for, for instance, sedimentation rates, current velocities or the size, depth and slope of the mudflat area. Therefore it has been estimated how the locations compare to one another by assessing how they score according to the information on the maps. If a location is situated relatively favourable in comparison to the other locations it gets 3 points, 2 points if the conditions seem moderate and 1 point if the location is situated in a more unfavourable environment. The reason for why each individual score has been assigned is given in appendix 4. The number between brackets behind the location number (in appendix 4) corresponds to the number of the criterion. The value between the brackets behind each individual score (in the MCA) is the result of multiplying this score with the weighting factor.

Criterion Location

1 (3)

2 (3)

3 (3)

4 (3)

5 (3)

6 (2)

7 (2)

8 (2)

9 (2)

10 (1)

11 (1)

Total

1A 1 (3) 1 (3) 2 (6) 1 (3) 3 (9) 1 (2) 1 (2) 1 (2) 1 (2) 1 (1) 1 (1) 14 (34)

1B 1 (3) 1 (3) 2 (6) 1 (3) 3 (9) 1 (2) 1 (2) 1 (2) 1 (2) 1 (1) 1 (1) 14 (34)

2A 2 (6) 2 (6) 3 (9) 1 (3) 3 (9) 1 (2) 1 (2) 1 (2) 2 (4) 2 (2) 1 (1) 19 (46)

2B 2 (6) 2 (6) 3 (9) 3 (9) 3 (9) 1 (2) 1 (2) 1 (2) 2 (4) 2 (2) 1 (1) 21 (52)

3A 3 (9) 2 (6) 3 (9) 3 (9) 3 (9) 3 (6) 3 (6) 3 (6) 3 (6) 3 (3) 1 (1) 30 (70)

3B 3 (9) 2 (6) 3 (9) 3 (9) 3 (9) 3 (6) 3 (6) 3 (6) 3 (6) 3 (3) 1 (1) 30 (70)

4A 3 (9) 2 (6) 3 (9) 3 (9) 3 (9) 3 (6) 3 (6) 2 (4) 3 (6) 3 (3) 1 (1) 29 (68)

4B 3 (9) 2 (6) 3 (9) 3 (9) 3 (9) 3 (6) 3 (6) 2 (4) 3 (6) 3 (3) 1 (1) 29 (68)

5A 3 (9) 3 (9) 3 (9) 3 (9) 3 (9) 2 (4) 3 (6) 3 (6) 1 (2) 1 (1) 1 (1) 26 (65)

5B 3 (9) 3 (9) 3 (9) 3 (9) 3 (9) 2 (4) 3 (6) 3 (6) 1 (2) 1 (1) 1 (1) 26 (65)

6A1 3 (9) 3 (9) 3 (9) 3 (9) 3 (9) 2 (4) 3 (6) 2 (4) 1 (2) 1 (1) 1 (1) 25 (63)

6A2 3 (9) 3 (9) 3 (9) 1 (3) 3 (9) 1 (2) 3 (6) 3 (6) 1 (2) 1 (1) 1 (1) 23 (57)

6B1 3 (9) 3 (9) 3 (9) 3 (9) 3 (9) 1 (2) 3 (6) 3 (6) 1 (2) 1 (1) 1 (1) 25 (63)

6B2 3 (9) 3 (9) 3 (9) 1 (3) 3 (9) 1 (2) 3 (6) 3 (6) 1 (2) 1 (1) 1 (1) 23 (57)

7A1 3 (9) 3 (9) 2 (6) 3 (9) 3 (9) 3 (6) 3 (6) 3 (6) 3 (6) 1 (1) 1 (1) 28 (68)

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7A2 3 (9) 3 (9) 2 (6) 1 (3) 3 (9) 3 (6) 3 (6) 3 (6) 3 (6) 1 (1) 1 (1) 26 (62)

7A3 3 (9) 3 (9) 2 (6) 3 (9) 3 (9) 3 (6) 3 (6) 2 (4) 3 (6) 1 (1) 3 (3) 29 (68)

7B 3 (9) 3 (9) 2 (6) 3 (9) 3 (9) 3 (6) 3 (6) 3 (6) 3 (6) 1 (1) 3 (3) 31 (70)

8A 1 (3) 2 (6) 1 (3) 3 (9) 1 (3) 1 (2) 3 (6) 2 (4) 1 (2) 1 (1) 3 (3) 19 (42)

8B 1 (3) 2 (6) 1 (3) 3 (9) 1 (3) 1 (2) 3 (6) 2 (4) 1 (2) 1 (1) 3 (3) 19 (42)

9A 3 (9) 3 (9) 3 (9) 1 (3) 3 (9) 2 (4) 3 (6) 3 (6) 3 (6) 1 (1) 3 (3) 28 (65)

9B 3 (9) 3 (9) 3 (9) 1 (3) 3 (9) 2 (4) 3 (6) 3 (6) 3 (6) 1 (1) 3 (3) 28 (65)

10A 3 (9) 3 (9) 2 (6) 3 (9) 1 (3) 3 (6) 3 (6) 3 (6) 3 (6) 1 (1) 3 (3) 28 (64)

10B 3 (9) 3 (9) 2 (6) 3 (9) 1 (3) 3 (6) 3 (6) 3 (6) 3 (6) 1 (1) 3 (3) 28 (64)

11A 1 (3) 3 (9) 2 (6) 3 (9) 1 (3) 3 (6) 3 (6) 3 (6) 1 (2) 1 (1) 3 (3) 24 (54)

11B 1 (3) 3 (9) 2 (6) 3 (9) 1 (3) 3 (6) 3 (6) 3 (6) 1 (2) 1 (1) 3 (3) 24 (54)

Figure 14: multi criteria analysis The highest possible score for a location in the MCA is 33 (75) and the lowest possible score is 11 (25). The ranking of the individual locations is more interesting than the score however in the sense that the score alone does not give much information about the location. The highest score was 31 (70) for location 7B and the lowest score was 14 for locations 1A and 1B. These figures seem to make sense based on what we have learned in this report because if one would compare these two locations to each other, by estimating how they would relate to the (important) criteria without giving (weighted) scores, one should get the idea that location 7B is much more promising than location 1A or 1B. Therefore it is clear that the point system that was used for the MCA did, at least for this example, what it was supposed to do, which is to separate the less suitable locations from the more suitable locations. Comparing the weighted totals shows that 5 of the 26 locations (1A, 1B, 2A, 8A and 8B) scored between 25 and 50 points. That low scores have been awarded to these locations does not seem very surprising either considering that they are located next to each other or are located next to the Punt van Reide which looks like the least sheltered area in the estuary. In that sense it is strange that the best scoring location borders one of the worst scoring locations however, the distance to nearby waterways could very well make a large difference here as is also evident in the comparison between locations 10A, 10B and 11A, 11B. When we look at the upper range of 65 points till 70 points we see that location 3A, 3B, 4A, 4B, 5A, 5B, 7A1, 7A3, 7B, 9A and 9B form the top 11 out of the 26. Looking at their locations in figure 13 makes clear that these areas are again situated close to one another. Apart from the locations between the harbour of Delfzijl and the Punt van Reide they are situated where the coastline slightly shows the shape of a cavity. That the locations closest to the Breedaartpolder score high does not seem very surprising either because of the shape of the coast and because of the recent creation of this intertidal nature area. Close to this score could be where the MCA shows a division between the most suitable locations and the less suitable locations because these two locations are for obvious reasons less suitable and location 5A and 5B, which have the same weighted total, also seem less convincingly promising as the higher scoring locations. Weighting the scores did not lead to large shifts in the ranking of the individual locations. One could argue that this renders the weighting useless however one could also argue that this shows that the differences between the locations are so distinct that they are already dominant in the unweighted scores. An explanation for this could be that the division of the locations has been made with the same criteria. This would mean that the outcome is more convincing since weighting the scores was not even necessary to separate the more suitable locations from the less suitable locations. Al of this had not been foreseen however.

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We conclude by saying that the MCA seems to provide a good idea of which locations are more and which locations are less suitable for the development of a salt marsh as a natural coastal defence system. As has been previously mentioned, the study for this report has shown that pointing out locations that are guaranteed to be suitable for the development of a salt marsh as a natural coastal defence system in the Ems-Dollard estuary is impossible. One of the reasons is that it is still uncertain for how long salt marshes in the estuary will remain to exist. During the study for this report no evidence for formation but only evidence for erosion of salt marshes in the estuary has been found. The accelerated sea level rise will only decrease their chances for survival, for as far as the information found for the study of this report shows. Luckily, it does not seem to be inevitable that they will disappear as well. In the next paragraph we will look at which methods to develop salt marshes as natural coastal defence systems on the locations that are shown in figure 13 seem to be the most promising.

5.4: The most promising methods for the development of salt marshes as a natural coastal defence system on the suitable locations From chapter 3 we have learned that the following methods can be applied to the locations which are shown in figure 13 to develop a salt marsh as a natural coastal defence system: 1: Scooping a large amount of sediment onto the mudflat 2: Stimulating sediment deposition by using sedimentation fields 3: Building a structure in the sea, for instance a dam 4: Protecting the salt marsh from erosion with groynes 5: De-embanking summer polders 6: Relocating the sea dike further inland and (partially) excavating the foreland 7: Building a new sea dike further inland and allowing sea water to flow through pipes (for instance) that are built in the former sea dike. 8: Creating oyster or mussel banks 9: Stimulating the settlement of halophytes In figure 14 a table is presented in which the most promising development methods have been marked for each location. In appendix 5 an explanation is given for why these methods are estimated to be more promising than the others. Estimating which salt marsh development methods are the most promising for each location is not essential for answering the main question, however it gives a better idea of what can be done with the outcome of the analysis in the former paragraph. The numbers of the methods correspond to the number in the upper row of the table. Method Location

1 2 3 4 5 6 7 8 9

1A 1B X X 2A 2B X X X 3A X X X X X 3B X X X X 4A X X X X X 4B X X X X

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5A X X X X X 5B X X X X

6A1 X X X X X 6A2 6B1 X X X X 6B2 7A1 X X X X X 7A2 7A3 X X X X X 7B X X X X 8A 8B 9A 9B

10A 10B 11A 11B

Figure 14: salt marsh development methods for the suitable locations For the reasons given in appendix 5, no methods to develop a salt marsh have been selected for 13 of the 26 locations. This means that for these locations no evidence has been found during the study for this report that they would be suitable for the development of a salt marsh that has to function as a coastal defence system. In the future these locations may become suitable, for instance when the currently present salt marshes disappear or in the unlikely event that build up areas would be broken down. Within this report however, these locations will be considered to be unsuitable. For the remaining 13 locations, several development methods have been selected. Why these combinations have been chosen will be explained in the next paragraph.

5.5: Imaginable combinations of salt marsh development methods in the Ems-Dollard estuary Looking at the table in the previous paragraph shows that there are 4 different combinations of salt marsh development methods that are expected to be the most promising for the development of a salt marsh as a natural coastal defence system on the separate locations in the Ems-Dollard estuary. In this paragraph we will discuss the differences between these combinations and why they are expected to be more promising than other combinations. Then some ideas about how these combinations could be put into practice will be given. -Combination 1 was selected for location 1B. This combination exists out of method 3 and method 9 of the salt marsh development methods mentioned in paragraph 4. Method 3 is the initial method and method 9 an additional method, which means that, the most promising method for the development of the salt marsh is expected to be method 3. If the deposition of sediment needs to be increased then it is advised to do this with method 9. In theory, increasing the deposition rate could also be done with methods 2, 4 and 8 but it seems logical that the construction of a dam in this turbulent location would be done in a way that excludes the need for these methods. If more deposition of sediment would turn out to be necessary then method 9 would be more logical than methods 2, 4 and 8 because it is estimated to be

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less costly while methods 2 and 4 more or less conflict with building a dam and method 8 is estimated to be less promising due to its distance to existing oyster and mussel banks and the experimental nature of this method. -Combination 2 was selected for location 2B. This combination is different from combination 1 because it is expected that a smaller dam will suffice in this location. Therefore looking at a combination with oyster or mussel banks seems more logical here. Due to the experimental nature of method 8 this is not advised till more knowledge about the chances for success with this method has been acquired. In the event that a higher sedimentation rate is required however, this seems a promising location to experiment with oyster and mollusc banks because of the presence of these banks in the vicinity. -Combination 3 was selected for location 3A, 4A, 5A, 6A1, 7A1 and 7A3. In this combination method 6 is proposed as the initial method and method 2, 4, 8 and 9 as additional methods to increase the sedimentation rate, if necessary. Method 2 is expected to be capable of increasing the sedimentation rate most drastically while method 8 is extra attractive when such a drastic increase is not estimated to be necessary and there is an opportunity to experiment with this especially nature friendly method. Experiments with molluscs located between the groynes and the (new) sea dike could be carried out as well although there does not seem to be a deficit of suspended silt in the estuary at the moment. Creating an embankment carries with it the risk that the salt marsh would develop into a climax stage (Esselink, 2000). Therefore method 9 may be considered as a first alternative to further increase the deposition in addition to method 8. No evidence has been found during the study of this report that method 6 has been carried out in Germany or the Netherlands. Therefore no information has been found on how to carry out such an operation. It is probably in some ways similar to a de-embankment of a summer polder (mentioned in paragraph 3.4.1). How these are developed has been well monitored and described (Beintema, 2007). Due to time constraints and because giving instructions on how to develop such a salt marsh is probably the work for an engineer it will not be described in this report how that operation should be performed. -Combination 4 was selected for location 3B, 4B, 5B, 6B1 and 7B. In this combination method 2 is the initial method although (a combination with) method 8 could be used as a starting point as well as soon as there is more certainty about its potential to form salt marshes in the Ems-Dollard estuary. For now methods 4, 8 and 9 are ascribed the same role they have in combination 3. Even though the selected combinations are expected to be the most promising ones, it is uncertain if a salt marsh would develop and be able to cope with the influences that have been discussed in chapter 1 (climate change and land subsidence) and chapter 4 (dredging, coastal squeezing and straightening of the coast). As was made very clear, more research is needed to make sound predictions about the “survival” of the salt marshes in the Ems-Dollard estuary. Therefore it is advised to monitor the development of the salt marshes intensively (if these combinations would be put into practice) to make alterations, for instance by carrying out one of the additional methods, when necessary. The analysis of the information that has been gathered during the study for this report has now been completed. As a result, the answer to the main question has been given in this chapter. In the next chapter it will be explained why the main question has been answered and a brief description of the answer will be presented as well.

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Chapter 6: Conclusion At the beginning of the study for this report, a set of sub-questions was formulated of which the answers, together, should deliver the answer to the main question. These answers have been summarized from the previous chapters and will be presented in the following subsections. After this, the answer to the main question is provided. How do nature and men create salt marshes? Salt marshes are created by nature on locations where the currents are slow enough and the amount of suspended sediment and silt in the coastal water is large enough for a sedimentation rate that exceeds the rate of the sea level rise. A relatively large amount of deposited silt is necessary to provide for a soil which is sufficiently fertile for halophytes like Salicornia species to settle. Bio-engineers, such as mussels and oysters, are able to increase the sedimentation rate by excreting depositable silt particles and by reducing the current velocity. Sediment particles are mainly transported towards the shore by wind driven currents. The higher the shore gets by this process, the less frequently it is submerged by the sea. As a result, less sediment particles will be deposited, but it offers the chance for halophytes to settle. These plant species reduce the current velocities and counteract the erosive forces of the sea by covering or holding on to the sediment. Throughout Dutch and German history, salt marshes have been artificially created by building structures in the sea that reduce the currents, such as sedimentation fields and dams but also by de-embanking summer polders to allow nature to take control again. Attempts to speed up the process by scooping large amounts of sediment on shore did not lead to salt marsh formation due to an absence of a natural creek system. Allowing the water to flow through parts of a former dike seems to lead to similar problems. Likely, the most nature-friendly way that has been found during the study for this report to stimulate salt marsh formation is to stimulate the development of shellfish banks. Creating these artificial reefs helps to slow down the currents, generate more depositable silt and prevent erosion. This approach is however still in an experimental phase. In which ways are salt marshes currently designed as a coastal defence system? Although salt marshes are used as coastal defence systems in Germany, no evidence has been found during the study for this report that this has led to changes in the way these salt marshes are created. The policy seems to have shifted towards improving the “naturalness” for instance by de-embanking former summer polders, discontinuing the creation of artificial ditches yet restoring former creeks but these activities do not enhance the salt marshes capabilities of defending the coast against floods. What are the differences between German and Dutch methods for salt marsh development? Actually, these methods are quite similar. The biggest difference that has been found during the study for this report is the use of sedimentation fields in the German part of the estuary while this form of salt marsh development is no longer practised at the Dutch side. Which influences of nature play a role in the development of a salt marsh? Besides the influences that have been mentioned in the answer to the first sub-question, the following aspects also play an important role (reasons are indicated after the colon).

- Accelerated sea level rise: Due to global warming the sea level is expected to rise at an accelerated rate. At this moment it is uncertain if the salt marshes in the Ems-Dollard estuary are capable of gaining height fast enough to stay above sea level.

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- Land subsidence: Due to natural processes, the sea floor in the estuary subsides at a rate slightly less than one millimetre per year. Gas extraction causes a descent of up to 3.2 millimetres per year however, which therefore has a more significant influence on the salt marshes’ capability to survive.

- Dredging: Dredging activities in the Ems-Dollard estuary have increased the tidal range, increased current velocities and clouded the water. They are likely to be the main reason why salt marshes in the estuary are disappearing.

- Coastal squeezing: Claiming land from the Wadden Sea had as a result that the shore is bordering deeper waters now, which is emphasized by the accelerating sea level rise. The conditions along the coast are therefore less favourable for the formation of salt marshes these days.

- Straightening of the coast: Constructing dikes along the coast has caused that the coastline has hardly any irregularities any more in which the water is calmer and salt marshes are more likely to develop.

- Waves: The longer the fetch seaward of a salt marsh, the bigger the waves that travel towards them can become. This has as a positive effect that more sediment is brought to the higher parts of a salt marsh. On the lower parts of the salt marsh the wave movements can lead to more erosion however.

- Zonation and succession: a salt marsh can be divided into several zones which differ in the plant species that grow on them, in their vulnerability to erosion, in the number of times they get flooded and in the composition of the soil.

- Referential acreage: The bigger a salt marsh is, the less vulnerable it is to a decline of species diversity and to developing into a climax stage.

- Bio-builders and bio-breakers: Certain species contribute to the formation of salt marshes by reducing the currents, holding on to the soil or increasing the amount of sediment for instance. There are also species that reduce these effects by destabilising the sediment or by diminishing populations of bio-builders.

- The creek system: Creeks improve the drainage of the salt marshes, which gives the deposited sediment the chance to dry faster.

- The depth of the foreland: Mud flats suppress the wave impact in the same way as salt marshes. It is therefore beneficial to the development of a salt marsh if the mudflat zone is relatively high and long.

To which extent can mussel and oyster banks contribute to the development of a salt marsh? They are able to increase the silt deposition by excreting silt and reducing the current velocities. Artificial development of oyster and mussel banks is still experimental however. Where are currents, wave movements and sediment transports located in the estuary and what are the threshold values for developing, maintaining and eroding a salt marsh? The currents and wave movements are shown in the maps called “Tidal maps”, “Current velocity” and “Wave velocity” in appendix 3. Sediment transportation is driven by (wind induced) currents in which sediment is picked up in places where the current velocity exceeds the bed shear strength of the soil. Most of the sediment that ends up in the salt marshes of the Ems-Dollard estuary comes from the Wadden Sea and the estuary itself. The rivers Westerwoldse Aa and Ems only deliver approximately 1% of the suspended sediment in the estuary. Exact threshold values for the accretion or erosion of a salt marsh have not been found during the study for this report. The current velocities that lead to deposition or erosion of fine sand

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particles that have been mentioned in subparagraph 3.1.2 give an idea however of which velocities allow or prevent the natural formation of salt marshes. Is the natural formation of salt marshes being held back in any locations in the Ems-Dollard region? For as far as the information retrieved during the study for this report has shown, the answer is: yes, in the entire estuary. This is mainly due to dredging, coastal squeezing, straightening of the coast, land subsidence and the influences of climate change. Which extra measures are necessary to develop a salt marsh on a location where it would not be created by nature itself? The following options for development of salt marshes have been found during the study for this report: Scooping a large amount of sediment onto mudflats, stimulating sediment deposition by using sedimentation fields, building a structure in the sea (for instance a dam), protecting the salt marsh from erosion with groynes, de-embanking summer polders, relocating the sea dike further inland and (partially) excavating the foreland, building a new sea dike further inland and allowing sea water to flow through pipes (for instance) that are built in the former sea dike, creating oyster or mussel banks and stimulating the settlement of halophytes. Some of these options are less promising than others. In general the extra measures that are necessary to develop a salt marsh in the Ems-Dollard estuary is to reduce the currents by either retreating inland or by building structures that slow down the water. If this is enough to prevent submersion by the accelerating sea level remains to be seen. Which locations in the estuary are suitable to develop a salt marsh that has to serve as a coastal defence system? The locations indicated by red numbers in figure 15 are the ones that are considered unsuitable for the development of a salt marsh that can serve as a coastal defence system (Figure 15 is a small version of the poster that has been handed in as one of the three products for the assignment). Locations with green numbers are more suitable. The ranking in figure 15 shows how they compare to each other. Due to many uncertainties, such as the capability of the salt marshes in the estuary to withstand the anticipated accelerated sea level rise, it can not be stated with certainty if these locations are fully suitable. A few unanticipated aspects turned out to be important for the development of salt marshes in the estuary as well. They are the presence of valuable buildings or other man made structures and the distance to nearby waterways. The presence of valuable buildings will probably rule out some areas that could be suitable for the development of salt marshes as a natural coastal defence system because the price for giving these locations back to nature would be high. The distance to nearby waterways is relevant because it seems logical to believe that areas in the vicinity will undergo the damaging effects of dredging for some time to come. There are also aspects that may influence the formation of salt marshes which have not been taken into account at all because they were considered to be less significant such as acidification of the sea. Now that the sub-questions have been addressed, the answer to the main question will be presented on the next page.

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The main question for the study for this report was the following: Which aspects should be taken into account when designing salt marshes that will have to function as natural coastal defence systems in the Ems-Dollard estuary and which locations in this region are suitable to develop a salt marsh that meets these demands? The first part of the main question, “which aspects should be taken into account when designing salt marshes that will have to function as natural coastal defence systems in the Ems-Dollard estuary?”, has already been addressed by the answer to the first 8 sub-questions. From these aspects, the ones that could be incorporated into a multi criteria analysis have been used to point out which locations in the estuary are more and which locations are less suitable for the development of a salt marsh that can serve as a natural coastal defence system. By this approach, the second part of the main question, “which locations in this region are suitable to develop a salt marsh that meets these demands?” has also been answered and the result has been presented in the map shown in figure 15. This map must be seen in proper context. It is important to understand the aspects that influence salt marsh formation and their detailed description in this report as well, if one would consider to develop a salt marsh at one of these locations. With a subject of this size, that is influenced by many factors containing uncertainties, one can only try to cover as much significant information as possible and to gain as much certainty as possible. The answer will never be 100% complete, nor is it possible, for example, to guarantee which locations are suitable as long as it is too early to predict the impact of certain influences like sea level rise. It is hoped, however, that the information and analyses provided in this report may contribute to future attempts to answer the main question of this report more completely.

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Figure 15: Small version of the poster in A3 format which, together with this report and the presentation, is handed in as the product of the assignment.

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Chapter 7: Discussion and recommendations Within this report the suitable locations and the most promising methods to develop salt marshes as natural coastal defence systems on these locations in the Ems-Dollard estuary have been presented. Unfortunately it was not possible to point out methods and locations with a (almost) guaranteed chance for success. The reason for this is that the survival of the present salt marshes is already uncertain and the changing climate, subsidence of the land, dredging, sediment transportation, etc. add even more uncertainties. It has been mentioned throughout this report that more knowledge is needed about the climate change or the ability of the salt marshes in the estuary to withstand the various conditions. Partially this needs to be done by monitoring. Therefore the combinations presented in chapter 5 carry with it a certain amount of risk and one might want to wait till some variables are better understood before developing a salt marsh based on this report. This report does however give an overview on the current knowledge and lack there of and takes a step in the right direction. Many uncertainties surrounding this subject are currently being addressed by the Dutch Government (Ministeries van V&W et al., 2010). The results will probably be made public on www.delta-programma.nl when they become available. It is probably needless to say that due to the many uncertainties and the importance of protecting the land against flooding the proposed coastal defence designs are not meant to be carried out without further research nor would it be realistic to think that it is possible without a high level of certainty about their ability to protect the land against flooding. For further research it could be interesting to study the effects of dredging on the salt marshes in more detail. Dredging has a large influence on the salt marshes because it changes the currents, tidal patterns and aquatic ecosystem. Dredging also increases the concentration of suspended sediment and therefore a change in the dredging policy could lead to a different rate of sediment deposition on the salt marshes. If a study on suitable locations for salt marshes as natural coastal defence systems in the Ems-Dollard estuary would be carried out when the ability of the salt marshes to cope with the changing conditions caused by land subsidence and climate change is certain, then one could study if the expected influence from dredging exceeds the capacity of the salt marshes’ ability to increase and needs to be altered. However, at this moment too many variables have a large margin of uncertainty. Therefore the advise is to solve this problem by creating a flowchart in order to research certain topics at the time when there is sufficient knowledge to determine their influence. One could start by acquiring more certainty about the deposition rates through monitoring. When this leads to believe that the salt marshes in the estuary are able to keep up with the upper boundary of a more extreme climate scenario as the one made by the delta commission then one could study if it can be determined with certainty if the salt marshes are able to keep up with one of the other influences etc. If it is still uncertain if the salt marshes are able to keep up with the climate scenario, then the plan of using them to (partially) take over the role of sea dikes to protect the coast from flooding will come to a halt until there is more certainty because the protection of the coast is too important to take chances. Something else that made it difficult to perform this study was the lack of maps for German salt marshes in the estuary. A possible reason for the absence of this information has been given in this report. Unfortunately this meant that the study could not be equally carried out for the German half of the estuary.

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The last comment concerning the information in this report that seems important to share is that the maps in appendix 3 are the best that have been retrieved during the study for this report. It would be better, however, if some of them were more recent, had a better description or if they would cover a longer period. All relevant knowledge about the maps that was delivered by the source has been incorporated in the figures in appendix 3. If one would find more informative maps, one could analyse the suitable locations based on the information provided in this report to check if a change in tidal currents for instance changes the ranking in figure 14. As the author of this report I realise that the main question can always be answered in more detail and in further depth but I hope that the information that is presented in this report is sufficient to meet the expectations of the initiator and my supervisors.

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Sources Altenhofen, D., Schwanken, J., 2010: Organisatie, Found on Januari the 25th 2011, on www.ems-eems.de Arcadis, 2006: Bouwsteen beheerplan kwelders Groningen noordkust en Dollard, Provincie Groningen, Groningen Atsma, J., R. Kruisinga, 2011: Nationaal Kader Kust; Naar een veilige, sterke en mooie Noordzeekust, Rijksoverheid, Den Haag Avifauna Groningen, 2009: Polder Breebaart, Found on May the 13th 2011, on www.avifaunagroningen .nl Baarda, B., M. De Goede. (2001). Basisboek Methoden en Technieken, Stenfert Kroese, Groningen Bakker, J., P. Esselink, K. Dijkema, 1997: Salt Marsh Management for Nature Conservation, the Value of Long-term Experiments, University Groningen and IBN-DLO, Texel Baptist, M., marine ecologist, IMARES, Personal communication, March the 30th 2011 Beintema, A., 2007: Van polder naar kwelder, proefverkweldering Noarderleech – een experiment, Altenburg en Wymenga, Veenwouden binnenland.nieuws.nl, 2010: Bodem Groningen-gasveld iets meer gedaald, Found on April the 16th 2011, on http://binnenland.nieuws.nl Common Wadden Sea Secretariat, 2011: Found on Februari the 8th 2011, on www.waddensea-secretariat.org Coumans, C., 2006: Milieudruk in het Eems-Dollard estuarium, RIKZ, Groningen

Deltacommissie, 2008a: FAQ Organisatie, Found on April the 2nd 2011, on www.deltacommissie.com

Deltacommissie, 2008b: Samen werken met water; Een land dat leeft, bouwt aan zijn toekomst, Deltacommissie, Den Haag Deltamagazine, 2011: De Japanse oester: Zeeuwse import schelp als samurai, Found on May the 7th 2011, on www.deltamagazine.nl de Ranitz, C., W. Huisman, S. Puijman, P.Esselink, 2009: Kwelderbeheer; Verslag veldwerkplaats Duin- en kustlandschap, Found on April the 10th 2011, on http://edepot.wur.nl Dijkema, K., A. Nicolai, J. de Vlas, C. Smit, H. Jongerius, H. Nauta, 2001: Van landaanwinning naar kwelderwerken, Grafisch Productiebedrijf Gorter b.v., Steenwijk

61

Dijkema, K., D. de Jong, M. Vreeken-Buijs, W. van Duin, 2005: Kwelders en schorren in de Kaderrichtlijn Water, ALTERRA, Texel Dijkema, K., W. van Duin, E. Dijkman, P. van Leeuwen, 2007: Monitoring van kwelders in de Waddenzee, Alterra, Wageningen Dijkstra, K., 2009: Wilde planten in Nederland en België, Found on April the 17th 2011, on http://wilde-planten.nl

Ecomare, 2011a: Engels slijkgras, Found on April the 17th 2011, on www.ecomare.nl

Ecomare, 2011b: Kwelders (schorren), Found on April the 26th 2011, on www.ecomare.nl Esselink, P., 2000: Nature Management of Coastal Salt Marshes, Koeman en Bijkerk bv – ecologisch onderzoek en advies, Haren Esselink, P., J. Petersen, S. Arens, J. Bakker, J. Bunje, K. Dijkema, N. Hecker, U. Hellwig, A. -V. Jensen, A. Kers, P. Körber, E. Lammerts, M. Stock, R. Veeneklaas, M. Vreeken, M. Wolters, 2009: Salt Marshes. Thematic Report No.8. In: Marencic, H. Vlas, J. de (Eds), 2009 Quality Status Report 2009. Wadden Sea Ecosystem No.25. Common Wadden Sea Secretariat, Trilateral Monitoring and Assessment Group, Wilhelmshaven (Germany) Google Maps, 2011: Found on Januari the 25th 2011, on http://maps.google.nl Kamps, L., 1956: Slibhuishouding en landaanwinning in het oostelijk Waddengebied, Rijkswaterstaat, Friesland en Groningen Klein Tank, A., G. Lenderink (red.), 2009: Klimaatverandering in Nederland; Aanvullingen op de KNMI ’06 scenario’s, KNMI, De Bilt KNMI, 2006: Eerste najaarsstorm 2006 bereikt windkracht 10, Found on April the 21st 2011, on www.knmi.nl KNMI, 2011: Zeespiegelstijging, Found on Januari the 20th 2011, on www.knmi.nl

Kwelderherstel Groningen, 2011a: Het ideaalbeeld van de kwelder, Found on March the 30th 2011, on www.kwelderherstelgroningen.nl

Kwelderherstel Groningen, 2011b: Home, Found on March the 30th 2011, on www.kwelderherstelgroningen.nl

Kwelderherstel Groningen, 2011c: Kwelderherstel Groningen werkt aan drie gebieden, Found on April the 17th 2011, on www.kwelderherstelgroningen.nl Ministerie van Onderwijs, Cultuur en Wetenschap, 2011: Nationaal archief; rijzendam, Found on May the 9th 2011, on www.nationaalarchief.nl

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Ministeries van V&W, LNV, Volkshuisvesting, Ruimtelijke Ordening en Milieubeheer, 2010: Deltaprogramma Waddengebied Samenvatting Plan van Aanpak, Found on March the 30th 2011, on www.deltacommissaris.nl Ministerie van V&W, 2011: Kwelderatlas, Found on May the 26th 2011, on www.rijkswaterstaat.nl Mulder, H., Engineer, Waterdienst Rijkswaterstaat, Personal communication, May the 10th 2011 NAM, 2005: Bodemdaling door Aardgaswinning, Found on April the 5th 2011, on www.waddenzee.nl Natuurinformatie, 2011: Kwelders (Schorren), Found on Februari the 2nd 2011, on www.natuurinformatie.nl Natuurkennis, 2010: Areaalbehoud, Found on May the 19th 2011, on www.natuurkennis.nl Natuur en Milieufederatie Groningen, 2010: Met verdieping vaargeul streeft Eemshaven Antwerpen voorbij, Found on April the 16th 2011, on www.nmfgroningen.nl NOM, 2010: Gasproductie uit Groningen-gasveld op hoogste niveau van deze eeuw, Found on April the 8th 2011, on www.nom.nl Packham, J., A. Willis, 1997: Ecology of Dunes, Salt Marsh and Shingle, Chapman & Hall, London Provincie Groningen, 2010: Briefnummer: 2010-60.520/49/A.8, LGW, Found on April the 7th 2011, on www.provinciegroningen.nl Raad voor de Wadden, 2010: Eems-estuarium: van een gezamenlijk probleem naar een gezamenlijke oplossing, Found on March the 28th 2011, on www.raadvoordewadden.nl Rijksoverheid, 2011: “Water en Veiligheid”, Found on Januari the 20th 2011, on www.rijksoverheid.nl Rijksoverheid, 2010: Nederlands-Duitse samenwerking voor getijdennatuur Eems-Dollard, Found on April the 16th 2011, on www.rijksoverheid.nl Rijkswaterstaat, 2010: Verslag werkconferentie; Helder over slib, Rijkswaterstaat, Leeuwarden

Rijkswaterstaat, 2011a: Vegetatie, Found on April the 14th 2011, on www.rijkswaterstaat.nl

Rijkswaterstaat, 2011b: Zeegras, Found on May the 7th 2011, on www.rijkswaterstaat.nl

Rijkswaterstaat, 2011c: Kustlijnzorg; Procedure, Found on May the 20th 2011, on www.rijkswaterstaat.nl

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RIKZ, 2004: Bodemdalingstudie Waddenzee 2004, Found on April the 4th 2011, on www.waddenzee.nl Schwartz, M., 2005: Encyclopedia of coastal science, Springer, Dordrecht Schorrenwerkgroep, 2011: Engels slijkgras – Spartina townsendii, Found on May the 19th 2011, on http://scheldeschorren.be Sips, H., C. de Leeuw, 2009: Natuurherstelplan Waddenzee; Bouwsteen thema 1: Wadbodem en waterkolom, Ministerie van LNV Stichting Het Groninger Landschap, IVN consulentschap, Landschapsbeheer Groningen, Natuurmonumenten, Natuur en milieufederatie Groningen, Staatsbosbeheer, 2011: Ecologie en Economie in de Eems-Dollard, Found on May the 9th 2011, on www.nmfgroningen.nl Talke, S., H. de Swart, 2006: Hydrodynamics and Morphology in the Ems/Dollard Estuary: Review of models, measurements, scientific literature and the effects of changing conditions, Found on April the 4th 2011, on www.depts.washington.edu USDA, 1996: Soil Quality Resource Concerns: Sediment Deposition on Cropland, USDA, Washington Van der Lee, W., 2000: The settling of mud flocs in the Dollard estuary, The Netherlands, University of Utrecht, Utrecht Van Duin, W., DLO Researcher, IMARES, Personal communication, March the 18th 2011 Van Duin, W., K. Dijkema, P. van Leeuwen, 2008: Jaarrapportage 2007: vegetatie en opslibbing in de Peazemerlannen en referentiegebied west-Groningen, IMARES, Wageningen Van Leeuwe, M., 2010: Sedimentatie en zeespiegelsijging, Found on April the 3rd 2011, on www.waddenacademie.knaw.nl Van Loon-Steensma J. M., 2011: Kweldervorming langs de Terschellinger Waddendijk, Alterra, Wageningen Van ’t Hof, P., 2006: Lange-termijn trends van fauna en biotopen in het Eems-Dollard gebied, Alterra, Texel Vastenhouw, B., 2010. De kwelders gezien vanaf de zeedijk, Found on March the 4th 2011, on http://graphics.tudelft.nl Vreeken-Buijs, M., 2002: Toelichting bij de vegetatiekartering Dollard & Punt van Reide 1999, RWS, Delft Waddenacademie, 2009: Naar een rijke Waddenzee, Waddenacademie, Leeuwarden Waddenvereniging, 2009: Nieuwsbrief voor het waddengebied; nummer 7, Found on May the 6th 2011, on www.waddenvereniging.nl

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Waddenvereniging, 2011a: Zeespiegelstijging, vragen, Found on April the 7th 2011, on www.waddenvereniging.nl

Waddenvereniging, 2011b: Red de Eems; Historie, Found on April the 17th 2011, on www.waddenvereniging.nl Waddenvereniging, 2007: Red de Eems, Found on May the 21st 2011, on www.watersportalmanak.nl WaddenZee.nl, 2008: Storm en hoog water, Found on April the 16th 2011, on www.waddenzee.nl WaddenZee.nl, 2001: Sedimentatlas Waddenzee, Found on May the 20th 2011, on www.waddenzee.nl Wadden Sea World Heritage, 2011: Ems-Dollard, Found on April the 13th 2011, on www.waddensea-worldheritage.org WOK, 2010: 50 jaar monitoring en beheer van de Friese en Groninger kwelderwerken: 1960-2009, IMARES, Wageningen ZOETZOUT, 2011: Eems-Dollard estuarium, Found on April the 13th 2011, on www.zoetzout.waddenloket.nl

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Appendix 1

land subsidence due to gas extraction from the Groningen-gasveld (NAM, 2005)

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Appendix 2

Vegetation map of the Dollard (Van ‘t Hof, 2006). Altered by removing the, for this report, irrelevant vegetation types from the legend.

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Appendix 3

Sedimentation and erosion

68

Tidal maps

69

70

71

72

73

74

75

Current velocity

Wave velocity

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Buildings

De-embanked Breebaartpolder

77

Primary coastal defence system

Most recent vegetation map of salt marshes in the estuary that has been found during the study for this report

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Depth

Duration of being above sea level

79

Intertidal areas

Sediment size

80

Silt concentration in the soil

Silt concentration in the water

81

Waterways

Oyster banks

82

Mussel banks

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Appendix 4 1A (1): Sedimentation on a few small areas surrounded by relatively many areas with erosion 1A (2): A relatively high current velocity 1A (3): Moderate to low wave velocity 1A (4): Ems Harbour 1A (5): No salt marsh 1A (6): A relatively deep and steep terrain 1A (7): Relatively low concentration of silt and high concentration of large sediment particles 1A (8): Relatively low concentration of silt 1A (9): Relatively short distance to the nearest waterway 1A (10): No oyster or mussel banks 1A (11): Not bordering a salt marsh 1B (1): Sedimentation on a few small areas surrounded by relatively many areas with erosion 1B (2): A relatively high current velocity 1B (3): Moderate to low wave velocity 1B (4): Ems Harbour 1B (5): No salt marsh 1B (6): A relatively deep and steep terrain 1B (7): Relatively low concentration of silt and high concentration of large sediment particles 1B (8): Relatively low concentration of silt 1B (9): Relatively short distance to the nearest waterway 1B (10): No oyster or mussel banks 1B (11): Not bordering a salt marsh 2A (1): As many areas with erosion as with sedimentation 2A (2): A moderate to low current velocity 2A (3): Low wave velocity 2A (4): Build up area 2A (5): No salt marsh 2A (6): A relatively deep and steep terrain 2A (7): Relatively low concentration of silt 2A (8): Relatively low concentration of silt 2A (9): Relatively moderate distance to the nearest waterway 2A (10): Hardly visible on the map but there seems to be a small area with mussel banks 2A (11): Not bordering a salt marsh 2B (1): As many areas with erosion as with sedimentation 2B (2): A moderate to low current velocity 2B (3): Low wave velocity 2B (4): No structures 2B (5): No salt marsh 2B (6): A relatively deep and steep terrain 2B (7): Relatively low concentration of silt 2B (8): Relatively low concentration of silt 2B (9): Relatively moderate distance to the nearest waterway 2B (10): Hardly visible on the map but there seems to be a small area with mussel banks 2B (11): Not bordering a salt marsh 3A (1): A relatively high sedimentation rate 3A (2): A moderate to low current velocity 3A (3): Low wave velocity 3A (4): No structures

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3A (5): No salt marsh 3A (6): A relatively large shallow terrain 3A (7): High concentration of silt, no evidence of high concentration of larger sediment particles 3A (8): Relatively high concentration of silt 3A (9): Relatively large distance to the nearest waterway 3A (10): Oyster and mussel banks present 3A (11): Not bordering a salt marsh 3B (1): A relatively high sedimentation rate 3B (2): A moderate to low current velocity 3B (3): Low wave velocity 3B (4): No structures 3B (5): No salt marsh 3B (6): A relatively large shallow terrain 3B (7): High concentration of silt, no evidence of high concentration of larger sediment particles 3B (8): Relatively high concentration of silt 3B (9): Relatively large distance to the nearest waterway 3B (10): Oyster and mussel banks present 3B (11): Not bordering a salt marsh 4A (1): A relatively high sedimentation rate 4A (2): A moderate to low current velocity 4A (3): Low wave velocity 4A (4): No structures 4A (5): No salt marsh 4A (6): A relatively large shallow terrain 4A (7): High concentration of silt, no evidence of high concentration of larger sediment particles 4A (8): Relatively moderate concentration of silt 4A (9): Relatively large distance to the nearest waterway 4A (10): Oyster and mussel banks present 4A (11): Not bordering a salt marsh 4B (1): A relatively high sedimentation rate 4B (2): A moderate to low current velocity 4B (3): Low wave velocity 4B (4): No structures 4B (5): No salt marsh 4B (6): A relatively large shallow terrain 4B (7): High concentration of silt, no evidence of high concentration of larger sediment particles 4B (8): Relatively moderate concentration of silt 4B (9): Relatively large distance to the nearest waterway 4B (10): Oyster and mussel banks present 4B (11): Not bordering a salt marsh 5A (1): Hard to see on the map but what is shown indicates a relatively high sedimentation rate 5A (2): Low current velocity 5A (3): Low wave velocity 5A (4): No structures 5A (5): No salt marsh

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5A (6): The depth and slope of the terrain are relatively moderate 5A (7): High concentration of silt, no evidence of high concentration of larger sediment particles 5A (8): Relatively high concentration of silt 5A (9): Relatively short distance to the nearest waterway 5A (10): No oyster or mussel banks 5A (11): Not bordering a salt marsh 5B (1): Hard to see on the map but what is shown indicates a relatively high sedimentation rate 5B (2): Low current velocity 5B (3): Low wave velocity 5B (4): No structures 5B (5): No salt marsh 5B (6): The depth and slope of the terrain are relatively moderate 5B (7): High concentration of silt, no evidence of high concentration of larger sediment particles 5B (8): Relatively high concentration of silt 5B (9): Relatively short distance to the nearest waterway 5B (10): No oyster or mussel banks 5B (11): Not bordering a salt marsh 6A1 (1): Hard to see on the map but what is shown indicates a relatively high sedimentation rate 6A1 (2): Low current velocity 6A1 (3): Low wave velocity 6A1 (4): No structures 6A1 (5): No salt marsh 6A1 (6): The depth and slope of the terrain are relatively moderate 6A1 (7): High concentration of silt, no evidence of high concentration of larger sediment particles 6A1 (8): Relatively moderate concentration of silt 6A1 (9): Relatively short distance to the nearest waterway 6A1 (10): No oyster or mussel banks 6A1 (11): Not bordering a salt marsh 6A2 (1): Hard to see on the map but what is shown indicates a relatively high sedimentation rate 6A2 (2): Low current velocity 6A2 (3): Low wave velocity 6A2 (4): Delfzijl 6A2 (5): No salt marsh 6A2 (6): A relatively deep and steep terrain 6A2 (7): High concentration of silt, no evidence of high concentration of larger sediment particles 6A2 (8): Relatively high concentration of silt 6A2 (9): Relatively short distance to the nearest waterway 6A2 (10): No oyster or mussel banks 6A2 (11): Not bordering a salt marsh 6B1 (1): Hard to see on the map but what is shown indicates a relatively high sedimentation rate 6B1 (2): Low current velocity 6B1 (3): Low wave velocity

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6B1 (4): No structures 6B1 (5): No salt marsh 6B1 (6): A relatively deep and steep terrain 6B1 (7): High concentration of silt, no evidence of high concentration of larger sediment particles 6B1 (8): Relatively high concentration of silt 6B1 (9): Relatively short distance to the nearest waterway 6B1 (10): No oyster or mussel banks 6B1 (11): Not bordering a salt marsh 6B2 (1): Hard to see on the map but what is shown indicates a relatively high sedimentation rate 6B2 (2): Low current velocity 6B2 (3): Low wave velocity 6B2 (4): Harbour of Delfzijl 6B2 (5): No salt marsh 6B2 (6): A relatively deep and steep terrain 6B2 (7): High concentration of silt, no evidence of high concentration of larger sediment particles 6B2 (8): Relatively high concentration of silt 6B2 (9): Relatively short distance to the nearest waterway 6B2 (10): No oyster or mussel banks 6B2 (11): Not bordering a salt marsh 7A1 (1): A relatively high sedimentation rate 7A1 (2): Low current velocity 7A1 (3): Moderate wave velocity 7A1 (4): No structures 7A1 (5): No salt marsh 7A1 (6): A relatively large shallow terrain 7A1 (7): High concentration of silt, no evidence of high concentration of larger sediment particles 7A1 (8): Relatively high concentration of silt 7A1 (9): Relatively large distance to the nearest waterway 7A1 (10): No oyster or mussel banks 7A1 (11): Not bordering a salt marsh 7A2 (1): A relatively high sedimentation rate 7A2 (2): Low current velocity 7A2 (3): Moderate wave velocity 7A2 (4): Build up area 7A2 (5): No salt marsh 7A2 (6): A relatively large shallow terrain 7A2 (7): High concentration of silt, no evidence of high concentration of larger sediment particles 7A2 (8): Relatively high concentration of silt 7A2 (9): Relatively large distance to the nearest waterway 7A2 (10): No oyster or mussel banks 7A2 (11): Not bordering a salt marsh 7A3 (1): A relatively high sedimentation rate 7A3 (2): Low current velocity 7A3 (3): Moderate wave velocity 7A3 (4): No structures

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7A3 (5): No salt marsh 7A3 (6): A relatively large shallow terrain 7A3 (7): High concentration of silt, no evidence of high concentration of larger sediment particles 7A3 (8): Relatively moderate concentration of silt 7A3 (9): Relatively large distance to the nearest waterway 7A3 (10): No oyster or mussel banks 7A3 (11): Bordering a salt marsh 7B (1): A relatively high sedimentation rate 7B (2): Low current velocity 7B (3): Moderate wave velocity 7B (4): No structures 7B (5): No salt marsh 7B (6): A relatively large shallow terrain 7B (7): High concentration of silt, no evidence of high concentration of larger sediment particles 7B (8): Relatively high concentration of silt 7B (9): Relatively large distance to the nearest waterway 7B (10): No oyster or mussel banks 7B (11): Bordering a salt marsh 8A (1): A large area in this location is eroding 8A (2): A moderate current velocity 8A (3): High wave velocity 8A (4): No structures 8A (5): Salt marsh 8A (6): A relatively deep and steep terrain 8A (7): High concentration of silt, no evidence of high concentration of larger sediment particles 8A (8): Relatively moderate to low concentration of silt 8A (9): Relatively short distance to the nearest waterway 8A (10): No oyster or mussel banks 8A (11): Bordering a salt marsh 8B (1): A large area in this location is eroding 8B (2): A moderate current velocity 8B (3): High wave velocity 8B (4): No structures 8B (5): Salt marsh 8B (6): A relatively deep and steep terrain 8B (7): High concentration of silt, no evidence of high concentration of larger sediment particles 8B (8): Relatively moderate to low concentration of silt 8B (9): Relatively short distance to the nearest waterway 8B (10): No oyster or mussel banks 8B (11): Bordering a salt marsh 9A (1): A relatively high sedimentation rate 9A (2): Low current velocity 9A (3): Low wave velocity 9A (4): Breebaartpolder 9A (5): No salt marsh 9A (6): The depth and slope of the terrain are relatively moderate

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9A (7): High concentration of silt, no evidence of high concentration of larger sediment particles 9A (8): Relatively high concentration of silt 9A (9): Relatively large distance to the nearest waterway 9A (10): No oyster or mussel banks 9A (11): Bordering a salt marsh 9B (1): A relatively high sedimentation rate 9B (2): Low current velocity 9B (3): Low wave velocity 9B (4): Breebaartpolder 9B (5): No salt marsh 9B (6): The depth and slope of the terrain are relatively moderate 9B (7): High concentration of silt, no evidence of high concentration of larger sediment particles 9B (8): Relatively high concentration of silt 9B (9): Relatively large distance to the nearest waterway 9B (10): No oyster or mussel banks 9B (11): Bordering a salt marsh 10A (1): A relatively high sedimentation rate 10A (2): Low current velocity 10A (3): Moderate wave velocity 10A (4): No structures 10A (5): Salt marsh 10A (6): A relatively large shallow terrain 10A (7): High concentration of silt, no evidence of high concentration of larger sediment particles 10A (8): Relatively high concentration of silt 10A (9): Relatively large distance to the nearest waterway 10A (10): No oyster or mussel banks 10A (11): Bordering a salt marsh 10B (1): A relatively high sedimentation rate 10B (2): Low current velocity 10B (3): Moderate wave velocity 10B (4): No structures 10B (5): Salt marsh 10B (6): A relatively large shallow terrain 10B (7): High concentration of silt, no evidence of high concentration of larger sediment particles 10B (8): Relatively high concentration of silt 10B (9): Relatively large distance to the nearest waterway 10B (10): No oyster or mussel banks 10B (11): Bordering a salt marsh 11A (1): Erosion 11A (2): Low current velocity 11A (3): Moderate wave velocity 11A (4): No structures 11A (5): Salt marsh 11A (6): A relatively large shallow terrain 11A (7): High concentration of silt, no evidence of high concentration of larger sediment particles

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11A (8): Relatively high concentration of silt 11A (9): Relatively short distance to the nearest waterway 11A (10): No oyster or mussel banks 11A (11): Bordering a salt marsh 11B (1): Erosion 11B (2): Low current velocity 11B (3): Moderate wave velocity 11B (4): No structures 11B (5): Salt marsh 11B (6): A relatively large shallow terrain 11B (7): High concentration of silt, no evidence of high concentration of larger sediment particles 11B (8): Relatively high concentration of silt 11B (9): Relatively short distance to the nearest waterway 11B (10): No oyster or mussel banks 11B (11): Bordering a salt marsh

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Appendix 5 1A: No method has been chosen because developing a salt marsh on this location would mean that a part of the Ems harbour has to be excavated which does not seem to be realistic. 1B: Method 3 has been selected because of the depth of the location and the surrounding area. If necessary, the deposition rate could be increased by incorporating method 9. 2A: No method has been chosen because developing a salt marsh on this location would mean that a relatively densely build up area has to be excavated which does not seem to be realistic. 2B: Method 3 has been selected because of the depth of the location and the surrounding area. Method 8 and 9 could increase the deposition rate, if necessary. The presence of oyster and mussel banks nearby seems to offer a good opportunity to use method 8. 3A: Method 6 has been selected because for the development of a salt marsh inland on a different type of location than a summer polder method 6 is more promising because the halophytic vegetations that arise as a result from method 7 are not believed to be an alternative for salt marshes. If necessary, the deposition rate could be increased by incorporating method 2, 4, 8 and 9. The presence of oyster and mussel banks nearby seems to offer a good opportunity to use method 8. 3B: Method 2 is estimated to be more efficient than building a dam in this location because the area is relatively shallow and method 2 is less likely to cause erosion in surrounding areas. If necessary, the deposition rate could be increased by incorporating method 4, 8 and 9. The presence of oyster and mussel banks nearby seems to offer a good opportunity to use method 8. 4A: See 3A 4B: See 3B 5A: Method 6 has been selected because for the development of a salt marsh inland on a different type of location than a summer polder method 6 is more promising because the halophytic vegetations that arise as a result from method 7 are not believed to be an alternative for salt marshes. If necessary, the deposition rate could be increased by incorporating method 2, 4, 8 and 9. The presence of mussel banks nearby seems to offer a good opportunity to use method 8. The silt concentration is relatively high however. 5B: Method 2 has been chosen for this location because the distance to the nearby waterway seems too short to build a dam. Method 1 is considered to be less promising because using this method would not lead to the formation of a salt marsh. If necessary, the deposition rate could be increased by incorporating method 4, 8 and 9. The presence of mussel banks nearby seems to offer a good opportunity to use method 8. The silt concentration is relatively high however. 6A1: Method 6 has been selected because for the development of a salt marsh inland on a different type of location than a summer polder method 6 is more promising because the halophytic vegetations that arise as a result from method 7 are not believed to be an alternative for salt marshes. If necessary, the deposition rate could be increased by incorporating method 2, 4, 8 and 9. 6A2: No method has been chosen because developing a salt marsh on this location would mean that a part of (the harbour of) Delfzijl has to be excavated which does not seem to be realistic 6B1: Method 2 has been chosen for this location because the distance to the nearby waterway seems too short to build a dam. Method 1 is considered to be less promising because using this method would not lead to the formation of a salt marsh. If necessary, the deposition rate could be increased by incorporating method 4, 8 and 9. 6B2: No method has been selected because it is unclear how the development of a salt marsh on this location would contribute to the protection of the coast against flooding

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7A1: See 6A1 7A2: See 2A 7A3: See 6A1 7B: Method 2 is estimated to be more efficient than building a dam in this location because the area is relatively shallow and method 2 is less likely to cause erosion in surrounding areas. If necessary, the deposition rate could be increased by incorporating method 4, 8 and 9. 8A: No method has been selected because it is unclear how the additional development of a salt marsh on this location would contribute to the protection of the coast against flooding 8B: No method has been selected because it is unclear how the development of a salt marsh on this location would contribute to the protection of the coast against flooding 9A: No method has been chosen because developing a salt marsh on this location would mean that a part of the Breebaartpolder has to be excavated which does not seem to be realistic 9B: No method has been chosen because developing a salt marsh on this location would mean that it would be situated in front of the Breebaartpolder which does not seem to be realistic 10A: See 8A 10B: See 8A 11A: See 8A 11B: See 8A

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Appendix 6 1: Sedimentation and erosion (“Sedimentation and erosion”) - The sedimentation or erosion rates give a good indication about the suitability of a location for salt marsh development because these numbers reflect the circumstances that are present and if it can be expected that a salt marsh would form. It gives a better indication than the current velocities for instance because they are more or less the outcome of a field test which takes into account various factors. Locations with higher deposition rates are also preferable because this increases the chance of the salt marsh to cope with the accelerated sea level rise. Long term changes in sediment transports and the tidal range can not yet be predicted however (Talke et al., 2006). Therefore we will point out these more suitable locations based on the sedimentation and erosion map in appendix 3. 2: Current velocity (“tidal maps” and “Current velocity”) - The current velocities are estimated to be one of the most influential factors on salt marsh development because, as has been explained in the previous chapters, the high currents seem to be the main reason why salt marshes are eroding and prevented from forming in the estuary. A lower current velocity is therefore preferable. 3: Wave velocity (“Wave velocity”. This criterium has not been incorporated in criterium 2 because this map and the map “Current velocity” came from the same source which leads to believe that the wave velocity has not been incorporated in the map “Current velocity” and is therefore not counted twice. Using it as a separate criterium also enables the possibility to divide the locations in conformity with this data). - The wave velocity is a form of current velocity and is expected to be less influential on the formation of salt marshes in the estuary than all the currents combined. The way in which it impacts the salt marshes is the same in the sense that it applies a force to the soil that could exceed the bed’s shear strength. A lower wave velocity is therefore preferable. 4: The presence of valuable buildings or other man made structures (“Buildings”, the map “De-embanked Breebaartpolder” and “Primary coastal defence system”. This last map also shows the dams belonging to the harbour of Delfzijl and the Ems harbour) - The structures which are mentioned in criterium 4 are of such high value for obvious reasons that they limit the possibilities for salt marsh formation on these locations. The dams of the harbor of Delfzijl also influence the locations more seaward because the contribution to the protection of the coast against flooding by developing salt marshes on these locations is unclear. These structures therefore make the locations less preferable. 5: The presence of salt marshes in the foreland (“Most recent vegetation map of salt marshes in the estuary that has been found during the study for this report ”) - The presence of salt marshes in the foreland reduces the role new salt marshes could have in protecting the coast against flooding because 100 meters of salt marsh seaward is expected to be enough. This criterium is less important than the previous one because the creation of additional salt marshes here would not have (such) a damaging effect. Locations in front of or behind existing salt marshes are therefore less preferable. 6: The depth of the terrain (“Depth”, “Duration of being above sea level” and “Intertidal areas”. Here three maps have been used instead of one because they fill up deficits in knowledge which are unanswered by either of the maps alone. For instance: the map called “depth” does not contain information about whether these are the heights in comparisment to NAP or the mean low sea level for instance. The map called “Duration of being above sea level” shows that the areas that are 1 meter or higher according to the map called “Depth” completely fall into the regions of the estuary which are flooded by the sea for a relatively short amount of time. The map called “Intertidal areas” shows that these regions are still flooded with sea water twice a day however. Because the tidal range in the estuary is close to

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3 meters we can conclude that the values of map called “Depth” are according to NAP or close to it and therefore provide a useful basis to estimate which locations are more or less suitable, presuming that the circumstances have not changed dramatically between the creations of the three maps). - The depth of the terrain corresponds with how promising the location is for salt marsh development because, in general, more silt is being deposited in shallow waters due to the lower current velocities. No limiting value has been found during the study of this report that tells which depth of the sea floor renders a terrain unsuitable for salt marsh development. Therefore we assume that deeper areas could be suitable, for instance by constructing sediment fields or a dam but that shallower areas are more promising. The depth of the surrounding terrain is also taken into account because a shallower foreland reduces the current velocities. 7: Sediment size (“Sediment size” and “Silt concentration in the soil”. This second map has been added because the first map alone does not explain how much silt is being deposited in each area. These maps together give a better indication of where the conditions are favourable for the development of a salt marsh) - In coordinance with criterium 6, the sediment size of the soil also gives an indication of which locations are situated in a more favorable environment for the formation of a salt marsh because the composition of the soil corresponds with factors like current velocity and the silt deposition. It is valued as being less important than the previous criterium because the restraining effects of this criterium on salt marsh development are estimated to be less evident. A high value for the sediment size makes a location less suitable for the development of a salt marsh as does a low value for the silt concentration in the soil. 8: Silt concentration in the water (“Silt concentration in the water”) - The silt concentration in the water is of importance because the formation of a salt marsh is dependable on the silt concentration. This criterium is estimated to be less important than the previous criteria however because the silt concentration in the estuary is said to be high, due to clouding. The map in appendix 3 shows a lot of variation in the concentration of several areas however, it is unclear when this map has been made and which time frame it represents. Therefore we will use it as an indication but add more value to the information from paragraph 4.1. This criterium is also of importance to the formation of oyster and mussel banks because at a certain level it makes it harder for these species to survive. The silt concentration does not translate one on one to the level of cloudiness or the amount of other sediments in the water however it seems safe to assume that 90% to 100% of silt in the water will make it difficult to survive. Therefore we will use this as an upper value to divide the different locations and as an indication for the chances of survival for oyster and mussel banks in the next paragraph. In the MCA, the silt concentration is only compared in relation to deposition and not in relation to molluscs and is awarded a higher score when the concentration is high. 9: The distance to nearby waterways (“Waterways”) - A relatively short distance to a nearby waterway presents a risk for the development of a salt marsh because it means that the currents in the environment are higher. It also means that the slope of the nearby mudflat is steeper which offers less protection to those currents and to wave action. This criterium has been placed lower in this list however because much of these influences are already taken into account by previous criteria. A large distance to the nearest waterway is more favourable then a short distance and therefore awarded with a higher score in the MCA. 10: Nearby presence of oyster or mussel banks (“Oyster banks”and “Mussel banks”) - When oyster or mussel banks are present in the environment it gives an indication that the conditions for the creation of new oyster or mussel banks are good. Because the influence of

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these banks to salt marsh formation (reducing current velocities, adding silt) is estimated to be less significant in comparisment with the previous criteria, this criterium has been placed low on the list. The presence of these banks makes a location more favourable. 11: Contribution to the total acreage of a bordering salt marsh (“Most recent vegetation map of salt marshes in the estuary that has been found during the study for this report”) - The potential to contribute to the size of the already existing salt marshes is considered to be an extra in the sense that it does not make such a large difference that a salt marsh could not survive when it has the worst possible score on this criterium. Scoring high on this criterium also does not seem to imply as much of an improvement in the chances for development of a salt marsh as a good score on the other criteria would. It would imply that the chances for rejuvenation and a higher species diversity of the new salt marsh and the bordering salt marsh are better.