master thesis - dhi · pdf filenumerical model was set up in mike 21 fm with a hydrodynamic...
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D E P A R T M E N T O F G E O S C I E N C E S A N D N A T U R A L R E S O U R C E M A N A G E M E N T
F A C U L T Y O F S C I E N C E
U N I V E R S I T Y O F C O P E N H A G E N
Master thesis
Anders Borregaard Stephensen
Numerical modelling of deposition of fine-
grained sediment in Fanø Marina and possible
remedial actions to reduce the annual
sedimentation rate
A case study from Fanø Marina, in the Danish Wadden Sea
Academic advisor: Jesper Bartholdy
Submitted: 01/06/16
Name of department: Department of Geosciences and Natural Resource
Management
Author: Anders Borregaard Stephensen Title: Numerical modelling of deposition of fine-grained sediment
in Fanø Marina and possible remedial actions to reduce the annual sedimentation rate
Academic advisor: Jesper Bartholdy External advisor: Klavs Eske Bundgaard External institution: Danish Hydraulic Institute (DHI) – Department of Water
and Environment Submitted: 1st of June 2016 Front page illustration: Photography of Fanø Marina (Taken by the author).
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Abstract Fanø Marina, Fanø Yacht Club and Fanø Municipality are collaborating to determine
the reasons for sedimentation within the marina basin in order to implement
possible remedial actions to minimize the sedimentation rate. In the present thesis a
numerical model was set up in MIKE 21 FM with a hydrodynamic module coupled
with a cohesive sediment transport module. The model was calibrated with field
data from the study area collected in the summer and autumn 2015. It was shown
analytically, from in situ measurements and from numerical modelling that the tidal-
induced and current-induced exchange were significantly contributing to the
sedimentation. On the basis of this finding 10 scenarios were tested with a view to
reduce the sedimentation within Fanø Marina. The approach of Keeping Sediment
Out (KSO) applied in Scenarios 1 - 9 was shown to reduce the annual sedimentation
rate by up to 15-30 %. This was achieved by establishing a current deflection wall
(Scenario 2), by establishing sheet pile walls along the jetties (Scenarios 1, and 6-9)
or narrowing the marina entrance (Scenarios 3, 5 and 6). The approach of Keeping
Sediment Moving (KSM) (Scenario 10) was not successful.
Keywords: Fanø Marina; Grådyb tidal area; Estuarine environment; Hydrodynamics;
Cohesive sediments; Numerical modelling; Reduction of marina sedimentation.
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Contents
Abstract .........................................................................................................................................................i
1 Introduction ..................................................................................................................................... 2
1.1 Aim of study ........................................................................................................................ 3
2 Scientific background................................................................................................................... 5
2.1 Estuary .................................................................................................................................. 5
2.2 Cohesive and non-cohesive sediment ...................................................................... 5
2.3 Pelagic processes .............................................................................................................. 7
2.3.1 Advection and dispersion ............................................................................................ 7
2.3.2 Flocculation and settling velocity ............................................................................. 7
2.4 Benthic processes ............................................................................................................. 8
2.4.1 Erosion and deposition ................................................................................................. 8
2.4.2 Consolidation and resuspension ............................................................................... 9
2.5 Sediment transport in relations to tidal currents .............................................. 10
2.5.1 Tidal asymmetry ............................................................................................................ 10
2.5.2 Lag effects ......................................................................................................................... 10
2.6 Sedimentation processes in harbor basins ........................................................... 11
2.7 Minimizing harbor sedimentation ........................................................................... 12
3 Study area ....................................................................................................................................... 15
3.1 Physical settings .............................................................................................................. 16
3.2 Fanø Marina ...................................................................................................................... 19
4 Methods ........................................................................................................................................... 24
4.1 MIKE 21 Flow Model FM .............................................................................................. 24
4.2 Model set-up ..................................................................................................................... 24
4.2.1 Bathymetry and domain ............................................................................................. 24
4.2.2 Simulation period ..................................................................................................... 29
4.2.3 Module selection ....................................................................................................... 29
4.2.3.1 Hydrodynamic module (HD) ................................................................................ 30
4.2.3.2 Mud Transport module (MT) ............................................................................... 31
4.3 Measured data ................................................................................................................. 32
4.3.1 Water column measurements ............................................................................. 34
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4.3.2 Water level .................................................................................................................. 34
4.3.3 Wind .............................................................................................................................. 35
4.3.4 Bed samples ................................................................................................................ 35
4.4 Data processing ............................................................................................................... 38
4.4.1 Current velocity ........................................................................................................ 38
4.4.2 Sediment transport calculations ........................................................................ 39
4.5 Calibration of model ...................................................................................................... 40
4.6 Minimizing harbor sedimentation ........................................................................... 45
5 Fieldwork campaigns ................................................................................................................. 47
5.1 Stations 1 and 2 June 3-8, 2015 ................................................................................ 47
5.2 Stations 3 and 4 July 20-22, 2015 ............................................................................ 51
5.3 Stations 5 and 6 October 30 to November 27, 2015 ......................................... 54
5.3.1 Stations 5 and 6 November 1-7, 2015 .............................................................. 57
5.3.2 Stations 5 and 6 November 12-17, 2015 ......................................................... 60
5.3.3 Stations 5 and 6 November 21-27, 2015........................................................ 63
5.4 Discussion of the hydrodynamics and sediment dynamics ........................... 65
5.4.1 Incident of large import on June 4, 2015 .............................................................. 67
5.5 Summarize of fieldwork campaigns ........................................................................ 69
6 Validation of model ..................................................................................................................... 71
6.1 Validation .......................................................................................................................... 71
6.2 Hypsography .................................................................................................................... 76
7 Analysis of sedimentation processes ................................................................................... 77
7.1 General sedimentation processes ............................................................................ 77
7.2 Bed shear stress .............................................................................................................. 78
7.3 Current-induced exchange.......................................................................................... 79
7.4 Sedimentation map ........................................................................................................ 80
7.5 Evaluation of model results ........................................................................................ 81
8 Evaluation of possible remedial actions ............................................................................. 87
8.1 Scenario 1 - Sheet pile walls along the jetties ..................................................... 87
8.2 Scenario 2 - Expansion of pier with a current deflection wall ...................... 90
8.3 Scenario 3 - Narrowing of the entrance ................................................................. 91
8.4 Scenario 4 - Expansion of the eastern pier ........................................................... 92
8.5 Scenario 5 - Narrowing of the entrance ................................................................. 94
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8.6 Scenario 6 - Narrowing of the entrance and establishment of sheet pile
walls along the jetties ..................................................................................................................... 95
8.7 Scenario 7 - Establishment of sheet pile walls along the jetties and
dredging in the southwest corner.............................................................................................. 96
8.8 Scenario 8 - Establishment of a truncated sheet pile wall along the
northern jetty ..................................................................................................................................... 98
8.9 Scenario 9 - Establishment of a sheet pile wall along the northern jetty . 99
8.10 Scenario 10 - Expansion of and establishment of a flow through the pier
100
8.11 Summary of tested scenarios .................................................................................. 102
8.12 Discussion of possible remedial actions ............................................................. 103
9 Conclusion ................................................................................................................................... 106
Acknowledgement ............................................................................................................................. 109
References ............................................................................................................................................ 110
Appendix ............................................................................................................................................... 115
Appendix A Statistical description of bed samples .................................................. 115
Appendix B Outline proposal by Sunke Arkitekter .................................................. 116
Appendix C Current patterns and sediment concentration patterns and data disc
with calculations ............................................................................................................................ 117
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1 Introduction Estuaries are semi-enclosed bodies with a connection to the open sea diluted with
fresh water draining into them and they are acting as depositional environments for
sediment exchanged between the ocean and the land (Pedersen, 2004). Estuaries of
the type formed behind barrier islands, like the Wadden Sea, are in general
importing fine-grained sediment from the adjacent sea (Pedersen & Bartholdy,
2006; Bartholdy & Madsen, 1985; Postma, 1981). Before the fine-grained sediment
becomes a part of the depositional environment within the estuary a complicated
cycle of erosional and depositional patterns are taking place. Among the most
important processes contributing to the complexity of fine-grained material are
flocculation, estuarine circulation, tidal asymmetry as well as and settling and scour
lag (van Maren & Winterwerp, 2013; Bartholdy, 2000; Masselink & Hughes, 2003).
Furthermore, the net annual import of fine-grained sediment is reported to be
exchanged within a few tidal cycles (Bartholdy & Anthony, 1998; Andersen & Pejrup,
2001).
The suspended sediment within many estuaries worldwide consists of fine-grained
sediment below 63 µm in grain size, which gives them a cohesive nature, and the
sediment grains tend to strongly adhere to each other. A quantification of the
accumulation of fine-grained sediment is therefore of great importance for both
economic and ecologic reasons. In terms of the environmental aspects nutrients,
pesticides, herbicides and heavy metals tend to stick strongly to cohesive sediments
and a knowledge about the transport pathways and the potential for erosion and
deposition are therefore of great importance (Andersen & Pejrup, 2001). For
economic reasons it is important to have knowledge about the deposition of fine-
grained sediment in relation to the future planning of a new port, an optimizing of
present marina designs as well as knowledge about the routine cost of dredging
(Whitehouse et al., 2000).
In general there is a lot of interest for estuaries as they may generate recreational
opportunities for a large population worldwide, transportation pathways for
harbors, high biodiversity and unique nature (Pedersen, 2004). However, estuaries
are also vulnerable due to future changes in natural processes that may occur over
time, which is why simulations models have become of great importance. There is a
great potential in numerical models such as MIKE 21/3, which is a useful tool in
future cohesive sediment management (Violeau et al, 2002). The dynamics behind
the cohesive sediments are primarily based on empirical observations as the
processes involved are interacting and affected by a lot of parameters, which are not
possible to determine theoretically. Field data is a very important factor when
adapting a model to a local environment (DHI, 2014b; Andersen, 2001; Andersen &
Pejrup, 2001).
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Fanø Marina, Fanø Yacht Club and Fanø Municipality, located in the Danish Wadden
Sea, are collaborating to determine the reasons for sedimentation within the harbor
basin with a view to implement remedial actions to reduce the sedimentation. The
harbor is seen in Figure 1-1.
Figure 1-1 Fanø Marina (From: www.fanoesejlklub.dk)
In this context a meeting was held in June 2012 at Fanø Marina in order to discuss
the issues related to the marina. The meeting resulted in an agreement where Hans
Jacob Vested from Danish Hydraulic Institute (DHI) and Jesper Bartholdy from
Department of Geosciences and Natural Resource Management, University of
Copenhagen, should conduct a project in order to examine the problem related to
sedimentation within the harbor and to make a recommendation for future remedial
actions. The project was initiated in 2015 and field data was collected during the
summer and autumn of 2015 with subsequent data processing and set-up of a local
hydrodynamic and cohesive sediment transport model in MIKE 21.
1.1 Aim of study The aim of the thesis is to investigate the hydrodynamic conditions and sediment
concentration processes in terms of transport, deposition and erosion of cohesive
sediment in Fanø Marina located in Grådyb tidal area, the Danish Wadden Sea. The
study was performed in connection with the project to evaluate and minimize the
sedimentation rate in Fanø Marina and was carried out by using numerical
modelling.
Description and analysis of the hydrodynamic and sediment dynamics in the
area of Fanø Marina during the field campaigns from June 3-8, July 20-22 and
October 30 to November 27, 2015.
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The collected data were used to set-up and calibrate a local MIKE 21 model in
order to describe the dynamics. Furthermore the modeled results were
validated and evaluated.
Simulation and evaluation of different scenarios proposed in terms of
minimizing the sedimentation rate within the harbor basin of Fanø Marina.
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2 Scientific background In the following section a definition of an estuary is given and the most important
characteristics of estuaries are outlined. In the second part of the section the most
important pelagic and benthic processes within the estuarine environment are
outlined, while in the last part, the most common anti-sedimentation techniques are
presented.
2.1 Estuary Perillo (1995) analyzed previous general definitions of estuaries and proposed the
following definition of an estuary:
"An estuary is a semi-enclosed coastal body of water that extends to the effective limit
of tidal influence, within which sea water entering from one or more free connections
with the open sea, or any other saline coastal body of water, is significantly diluted
with fresh water derived from land drainage, and can sustain euryhaline biological
species from either part or the whole of their life cycle"
This definition succeeded with including physical, chemical and biological processes
within the estuary compared to previous definitions, which primarily defined an
estuary as a hydrographical phenomenon (Perillo, 1995).
One of the most common characteristics of an estuary is the large suspended
sediment concentration (SSC) in the water column ranging from typical values of 20-
1000 mg l-1 and the fact that the SSC is temporally varying a lot within each tidal
period. The concentrations found in estuaries are typical much higher than those
found in the marine environment and the fluvial environment (Eisma, 1993).
Estuarine circulation is another characteristic of most typical estuaries. The
circulation is caused by density-driven currents due to contribution of freshwater
inflow and saltwater from the open sea. This will cause an inward bottom flow and
an outward freshwater flow on top. In well-mixed and partially mixed estuaries the
magnitude of the tidal currents at the bottom is strong enough to mix or partially
mix the water column, thus causing a vertical circulation. At the point where the
residual inward tidal current is almost zero a turbidity maximum is usually present
at the bottom (Eisma, 1993).
2.2 Cohesive and non-cohesive sediment Sediment refers to organic and inorganic loose fragments of rocks or minerals
broken off bedrock, shells or minerals precipitated directly out of the water. Physical
and chemical weathering break intact rock into smaller parts and physical agents
such as wind, currents, waves and gravity move the material from time to time.
Sediments can either be allochtonous or autochtonous referring to the origin of the
sediments being produced non-locally or locally, respectively (Marshak, 2008).
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The grain size of the sediment is an important parameter to determine in a wide
range of coastal processes including deposition, transport, erosion, flocculation etc.
Sediments are described according to their grain size which can vary from very large
boulders (>2048mm) to clay (<2µm). The medium-grained sediment is often
referred to as sand (<2mm to >63µm) and the fine-grained sediment is referred to
as silt (<63µm to >2µm) and clay (<2µm) (Blott & Pye, 2001). Cohesive sediments
are defined as grain sizes <63µm and non-cohesive sediments are >63µm.
The main difference between cohesive and non-cohesive sediments is the complex
nature of cohesive sediments. Unlike sand, which can be fully characterized by its
grain size distribution, cohesive sediments are much more difficult to characterize
as complex processes like flocculation, consolidation and biological activity play a
major role on the behavior of the cohesive sediment (Berlamont et al., 1993).
The fundamental processes acting to constitute sediment dynamics are represented
in Figure 2-1. The fluid-induced stresses and forces acting on the seabed cause
sediment to be entrained up into the flow. The transport of sediment can either be
as suspended load or bed load. At some point when the moment transfer from the
fluid to the sediment is too weak to keep the material in suspension the sediment
starts to settle as gravitational forces are acting on the sediment. Depending on the
settling velocity and the change of the hydrodynamic conditions, such as e.g. the
return of a tide, the sediment will either be re-entrained from the bed or remobilized
within the water column (Masselink & Hughes, 2003).
Figure 2-1 Illustration of the processes that together constitute sediment dynamics (Modified from
Masselink & Hughes, 2003).
The processes acting on the fine-grained sediment can be divided in pelagic and
benthic processes, respectively. The pelagic processes comprise the ones that take
place in the water column and are often referred to as advection, dispersion,
diffusion or flocculation. The benthic processes are referring to the mass exchange at
the boundary between the water column and the seabed e.g. through deposition,
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erosion and consolidation (Kombiadou & Krestenitis, 2013). The fundamental
pelagic and benthic processes are listed in Table 2-1.
Table 2-1 Fundamental pelagic and benthic processes in the estuarine environment.
Pelagic processes Benthic processes
Advection Erosion
Dispersion Deposition
Flocculation Consolidation
Settling Resuspension
2.3 Pelagic processes
2.3.1 Advection and dispersion
The pelagic processes of advection and dispersion refer to the movement of particles
within the fluid. The advective component refers to the mass transport of particles
with the mean fluid flow. For example, if the tidal current is ebb-directed, advection
will transport particles towards the sea. When a fluid moves it tend to spread out
from its advection path, which is referred to as dispersion. Dispersion refers to the
random motion of particles due to a velocity gradient in the fluid. The velocity is
variable with depth and along the direction (e.g. difference in cross sectional area in
a tidal channel) causing turbulent dispersion. Furthermore turbulent diffusion is
occurring when a gradient in suspended matter is present (Kombiadou &
Krestenitis, 2013).
2.3.2 Flocculation and settling velocity
Flocculation is an important process when dealing with cohesive sediment in
estuarine environments (Pejrup, 1988) and the size of a given floc are constituting a
much greater settling velocity compared to individual particles. Flocculation is the
process where suspended material coagulates and forms larger aggregates or flocs
in the water column (Pejrup, 2003). The flocs can contain thousands of small
particles and have settling velocities of several magnitudes higher than the
individual particles (Pejrup, 1988). Thereby fine-grained sediment can be deposited
in intertidal areas during tide slack when the current velocity is very low
(Kystdirektoratet, 2006).
The flocculation process is mainly governed by three mechanisms; random
brownian movement, turbulence within the flow and differential settling. The
random brownian movement is the collision between particles and their attachment
to a larger coagulant. The brownian motion is only important for small particles
(~1-3µm). Furthermore turbulence within the water column causes particles to
collide and aggregate, however if the turbulence is too large it can induce a floc
breakup which results in a lower settling velocity. Differential settling is the process
8
of larger particles settling through the water column, colliding with smaller particles
with a lower settling velocity, which attach to each other forming larger aggregates
(Kombiadou & Krestenitis, 2013).
The flocculation process is governed by several different factors and among these it
has been shown that the formation of flocs is dictated by salinity, temperature,
aggregation by organism, sticky organic compounds adsorbed to the particles
surface and the concentration of particles, which increases the possibility of collision
(Eisma, 1993). In the following the most important factors causing flocculation are
described.
Salt flocculation
Clay and small silt particles are generally negatively charged and repulse each other
in natural waters. When the salinity increases e.g. in estuaries where fresh water
mixes with sea water the negatively charged particles becomes neutralized by a
cloud of positive ions in the solution, within the so-called electrical double layer
(Pejrup, 2003). The double layer is compressed at increasing salinity as the
negatively charged surface is neutralized and the repulsing forces diminish, and
when the particles come very close to each other the attraction by Van der Vaals
forces or intermolecular forces dominate. These forces allow the particles to come
very close together, collide and form flocs (Eisma, 1993).
Bioflocculation
Bioflocculation is an important and complex process where microorganism plays an
important role (Huiming et al., 2011). Bioflocculation is a process where sediment
binds together by organic polymers that are adsorbed to the surface of the particles
(Pejrup, 2003).
Fecal pellets
Benthic organisms are known to induce flocculation in various ways. Filter- and
deposit feeders (eg. snails and mussels) feed on fine-grained sediment particles and
excrete these as fecal pellets or pseudo pellets. Both pellets have settling velocities
which are much higher than the individual particles. The process binds the sediment
pellets tightly together by organic glue and results in very strong aggregates with
characteristic shapes for different species (Pejrup, 2003). However, the presence of
benthic animal might cause the bed more easily eroded, even though the settling
velocity is larger (Andersen, 2001).
2.4 Benthic processes
2.4.1 Erosion and deposition
Erosion and deposition are vertical exchange processes between the sea bed and the
overlying water column. The deposition term refers to the addition of sediment to
the bed and the erosion term refers to the movement of sediment from the bed.
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Deposition and erosion are often represented by the bed shear stress (b), which is
the frictional force exerted by the flow per unit area of the bed. The critical bed
shear stress for erosion (ce) and the critical bed shear stress for deposition (cd)
refer to the forces needed to act on a bed in order for erosion or deposition to occur.
The value of ce is larger than cd as the force needed to mobilize a particle from the
bed is larger than the force needed to deposit the particle (Whitehouse et al., 2000).
The erodibility of cohesive sediments are influenced by a lot of parameters including
bulk density, grain size, bed roughness and the biology, e.g. the presence of
macrofauna and benthic diatoms, thus also being seasonal varying (Andersen,
2001). Furthermore induced wind-wave activity across shallow areas has shown to
increase the resuspension quite significantly (Andersen & Pejrup, 2001;
Christiansen et al., 2002). The deposition threshold is also varying in time due the
hydrodynamic conditions and is also governed by the settling velocity, water
properties and the material in suspension (Whitehouse, 2000).
2.4.2 Consolidation and resuspension
When sediment has settled it is usually of a very loose structure and over time self-
weight consolidation takes place. After a few hours of consolidation the sediment is
already to some degree compacted and the bed level lowered as a consequence
thereof. As the sediment layer becomes compacted the water is expelled from the
pores and the density of the layer is increasing with time and depth. The
consolidation process leads to a density profile within the bed, as the top layers are
typically partially to under-consolidated with a lower density, compared to the more
consolidated part of the profile with a higher density. Typical surface dry densities
of intertidal mudflats are in the range of 500 to 1000 kg m-3 and beneath the top
layer the dry density might be as high as 1000 to 1600 kg m-3 (Whitehouse et al.,
2000).
Resuspension is when bed material is eroded or particles settling within the water
column are put in suspension again. This is a well-known process in estuarine tidal
areas, where the current speed is varying with the tidal period. Resupension will
often occur after a slack period, where particles are poorly consolidated and the
current speed is increasing, thereby inducing a larger bed shear stress and at last
exceeding the threshold for erosion (Webster & Lemckert, 2002; Bartholdy &
Anthony, 1998).
Resuspension and consolidation are highly dependent on the hydrodynamic
conditions, biological activity and the sediment properties. In sheltered harbor and
marinas there will typically be no or little resuspension as the current speed is low
and the area is sheltered from waves. However the possibility of propellers and
hulls of moving vessels may cause resuspension of the bed (Whitehouse et al., 2000).
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2.5 Sediment transport in relations to tidal currents The net transport of fine grained sediment is related to asymmetry in the
hydrodynamic and sedimentological conditions. According to van Maren and
Winterwerp (2013) the two types of asymmetries can be divided in (1) asymmetry
in the water movement and (2) asymmetry in the sediment behavior.
2.5.1 Tidal asymmetry
During the flood cycle the tidal wave might propagate with, a velocity of 𝑐 = √ℎ ∙ 𝑔
in a frictionless estuary, whereas the tidal area is drained by the bed gradient during
the ebb cycle. As the water depth is varying from the waves crest to the trough, the
bottom friction is significantly greater at the trough, at the incoming tide. Due to
bottom friction, the wave crest moves faster than the trough, and the crest of the
tide may partially overtake the trough. This results in a shorter duration, but
stronger current velocity at flood and a longer duration, but weaker current velocity,
at ebb tide (Friedrichs & Aubrey, 1988). Another form of tidal asymmetry is
occurring when the tidal amplitude is decreasing as the tidal waves propagate
towards land. The decrease in the current velocity is faster around high tide slack
than during low tide slack, thus favoring a longer period with slack time just before
flood compared to ebb, which results in a greater current velocity during flood,
which in turn favors a landward transport of fine-grained sediment (Ridderinkhof,
1998). This is also referred to as deformation of the shoaling tidal wave and causes
the asymmetric distribution of current velocities and sediment concentration
observed in many estuaries, also known as tidal pumping (Christiansen et al., 2006).
The asymmetries are often referred to as horizontal and vertical asymmetry (van
Maren & Winterwerp, 2013). Both types of asymmetry can occur and it is difficult to
determine which component is the most dominating in an estuary. An estuary can be
flood dominated in some parts of the estuary and ebb dominated in other parts of
the estuary. The flood dominated area favors landwards sediment transport
whereas the ebb dominated areas favor seawards sediment transport (Ridderinkhof,
1998). Temporally and spatially variation in the tidal asymmetry is found in the
Grådyb tidal area with an overall effect on the sediment transport. In general the
longer ebb period the more landward you are from the tidal inlet due to the friction
with the bottom and the sides of the tidal channels. This cause the flood period in the
northern part of Ho Bugt to be 1-2 hours shorter that the ebb period
(Kystdirektoratet, 2006). Ebb dominance is often occurring in tidal inlets as the
intertidal areas and flood areas are often drained and emptied during low water,
thus allowing a faster water exchange in the relatively deeper channels (Friedrichs
& Aubrey, 1988).
2.5.2 Lag effects
The water movement is not the only factor contributing to a net import of fine-
grained material. Other mechanisms contributing to a net import is referred to as lag
effects. The lag effects is caused by the fact that the energy required to mobilize
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cohesive sediment from the bed is much greater than the force needed to bring and
keep cohesive sediment in suspension (van Maren & Winterwerp, 2013).
Suspended sediment will start to settle when the current speed has reached the
critical bed shear stress for deposition. However, due to the very low settling
velocity of fine-grained material the particle will travel a distance before it is
deposited at the bed. This process is referred to as settling lag. As the current speed
is normally accelerating at a lower speed during ebb than flood the net-effect of fine-
grained material will be landward in the Wadden Sea (Kystdirektoratet, 2006).
The term scour lag refers to the cohesive sediments tendency to adhere to each
other. When particles are deposited, consolidation processes makes resuspension
more difficult, as the critical shear stress for erosion to bring particles in suspension
is now larger (Eisma, 1993).
Bartholdy (2000) investigated the effect of settling and scour lag by running a
simulation model from in situ measurement in Grådyb tidal area, and it was found
that scour lag is by far the most important lag effect contributing to the import of
fine-grained sediment in Grådyb tidal area. It was found that, with grain sizes
between 50 µm - 80 µm and an initial concentration of 0.030 kg m-3, settling lag
constituted approximately 10 %, scour lag approximately 85 % and resuspension
lag approximately 5 %. The contribution from settling lag was also shown to be
more significant with increasing mean grain-size.
2.6 Sedimentation processes in harbor basins Figure 2-2 displays a simplified schematization of the typical flow patterns within
semi-enclosed marinas:
Figure 2-2 General flow conditions within semi-enclosed basins (PIANC, 2008).
The sedimentation within harbors is often related to a current-induced exchange, a
tidal filling and a density-driven current.
It is not possible to reduce the tidal prism, unless costly lock mechanisms are
established, where the tide cannot enter the harbor. However, it is possible to
reduce the sedimentation processes associated with current-induced exchanges and
density-driven exchanges (PIANC, 2008).
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A small change in the density can exchange a tremendous water volume. The
horizontal pressure gradients are induced by differences in salinity, temperature
and/or sediment concentration within the harbor basin and the ambient water. The
effect of the gradient differences can induce the gravitational circulation pattern
illustrated in Figure 2-2c. The density-driven exchange is often related to gradients
in the salinity and water temperature. However, it was recently discovered that
sediment-induced density currents are of significant importance, and it was seen
that the sediment fluxes entering the Port of Rotterdam were increased by at least a
factor three due to density-driven currents (PIANC, 2008).
The current-induced exchange is often associated with the flow pattern illustrated in
Figure 2-2a. The flow is separated in two as the fluid passes the harbor entrance.
The horizontal entrainment causes an eddy in the harbor entrance, thus leading to
an increased water volume entering the harbor basin per tidal period (PIANC, 2008).
In order to reduce the volume exchange various anti-sedimentation techniques can
be used which will be presented in the following section.
2.7 Minimizing harbor sedimentation There is a clear economic and environmental interest in reducing sedimentation
within harbor or marina basins as dredging can be quite costly. In 2010 the Danish
part of the Wadden Sea was appointed National Park and with this followed an
increased attention to the environmental aspects of the National Park as well as the
implementation of the Water Framework Directive (Nationalpark Vadehavet, n.d.).
As harbor basins are often acting as sediment traps for cohesive sediment, these
environments are often associated with emissions of xenobiotics (e.g. TBT, PCB,
PAH) (Kystdirektoratet, 2006). There is a clear interest in adopting more sustainable
solutions for environmental and economic reasons and successful management
must be based on knowledge of the natural system (Kirby, 2011).
The Permanent International Association of Navigation Congresses (PIANC)
conducted a study in 2008 where failures of past time implementations of
minimizing siltation in the muddy ports world-wide was investigated in an attempt
to provide a modern method to minimize harbor siltation (Kirby, 2011). In the
following some of this work, with relevance to this thesis will be presented.
In general the basin sedimentation can be governed by the simplified equation
(PIANC, 2008):
𝑆𝑒𝑑𝑖𝑚𝑒𝑛𝑡𝑎𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 = 𝑝 ∙ 𝑄 ∙ 𝑐𝑎 (2.1)
where p is the basin trapping efficiency, Q is the water exchange between the
surrounding water and the harbor basin and ca is the concentration of suspended
matter in the water outside the basin.
13
Sedimentation within marinas can be diminished by reducing the parameters in the
above mentioned equation. The water exchange (Q) and suspended sediment (ca)
outside the harbor can be diminished by keeping the sediment out (KSO) and the
basin trap efficiency (p) can be reduced by keeping the sediment movement (KSM)
(Kirby, 2011).
Management methods can be categorized in the following three groups where the
essence is to control and manipulate the flow conditions (PIANC, 2008):
Keep Sediment Moving (KSM)
Keep Sediment Out (KSO)
Keep Sediment Navigable (KSN)
KSO techniques are based on minimizing the inflow of sediment-rich waters into the
harbor basin, thus reducing the water exchange and the SSC within the water
entering the basin. One KSO anti-sedimentation technique is a current deflection
wall (CDW), which has shown to be successful in several cases (Kirby, 2011). A
schematized situation with and without a CDW is shown in Figure 2-3.
Figure 2-3 Hydrodynamic and sediment transport conditions with and without a CDW (PIANC,
2008).
The primary purpose of implementing a CDW is to control the flow within the
system. The wall deflects the current and reduces the flow entrainment.
Furthermore the "new" channel between land and the wall leads the tidal filling
through the CDW instead of the harbor entrance. Thereby the near bed transport is
trapped within the sill and accordingly the concentration is reduced (PIANC, 2008).
14
The Implementation of a CDW in the Port of Hamburg has been studied over several
years and it was found that the yearly siltation rate was reduced by up to 45 %
(PIANC, 2008). Another study from the Port of Antwerp showed a reduction of 10-
20 % after the implementation of a CDW (Decrop et al., 2013).
Other techniques related to KSO are the modification of the harbor entrance and
establishment of different structures such as gates, dikes and sediment traps
(PIANC, 2008). Furthermore an optimized orientation of the marina design is also an
effective way to reduce the sedimentation within the basin. However, the most
optimal orientation are not necessarily fitting with other parameters such as the
maneuverability, water velocities, etc. which parameters are also important to take
into account when providing the most sustainable solution (PIANC, 2015).
The primary anti-sedimentation approach used in the present thesis is KSO, which is
why KSN and KSM are only briefly described herein. The strategy of KSM is to
increase the flow and thereby preventing the sediment from settling. This technique
is often applied where a small increase in flow velocity can reduce the siltation rate.
Finally, the strategy behind KSN is to take advantage of ships sailing close to or
through low density fluid in areas whit high turbidity located at the near-bottom,
thereby preventing settling to take place (PIANC, 2008).
15
3 Study area The European Wadden Sea is forming the area from Den Helder in the Netherlands
to the south to Blåvandshuk in Denmark to the north. The European Wadden Sea
seen in Figure 3-1 is a coastal wetland and it is the largest coherent tidal flat area in
the world (Wang et al., 2012).
Figure 3-1 Map showing the European Wadden Sea (From: www.waddensea-
secretariat.org).
The area is unique in terms of the dynamic behavior with barrier islands interrupted
by tidal channels protecting the lagoon waters and the nearby land with a unique
biotic system. The flow of water, air and transport, erosion and sedimentation of
sand and fine-grained sediment result in a fast and ever-changing morphology in the
Wadden Sea(Wang et al., 2012). The study area of the present study is within
Grådyb tidal area, located in the Danish part of the Wadden Sea seen in Figure 3-2.
The Danish part of the Wadden Sea consist of an area of approximately 730 km2 with
the barrier islands from south - Rømø, Mandø, Fanø and the barrier spit island
Skallingen (Pedersen, 2004).
16
Figure 3-2 Map showing the Danish part of the Wadden Sea with the barrier islands Rømø, Mandø,
Fanø and the barrier spit island Skallingen (Pedersen & Bartholdy, 2006).
3.1 Physical settings In this section the physical settings at the study area will be presented. The study
area is located in the southern part of Grådyb tidal area sheltered from the North
Sea by the barrier island Fanø and the marina is located on the northeastern coast of
Fanø in Nordby. The study area and the location of Fanø Marina are shown in Figure
3-3.
17
Figure 3-3 The Grådyb tidal area and Fanø Marina are located within the red square (Adapted from
Pedersen, 2004)
The Grådyb tidal inlet is situated at the microtidal part of the Wadden Sea between
Fanø and Skallingen and is connected to the North Sea through two tidal channels
draining to the north into Ho Bugt through Hjertling Løb and Hobo Dyb, and
draining to the south through one tidal channel, Havneløbet, which is separated to
the south by a tidal divide located at the head of Sneum Å to Klingebjerg at Fanø
(Pedersen, 2004). The area has a semi-diurnal tide with an average mean tidal range
of 1.6 m (Kystdirektoratet, 2006) with an average neap spring tide of 1.3 m and an
average spring tide of 1.7 m (Vinther et al., 2004). During storms the sea level can
reach above 4 m DNN (Danish Ordnance Datum) (Bartholdy & Aagaard, 2001). The
tidal inlet is flood-dominated in the inner parts with current flood velocities of 1-2 m
s-1 and is ebb-dominated in the outer parts (Bartholdy & Anthony, 1998).
The tidal prism in Grådyb is in the order of 165·106 m3 (Kystdirektoratet, 2006). The
total contribution of freshwater consists of less than 1 % of the total tidal prism and
the estuary is classified as well-mixed as no strong salt stratification is present
(Edelvang et al., 1992). The area is almost exclusively supplied by two rivers. Hobo
Bugt is supplied by Varde Å and the southern basin is supplied by Sneum Å
(Bartholdy & Anthony, 1998). The salinity close to Sneum Å is in the range of 25-
30‰, whereas the salinity is between 5-20‰ close to Varde Å. On a yearly basis the
salinity varies between 27-31‰ in the tidal inlet (Pejrup et al., 1993).
18
The Grådyb tidal area is 168 km2 including the salt marsh areas. Of this
approximately 37% is permanently water-covered, 45 % are intertidal flats and 18
% are salt marshes above mean high water level (Bartholdy & Folving, 1986). The
southern part of Grådyb tidal area is 68 km2, excluding salt marshes, of which
approximately 58 % is permanently water-covered, and 42 % of the area is
intertidal flats. Classification suggests that the tidal flats consists of approximately
58 % sand flats, 39 % mixed flats and 3 % mud flats (Sørensen et al., 2006).
Fine-grained sediment is primarily transported to the area through the tidal inlet
from the North Sea (Bartholdy & Anthony, 1998; Bartholdy & Madsen, 1985;
Pedersen, 2004). Pedersen (2004) estimated an annually net accumulation of fine-
grained sediment in the order of 90.21 · 103 ty-1 of which the North Sea contributed
to 60.7 % of the total budget. Other sources contributing to the annual accumulation
are atmospheric deposition (0.5 %), fluvial sources (10.4 %), primary production
(11.3 %) and local coast erosion (16.2 %).
The hydrodynamic forces within the area are setting the transport pathway of fine-
grained sediment within Grådyb. Waves are bringing sediment in resuspension, and
the tide is bringing water and fine-grained sediment back and forth in a constant
exchange with the North Sea (Kystdirektoratet, 2006). Thus, wind is also an
important transport agent as the bed shear stress can be enlarged by a factor four at
the intertidal flat at short periods with wind-wave activity (Andersen & Pejrup,
2001). Furthermore, a turbidity maximum is located at the border of the tidal divide
to the south and wind-induced currents can bring turbid water from south to north,
or relatively clean water from north to south (Bartholdy & Anthony, 1998).
The sediment within Grådyb tidal area consists of mainly two types of sediment
which is fine-grained sedimemt (<63µm) and fine sand. The mineralogy of the sand
grains is primarily quartz with a mean grain size of 135 to 235 µm. The fine-grained
sediment has a mean grain size between 3 to 55 µm (Bartholdy & Madsen, 1985).
The major clay minerals consist of illite (57 %), kalolinite (20 %), chlorite (16 %)
and smectite (7 %) (Bartholdy & Anthony, 1998).
The sea level rise in the area has on average been at 1.3 mm y-1 since the
establishment of the first tidal gauge in the late 19th century (Vinther et al., 2004).
However, the rate of sea level rise has increased during the late 20th and 21st
century to a present sea level rate of 3-4 mm y-1 (Fruergaard et al., 2015). The salt
marshes located on the backbarrier of Skallingen have on an average annual
accretion of 2-4 mm y-1 (Bartholdy et al., 2004). However, accretion rates of 8 mm y-
1 was found on locations with Spartina vegetation on the edge of the salt marsh
which vegetation is characteristic for recently developed salt marsh in Skallingen
(Nielsen & Nielsen, 2002). Thus, the yearly accretion of the salt marsh is keeping
pace with the mean sea level rise at some places, while others places fall behind
(Vinther et al., 2004; Bartholdy et al., 2004).
19
Grådyb tidal area is influenced by and subject to anthropogenic activities. Esbjerg
city is the largest city in the area with its 80.000 inhabitants, and Esbjerg harbor is
the largest export harbor in Denmark (Pejrup et al., 1993). The tidal inlet is
approximately 1000 m wide and the channel is dredged to maintain a navigational
depth of 10.3 m. South of Grådyb similar undredged channels passing through the
ebb tidal deltas would under natural conditions be approximately 5 m (Bartholdy &
Anthony, 1998). In the navigational channel towards Esbjerg harbor dredged
material is in the order of 1.2 million m3 annually (Kystdirektoratet, 2006).
3.2 Fanø Marina Fanø Marina is located on the northeastern coast of Fanø (see Figure 3-3) and the
present day marina design inaugurated in 1990 is seen in Figure 3-4. The marina
basin is approximately 21000 m2.
Figure 3-4 Fanø Marina (From: www.fanoesejlklub.dk)
Fanø Marina can be approached through Slunden from Esbjerg, which was dug in
1994 to improve the infrastructure between Fanø and Esbjerg (Slunden sejlrende
1994, n.d.). The marina is navigable throughout the day, but with consideration to
the tide as seen in Figure 3-5. It is seen that the marina is almost completely drained
from water and the mud flats are exposed during low tide. The marina was last
dredged in 2007 to a navigational depth of 2.2 m (Fanø Sejlklub, n.d.)
20
Figure 3-5 Fanø Marina during neap tide (From: Per Hansen)
The board of Fanø Marina delivered information about past dredged material within
the marina.
In 1988/89 when the marina was renovated the marina basin was dredged
according to the planned levels.
In 1997/1998 a dredged volume of 18,300 m3 was removed from the marina.
Furthermore a volume of 1,500 m3 was removed from a sediment trap next
to Svenskerbroen (Located next to Station 5, cf. Figure 4-8).
In 2007 a volume of 16,000 m3 was dredged from the marina basin. This was
also the last time the marina was dredged.
From the above mentioned dredged volumes a yearly net deposition of
approximately 8-10 cm y-1 averaged over the entire marina basin is expected.
Besides the above mentioned information the entrance was flushed with a propeller
from a tug during ebb for approximately 4 hours in total during the past 10 years.
The effect of eroded material within this process is hard to estimate, however it
indicates that the natural annual net deposition is expected to be larger than the
rough calculations of 8-10 cm y-1. An annual net deposition of 20-30 cm y-1 based on
local observations was later estimated by the board of Fanø Marina. The board of the
marina also reported that the primary sedimentation is taking place within the
entrance and immediately west of the entrance (Per Hansen, personal
communication, spring 2016).
21
Figure 3-6 to 3-10 show historical orthophotos and satellite images of the study
area.
Figure 3-6 Orthophoto from1945. (www.kb.dk/danmarksetfraluften/)
Figure 3-7 Orthophoto from 1954 (www.kb.dk/danmarksetfraluften/).
22
Figure 3-8 Orthophoto from 1995 (www.kb.dk/danmarksetfraluften/).
Figure 3-9 Orthophoto from 2006 (www.kb.dk/danmarksetfraluften/).
23
Figure 3-10 Satellite image from 2014 (Google Earth).
Judged from the images above, the natural navigational channel pathway running
along the waterfront of Nordby towards and around the marina has not undergone
any rapid changes in the period from 1945-2014.
24
4 Methods In this section the methods used in the current thesis will be presented. The analysis
of the inter- and subtidal areas within the study area is based on numerical
modelling (MIKE 21 FM) of the hydrodynamics and cohesive sediment dynamics
together with field data collected in the area. The model is set-up and calibrated to
correctly represent the currents in Fanø Marina, the navigational channel and the
adjacent tidal flats, respectively. At first the model was used to investigate and
understand the hydrodynamics and the cohesive sediment dynamics, whereupon
the focus was moved towards investigating possible remedial actions to reduce the
sedimentation rate within the marina.
4.1 MIKE 21 Flow Model FM The numerical model used in the present study is MIKE 21 Flow Model FM
developed by the DHI (Danish Hydraulic Institute). MIKE 21 Flow Model FM is a
program developed to simulate in two dimensions and is suitable for investigating
e.g. fully mixed ocean areas, lakes and estuaries etc. However, MIKE 21 is not
suitable in areas with a stratified water column. FM is referring to a flexible mesh,
which means that the software operates with an unstructured mesh of triangles and
quadrilateral elements of different sizes. This makes it possible to optimize the
resolution in the area of interest while still holding the computation time on a
reasonable level (DHI, 2015a).
4.2 Model set-up The following approach which was used to set-up the MIKE 21 FM can be divided
into some main inputs to the model:
A bathymetry and a mesh representing the area of interest
A simulation period with a selected time interval
A module selection of the preferred numerical modules and measured data
4.2.1 Bathymetry and domain
The bathymetry is extremely important when modelling in an estuarine and marine
environment as the accuracy of the simulated results rely on the quality of the
bathymetry. The purpose of creating a bathymetry in MIKE 21 is to describe the
water depth within the modeled domain. In the following the bathymetry based
data, from surveys carried out by the Danish Coastal Authorities in 1967 and 2002,
will be presented.
A survey of Grådyb tidal basin was carried out between 1966 and 1969 and is also
referred to as the 1967 survey. The survey was carried out with a traditional
singlebeam echosounder and the measurements were collected in non-uniform lines
with a standard deviation of ± 20 cm (Kystdirektoratet, 2006).
25
Grådyb tidal basin was surveyed again in 2002 with a multiplebeam echosounder
and the measurements were collected in lines with a spatial resolution of
approximately 200 m. This survey is also referred to herein as the 2002 survey. The
estimated standard deviation of the 2002 survey was ± 5 cm. The 2002 survey is the
most comprehensive and valid survey carried out in Grådyb tidal area
(Kystdirektoratet, 2006). However, as explained below the combination of these
data gave the best result.
Light Detection and Ranging (LIDAR) data from 2007 were also used in order to
expand the model domain to include the marsh area at Skideneng and the eastern
part of Grønningen located at the northeastern coast of Fanø (cf. Figure 3-3). A
delineation of 3 m above DVR90 was used and the LIDAR 2007 data were resampled
from a resolution of 1.6 m to a resolution of 25 m, as a coarser resolution in the area
was assumed to be sufficient, taking the bathymetry surveys into account.
The data from 1967, 2002 and 2007 were imported to a mesh generator and the
generated model domain, mesh-grid and scatter data are seen in Figure 4-1:
Figure 4-1 Model domain and mesh with water level data from 1967, 2002 and 2007.
26
In order to obtain a more smooth interpolation of the bathymetry it was necessary
to use data from 1967 together with data from 2002. The data was carefully
examined in the years 1967 and 2002. This included examination of 1967 and 2002
data within the navigational channel as seen in Figure 4-2.
Figure 4-2 Bathymetry measurements from 1967 and 2002. The measurements from 2002 (circles) were
collected in straight lines whereas data from 1967 (squares) were collected in lines as well as
scattered points. The combination of using data from 1967 and 2002 gave the best results of the
bathymetry.
Only a few measurements were collected within the marina during the 1967 survey
as the Fanø Marina basin was first established in its present design in 1990,
meanwhile the survey from 2002 did not include the marina. The measurements
within the marina were deleted as the water level was within the range of 0.019 to -
27
0.318 m, which does not correspond to the present day level maintained in the
marina. The marina was last dredged in 2007 to a navigational depth of 2.2 m (Fanø
Sejlklub, n.d.).
Furthermore water column measurements from a pressure transducer were
installed at the bed adjacent to one of the piers within the marina. The data are seen
in Figure 4-3.
Figure 4-3 Water level as a function of water depth. The linear equation is given as Y = 1 * X - 1.1095 and the
water depth is 1.1 m.
It can be seen that the water depth is 1.1 m DVR90 at mean water level. On the basis
of this and the previous dredging operation in 2007, the water depth was set to 2.2
m within the inner part of the basin and decreased gradually towards the edge of the
basin to 1.3 m.
In general the measurements were closely examined and where the water depth had
changed between 1967 and 2002 the measured data from 2002 were used, e.g. the
dredged navigational channel Slunden was first established in 1994 and thus the
2002 data were used in this area.
On the basis of the approach described above, the bathymetry used in the numerical
model is seen in Figure 4-4 and the mesh size and bathymetry in Fanø Marina are
shown in Figure 4-5.
Thus, there are used more than 45,000 data points distributed within the model
domain. The data is interpolated to a computational mesh with approximately
19,000 elements of varying size. In the navigational channel and the marina the
finest resolution of approximately 5 m between each computational element is used.
This was the finest possible resolution along with a reasonable computation time.
The coarsest resolution is used across the tidal flats with approximately 50 m
between each element. The total model area is 9.45 km2.
28
Figure 4-4 Bathymetry of model domain.
Figure 4-5 Bathymetry of Fanø Marina area. It is seen that the channel resolution is made in squares
in order to simulate the currents as correctly as possible and the finest resolution was
used within the marina.
The model is operated by time series of water levels which are added to the open
boundary. The open boundary is seen in Figure 4-6 and is set to be along Havneløbet
29
(cf. Figure 3-3). The boundary is closed to the south as a tidal divide is present
further south and it is assumed that water from south of the model domain is only
contributing to a minor part of the total tidal prism in the model domain.
Figure 4-6 Boundary conditions in the model. Water level measurements were added to the open
boundaries.
4.2.2 Simulation period
Three simulation periods were used in order to calibrate and validate the model.
The measurements from June 3-8 2015 were used as the calibration period.
The validation periods were from July 20-22 and October 30 to November 5, 2015
The results were saved in time intervals of 30 minutes.
4.2.3 Module selection
The MIKE 21 Flow Model FM is a shell including 8 different modules. For the
purpose of the present thesis the hydrodynamic and mud transport module was
selected and was assumed to be sufficient, as these are the key factors influencing
sedimentation within Fanø Marina. It was assumed that the marina basin is
sheltered from wave activity, as it is located behind a pier, even though the wave-
activity has been reported to promote resuspension of fine-grained sediment in the
Danish Wadden Sea (Andersen & Pejrup, 2001).
30
The inputs to the modules can be divided in different groups and an overview of the
various model inputs and the following outputs are seen in Figure 4-7 and will be
outlined in the following sections. A lot of fieldwork and calibration are needed in
order to run a cohesive sediment model due to its generic nature (DHI, 2015a; DHI,
2015b). Furthermore, literature was reviewed in order to set realistic values for the
mud transport module (cf. Table 4-4).
Figure 4-7 Overview of MIKE 21, module selection, model inputs and outputs
4.2.3.1 Hydrodynamic module (HD)
The Hydrodynamic (HD) module is the fundamental module in MIKE 21 Flow Model
FM and provides the basis for other modules, but can also be used on its own. It is
important to set-up and calibrate the HD module before the sediment module can be
used. The HD module can also be used to simulate flow patterns and current
velocities to for example in Environmental Impact Assessment or assessment for
hydrographic conditions for design and structures etc. (DHI, 2015a).
The HD module is used to simulate the water level and the flow within the domain.
The calculations are based on bathymetry and topography (marsh) of the area, the
water level and the bed resistance. The inputs to the HD module are based on
previous surveys of the area (bathymetry), in situ data from field campaigns in the
summer 2015 and the autumn 2015 (water level), as well as calibration parameters,
such as the bed resistance.
The HD module is based on numerical solutions in two horizontal dimensions in one
layer taking the Reynolds averaged Navier-Stokes equations invoking the
31
assumptions of Boussinesq and of hydrostatic pressure. Thus, the model vertically
integrates equations of continuity and momentum in two horizontal dimensions is
solved in the HD module by cell-centered finite volume method with the variables
defined on a spatially unstructured mesh grid (DHI, 2015a).
The model is furthermore taking flooding and drying into consideration as a
computational feature. This feature is suitable in tidal areas where the element/cell
is removed from the simulation when the water depth is less than the drying values
(DHI, 2014a).
4.2.3.2 Mud Transport module (MT)
The HD module can be coupled with a sediment transport module. The Mud
Transport module (MT) is used to describe erosion, deposition, transport of fine-
grained cohesive sediment (<63µm) and a mixture between cohesive and non-
cohesive sediments under the action of currents and waves. The MT module is
typically applied in areas where it is desired to investigate sediment transport,
harbor siltation, channels, dredging in coastal, marine and estuarine environments
(DHI, 2014b).
The cohesive sediment transport is generic, which means that it is based on
empirical formulas. Therefore it is of great importance to obtain several good
calibration factors in order to make a satisfactorily description of transport, erosion,
deposition and settling of the cohesive sediment (Lumborg & Pejrup, 2005).
Some of the important processes incorporated in the MT-module are presented in
the following.
The bottom shear stress (𝜏𝑏) is computed on the basis of the HD module. The bottom
shear stress in numerical models without waves is given as (DHI, 2014b):
𝜏𝑏 = 0.5𝜌 ∙ 𝑓𝑐 ∙ 𝑉2 (4.1)
wherein ρ is the water density, fc is the current friction factor, and V is the current
velocity. The 𝜏𝑏 will in nature differ spatially and temporally within the area.
Erosion of the bed is occurring in the model when the bed shear stresses exceed the
user-defined critical bed shear stress for erosion (𝜏𝑐𝑒). The erosion (SE) for soft and
partly consolidated beds is computed as follows (DHI, 2014b):
𝑆𝐸 = 𝐸 · 𝑒(𝛼∙√𝜏𝑏−𝜏𝑐𝑒)½, 𝜏𝑏 > 𝜏𝑒 (4.2)
wherein E is the erodibility of the bed and is a coefficient. The coefficient describes
the exponential rise in erosion when the bed shear stress rises. The coefficient used
was 6.5 m N-0.5 which was also used in a study in Listerdyb (Lumborg & Pejrup,
2005).
32
The deposition of sediment occurs when the bed shear stress is below the user-
defined bed shear stress for deposition (𝜏𝑐𝑑). The deposition is given by (DHI,
2014b):
𝑆𝐷 = 𝑤𝑠 ∙ 𝑐𝑏 ∙ 𝑝𝑑 (4.3)
wherein cb is the near bed concentration, pd is the probability of deposition and ws is
the settling velocity.
4.3 Measured data In this section the collected data and the data processing are presented. The
fieldwork was conducted in order to calibrate the model and was carried out during
the summer and autumn 2015. The flow model is site-specific and measured data of
the hydrodynamic and sediment concentration are of great importance in order to
obtain a well calibrated model.
During three field campaigns two Aanderaa RCM9 sensors were used at different
locations from June 3-8 (Station 1 and 2), July 20-22 (Station 3 and 4) and October
30 to November 27, 2015 (Station 5 and 6) in the area around Fanø Marina. An
overview of the three main fieldwork campaigns are schematized in Table 4-1 and
the positions of the RCM9 Aanderaa sensors are seen in Figure 4-8.
Table 4-1 Overview of fieldwork campaigns.
Fieldwork campaigns
RCM9 Aanderaa Bed samples D-GPS-coordinates
June 3-8, 2015 X (Station 1 and 2) X
July 20-22, 2015 X (Station 3 and 4)
October 30 to November 27, 2015
X (Station 5 and 6) X
33
Figure 4-8 Position of RCM9 Aanderaa sensors during fieldwork carried out on June 3-8 (St. 1-2), July 20-22 (St.
3-4) and October 30 to November 27 (St. 5-6)
34
4.3.1 Water column measurements
Measurements of salinity, temperature, current velocity, current direction and SSC were
obtained by means of Anderaa RCM9 sensors in intervals of 5 minutes.
Figure 4-9 Aanderaa RCM9 sensor
The SSC was measured with an optical backscatter sensor (OBS). The OBS is automatically
emitting short pulses of infrared light in the water column, and the sensor records the
backscattered light. The OBS measures in turbidity units (NTU), and these units were
transformed into concentrations of suspended matter by an in situ calibration carried out
during earlier measurements in the area (Bartholdy & Aagaard, 2001). The transformation
equation is as follows:
𝐶𝑚𝑔/𝑙 = 2.289 ∙ 𝑁𝑇𝑈𝑂𝐵𝑆 (4.4)
Similar calibration formulas were found in Vinther et al. (2004) in Ho bight (𝐶 = 2.1785 ·
𝑁𝑇𝑈) and from a study of a microtidal flat at Kongsmark in Andersen & Pejrup (2001). No
surface film was observed on the OBS when they were collected at the end of the field
campaigns. Furthermore the datasets were closely inspected and evaluated, and outliers in
the dataset were removed. In situ measurements from an ISCO sampler were also carried out
in time intervals of 12.5 hours in order to calibrate the OBS. However, the results were shown
not to be reliable as the suction length was too long.
4.3.2 Water level
Water level measurements were obtained in the period from June 1 to July 30 from a pressure
transducer in Port of Esbjerg and were used in the summer period simulations. However, the
pressure transducer maintained by the Port of Esbjerg was out of order during the autumn
2015, for which reason it was decided to use water level in situ measurements from Station 5
in the simulated period in the autumn.
35
The water level was measured with pressure transducers of the type DI240. At Station 1, 2
and 5 the pressure transducers were mounted to a tripod together with the Aanderaa RCM9
sensors. At station 3, 4 and 6 the pressure transducers were attached to a fixed pole in the
entrance to the marina. The pressure transducers measured in intervals of 5 minutes.
The water column (m) can be found from the Diver Manual, 2014 and is given as follows:
𝑊𝐶 = 9830.36 ∙(𝑃𝑑𝑖𝑣𝑒𝑟−𝑃𝑏𝑎𝑟𝑜)
(𝜌∙𝑔) (4.5)
Wherein 𝑃 𝑏𝑎𝑟𝑜 is the barometric pressure (m), 𝑃 𝑑𝑖𝑣𝑒𝑟 is the pressure above the diver (m), g
is the gravitational acceleration (m s-2) and ρ is the water density (kg m-3). The barometric
pressure was obtained from a weather station in Esbjerg Airport and a pressure transducer
installed in Nordby.
The water density, , can be calculated by means of the following empirical formula
(Bartholdy, 2013):
𝜌 = 1000.92 + 7.73 ∙ 10−1𝑆 − 2.54 ∙ 10−2 ∙ 𝑇1.5 − 2.12 ∙ 10−3 ∙ 𝑇2 (4.6)
wherein T is the water temperature (oC) and S is the salinity (‰).
In order to convert the measurements into Danish Vertical Reference (DVR90) the water level
was noted on a water table board installed in Fanø Marina over a tidal period and was later on
measured with a D-GPS Trimble R8.
4.3.3 Wind
Wind speed and direction from May 1to December 12, 2015 was obtained from a measuring
station in the Port of Esbjerg.
4.3.4 Bed samples
Bed samples were collected during October 29-30 and December 7with a Van Veen Grab
sampler seen in Figure 4-10. The samples were immediately stored in darkness. The
instrument was used to sample sediment around the marina in order to obtain information
about the bed material. As the instrument is put in the water, the two levers are spread open
and are unlocked when hitting the bed. When the instrument is pulled up the bucket closes
and grabs a sample from the bed.
36
Figure 4-10 A Van Veen Grab used to collect bed samples
The location of the collected bed samples, with selected photos of the bed samples are seen in
Figure 4-11.
Figure 4-11 Position of collected bed samples with selected photos of sample 5, 8, 12, 14, 16 and 17.
The bed samples from the marina consisted of sticky fine-grained material with a high water-
content and a dark appearance. Bed samples just outside the marina were of the same
consistency but with a brown to light dark appearance. In the navigational channel a lot of
benthic animal was present and the bed consisted primarily of sand with a grey-brown color.
Further seaward the bed samples consisted of a mixture between fine-grained material and
sand with a brown color.
37
The grain size distribution was analyzed by using a Malvern Mastersizer 2000. The bed
samples were dispersed and sieved through a 1.40 mm sieve. Hereby mussels and coarse
grain sizes were excluded from the analysis. The grain size distribution divided in sand (>63
µm), silt (2-63 µm) and clay (<2 µm) are shown in Figure 4-12.
Figure 4-12 Grain size distribution of bed samples divided in clay (<2µm), silt (2-63µm) and sand (>63µm) of 18
samples collected 29-30 October 2015 and 7 December 2015.
Furthermore statistical descriptors were calculated using the logarithmic method of moment
measures based on the grain size distribution. The mean, sorting, skewness is calculated from
the following equations (Blott & Pye, 2001):
�̅�ᶲ =∑ 𝑓𝑚ᶲ
100 (4.7)
𝜎ᶲ = √∑ 𝑓(𝑚ᶲ−�̅�ᶲ)2
100 (4.8)
𝑆𝑘ᶲ =∑ 𝑓(𝑚ᶲ−�̅�ᶲ)3
100∙𝜎ᶲ3
(4.9)
wherein �̅�ᶲ is the mean grain size in phi-units, 𝜎ᶲ is the sorting, 𝑆𝑘ᶲ is the skewness and 𝑚ᶲ is
the midpoint of the class interval.
The mean grain size D (mm) was determined from the following equation:
ф =− 𝐿𝑜𝑔(𝐷)
𝐿𝑜𝑔(2)=> 𝐷(𝑚𝑚) = 2−ф (4.10)
38
A table with the mean grain size, sorting, skewness and description of the bed samples are
listed in Appendix A.
4.4 Data processing
4.4.1 Current velocity
In order to compare the current velocity obtained during the field campaigns, with the
simulated current velocity averaged over depth in MIKE 21, the current velocity was averaged
over depth using the following approach.
The depth averaged current velocity, V, and the friction velocity, uf, is based on the
logarithmic velocity distribution and is found from (Bartholdy, 2013):
𝑉 = (6 + 2.5ln (𝐷
𝑘)) ∙ 𝑢𝑓 (4.11)
𝑢𝑓 =𝑢
(8.5+2.5ln (𝑧
𝑘))
(4.12)
wherein D is the depth (m), k is the bed roughness (m), uf is the friction velocity (m s-1), z is
the measuring height (m), and u is the current velocity (m s-1) at the measuring height.
The measuring heights of the current velocities, pressure transducers and the water depth at
Stations 1-6 are shown in Table 4-2.
Table 4-2 The measuring height of the current velocity, the pressure transducer and the water level (DVR90) at Station 1-6.
Station number Current velocity sensors above bed [m]
Pressure transducer above bed [m]
Water depth [m]
Station 1 0.94 0.42 3.67
Station 2 0.92 0.4 1.35
Station 3 0.25 0.1-0.2 1.22
Station 4 0.25 0.1-0.2 1.22
Station 5 0.87 0.62 2.44
Station 6 0.25 0.1-0.2 1.22
The bed roughness used is set at 0.02 m, which number is based on previous studies in the
adjacent Wadden Sea (Bartholdy, 2000; Bartholdy & Aagaard, 2001). A sensitivity analysis of
the bed roughness parameter was carried out in order to test the effect on the depth averaged
current velocity. The bed roughness was multiplied and divided by a factor 10. The results can
be seen in Figure 4-13, and they did not show any significant change in velocity.
39
Figure 4-13 Depth-averaged current velocity from Station 1 calculated with different bed roughness values
4.4.2 Sediment transport calculations
The net suspended sediment transport was calculated as the net flux for each tidal period,
from low water to low water. The method is based on the work by Bartholdy & Anthony
(1998), and the net transport of suspended sediment transport (QSn) during each tidal period
from low water to low water is given by a stepwise integration of the current velocity and SSC:
𝑄𝑆𝑛 = ∑ 𝐶𝑖 ∙ 𝑉𝑖 ∙ ∆𝑖𝑛𝑖=1 (4.13)
wherein ∆i is the time interval, which is 300 s between each measurement, n is the number of
measurements over a tidal period, Ci is the suspended sediment concentration (kg m-3), and Vi
is the corresponding depth-averaged current velocity. The sediment flux is positive during
flood and negative during ebb.
If the tidal excursion differs between the flood and ebb period the tidal displacement
compromises an advective component. The corrected net suspended sediment flux is
corrected by the following equation:
𝑄𝑆𝑐𝑛 = 𝑄𝑆𝑛 − (𝐶𝑚 ∙ 𝑇𝑑) (4.14)
wherein QScn is the corrected net suspended transport (kg m-2 per tidal period), Cm is the
transport weighted mean concentration (kg m-3), and Td is the tidal displacement (m).
The transport weighted mean concentration can be found by:
𝐶𝑚 = ∑ 𝐶𝑖 ∙ |𝑈𝑖| ∙ ∆𝑖𝑛𝑖=1 / ∑ |𝑈𝑖| ∙ ∆𝑖𝑛
𝑖=1 (4.15)
And the tidal displacement can be found by:
𝑇𝑑 = ∑ 𝑈𝑖 ∙ ∆𝑖𝑛𝑖=1 (4.16)
40
It is assumed that the depth-averaged current velocity and the SSC was representative in the
vertical direction for average water column values.
4.5 Calibration of model Calibration is an approach to compare simulated results with measured data collected in the
field to improve the model. In estuarine and marine studies the calibration of the model
involves changing the sediment properties or the bed resistance ( represented as the manning
number). The tuning of calibration parameters is done until the simulated results are
comparable with the measured data, but with consideration to the physical limits of the
values. The calibration did also include tuning on the resolution of the mesh grid.
In Figure 4-14 and 4-15 the modeled results are calibrated against measured data for the
water level, current speed and suspended sediment concentration at Station 1 and 2.
41
Figure 4-14 Calibration of model at Station 1.
42
Figure 4-15 Calibration of model at Station 2.
In order to calibrate the hydrodynamics a varying bed resistance was used in the model
domain. The bed resistance is expressed as the manning number and a varying manning
within the domain is seen in Figure 4-16. The manning number at the tidal flats was set to 16
as the resistance across the flats are expected to be larger compared to the channels. The
manning number is set to 42 in the main channels and the deeper parts of the model domain.
43
Figure 4-16 Varying manning number across the model domain.
The calibration parameters and values for the cohesive sediment module are listed in Table 4-
3.
44
Table 4-3 List of calibration parameters and inputs to the Mud Transport module.
Parameter Value
Initial conditions
Boundary conditions
Bed resistance
Critical shear stress for deposition (cd)
Critical shear stress for erosion (ce)
Erodibility (E)
Density of bed
Settling velocity
Bed layer thickness
Number of layers
Number of fractions
0.09 [kg m-3]
Timeseries of SSC at Station 1 [kg m-3] b)
0.001 [m]
0.06 [N m-2]
0.1 [N m-2]
5·10-5 [kg·m-2·s-1]
650 [kg·m-3]
0.0004 [m s-1]a)
0.2 [m]
1
1
a) The settling velocity varies within the model domain. The settling velocity is set to 0.0001 m
s-1 within the marina basin and 0.0004 m s-1 in the rest of the area. The settling velocity was
based on fortnightly sediment samples during a study by Bartholdy & Anthony (1998) in
Grådyb Tidal area. The mean value of the samples was found to be 26 µm with a
corresponding settling velocity of 0.0004 m s-1 at 1°C and 0.0006 m s-1 at 20°C (Bartholdy &
Anthony, 1998). Due to the very fine-grained material of the bed samples collected in the
marina basin, the settling velocity was extrapolated to 0.0001 m s-1 as its minimum (Jesper
Bartholdy, personal communication, spring 2016).
b) The same time series of SSC is used in periods without measurements at Station 1. The time
series is set to the respective time within the tidal cycle. It is assumed that the concentration
at Station 1 represents a typical sediment concentration pattern at the boundary. It should
also be noted that the incident of large SSC during 4 June was not included in the simulations,
as the event were not assumed to be typical for a summer situation (cf. Figure 5-2).
The calibration parameters were primarily based on previous studies carried out in the
Wadden Sea and is listed in Table 4-4.
45
Table 4-4 Parameters used to calibrate the model in the present study.
Reference ws [m s-1]
ce
[N m-2] cd
[N m-2] E
[kg·m-2·s-1] Density of bed
[kg m-3]
(Austen et al., 1999) 0.16-3
(Bartholdy & Anthony, 1998) 0.0001-0.0021
(Bartholdy, 2000) 0.3a) 0.12a)
(Lumborg & Windelin, 2003) 3.96 · 10-6 · SSC1.19
0.2-1.5 0.002-0.02 5·10-6
(Lumborg & Pejrup, 2005) 3.96 · 10-6 · SSC1.19
0.19-5 0.05-0.3 5·10-5 - 1.5·10-4
90-1000
(Whitehouse et al., 2000) 0.06-0.10 500-1000b)
a) Bartholdy (2000) used a ce of 0.3 N m-2, but did also cite values from "the real world" to
range from 0.1 N m-2 to 1.3 N m-2 based on data from American Geophysical Union. The value
of cd = 0.12 N m-2 was for grain size with a diameter close to 0.06 mm.
b) Whitehouse et al. (2000) are referring to typical dry density values of intertidal mudflats.
4.6 Minimizing harbor sedimentation As described in section 2.7 different approaches exists to minimize harbor siltation (PIANC,
2008). In the present study the approach of Keeping Sediment Out (KSO) was primarily used
except in scenario 10 which was based on Keeping Sediment Moving (KSM).
Besides from the simulation of the existing conditions a list of the tested scenarios is shown
below:
Scenario 1: The scenario is a simulation of the existing condition with established sheet
pile walls along the two jetties (cf. Figure 8-1). The purpose of this scenario is to break
the current-induced exchange, which is done by stopping the water flow beneath the
jetties.
Scenario 2: The scenario includes a construction of a CDW north of the eastern pier (cf.
Figure 8-7). The purpose of establishing a CDW is to manipulate the flow further around
the entrance and thereby reduce the sedimentation.
Scenario 3: The scenario includes narrowing of the entrance by enlargement of the
westerly part of the entrance (cf. Figure 8-9). The purpose of the scenario is to reduce the
water volume exchange between the marina and the ambient water.
46
Scenario 4: The scenario includes an extension of the eastern pier (cf. Figure 8-11). The
desired effect is to bend the flow around the pier and thereby reducing the total water
exchange during each tidal period.
Scenario 5: In this scenario an enlargement of the western part of the entrance were
constructed (cf. Figure 8-13) with the same purpose as for Scenario 2.
Scenario 6: The scenario is an expansion of Scenario 1, where the construction of sheet
pile walls along the jetties is combined with an enlargement of the western part of the
entrance (cf. Figure 8-15). The scenario is inspired by an outline proposal prepared by
Sunke Arkitekter (see Appendix B) The purpose of the scenario is to test the combination
of narrowing the entrance along with stopping the flow beneath the jetties.
Scenario 7: The scenario is based on scenario 1. The established sheet pile walls along the
jetties are combined with dredging of the southwestern corner of the marina basin to a
navigational depth of 2.2 m. The purpose of this scenario is to control the sedimentation
towards the dredged area (cf. Figure 8-17).
Scenario 8: The scenario is based on scenario 1, where there is only established one
truncated sheet pile wall along the northernmost jetty (cf. Figure 8-19). The purpose of
the scenario is to highlight the expenses related to the establishment of sheet pile walls in
order to reduce the sedimentation - If there is any benefit of establishing one wall instead
of two.
Scenario 9: In this scenario one sheet pile wall is established along the northernmost jetty
(cf. Figure 8-21). The purpose is the same as for scenario 8.
Scenario 10: The purpose of this scenario is to increase the flow within the marina in
order to prevent the deposition of sediments. This is done by an extension of the eastern
pier plus an opening of the pier in the southeastern part in order to create flow (cf. Figure
8-23). This scenario is included at the request of the board of Fanø yacht club.
The structures established in the tested scenarios are implemented in the model within the
HD module. The structures are added as "Gates" and the geometry is selected to cover the
full water column. Furthermore the gates are controlled by a factor between 0-1. A factor of 1
is describing an open gate and a factor of 0 is describing a closed gate. The factors between 0
and 1 are describing a partly opened gate. Within the current study a closed gate was used
with a controlling factor of 0 (DHI, 2014a).
47
5 Fieldwork campaigns
5.1 Stations 1 and 2 June 3-8, 2015 In Figure 5-1 the wind, tidal, and transport dynamics are shown in the period between June 3-
8, 2015 at Stations 1 and 2.
In the period the wind speed varied between 2-12 m s-1 with a westerly wind direction
interrupted by south to southeast directed winds during the June 5. The maximum current
velocity varied between 0.7-0.8 m s-1 in the flood period and 0.6-0.7 m s-1 in the ebb period at
Station 1, whereas the maximum current velocity at Station 2 varied from 0.4-0.5 m s-1 in the
flood period and 0.35-0.4 m s-1 in the ebb period before the sensor dried out. The difference in
maximum current velocity is linked to the difference in cross-sectional areas between the two
locations where a larger cross-sectional area at Station 2 causes a slower current velocity and
vice versa. The median (V50%) and the highest 1 percentile (V1%) of the current velocity are
listed in Table 5-1.
Table 5-1 The median and the highest 1 percentile of the current velocity at Station 1
At Station 1 the current velocity is approximately of the same magnitude in both ebb and
flood, with slight flood dominance. Thus, an evaluation and analysis of the fine-grained
sediment behavior should provide some good and valid results for the area.
The typical sediment concentration at Station 1 varied between 0.02-0.400 kg m-3 and the
variation of SSC was easily recognized, except during June 4 where the SSC reached values
above 1.2 kg m-3. However, in the beginning of a typical flood period the SSC increased quite
rapid from SSC in the range of 0.02-0.06 kg m-3 to a maximum peak between 0.25-0.40 kg m-3.
As the tide continued to fill the area the SSC dropped to a level of 0.02-0.04 kg m-3 with a
minimum around high tide slack, as turbid water from the inner tidal areas was replaced with
relatively clean water from the North Sea. At the return of the tide the SSC gradually increased
towards low tide slack, whereby the SSC dropped as a result of settling during the low tide
slack. A resuspension peak was formed at the return of the tide, whereupon the SSC dropped
towards the next high tide. The resuspension peak that followed a high tide was relatively
weak compared to that after a low tide because of the general low SSC in the water entering
from the North Sea (Bartholdy & Anthony, 1998). The RCM9 Aanderaa sensor at Station 2
dried out during each ebb period and records from the period are therefore not available and
the sediment flux is not calculated for Station 2. The SSC at Station 2 followed the same
pattern and reached the same concentrations as Station 1 both in the flood cycle and the
recorded part of the ebb cycle.
Station 1 Flood [m s-1] Ebb [m s-1]
V (50 %) 0.33 0.25
V (1 %) 0.78 0.72
48
Figure 5-1 Wind, tidal, and transport dynamics at Stations 1 and 2 from 3-8 June, 2015. From top to bottom: Wind speed and wind direction, suspended sediment concentration, depth-averaged current velocity, water level and tidal displacement, net flux of fine-grained sediment and corrected net flux of fine grained sediment (Note: Scales at SSC and Sediment flux are lower in following plots)
49
The sediment concentration pattern stands out during June 4 compared to the rest of the
period. The concentrations reached levels between 0.50-1.00 kg m-3 at low tide, followed by
settling during low tide slack and a resuspension peak at the return of the tide. The maximum
ebb current remained at 0.7 m s-1. However, as the maximum current velocity of 0.75 m s-1
was reached during high tide a sudden increase in SSC of above 1.20 kg m-3was observed. The
duration of the incident occurred between 13:25-16:35. The high concentrations were
interrupted by a sudden drop in SSC at the return of the tide. The recorded data at Station 2
did not follow the same pattern during June 4 as for the rest of the period and the sediment
concentration varied between 0.03-0.17 kg m-3. The incident is interesting as such events can
contribute to a large amount of the annual import of fine-grained material to the Wadden Sea
(Bartholdy & Anthony, 1998; Andersen & Pejrup, 2001). The incident is discussed in further
details in section 5.4.1.
The tidal displacement, net flux of fine-grained sediment and corrected net flux of fine-grained
sediment at Station 1 are shown in Figure 5-1 and the values and the average sediment flux
for the period are listed in Table 5-2. Excel sheets contacting the calculations are attached to a
data disc, cf. Appendix C.
Table 5-2 The net flux of fine-grained sediment, the corrected net flux of fine-grained sediment and tidal
displacement for 9 tidal periods at Station 1.
Tidal period Net sediment flux
[kg m-2] Corrected sediment flux
[kg m-2] Tidal displacement
[m]
1 -311.6 -348.4 273.0
2 4380.8 3761.9 1473.0
3 129.2 143.4 -129.0
4 -14.8 -24.4 51.0
5 154.2 -33.5 2043.0
6 356.6 -329.4 5244.0
7 268.3 -238.4 4722.0
8 336.0 -0.7 2748.0
9 -349.9 -490.6 1284.0
Average for period 549.9 271.1 1967.7
Average for period without the event 4 June
124.2 -116.8 2022.6
The net flux of fine-grained sediment followed the tidal displacement, as a large landward
tidal displacement showed a positive, landward sediment flux in the navigation channel. The
typical net sediment flux varied between -350 and 350 kg m-2 per tidal period with an average
import of 124 kg m-2 per tidal area, whereas the opposite was valid for the corrected net
transport with an average export of 116.8 kg m-2. In a typical summer situation the overall
circulation of fine-grained material was dominated by an approximate balance between
import and export of fine-grained sediment. However, it was seen that the incident on June 4
50
contributed to a tremendous part of the overall net transport, with a sediment flux of
approximately 4000 kg m-2 during one single tidal cycle. The incident on June 4 had an overall
effect of importing between 270-550 kg m-2 per tidal cycle during the entire period.
The tidal displacement was in general directed south with 2000 m on average. The water
volume flowing past Station 5 during flood was larger than the volume returned during ebb.
According to the continuity equation this has to be compensated for by an advective
component at another place in the tidal area. This is compensated for in the corrected net flux
of fine grained sediment. However, as settling and scour lag mechanisms are important
processes for the net landward deposition of fine-grained sediment it is expected that the net
flux of suspended sediment will be somewhat between the corrected sediment flux and the
non-corrected sediment flux. The corrected term is calculated as the tidal displacement (eq.
4.16) multiplied by the transport weighted mean concentration (eq. 4.15) of the
corresponding tidal period, which is an estimate of the landward effect of the tidal asymmetry
and settling and scour lag mechanisms.
51
5.2 Stations 3 and 4 July 20-22, 2015
Two RCM9 Aanderaa sensors were installed on opposite sides of the marina entrance,
referred to as respectively Station 3 in the western part of the entrance and Station 4 in the
eastern part of the entrance from July 20-22. The wind-, tidal- and sediment dynamics are
seen in Figure 5-2. The wind speed varied between 1-12 m s-1 with a primary wind direction
from southwest.
Typical maximum current velocities at Stations 3 and 4 were 0.1-0.2 m s-1 in both the ebb and
flood cycle. It was seen that opposite directed currents were present on either side of the
entrance during the first and third tidal period. This observation indicated that a horizontal
current-induced exchange was present in the marina. The phenomenon exclusively occurred
when the basin was filled up close to high tide. It is well known that at larger water exchange
within a harbor basin results in a larger sedimentation. The basin’s tidal prism is the
minimum water volume that can contribute to harbor sedimentation within a tidal cycle.
However, the water volume can be increased quite significantly if a current-induced exchange
is present (PIANC, 2015).
The SSC varied between 0.015-0.15 kg m-3 with a mean concentration of approximately 0.030
kg m-3 at both stations. The SSC was observed to increase during flood to peaks between 0.08-
0.15 kg m-3 at Station 3 and peaks between 0.04-0.12 kg m-3 at Station 4. The SSC dropped due
to the relatively clean water coming in from the North Sea. At the return of the tide no
resuspension peak was formed as the current-induced bed shear stress within the marina was
too weak to erode the bed.
52
Figure 5-2 Wind, tidal and transport dynamics at Stations 3 and 4, July 20-22, 2015. From top to bottom: Wind
speed and wind direction (A), suspended sediment concentration, depth-averaged current velocity,
water level, current direction.
53
On the basis of the in situ measurements of the SSC from the entrance of the marina a rough
calculation of the contribution to the annual sedimentation from tidal filling was carried out.
The results are seen in Table 5-3:
Table 5-3 Rough calculation of the contribution from the maximum tidal-induced exchange to the annual
sedimentation in Fanø Marina. It is assumed that a year consist of 730 tidal periods.
Tidal range [m]
Marina basin [m2]
Tidal prism [m3]
Mean SSC in entrance [kg ,m-3]
Dry density [kg ,m-3]
Annual sedimentation
[cm]
1.6 21,000 33,600 0.03 650 5.4
1.6 21,000 33,600 0.03 1000 3.5
1.6 21,000 33,600 0.03 350 10.0
1.6 21,000 33,600 0.02 650 3.6
1.6 21,000 33,600 0.04 650 7.2
The maximum contribution from the tidal-induced exchange to the annual sedimentation was
calculated to approximately 3-10 cm. The used approach is of course a rough simplification, as
it is assumed that all suspended sediment entering the basin is deposited during each tidal
cycle. This is far from what is happening in nature. The tidal-induced sedimentation
contributes to 10-50 % of the observed maximum annual sedimentation (local observation
20-30 cm y-1). There is a relative large uncertainty related to the observed amounts and to the
calculations. However, the essence of the calculations is that the tidal-induced contribution to
the sedimentation is less than the observed amounts, and that the contribution from current-
induced exchange is not insignificant.
The calculations are a good first-hand estimate of the sedimentation rate within the marina
and useful when evaluating the modeled results later on.
54
5.3 Stations 5 and 6 October 30 to November 27, 2015 Wind, tidal and transport dynamics from October 30 to November 27, 2015 at
Station 5 are seen in Figure 5-3.
Station 5 was located in the navigational channel approximately 200 m from the
marina entrance and Station 6 was located at the most seaward harbor pole in the
marina entrance.
The mean wind speed, wind direction and water level varied a lot in the period from
gentle breezes to fresh gales and the water level reached a maximum above 2.5 m
DVR90 during November 14. A period with light breezes to strong breezes occurred
from October 30 to November 7 interrupted by Storm Freja that hit Denmark on
November 8 with mean wind speed of up to 16 m s-1 and a water level reaching
above 2 m DVR90. In the rest of the period the wind speed varied a lot with two
peaks, respectively, on November 14 and again on November 20 with mean wind
speeds above 16 m s-1. In all three periods with high wind speeds the wind direction
was westerly and wind set-up forced water into Grådyb tidal area causing elevated
water levels above 2 m DVR90.
55
Figure 5-3 Wind, tidal and transport dynamics at Station 5. From bottom to top: Wind speed and wind
direction, suspended sediment concentration, depth-averaged current velocity, water level and net
flux of fine-grained sediment, corrected net flux of fine-grained sediment and tidal displacement.
56
In Figure 5-4 two rose plots are presented of the current velocity and the current
direction at Stations 5 and 6, and the median (V50%) and the highest 1 percentile
(V1%) of the current velocity are listed in Table 5-4. It can be seen that both Stations
are asymmetric in the ebb and flood cycle. Station 5 is flood dominated whereas
Station 6 is ebb dominated.
Figure 5-4 Rose plots of the current velocity and current direction at Station 5 and 6 in the period
from October 30 to November 27, 2015.
Table 5-4 The median and the highest 1 percentile of the current velocity at Station 5 and 6
Station 5 Flood Ebb Station 6 Flood Ebb V(50%) 0.19 0.06 V(50%) 0.10 0.12 V(1%) 0.58 0.39 V(1%) 0.45 0.54
The following results will be divided into three sections in order to create a better
and more detailed overview of the results. The purpose of dividing the results in
three sections was also to analyze the dynamics and the seasonal variability of
import and export of fine-grained sediment within the autumn fieldwork campaign
as different weather conditions were present.
The first period is from November 1-7 and was chosen due to fairly calm weather
conditions. The wind direction was primarily southeasterly with a wind speed
between 1-8 m s-1. The water level varied between -0.75 to 1.2 m DVR90.
The second period is from the November 12-17 and represented a period with
rough weather conditions. The maximum mean wind speed peaked at 18 m s-1 from
a westerly direction during 14 November. Water levels reached above 2.5 m DVR90
in this period.
The third period is from November 21-27 and represented a period with wind
directions from north and a wind speed that varied between 4-10 m s-1.
57
5.3.1 Stations 5 and 6 November 1-7, 2015
Wind, tidal and transport dynamics at Station 5 and 6 during November 1-7 are seen
in Figure 5-5.
The median (V50%) and the highest 1 percentile (V1%) of the current velocity are
listed in Table 5-5.
Table 5-5 The median and the highest 1 percentile of the current velocity at Station
Station 5 Flood Ebb Station 6 Flood Ebb
V(50%) 0.19 0.09 V(50%) 0.14 0.14
V(1%) 0.51 0.36 V(1%) 0.48 0.54
The current velocity at Station 5 was slightly asymmetric with typical maximum
current velocities between 0.4-0.5 m s-1 in the flood period compared to 0.3-0.4 m s-1
in the ebb period. The asymmetry is also reflected in the median current velocity of
0.19 m s-1 and 0.09 m s-1 in the flood and ebb period, respectively.
The current velocity at Station 6 reached typical maximum velocities of 0.4-0.5 m s-1
during flood and of 0.45-0.55 m s-1 during ebb. It was also observed that the current
velocity was not bidirectional like the current velocity observed at Station 5. This
was due to a current-induced exchange within the marina or just outside the marina
as was also present at Stations 3 and 4 mentioned earlier. When the marina was
filled up during high tide, water was forced back around and poured out in one of
the sides in the marina entrance. This claim is also supported as the current velocity
at Stations 5 and 6 in general were in the same direction during ebb whereas the
opposite occurred in most of the flood periods.
The SSC was in the range of 0.015-0.12 kg m-3 at both Stations 5 and 6. At Station 5
the concentration was low around high tide slack and increased towards low tide
slack as water from the inner tidal area was more turbid compared to the relatively
clean water from the North Sea. Particles settled during low water slack at Station 5,
which was seen as a small drop in SSC of 0.005-0.010 kg m-3. The decrease was low
compared to the concentration at Station 1 as the mean grain size of the primary
particles of the bed samples decreased by approximately 300 µm between the two
stations, which is the reason why a lower settling velocity was expected at Stations 5
and 6 compared to Station 1 (cf. Appendix A). On the return of the tide a
resuspension peak was observed at Stations 5 and 6.
The SSC at Station 6 was clearly asymmetric between the flood and ebb cycles. This
was due to the position of the sensor being sheltered from turbid water coming from
the intertidal areas during the ebb cycle. Furthermore the current velocity within
the marina was too low to erode the bed which is seen as a continuing decrease in
SSC at Station 6. At the return of the tide a resuspension peak was observed and
afterwards the SSC decreased towards the next high tide slack.
58
Figure 5-5 Wind, tidal and transport dynamics at Stations 5 and 6. From top to bottom: Wind speed and wind
direction, suspended sediment concentration, depth-averaged current velocity, water level and net
flux of fine-grained sediment, corrected net flux of fine-grained sediment and tidal displacement.
59
In the 6-day period the variation in SSC was in general small, with the largest
sediment concentration of approximately 0.12 kg m-3 during two tidal periods from
4-5 November. The two peaks coincided with an increase in the mean wind speed to
8 m s-1 with a southeastern wind direction. During onshore winds the adjacent tidal
flats in Fanø Lo are prone to wind-induced wave action. Tidal flats are very sensitive
to wind-induced wave action because of the low water depths. Even small waves
across the tidal flats favor the generation of high bed shear stresses, thus causing
sediment resuspension (Andersen & Pejrup, 2001).
The average tidal displacement, net flux and corrected net flux of fine-grained
sediment for Stations 5 and 6 are seen in Table 5-6.
Table 5-6 The average net flux of fine-grained sediment, the corrected net flux of fine-grained
sediment and tidal displacement for Stations 5 and 6 from November 1-7, 2015.
Net sediment flux
[kg m-2] Corrected sediment
flux [kg m-2] Tidal displacement
[m]
Average (Station 5) 84 -21.4 2887.4
Average (Station 6) -30.7 -60.6 -1299.8
The net flux of fine grained sediment and corrected net flux of fine-grained material
at Stations 5 and 6 were in general low with average fluxes between -100 to 100 kg
m-2. The net flux of fine-grained sediment followed the tidal displacement, as a large
landward tidal displacement showed a positive and landward sediment flux at
Station 5. This was also the case at Station 6, where a seaward tidal displacement
was followed by an export of fine-grained material at Station 6. The tidal
displacement at Station 6 was in general seaward, which is speculated to be caused
by a current-induced exchange within and outside the marina entrance. A
corresponding import might have occurred in the opposite side of the marina
entrance.
In an autumn period with calm weather conditions the overall circulation of fine-
grained material was dominated by an approximate balance between small import
and export of fine-grained sediment.
60
5.3.2 Stations 5 and 6 November 12-17, 2015
The wind, tidal, and transport dynamics at Station 5 and 6 from November 12- 17
are seen in Figure 5-6.
In the beginning of the period the mean wind speed varied between 4-10 m s-1 with
a west to south directed wind. The wind, hydrodynamic and transport conditions
were interrupted by a moderate to fresh gale that hit Denmark on 14 November.
During the gale a mean maximum wind speed of 18 m s-1 and maximum water levels
of 2.5 m DVR90 were recorded. The pressure transducers range at Station 5 was
exceeded at 2.5 m DVR90, for which reason the water level curve flattened, and the
pressure transducer at Station 6 was probably lifted upwards in the water column
during this period. In general the water level varied between -0.5 m and 3 m DVR90
from November 12-17 and the water level was above 0 m DVR90 at low tide as
wind-setup forced water into Grådyb tidal area thereby causing a landward tidal
displacement in this period.
From November 12-13 the SSC reached maximum values of 0.18-0.35 kg m-3 during
the resuspension peak. The largest peak was observed in the preface of the gale
around 12:00 November 13, which corresponded with the largest current velocity of
0.6 m s-1. During the gala the SSC was below 0.10 kg m-3 as relatively clean water
from the North Sea entered Grådyb tidal area. The water level was above 1 m DVR90
during the gale and the current speed decreased to 0.2 m s-1. The current velocity
decreased as a consequence of the increased cross-sectional area within the tidal
area. In the aftermath of the gale the SSC reached levels equal to the SSC observed
from November 1-7.
61
Figure 5-6 Wind, tidal and transport dynamics at Stations 5 and Station 6. From bottom to top: Wind speed and
wind direction, suspended sediment concentration, depth-averaged current velocity, water level,
and net flux of fine-grained sediment, corrected net flux of fine-grained sediment and tidal
displacement.
62
The average tidal displacement, net flux and corrected net flux of fine-grained
sediment at Stations 5 and 6 are seen in Table 5-6.
Table 5-7 The net flux of fine-grained sediment, the corrected net flux of fine-grained sediment and
tidal displacement for Station 5 from November 12-17, 2015.
Tidal period Net sediment flux
[kg m-2] Corrected sediment
flux [kg m-2] Tidal displacement
[m] 1 355.9 72.7 5598 2 301.6 66.3 3204 3 464.0 -50.5 6324 4 1091.0 41.5 8457 5 172.5 17.1 3765 6 71.1 16.4 1962 7 -12.8 -91.8 1581 8 289.6 103.2 3120 9 247.5 8.6 6246
10 230.7 51.4 4209 Average for period (St.
5) 321.1 23.5 4447
During a period with an autumn gale the net flux of fine-grained sediment was on
average importing 320 kg m-2 per tidal period at Station 5, which was balanced by an
advective component in the opposite direction according to the corrected net flux.
This was accompanied by a landward tidal displacement of up to 8.5 km.
63
5.3.3 Stations 5 and 6 November 21-27, 2015
The wind, tidal and transport dynamics from November, 21-27 are seen in Figure 5-
7. The battery at Station 6 ran out of power during November 24.
The period is characterized by a north directed wind from November 21-23
interrupted by periods with southwest and northwest directed winds. The wind
speed varied between 0-14 m s-1 with maximum peaks of 12 and 14 m s-1 during
November 22 and 24, respectively. The water level varied between -1.5 m to 2 m
DVR90.
The variation in SSC followed in general the same pattern as described above during
November 1-7, although now with much higher concentrations. The SSC reached
maximum concentrations above 0.20 kg m-3 in most of the tidal periods. The largest
peaks observed were between 0.50-0.60 kg m-3 at Stations 5 and 6 during November
22. In this incident the tidal displacement was seaward with an ebb maximum
current velocity of 0.5 m s-1 at Station 5 and the water level was below -1 m DVR90.
This favored an export of fine-grained sediment.
The average tidal displacement, net flux and corrected net flux of fine-grained
sediment for Station 5 are listed in Table 5-8.
Table 5-8 The average net flux of fine-grained sediment, the corrected net flux of fine-grained
sediment and tidal displacement for Station 5 from 21-27 November, 2015.
Tidal period Net sediment flux
[kg m-2] Corrected sediment
flux [kg m-2] Tidal displacement
[m] 1 -261.4 -326.5 627 2 -164.5 -252.3 885 3 294.2 127.5 1779 4 -593.4 -388.0 -1458 5 180.2 189.5 -72 6 721.1 307.1 3366 7 907.8 80.4 7278 8 1029.9 57.2 9690 9 312.7 145.5 2139
10 422.1 101.0 4146 11 332.9 127.4 2580 12 491.6 240.1 2481 13 326.8 -219.7 4638
Average for period 307.7 14.6 2929.2
The net flux of fine-grained sediment varied between -600 kg m-2 to 1000 kg m-2
within this period at Station 5. An export of suspended sediment was observed in
the beginning of the period followed by subsequent import of fine grained sediment
and it was seen that the net flux of fine grained sediment followed the tidal
displacement. The corrected net flux of fine grained sediment was balanced by a
variation between import and export.
64
Figure 5-7 Wind, tidal and transport dynamics at Station 5 and Station 6. From top to bottom: Wind speed and
wind direction, suspended sediment concentration, depth-averaged current velocity, water level,
and net flux of fine-grained sediment, corrected net flux of fine-grained sediment and tidal
displacement.
65
5.4 Discussion of the hydrodynamics and sediment dynamics
In this section the measurements of the hydrodynamics and sediment dynamics,
respectively, and the calculations of the suspended sediment transport in the
adjacent area to Fanø Marina will be discussed. It will be discussed if there is a
reasonable explanation for the variability observed in the hydrodynamics and
sediment dynamics.
The calculations of fine-grained sediment transport in the present study were based
on approximately 66 tidal periods from the summer and autumn 2015, with 54 tidal
periods during the autumn and 12 tidal periods during the summer.
In general it was seen that the net flux of fine-grained sediment was highly
dependent on the residual water flux. A tidal displacement towards the south caused
an import of fine-grained sediment, whereas a tidal displacement towards the north
caused an export. It is important to distinguish between the real net transport and
the advective component. The latter might cause large errors as a large import might
occur at the position of the RCM9 Aanderaa sensor, while a similar or larger export
is occurring at the other end (Bartholdy & Anthony, 1998). This was compensated
for in the corrected net flux of fine-grained sediment. The real net transport is
believed to be somewhere between the calculated net flux of fine-grained sediment
and the corrected transport, as mechanisms, such as settling and scour lag are
favoring a landward transport of fine-grained sediment, which mechanisms are not
taken into account in the transport calculations. However, the corrected term where
the tidal displacement (eq. 4.16) is multiplied with the transport weighted mean
concentration (eq. 4.15) is an actual estimate of the total effect of settling and scour
lag.
The area exported fine-grained sediment during two tidal periods from 30-31
October, which was due to a mean wind speed of up to 13 m s-1 and a south-easterly
wind (cf. Figure 5-3). During a strong south-easterly wind, the water was moved
further seawards, bringing turbid water from the turbidity maximum further north,
with a larger SSC as a result. Furthermore wave activity across the tidal flats
resuspended sediment. The transport of fine-grained sediment is very dependent on
the weather conditions and is not only a consequence of the tide and wave action
(Bartholdy & Anthony, 1998). However, the wind speed and wave action cannot be
separated, as the wind-induced wave action can generate high bed shear stresses
across the tidal flats, which favors resuspension of sediment (Andersen & Pejrup,
2001). Throughout the entire period the corrected fine-grained sediment was
exported during periods with wind directions from south to southeast, but was not
as prominent compared to the export observed on October 30-31, during which
period the wind speed was very high.
66
At Station 5 the corrected net flux of fine-grained sediment suggested a balance
between small import and export events from 1-20 November. An export was
expected during the fresh gale from 13-15 November, as export events have been
reported several times during stormy periods in the Wadden Sea (e.g. Bartholdy &
Anthony, 1998; Bartholdy, 2000). However, at Station 5 the corrected net flux of
fine-grained sediment was only slightly landward, and the measured net flux of fine-
grained sediment was strongly landward with 1000 kg m-2 per tidal period, part of
which was due to the residual water flux. In the following tidal periods a small
corrected net flux of fine-grained sediment was observed. The same pattern was
observed during a similar gale approximately one week later.
On the basis of measurements from 180 tidal periods covering all seasons in Grådyb
tidal area, Bartholdy & Anthony (1998) concluded that the area was exporting fine-
grained sediment during stormy periods with large amounts of fine-grained
sediment being mobilized and exchanged with the relatively clean water from the
North Sea. The tidal area was importing material during longer periods with calm
weather conditions following periods with more rough weather conditions,
especially during the summer period. The import during one such period was
estimated to cover about 25 % of the expected yearly net-import (Bartholdy &
Anthony, 1998). This was supported by a study by Andersen & Pejrup (2001) in
Listerdyb tidal area, where an extremely strong landward net-transport was
observed after a strong storm, where five tidal periods were estimated to be
equivalent to 40 % of the annual accretion. However, a small landward net transport
was observed during the storm surge, which is in line with the findings in this study.
It should be noted that in the current study the small import of suspended matter
during the fresh gale and in the aftermath of the storm only was measured within a
restricted part of the tidal basin. It is likely that erosion occurred in other parts of
the tidal basin. Fanø Lo is sheltered from the North Sea during westerly winds and
erosion probably occurred in the eastern part where wind-induced wave action
favored local erosion.
It was also seen that during northern winds the fine-grained sediment was exported
from the area from November 21-23. The largest SSC peak of 0.6 kg m-3 throughout
the entire period was measured during these days. Fanø Marina is sheltered during
winds from the north, but the fetch from e.g. Skideneng to the mainland is
approximately 5.5 km. This distance was probably sufficient to generate small wind-
induced waves across the tidal flats, promoting resuspension of sediment. During
the ebb period the resuspended sediment was transported seaward, thus causing an
export. The residual water flux was also directed seaward as the high concentrations
of suspended sediment coincided. The high concentrations observed is related to
turbid water being moved from south to north. A tidal divide between Knudedyb
and Grådyb tidal area is located to the south with a related turbidity maximum (cf.
67
Figure 3-2) (Bartholdy & Anthony, 1998). In the following tidal periods from 23-27
November the corrected net fine-grained sediment transport was landward.
The high concentrations could partly be explained by dredging operations in
Slunden and Fanø Lo during 20-28 November. The dredged sediment volume was
maximum 11,300 m3 within this period (Signe Ingvardsen, personal communication,
March 11, 2016). The dredging operation could have disturbed the bed in the area,
favoring high suspended sediment concentration, which was observed during the
period.
5.4.1 Incident of large import on June 4, 2015 The large landward net flux and corrected net flux of fine-grained sediment at
Station 1 during June 4 will be discussed in this section. It is not clear what caused
the large import and the high suspended sediment concentrations observed. The
hydrodynamic conditions did not stand out in the period, as typical maximum
current velocities of 0.7-0.8 m s-1 were observed in the ebb and flood cycles. Also,
the weather conditions were not extreme, with a mean wind speed of 6 m s-1 and a
wind direction from west. Furthermore no dredging operations were taking place
during June 4 and the latest dredging operation was carried out during April 2016
(Signe Ingvardsen, personal communication, March 11, 2016).
The incident may have been caused by an increasing availability of fine-grained
material in the northern part of the shelf area. This working hypothesis is under
investigation (Jesper Bartholdy, personal communication, spring 2016) and is based
on observations of episodic import events in Grådyb (Bartholdy & Anthony, 1998).
The assumption is that the submerged reef Horns Rev (cf. Figure 3-2), stretching
about 40 km out in the North Sea, is acting as groin making a vortex trapping fine-
grained sediment during calm conditions. The north-directed Jutland current
bringing sediment from south to north is assumed to be affected by the morphology
of Horns Rev, hence favoring deposition of fine-grained material. The deposition is
thought to take place during long periods with calm weather and the material is
resuspended during rough weather conditions. During such events fine-grained
sediment with large settling velocity has been mobilized at the shelf, and large
import events have been reported in Grådyb tidal area (Bartholdy & Anthony, 1998;
Pedersen & Bartholdy, 2006). In order to test the hypothesis the wind conditions
from a measuring station in the Port of Esbjerg are shown in Figure 5-8.
68
Figure 5-8 Mean wind speed and wind direction from May 5 to June 6, 2015. The red square
highlights the duration of the incident during June 4, where a large import of fine-
grained sediment was observed.
It can be seen that the mean wind speed during June 2-3 is up to 12-16 m s-1 for
almost a whole day. The last time the mean speed reached 16 m s-1 was during 10
May, but only for a short period as the wind speed dropped relatively fast below 3 m
s-1. This is consistent with a cohesive modelling study in Listerdyb by Lumborg &
Windelin (2003). They found that strong winds were eroding more sediment from
the bottom, but in the investigated period, these winds did not last long enough to
mobilize the bed. The duration of the high wind speeds is therefore of importance.
However, the wind speed during June 4 when the incident occurred was quite low. It
is possible that there is some response time, from the high wind speeds across
Horns Rev, until the high concentrations of suspended sediment is entering Grådyb
tidal area. It should be noted that this is just speculations, since no clear signs were
revealed as to why the large import occurred during June 4 in terms of the SSC at
Station 2 and the hydrodynamic and wind conditions. Another possible explanation
of the incident could be seaweed or other things blocking the signal from the OBS
sensor causing the data to be biased. However, it should be noted that the sediment
concentration pattern during June 4 seems reliable, as why the latter explanation
seems unlikely.
69
5.5 Summarize of fieldwork campaigns Through the analysis and evaluation of hydrodynamic and sediment concentration
measurements at Stations 1-6 during the summer and autumn 2015 it was seen that:
The suspended sediment transport was dependent on the wind conditions.
The SSC was seen to be largest during wind directions from north and south
to southeast.
An export of fine-grained sediment was in general observed during wind
directions from south- to southeast.
The net flux of fine-grained sediment was dependent on the residual water
flux
An import of fine-grained sediment was observed during gales in the autumn
The incident of a large sediment concentration on June 4 contributed to a tremendous part of the overall net flux of fine-grained sediment in the period June 3-8. A reasonable explanation of the incident could only be speculated.
A current-induced exchange was observed at Stations 3 and 4 between July
20-22 and at Station 6 in the autumn.
No natural resuspension was observed within the marina.
Not all sediment entering the marina was deposited, as a very fine-grained
size fraction is present with slow settling velocities which are not deposited.
Fanø Marina is a sink for fine-grained sediment.
Fanø Marina is acting as a sediment trap for fine-grained sediment. Two processes
related to marina sedimentation are illustrated in Figure 5-9:
70
Figure 5-9 Illustration of sedimentation processes observed during field campaigns in the present
study. Situation A is due to tidal filling and situation B is due to a current-induced
exchange within the marina basin and at the mouth of the entrance.
The illustrations are related to two flow processes causing harbor siltation in Fanø
Marina. Situation A represents the tidal filling of the marina. During the ebb cycle
sediment-rich water from the inner tidal area were passing by the sheltered marina,
whereas no natural resuspension occurred within the marina. At the return of the
tide a resuspension peak was formed bringing turbid water into the marina. As the
water entered the marina a settling process started. The sediment concentration
decreased gradually towards high tide slack as relatively clean water entered from
the North Sea. This pattern is more or less repeated during each tidal. However, it
should be noted that not all sediment entering the marina are deposited, as a very
fine fraction of the sediment were present having a very slow settling velocity.
Situation B is related to two types of current-induced exchange. The large circulation
pattern observed within the marina is happening due to tidal filling close up to high
tide. Water is forced into the marina in one side of the entrance and exiting the
marina in the other side. The other type of current-induced exchange is often related
to the marina entrance, where the exchange is induced by a flow separation of the
main flow in the navigational channel with the more calm flow within the marina (cf.
Figure 2-2a).
71
6 Validation of model
6.1 Validation The configured model was calibrated in section 4.5 and will be validated against
measurements from Stations 3, 4, 5 and 6. This is done in order to evaluate how well
the model is able to describe the hydrodynamics and the cohesive sediment
transport in the area after the calibration period from June 3-8.
The water level, current speed and SSC are seen in Figures 6-1 to 6-4:
Figure 6-1 Validation of model for Station 3
Figure 6-2 Validation of model for Station 4
72
Figure 6-3 Validation of model for Station 5
73
Figure 6-4 Validation of model for Station 6
It can be seen that there is a reasonable match between the model results and the
measurements in the essential points, and the model is considered sufficiently
calibrated to describe the hydrodynamics, sediment transport, deposition and
erosion within Fanø Marina. However, it should be noted that the simulated current
speed at Station 6 is underestimated by a factor 5. The current speed is illustrated in
Figure 6-5:
74
Figure 6-5 Current pattern low tide +4.5 hours
It is speculated that it was not possible to capture the details of the stronger outer
current that is forced around the eastern pier, which will reach further into the
entrance in reality. The current speed fits quite well at Station 3 and 4 and the water
volume entering the marina is assumed to be correct.
At Station 5 the simulated SSC were in general overestimated. This is due to a
mismatch between the current-induced bed shear stress and the critical bed shear
stress for deposition (cd) within this point. As a result the suspended sediment will
first be deposited, as the current speed is slowed down towards the marina entrance
within the model. The calibration and modeled results will be discussed in further
details in section 7.5 and 8.12.
The following 4 figures are illustrating typical in- and outflow patterns:
Figure 6-6 Current pattern high tide +3 hours
75
Figure 6-7 Current pattern high tide +5 hours
Figure 6-8 Current pattern low tide +3 hours
76
Figure 6-9 Current pattern low tide +5 hours
6.2 Hypsography
The hypsography of the model domain is seen in Figure 6-9.
Figure 6-9 Hypsographic curves used to determine the tidal prism for the model domain.
According to a report prepared by the Danish Coastal Authority in Grådyb tidal area
the mean water level in Esbjerg harbor was 0.12 m DVR90 in 2005 with an expected
rate of 0.022 m y-1 (Kystdirektoratet, 2006). It is seen that the modelled area is
approximately 6 km2 during mean water level which corresponds to 3.6 % of Grådyb
tidal area. The tidal prism in the model domain has been calculated in two ways. The
first one has been calculated on basis of the bathymetry in 0.1 m elevation intervals
and the second one has been simulated in the model. It is seen that the cumulated
water volume of the simulation are in good agreement with the calculated water
volume. The model complies with the continuity equation.
77
7 Analysis of sedimentation processes
7.1 General sedimentation processes The sedimentation in Fanø Marina can be related to a number of sedimentation
processes, which are coupled to the following flow phenomenon in the entrance:
Tidal-induced exchange
Density-induced exchange (vertical circulation)
Current-induced exchange (horizontal circulation)
Wave-induced exchange
Turbulence generated by the wash of ship’s propellers
The combination of these flow phenomenons contribute to the net sedimentation.
The density-driven, ship generated turbulence and wave-induced exchange are not
assumed to contribute to a significant amount of the overall sedimentation
compared to the contribution from the tidal-induced and current-induced exchange.
This will be explained in the following.
Waves are reported to play an important role in terms of the amount of suspended
fine-grained sediment in the Danish Wadden Sea, (Bartholdy & Anthony, 1998;
Andersen & Pejrup, 2001). However, it was assumed that Fanø Marina is not in
particular exposed to wave activity as the Marina is sheltered by a pier.
In Figure 7-1 the difference in temperature and salinity between Stations 5 and 6 are
shown. The salinity and temperature from Station 5 is subtracted from Station 6.
Figure 7-1 Difference in salinity and temperature between Stations 5 and 6. The values at Station 5
is subtracted from Station 6 and averaged in 3-hour intervals.
The average difference in temperature is 0.004 °C and salinity is -0.12 ‰. The
difference in salinity and temperature were in general low and were balanced by
small out- and inward density differences. The density-driven exchange is also more
78
often related to deep basins. Furthermore, it was seen that the horizontal difference
in SSC, between Stations 5 and 6, at the marina and the area in front of the marina in
general were low. On basis of this the density-driven exchange is not assumed to
have a significant importance for the sedimentation in the marina basin.
It is hard to estimate the overall contribution from vessels. An approach developed
by Hamill & Johnston (1999) was used to test if propellers from moving vessels were
able to disturb the bed. In the test a propel diameter of 0.4 m, a propeller rotation of
2 per second and a depth of 0.3 m above the bed were used. From these parameters
the generated bed shear stress of 0.6 N m-2 was not enough to erode the bed in the
entrance. However, it is expected that vessels can disturb the bed and bring
sediment in suspension - especially during low tide - and once in suspension some of
the sediment may escape the harbor. But more often vessels are redistributing
sediment into less active parts of the marina (Whitehouse et al., 2000). The overall
effect is assumed to be low compared to the tidal-induced exchange and current-
induced exchange.
7.2 Bed shear stress Deposition of fine-grained sediment is greatly influenced by the bed shear stress
(b). The bed shear stress at Station 4 located in the marina entrance is seen in
Figure 7-2:
Figure 7-2 Bed shear stress at Station 4, located in the entrance. The used bed roughness is 0.001 m.
The bed shear stress is not exceeding 0.004 N m-2 during July 20-22 as the current
speed is very low in the entrance. The bed shear stress is dependent on the bed
roughness, the water depth and the current speed. Fine-grained material can
normally be deposited when the bed shear stress is below 0.1 N m-2 (cf. Table 4-4).
It is therefore reasonable to assume that all of the sediment entering the marina will
start a settling process. However, not all the sediment is deposited during each tidal
period, as a proportion of the sediment is either not flocculated, is in small flocs or is
of a very fine fraction, thus having a very slow settling velocity. The typical bed shear
stress needed in order to erode the bed (ce) is given for different sediment types in
79
Table 7-1. As shown in the table, the deposited material will not be eroded by the
bed shear stress generated by the tide.
Table 7-1 Overview of the critical bed shear stress for erosion of different sediment types (DHI,
2008)
Sediment type ce [N m-2]
Fine grained sediment immediately after deposition 0.1
Fine grained sediment 2-4 days after consolidation 0.25
Fine grained sediment several weeks after consolidation 0.5-1
Fine sand 0.5-1
Coarse sand >1
7.3 Current-induced exchange In section 5.2 it was stated that current-induced exchange can enhance the water
volume quite significantly, with an increased sedimentation as a result. In order to
identify the presence of a current-induced exchange two RCM9 sensors were
installed for a short period, in opposite sides of the entrance. During the
measurements on July 20-22 at Stations 3 and 4, a current-induced exchange was
observed. The current direction was opposite directed in either side of the entrance
during flood as the marina basin was filled up close to high tide. The current-
induced exchange is simulated and illustrated in Figure 7-3:
Figure 7-3 Current-induced exchange in Fanø Marina. Current pattern +3 hours before high tide.
It is seen that two possible current-induced exchanges are taking place within the
marina. One takes place within the entire harbor basin and is responsible for the
80
distribution of sediment and thereby increases the sedimentation. The second type
of current-induced exchange (cf. Figure 2-2), is related to the mouth of the marina,
where the exchange takes place due to the strong current in the external water and
the more calm water within the marina. This type of current-induced exchange is
often associated with harbors, where the entrance is located more or less orthogonal
to the main flow.
7.4 Sedimentation map A sedimentation map after 10 tidal periods from 3-8 June is seen in Figure 7-4. It is
seen that the primary deposition is taking place in the entrance of the marina and
that the sedimentation decreases gradually towards land. Furthermore deposition
takes place east of the pier. This is also in accordance with local observations (Per
Hansen, personal communication, spring 2016).
Figure 7-4 Sedimentation map after 10 tidal periods
The annual input of sediment to Fanø Marina is calculated and seen in Table 7-2.
Table 7-2 Annual sedimentation in Fanø Marina. Calculated with a dry density of 650 kg m-3
Annual deposition 1980 tons
Annual sedimentationrate 15 cm
The average modelled deposition across the entire harbor basin is 15 cm y-1. The
sedimentation rate is below the local observations of 20-30 cm y-1, but larger than
the rough calculations of 3-10 cm y-1. In section 8.1 to 8.10 the modelled scenarios of
81
possible remedial actions are tested and the calculations will be made relative to the
baseline result above. The model results will be discussed and evaluated in the
following section.
7.5 Evaluation of model results On the basis of the simulated period from June 3-8, 2015 the annual sedimentation
rate was found to be 15 cm y-1 or 1980 t y-1. This corresponds to 2.2 % of the
estimated annual net accumulation of fine-grained sediment in Grådyb tidal area
(Pedersen & Bartholdy, 2006). The sedimentation rate seems quite high taking the
relative size of the marina into account. The marina basin corresponds to 0.013 % of
Grådyb tidal area. However, it should still be kept in mind, that the simulated annual
sedimentation is lower than the local observations of up to 20-30 cm y-1.
A similar net annual accumulation rate within the Danish part of the Wadden Sea
was not found. Pedersen & Bartholdy (2006) sampled monolith and piston cores and
estimated the accretion rates by 210Pb-dating of cores from the three tidal areas in
the Danish Wadden Sea. On average the annual accretion rate was largest in Juvre
Dyb tidal area corresponding to 7 mm y-1, intermediate in Knudedyb tidal area
corresponding to 4 mm y-1, and smallest in Grådyb tidal area corresponding to 2.6
mm y-1. However, the average values did have internal variations. The samples GM1
(marsh), GM2 (marsh) and GP1 (mixed flat) were collected closest to Fanø Marina
with an annual accretion rate of 11.9 mm y-1, 3.6 mm y-1 and 3.2 mm y-1,
respectively.
The annual accretion of a mudflat transect at Kongsmark in Listerdyb tidal area was
studied by Andersen & Pejrup (2001). Three stations were investigated with bed
level measurements along the mudflat transect in the period from 1997-2000. The
most landward station had an annual accretion rate of 3.3 cm y-1 and the most
seaward station had a rate of 1.0 cm y-1. Corrected with 210Pb-dating the rates were
1.9 cm y-1 and 0.6 cm y-1, respectively. However, the bed level measurements
showed that the most landward station 20 m from the marsh accumulated with up
to 10-14 cm in periods of just a few months. This occurred during winter term,
which was speculated to be caused by ice floes.
The referred studies and the collected samples were carried out in more or less open
environments, where import and export events are seasonally variable and the net
effect is often within ranges of a few millimeters to a few centimeters. Within Fanø
Marina, a semi-enclosed basin being sheltered by a pier, no natural or only very
modest natural resuspension is expected. Thus, a high annual deposition is favored.
In Figure 7-5 the simulated sedimentation map is compared with a historical Google
Earth image from 2014. It can be seen that the simulated areas experiencing greatest
deposition correspond quite well with the exposed flats in real life. Thus, even
though the annual deposition might be over- or underestimated the tested scenarios
might give a valid result relatively as the sedimentation pattern seem reliable.
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Figure 7-5 Sedimentation map compared with Google Earth map from 2014. It can be seen that the
simulated results correspond quite well with the exposed areas in the marina during low
tide.
Settling lag and scour lag
With inspiration from a cohesive sediment transport modelling study by Lumborg
(2004), carried out in Listerdyb tidal area, the following Figure 7-6 was prepared,
representing time series of two days, showing the water level, SSC, current velocity
and bed level change at the calibration site (Station 1). The figure highlights several
features characteristic for an estuary in the Danish Wadden Sea, some of them
already represented in the results section. However, what have not been discussed
in detail is the settling and scour lag mechanisms.
When the sediment particle starts settling during high tide slack, it will not be
deposited directly beneath, as the slow settling velocity causes it to move further
landward before it reaches the bed. When the tide returns as an ebb current, i.e. the
same water parcel that carried the sediment particle during flood, it is not strong
enough to resuspend the particle. The sediment particle will first be resupended by
water parcels situated further inward, thus it is not able to bring the particle as far
seaward as it was eroded. The net effect of this mechanism is a landward movement
of fine-grained sediment, also referred to as settling lag. The settling lag is illustrated
in Figure 7-6, letter A. The lag is measured as the time between high water slack and
the time with minimum SSC (Lumborg, 2004). The settling lag within this tidal
period is approximately 2 hours with an average current velocity of 0.12 m s-1,
which means that the sediment can be transported approximately 850 m landward
during such settling period.
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Figure 7-6 Simulated water level, current velocity, suspended sediment concentration and bed level
change at the calibration site at Station 1. The letters A and B refer to settling and scour
lag, respectively.
Scour lag is the process where a higher current velocity is needed to erode a particle
from the bottom compared to the velocity needed to deposit it. This means, that if a
flood current deposits a particle, a higher ebb current velocity is needed to
resuspend it from the bed. The process is included in the model by setting values of
the critical bed shear stress for erosion (𝜏𝑐𝑒) and deposition (𝜏𝑐𝑑) at 0.1 N m-2 and
0.06 N m-2, respectively.
The scour lag is illustrated in Figure 7, letter B. The first vertical line represents the
point when deposition starts and the current velocity is approximately 0.15 m s-1.
The second vertical line represents when the same velocity is reached during ebb,
however the resuspension takes place approximately 0.5 hours later which is
indicated by the third vertical line. The velocity is approximately 0.2 m s-1 at this
point of time and it can be seen that the scour lag ensures that fine-grained sediment
is moved further inward. During this scour lag period sediment was prevented from
84
being moved approximately 350 m further seaward. The contribution of each
mechanism seems to be the reverse of the findings by Bartholdy (2000), who found
that the contribution from scour lag was much more important than that from the
settling lag.
This reverse relationship might be due to uncertainties related to cohesive sediment
transport modelling of natural systems. Despite scientific progress a lack of
understanding and a correct parametrization of sediment processes still remain
(Vested et al., 2013). Furthermore it is seen from Figure 7-6 that the bed level
change was decreasing by approximately 7 mm during 4 tidal periods. This is of
course not expected in real life and it was seen that the simulated resuspension peak
in general was not simulated satisfactorily in the resuspension peak, but the overall
suspended sediment pattern seemed reasonable (cf. Figure 4-14). The
underestimated flood current seems to affect the size of the resuspension peak
during flood within the model.
In this thesis a relatively simple model has been set-up and the main differences
between the model used in the current study and the nature is portrayed in Figure
7-7. The development of a simple model is in accordance with the modelling
philosophy of Roelvink and Reniers (2012) (Vested et al., 2013). It is argued that
adding more physics will improve the ability of the model to represent certain
processes, but the overall model is not necessarily improved by the inclusion of
physical processes, which depend on uncertain coefficients. Every time another
process is added to the model, one more uncertain coefficient is added as well. These
arguments are also valid for the simulation in this thesis, where the model is
calibrated towards matching the hydrodynamics and cohesive sediment dynamics
within the marina, hence with possible errors in other parts of the model domain.
The development of a cohesive sediment model must in any case be defined on an
analysis of the most important key processes controlling the morphodynamics
(Vested et al., 2013).
85
Figure 7-7 Top illustration portray conditions in nature and the illustration below portray the
simplified model (Modified after Forsberg, 2014)
Settling velocity
One key parameter to determine in depositional studies is the settling velocity of
sediments. Due to time limitations the settling velocity was not measured with a
LISST within the study area. Hereby a constant settling velocity map of 0.0001 m s-1
within the marina and 0.0004 m s-1 in the rest of the model domain, respectively,
were used based on sediment samples collected in a comprehensive study by
Bartholdy & Anthony (1998), thereby having a good approximation of the settling
velocity. A sensitivity analysis was carried out by running simulations using
different settling velocities, with the existing conditions of 0.0004 m s-1 compared
with simulations of a constant velocity of 0.0003 m s-1 and 0.0005 m s-1. The results
are seen in Figure 7-8.
86
Figure 7-8 Sensitivity analysis of different settling velocities at Station 1.
On basis of the sensitivity analysis it is clear that the settling velocity is a very
important parameter to determine, as the reliability of the results is determined by
this. It is seen that a faster settling velocity will cause an increase in the annual
sedimentation and vice versa.
The settling velocity is also seasonally variable. The settling velocity during cold
seasons is lower compared to settling velocities in the summer term (Lumborg &
Windelin, 2003). This is partly due to the water temperature, salinity and the
seasonal variation in biological activity. A study from the Danish Wadden Sea carried
out at intertidal mudflats in Listerdyb tidal area by Andersen & Pejrup (2002)
showed that the settling velocity was highly dependent on the content of fecal
pellets present. The study showed that the settling diameter varied between 20 µm
to 80µm, depending on low or high fecal pellet contents. The settling velocity was
shown to be highly seasonally and locally dependent on the presence of Hydrobia
ulvae which caused a more active pelletization of the bed material during the
summer period compared to the winter period. Even though the presence of
Hydrobia ulvae made the bed material erode more easily, the larger settling velocity
and associated settling and scour lag did not cause the material to escape (Andersen &
Pejrup 2002). Flocculation is indeed a process that makes it more difficult to compute
cohesive sediment transport in estuarine environments as the settling velocity will
vary spatially and temporally within the modelled domain (Manning et. al, 2010).
87
8 Evaluation of possible remedial actions
8.1 Scenario 1 - Sheet pile walls along the jetties The establishment of sheet pile walls along the jetties is seen in Figure 8-1 and the
corresponding sedimentation map is shown in Figure 8-2. The annual modelled
reduction is 27.6 %. It is seen that the primary sedimentation takes place in the
entrance and is gradually decreasing towards the southwestern corner
Figure 8-1 Scenario 1 - Established sheet pile walls along the jetties
Figure 8-2 Scenario 1 - Sedimentation map after 10 tidal periods
The annual sedimentation is compared with the baseline result in Table 8-1.
Table 8-1 Annual sedimentation in Fanø Marina for Scenario 1. Calculated with a dry density of 650
kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m]
Reduction [%]
Normal 1980.22 0.15 Baseline Scenario 1 1434.15 0.11 27.6
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For scenarios with established sheet pile walls along the jetties the current pattern are investigated in further details and typical current patterns are seen in Figure 8-3 to 8-6. The corresponding suspended sediment concentration patterns are referred to in Appendix C. In the following the typical current patterns and sediment concentration patterns are referred to in Appendix C.
Figure 8-3 Current pattern high tide +3 hours
Figure 8-4 Current pattern high tide +5 hours
89
Figure 8-5 Current pattern low tide +3 hours
Figure 8-6 Current pattern high tide +5 hours
It can be seen that the current pattern has changed due to the establishment of sheet
pile walls along the jetties. The current-induced exchange, which under the existing
conditions, was flowing around the entire marina basin is interrupted by smaller
circulation cells next to and between the jetties in scenario 1.
90
8.2 Scenario 2 - Expansion of pier with a current deflection
wall The expansion of the pier with a current deflection wall is shown in Figure 8-7. The
purpose of establishing a current deflection wall is to manipulate the flow around
the outer side of the pier. The scenario succeeded in reducing the annual
sedimentation by 25.6 % due to a deflection of the current around the wall. A
sedimentation map is seen in Figure 8-8 and it is seen that the deposition pattern is
similar to the existing conditions, just with a reduced deposition. It is furthermore
seen that the material is deposited on the leeside and eroded in the "channel"
between the pier and the current deflection wall.
Figure 8-7 Scenario 2 - Expansion of pier with a current deflection wall
Figure 8-8 Scenario 2 - Expansion of pier with a current deflection wall
The annual sedimentation is compared with the baseline result in Table 8-2.
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Table 8-2 Annual sedimentation in Fanø Marina for Scenario 2. Calculated with a dry density of 650
kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m]
Reduction [%]
Normal 1980.22 0.15 Baseline Scenario 2 1473.99 0.11 25.6
.
8.3 Scenario 3 - Narrowing of the entrance The narrowing of the entrance in scenario 3 and the corresponding sedimentation
map is seen in Figure 8-9 and Figure 8-10. The effect of the scenario is an annual
reduction of sedimentation by 27.7 %. The reduction is due to a reduced current-
induced exchange in the entrance of the marina, thus resulting in a lower water
volume exchange. The deposition pattern within the marina is comparable with the
existing conditions, just with a decreased deposition.
Figure 8-9 Scenario 3 - Narrowing of the entrance
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Figure 8-10 Scenario 3 - Sedimentation map after 10 tidal periods
The annual sedimentation is compared with the baseline result in Table 8-3.
Table 8-3 Annual sedimentation in Fanø Marina for Scenario 3. Calculated with a dry density of 650
kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m]
Reduction [%]
Normal 1980.22 0.15 Baseline Scenario 3 1641.11 0.12 27.7
8.4 Scenario 4 - Expansion of the eastern pier In scenario 4 the eastern pier was expanded and a sedimentation map is seen in
Figure 8-11 and 8-12, respectively. The annual reduction of deposition is 25.7 % and
the effect is similar to Scenario 2, where a current deflection wall was built, and the
main flow is forced around the pier with a reduced sedimentation as a result.
93
Figure 8-11 Scenario 4 - Expansion of the eastern pier
Figure 8-12 Scenario 4 - Sedimentation map after 10 tidal periods
The annual sedimentation is compared with the baseline result in Table 8-4.
Table 8-4 Annual sedimentation in Fanø Marina for Scenario 4. Calculated with a dry density of 650
kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m]
Reduction [%]
Normal 1980.22 0.15 Baseline Scenario 4 1471.05 0.11 25.7
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8.5 Scenario 5 - Narrowing of the entrance The entrance was narrowed as seen in Figure 8-13. The idea of scenario 5 was to
narrow the entrance opposite to the pier, thus the western "front" was on same
latitude as the pier.
The scenario succeeded in reducing the annual sedimentation by 28.1 % and the
corresponding sedimentation map is seen in Figure 8-14. The current-induced
exchange within the entrance and at the mouth of the entrance was slowed down,
which resulted in a reduced sedimentation.
Figure 8-13 Scenario 5 - Narrowing of the entrance
Figure 8-14 Scenario 5 - Sedimentation map after 10 tidal periods
The annual sedimentation is compared with the baseline result in Table 8-5.
95
Table 8-5 Annual sedimentation in Fanø Marina for Scenario 5. Calculated with a dry density of 650
kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m]
Reduction [%]
Normal 1980.22 0.15 Baseline Scenario 5 1423.03 0.10 28.1
8.6 Scenario 6 - Narrowing of the entrance and
establishment of sheet pile walls along the jetties Scenario 6 is seen in Figure 8-15. The scenario is inspired by an outline proposal
prepared by Sunke Arkitekter (cf. Appendix B). A sedimentation map is seen in
Figure 8-16 and the annual reduction of the scenario is 17.4 %. The combination of
narrowing the entrance and establishing sheet pile walls was shown to
counterbalance each other and no further reduction was obtained. As a consequence
of the narrowed entrance the flow was enhanced within the entrance, thus causing a
stronger current-induced exchange, cf. Appendix C.
Figure 8-15 Scenario 6 - Narrowing of the entrance and establishment of sheet pile walls along the
jetties
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Figure 8-16 Scenario 6 - Sedimentation map after 10 tidal periods
The annual sedimentation is compared with the baseline result in Table 8-6.
Table 8-6 Annual sedimentation in Fanø Marina for Scenario 6. Calculated with a dry density of 650
kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m]
Reduction [%]
Normal 1980.22 0.15 Baseline Scenario 6 1635.05 0.12 17.4
8.7 Scenario 7 - Establishment of sheet pile walls along the
jetties and dredging in the southwest corner Scenario 7 is seen in Figure 8-17. Sheet pile walls were established along the jetties
together with a dredged depth of 2.2 m in the southwestern corner. The modelled
sedimentation map is seen in Figure 8-18 and the annual reduction is 21.1 % in this
scenario. The main reason for the simulated reduction is the establishment of sheet
pile walls.
97
Figure 8-17 Scenario 7 - Establishment of pile sheet walls along the jetties and dredging in the
southwest corner
Figure 8-18 Scenario 7 - Sedimentation map after 10 tidal periods
The annual sedimentation is compared with the baseline result in Table 8-7.
Table 8-7 Annual sedimentation in Fanø Marina for Scenario 7. Calculated with a dry density of 650
kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m]
Reduction [%]
Normal 1980.22 0.15 Baseline Scenario 7 1562.93 0.11 21.1
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8.8 Scenario 8 - Establishment of a truncated sheet pile wall
along the northern jetty The establishment of a truncated sheet pile wall along the northern jetty is seen in
Figure 8-19 and the sedimentation map is seen in Figure 8-20. An annual reduction
of 26.2 % was achieved in this scenario. The scenario was successful to stop a part of
the current-induced exchange. In the existing conditions the centre of the large
current-induced exchange is located in the middle of the marina basin. This is not
the case for Scenario 8 as the centre is relocated around the established truncated
sheet pile wall, cf. Appendix C. The effect of this is probably a reduced current-
induced exchange, thus reducing the water volume exchange within each tidal
period.
Figure 8-19 Scenario 8 - Establishment of one truncated sheet pile wall along the northern jetty
Figure 8-20 Scenario 8 - Sedimentation map after 10 tidal periods
99
The annual sedimentation is compared with the baseline result in Table 8-8.
Table 8-8 Annual sedimentation in Fanø Marina for Scenario 8. Calculated with a dry density of 650
kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m]
Reduction [%]
Normal 1980.22 0.15 Baseline Scenario 8 1460.46 0.11 26.2
8.9 Scenario 9 - Establishment of a sheet pile wall along the
northern jetty The established structure is seen in Figure 8-21 and the corresponding
sedimentation map is seen in Figure 8-22. The annual reduction is 27.1 %. The effect
of Scenario 9 is similar to Scenario 1, but Scenario 9 is a better solution in terms of
costs.
Figure 8-21 Scenario 9 - Establishment of a sheet pile wall along the northern jetty
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Figure 8-22 Scenario 9 - Sedimentation map after 10 tidal periods
The annual sedimentation is compared with the baseline result in Table 8-9.
Table 8-9 Annual sedimentation in Fanø Marina for Scenario 9. Calculated with a dry density of 650
kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m]
Reduction [%]
Normal 1980.22 0.15 Baseline Scenario 9 1442.86 0.11 27.1
8.10 Scenario 10 - Expansion of and establishment of a flow
through the pier Scenario 10 is seen in Figure 8-23. The scenario belongs to another approach in
terms of anti-sedimentation techniques, where small changes in the flow can lead to
a reduced sedimentation. This was done by expanding the pier and establishing a
flow through the pier. A sedimentation map is seen in Figure 8-24 and the annual
deposition related to the scenario was increased by 8.3 %. The scenario was
successful in increasing the flow through the marina. However, this resulted in an
increased current-induced exchange with an increased sedimentation as a
consequence.
101
Figure 8-23 Scenario 10 - Expansion and flow through the pier
Figure 8-24 Scenario 10 - Sedimentation map after 10 tidal periods
The annual sedimentation is compared with the baseline result in Table 8-10.
Table 8-10 Annual sedimentation in Fanø Marina for Scenario 10. Calculated with a dry density of
650 kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m]
Reduction [%]
Normal 1980.22 0.15 Baseline Scenario 10 2144.50 0.16 +8.3 %
102
8.11 Summary of tested scenarios The model has been validated with hydrodynamic and sediment concentration
measurements from the summer and autumn 2015.
It was shown by both analytical, numerical and by field measurements, that tidal-
induced and current-induced flows were contributing to the sedimentation within
the marina. It was furthermore identified that the current-induced exchange was not
insignificant.
On the basis of this, 10 different scenarios with possible remedial action were tested
in order to reduce the sedimentation in Fanø Marina.
From the results it was seen that it is possible to reduce the annual sedimentation
within Fanø Marina by 15-30 %.
The annual sedimentation is compared with the baseline result in Table 8-11.
Table 8-11 Annual sedimentation in Fanø Marina. Calculated with a dry density of 650 kg m-3
Design Annual sedimentation [t]
Annual sedimentation [m] Reduction [%]
Normal 1980.22 0.15 Baseline
Scenario 1 1434.15 0.11 -27.6
Scenario 2 1473.99 0.11 -25.6
Scenario 3 1641.11 0.12 -27.7
Scenario 4 1471.05 0.11 -25.7
Scenario 5 1423.03 0.10 -28.1
Scenario 6 1635.05 0.12 -17.4
Scenario 7 1562.93 0.11 -21.1
Scenario 8 1460.46 0.11 -26.2
Scenario 9 1442.86 0.11 -27.1
Scenario 10 2144.50 0.16 +8.3
The reduction can be achieved by the following anti-sedimentation approaches:
Establishment of sheet pile walls along the jetties (reduction of 26.2-27.6 %)
Narrowing of the entrance (reduction of 17.4 - 28.1 %)
Deflection of the flow around the marina basin (reduction of 25.6 - 25.7 %)
Establishment of a flow through the pier is not recommended, as it will
increase the sedimentation
The analysis and evaluation of the tested scenarios will be discussed in further
details in the following section.
103
8.12 Discussion of possible remedial actions In the following the tested scenarios will be discussed and a few suggestions for
further improvements will be mentioned as well.
Within the model it was seen that the current speed and SSC were highly dependent
on each other. This became apparent from the fact that a high current speed favored
a greater SSC and vice versa, which is due to the relation between the current speed
and the current-induced bed shear stress in the model. This was also noted at the
calibration of Station 5, where at the current speed fitted quite well, but the SSC was
overestimated. This means that within the tested scenarios where the main flow was
manipulated further around the pier, the SSC entering the marina was drastically
decreased. Judged from the in situ measurements at Stations 5 and 6 the SSC did not
differ very much in the horizontal direction even though the current speed varied
quite a lot in some periods (cf. Figure 5-7), and it is assumed that the vertical
difference in the measuring height of the sensors may be disregarded, as the sensor
were located 200 meters apart.
To sum up, the SSC entering Fanø Marina is highly dependent on the current speed
in the model, whereas in real life the SSC entering Fanø Marina was less dependent
on the current speed based on the observed SSC differences between Stations 5 and
6 and due to a slow settling velocity of fine-grained sediment. The overall effect of
this might be an overestimation of the annual sedimentation reduction in the
various Scenarios. The effect is illustrated in Figure 8-25 and Figure 8-26, where the
SSC pattern from the existing conditions is compared with Scenario 2.
In Scenario 2 it is seen that the pattern of the SSC is moved further east as a
consequence of the established CDW. However, this is mainly due to an increased
current speed in this area, which causes a decrease in the sediment plume entering
the marina.
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Figure 8-25 Suspended sediment concentration pattern at existing conditions +3 hours low tide
Figure 8-26 Suspended sediment concentration pattern at Scenario 2 + 3 hours low tide
In order to improve the model results of the annual sedimentation rate a few further
things could have been done:
Including a broader seasonal variation in the simulation period. It is a
weakness that the calculated annual sedimentation rates were based on a
short simulation period consisting of 10 tidal periods within a summer
period. However, the validation of the model seemed reliable during periods
from July 20-22 and October 30 to November 5.
It is speculated that the model results could have been improved by coupling
a Near Shore Wave (NSW) module to the HD module. It was shown that the
wind speed and wind direction had a significant effect on the SSC in the area.
Even though the marina is sheltered from waves in terms of erosion of the
marina bed, measurements from the fieldwork campaigns revealed that the
SSC entering the marina was highly dependent on winds from south to
southeast and from north.
105
More emphasis could have been placed on investigating the different model
inputs, such as the settling velocity, erosion threshold, dry density of the bed,
etc. This would have provided more reliable results.
Furthermore field investigations of the southern boundary in terms of
sediment transport, would improve the reliability of the results. It was shown
that during periods with winds from south to southeast that the SSC in
general was larger and the water volume contribution from this part could be
interesting to determine
An updated bathymetry would also generate more reliable results, especially
within the marina.
Finally, the model operated with just one grain size fraction. The model could
have been improved by adding at least two fractions, as it was seen from the
measurements that not all sediment entering the marina was deposited, as
the sediment fraction also consisted of some very fine-grained material with
very slow settling velocities.
106
9 Conclusion In situ measurements of the hydrodynamics and sediment concentration covering a
total of 66 tidal periods were conducted in Grådyb tidal area in the summer and
autumn 2015 with two RCM9 Aanderaa sensors located at different spots (cf. Figure
4-8). The in situ measurements were used to analyze and describe the variation
observed in the hydrodynamics and sediment dynamics within the study area with
particular emphasis on Fanø Marina.
Overall, an approximate balance between import and export was observed in the
studied area as a whole.
It was seen that the net flux of fine-grained sediment was highly dependent on the
residual water flux. It was seen that a landward residual water flux favored a
landward transport of suspended sediment and vice versa. The suspended sediment
concentration was dependent on the wind conditions. The concentration was
observed to be larger during periods with a wind direction from south to southeast
and from north, respectively. The increased suspended sediment concentration
during these periods was speculated to be a result of induced wind-wave activity
across the tidal flats, promoting resuspension. The concentration was furthermore
increased during periods with higher wind speeds. During periods with wind from
south to southeast the net transport of fine-grained sediment was exported. This
was speculated to be caused by water from the turbidity maximum located at the
tidal divide being pushed further to the north, thereby bringing water further north
in Grådyb tidal area.
Generally, import of suspended sediment was observed in calm wind periods due to
settling and scour lag processes, favoring a landward sediment transport of fine-
grained sediment.
Furthermore, an incident during June 4 contributed to a tremendous part of the
overall net import of fine-grained sediment within the summer period in the area as
a whole. However, a reasonable explanation of the incident could only be speculated,
as the hydrodynamics, wind conditions or sediment dynamics did not reveal any
clear sign.
Of particular interest for deposition in Fanø Marina a current-induced exchange was
observed during July 20-22 at Stations 3 and 4 located at the opposite sides of the
marina entrance. This observation was interesting as such phenomenon is known to
increase the sedimentation within marina basins as the current-induced exchange
will increase the total water flow entering the marina basin during each tidal period..
Based on measurements from Stations 3, 4 and 6 no natural resuspension of
sediment was observed within the marina, which is one of the reasons why Fanø
Marina acts as a sediment trap.
107
The hydrodynamic and sediment concentration measurements were furthermore
used to set-up, calibrate and validate a numerical modelling in MIKE 21. A
hydrodynamic module was coupled with a mud transport module in order to
investigate the deposition of sediment within Fanø Marina and to test possible
scenarios with different remedial actions.
On the basis of the calibration and validation of the model the deposition was
simulated satisfactorily within Fanø Marina. An annual sedimentation of 15 cm y-1
averaged over the marina basin was found on the basis of the simulation of the
existing conditions. The primary deposition was taking place within the entrance of
the marina and gradually decreased towards land. Furthermore, deposition took
place east of the pier. These findings are in agreement with local observations, but
the annual sedimentation rate was slightly underestimated compared to local
observations estimating an annual sedimentation rate of 20-30 cm y-1. The annual
sedimentation found within Fanø Marina in this thesis, was quite large compared to
other studies carried out in the Danish Wadden Sea. The main reasons for this are
speculated to be that Fanø Marina is sheltered by a pier, and that no natural or very
modest resuspension takes place within the marina, compared to the referred
studies that were carried out in more or less open environment and thereby being
balanced by import and export events throughout the year.
In general it was also found that uncertainties were related to cohesive sediment
transport modelling of natural systems. A sensitivity analysis of the settling velocity
showed that the model results are highly dependent on the input to the model.
It was shown by both numerical modelling and field measurements that tidal-
induced and current-induced exchanges were contributing to the sedimentation
within the marina. It was furthermore identified that the current-induced exchange
was not insignificant as it increases the tidal prism during each tidal period.
On the basis of this, 10 different scenarios with possible remedial actions were
tested in order to reduce the sedimentation in Fanø Marina. The approach of
Keeping Sediment Out (KSO) was primarily used in the design of Scenarios 1-9, and
the approach of Keeping Sediment Moving (KSM) was used in Scenario 10. The KSO
approach applied in Scenarios 1 - 9 was shown to be successful and reduced the
annual sedimentation rate by up to 15-30 %. This was achieved by establishing a
current deflection wall (Scenario 2), by establishing sheet pile walls along the jetties
(Scenarios 1, and 6-9) or narrowing the marina entrance (Scenarios 3, 5 and 6). The
approach of Keeping Sediment Moving (KSM) (Scenario 10) was not successful, as
the current-induced exchange was increased as a result of the increased current
speed in the marina entrance.
From the model results it was seen that it is possible to reduce the annual
sedimentation within Fanø Marina by 15-30 %. However, the reduction might be
overestimated as there is a mismatch between the current speed and the related
108
suspended sediment concentration within the model so further improvements of the
model may be desired to give more reliable results. One way of improving the model
could therefore be to calibrate the model with at least two grain size fractions.
109
Acknowledgement The thesis is made in relation to the Master of Science (MSc) in Geography and
Geoinformatics at Department of Geosciences and Natural Resource Management,
University of Copenhagen. Jesper Bartholdy from Copenhagen University has been
the main supervisor and Klavs Bundgaard from Danish Hydraulic Institute (DHI) has
been the co-supervisor. I would like to express my gratitude for great supervision
and guidance throughout the process. Also, I would like to thank Hans Jacob Vested
from DHI for great input during the process. A sincerely thank you to the board
members and users of Fanø Marina. Especially, a great thank you to Regnar
Skelmose, Iben Christensen and Per Hansen who provided help during fieldwork
campaigns in the summer and autumn 2015, and subsequently provided photos of
Fanø Marina. Finally, I would like to thank Signe Ingvardsen from the Danish Coastal
Authorities for sending data of dredged sediment in Grådyb tidal area during 2015,
and a great thank to the Port of Esbjerg for providing water level and wind data.
Copenhagen, May 2016. Anders Borregaard Stephensen
110
References Andersen, T. J. (2001). Temporal variation in erodibility at an intertidal mudflat -
The Danish Wadden Sea. Estuarine, Coastal & Shelf Science, (53 (1)), 1-12.
Andersen, T. J. & Pejrup, M. (2001). Suspended sediment transport on a temperate
microtidal mudflat, the Danish Wadden Sea. Marine Geology, 173, 69–85
Andersen, TJ & Pejrup, M. (2002).Biological mediation of the settling velocity of bed
material eroded from an intertidal mudflat, the Danish Wadden Sea. Estuarine,
Coastal and Shelf Science, nr. 54, s. 737-745.
Austen, I., Andersen, T. J. & Edelvang, K. (1999).The Influence of Benthic Diatoms
and Invertebrates on the Erodibility of an Intertidal Mudflat, the Danish Wadden
Sea. Estuarine, Coastal and Shelf Science, Volume 49, 99-111
Bartholdy, J. (2000). Processes controlling import of fine-grained sediment to tidal
areas: A simulation model. Geological society, special puplication, (175), 13-29.
Bartholdy, J. (2013). Short Course Note On: Sediment Transport in Steady Flowing
Water. Department of Geosciences and Natural Resource Management, University of
Copenhagen, pp. 1-34
Bartholdy, J., & Aagaard, T. (2001). Storm surge effects on a backbarrier tidal
flat. Geo-Marine Letters, 20, 133-141.
Bartholdy, J., & Anthony, D. (1998). Tidal dynamics and seasonal dependent import
and export of fine-grained sediment through a back-barrier tidal channel of the
Danish Wadden Sea. I C. R. Alexander, R. A. Davis, & V. J. Henry (red.), Tidalites:
Processes & Products. (s. 43-52). (Society for Sedimentary Geology, Special
Publication, Vol. 61).
Bartholdy, J., Christiansen, C., & Kunzendorf, H. (2004). Long term variations in
backbarrier salt marsh deposition on the Skallingen peninsula - the Danish Wadden
Sea. Marine Geology, (203), 1-21.
Bartholdy, J. & Folving, S. (1986). Sediment classification and surface type mapping
in the Danish Wadden Sea by remote sensing. Netherlands journal of Sea Research,
20, 337–345.
Bartholdy, J., & Madsen, P. P. (1985). Accumulation of Fine-Grained Material in a
Danish Tidal Area. Marine Geology, 68, 121-137.
Berlamont, J., Ockenden, M., Toorman, E., & Winterwerp, J. (1993). The
characterisation of cohesive sediment properties. Coastal Engineering, Volume 21,
105–128
111
Blott, S.J. & Pye, K. (2001). GRADISTAT: a grain size distribution and statistics
package for the analysis of unconsolidated sediments. Earth Surface Processes and
Landforms 26, 1237-1248
Christiansen, C., Vølund, G., Lund-Hansen, L. C., & Bartholdy, J. (2006). Wind
influence on tidal flat sediment dynamics, Ho Bugt, Danish Wadden Sea. Marine
Geology, (235(1-4)), 75-86.
Decrop, B., Roose, F., Zimmermann, N. & Sas, M. (2013). Monitoring the siltation rate
at Deurganckdok, Port of Antwerp, and its reduction by a current deflecting wall. in:
CEDA (2013). 20th World Dredging Congress and Exhibition 2013 (WODCON XX). The
Art of Dredging. Brussels, Belgium, 3-7 June 2013. 622-634.
DHI (2008). Vurdering af rentabilitet for genåbning af Sønderho Havn Hydrografisk
modellering og vurdering Del I. Rapport til Foreningen Sønderho Havn udarbejdet af
DHI.
DHI (2014a). MIKE 21 Flow Model. Hydrodynamic Module. User Guide. MIKE by DHI
DHI (2014b). MIKE 21 & MIKE 3 FLOW MODEL FM. Mud Transport Module.
Scientific Documentation. MIKE by DHI
DHI (2015a). MIKE 21 and MIKE 3 Flow Model FM. Hydrodynamic Module. Short
Description. MIKE by DHI
DHI (2015b). MIKE 21 and MIKE 3 Flow Model FM. Mud Transport Module. Short
Description. MIKE by DHI.
Diver Manual, 2014. Schlumberger Water Services. Version November 2014.
Edelvang, K., Larsen, M. and Pejrup, M. (1992). Tidal Variation in Field Settling
Velocities of Suspended Sediment in a Tidal Channel. Geografisk Tidsskrift, Bind 92,
116-121.
Eisma, D. (1993). Suspended Matter in the Aquatic Environment. Berlin: Springer
Verlag.
Fanø Sejlklub (n.d.). Retrieved from:
http://www.vadehavetsbaadklubber.dk/default.asp?Id=21. Visited 26-05-2016
Forsberg, P. L. (2014). The influence of blue mussels (Mytilus edulis) on the
suspension of fine grained sediments: a case study from Rødsand lagoon, Denmark.
Vejleder: Verner Brandbyge Ernstsen. Speciale, København: Københavns Universitet.
Friedrichs, C.T. & Aubrey, D.G. (1988). Non-linear tidal distortion in shallow well-
mixed estuaries: a synthesis. Estuarine, Coastal and Shelf Science 27 (5), 521-545.
112
Fruergaard, M., Møller, I., Johannessen, P., Nielsen, L. H., Andersen, T. J., Nielsen, L., ...
Pejrup, M. (2015). Stratigraphy, evolution, and controls of a Holocene transgressive-
regressive barrier island under changing sea-level: Danish North Sea coast. Journal
of Sedimentary Research, 85(7), 820-844.
Hamill, G., Johnston, H., & Stewart, D.P. (1999). Propeller Wash Scour near Quay
Walls. Journal of Waterway, Port, Coastal, and Ocean Engineering, Volume 125, 170-
175
Huiming, Z, Hongwei, F. and Minghong, C. (2011). Floc architecture of bioflocculation
sediment by ESEM and CLSM. The Journal of Scanning Microscopies, Volume 33, Issue
6, 437-445
Kirby, R. (2011). Minimising harbour siltation - Findings of PIANC Working Group
43. Ocean Dynamics, Volume 61, 233-244.
Kombiadou, K. & Krestenitis, Y. N. (2013). Sediment transport processes and their
modelling applications. In Manning, A. J., editor, Modelling Cohesive Sediment
Dynamics in the Marine Environment (pp. 213-246). Intech.
Kystdirektoratet (2006). Morfologisk udvikling i Vadehavet. Grådyb
Tidevandsområde og Skallingen. Kystdirektoratet.
Lumborg, U. (2004). Cohesive sediment transport modelling - application to the
Lister Dyb tidal area in the Danish Wadden Sea. Journal of Coastal Research, Volume
41, 114-123
Lumborg, U. & Pejrup, M. (2005). Modelling of cohesive sediment transport in a tidal
lagoon - an annual budget. Marine Geology, Volume 218, 1-16.
Lumborg, U. & Windelin, A. (2003). Hydrography and cohesive sediment modelling:
application to the Rømø Dyb tidal area. Journal of Marine Systems, Volume 38, 287–
303.
Manning, A.J., Langston, W.J. & Jonas, P.J.C. (2010). A review of sediment dynamics in
the Severn Estuary: Influence of flocculation. Marine Pollution Bulletin, Volume 61,
37-51.
Marshak, S. (2008). Earth: Portrait of a Planet (Ed. 3). New York: W.W. Norton &
Company. pp. 184-187.
Masselink, G. and Hughes, M. G. (2003). Introduction to Coastal Processes and
Geomorphology. Great Britain, London: Hodder Arnold
Nationalpark Vadehavet (n.d.). Retrieved from:
http://nationalparkvadehavet.dk/om-nationalpark-vadehavet/
113
Nielsen, N. & Nielsen, J. (2002). Vertical growth of a young back barrier salt marsh,
Skallingen, SW Denmark. Journal of Coastal Research, Volume 18, 287-299.
Pedersen, J. (2004). Fine-grained sediment budgets for the Grådyb, Knudedyb and
Juvredyb tidal areas, the Danish Wadden Sea. Upubliceret prisopgave fra Geografisk
Institut, Københavns Universitet.
Pedersen, J.B.T. & Bartholdy, J., 2006. Budgets for fine-grained sediment in the
Danish Wadden Sea. Marine Geology, Volume 235, 101–117.
Pejrup, M. (1988). Suspended sediment transport across a tidal flat. Marine Geology,
Volume 82, 187-198.
Pejrup, M. (2003). Flocculation. In Encyclopedia of sediments and sedimentary
rocks. (pp. 284-285). Dordrecht: Kluwer Academic Publishers.
Pejrup, M., Bartholdy, J., & Jensen, A. (1993). Supply and Exchange of water and
nutrients in the Grådyb Tidal Area, Denmark. Estuarine, Coastal and Shelf Science, 36,
221-234.
Perillo, G.M.E. (Ed.) (1995). Definitions and geomorphologic classifications of
estuaries. In: Geomorphology and Sedimentology of Estuaries (pp. 17-47). Elsevier.
PIANC (2008). Minimising harbour siltation. Report no. 102, pp. 75.
PIANC (2015). Anti-Sedimentation system for marinas and yacht harbours. Report
no. 130, pp. 75.
Postma, H. (1981). Exchange of Materials between the North Sea and the Wadden
Sea. Marine Geology, Volume 40(1/2), 199-213
Ridderinkhof, H. (1998). Sediment Transport in Intertidal Areas. In Eisma, D., &
Boer, P. L., Intertidal deposits: River mouths, tidal flats, and coastal lagoons (pp. 363-
381). Boca Raton: CRC Press.
Roelvink, J.A. & Reiners, A.J.H.M. (2012). Modelling Philosophy. In: Advances in
Coastal and Ocean Engineering: Guide to Modelling Coastal Morphology (pp. 255-259).
University of Miami. USA: World Scientific Publishing Co.
Slunden sejlrende 1994 (n.d.). Retrieved from:
http://www.mitfanoe.dk/index.php/da/fanos-historie/havne-og-
besejlingsforhold/nordby-havn/706-slunden-sejlrende-1994. Visited 23-05-2016.
Sørensen, T. H., Bartholdy, J., Christiansen, C., & Pedersen, J. B. T. (2006). Intertidal
surface type mapping of the Danish Wadden Sea. Marine Geology,235(1-4), 87-99.
114
van Maren, D. S. and Winterwerp, J. C. (2013). The role of flow asymmetry and
sediment properties on intertidal mudflat sedimentation. Continental Shelf Research,
Volume 60, 71-84
Vested, H.J., Tessier, C., Christensen, B. C. & Goubert, E. (2013). Numerical modelling
of morphodynamics—Vilaine Estuary. Ocean Dynamics, Volume 63(4), 423-446.
Vinther, N., Bartholdy, J., Christiansen, C., Sørensen, C., & Lund-Hansen, L. C.
(2004). Sediment transport across a tidal divide in the Danish Wadden Sea.
Geografisk Tidsskrift, (104(1)), 71-86.
Violeau, D., Bourban, S., Cheviet, C., Markofsky, M, Petersen, O., Roberts, W.,
Spearman, J., Toorman, E., Vested, H.J. & Weilbeer. Numerical simulation of cohesive
sediment transport: intercomparison of several numerical models. Proceedings in
Marine Science, Volume 5, 75-89.
Wang, Z.B., Hoekstra, P., Burchard, H., De Swart, H.E. & Stive, M.J.F., (2012a).
Morphodynamics of the Wadden Sea and its barrier island system. Ocean & Coastal
Management, Volume 68, 39-57.
Webster, T. & Lemckert, C (2002). Sediment resuspension within a microtidal
estuary/embayment and the implication to channel management. Journal of Coastal
Research, Special Issue 36, 753-759.
Whitehouse, R.J.S., Soulsby, R.L., Roberts , W. & Mitchener, H.J. (2000). Dynamics of
estuarine muds - A manual for practical applications. London: Thomas Telford
Publications.
115
Appendix
Appendix A Statistical description of bed samples Appendix A - statistical description of the bed sample using method of moment measures
Sample no.
Moment mean [ф]
Moment mean [µm]
Moment sorting [ф]
Moment skewness [ф]
Description
1 6.38 12.00 2.04 -0.06 Silt. very poorly
sorted. symmetrical
2 6.06 14.99 2.22 -0.10 Silt. very poorly
sorted. symmetrical
3 5.39 23.85 2.29 0.36 Silt. very poorly
sorted. symmetrical
4 1.55 341.51 1.54 2.71 Medium sand. poorly
sorted. very fine skewed
5 6.62 10.17 1.97 0.07 Silt. poorly sorted.
symmetrical
6 6.90 8.37 1.80 0.11 Silt. poorly sorted.
symmetrical
7 6.95 8.09 1.81 0.19 Silt. poorly sorted.
symmetrical
8 5.39 23.85 2.25 0.55 Silt. very poorly
sorted. fine skewed
9 4.71 38.21 2.26 0.77 Silt. very poorly
sorted. fine skewed
10 4.71 38.21 2.17 0.93 Silt. very poorly
sorted. fine skewed
11 1.52 348.69 1.24 3.06 Medium sand. poorly
sorted. very fine skewed
12 2.27 207.33 1.92 1.93 Fine sand. poorly sorted. very fine
skewed
13 2.77 146.60 1.25 2.44 Fine sand. poorly sorted. very fine
skewed
14 3.82 70.81 0.37 0.12 Very fine sand. well sorted. symmetrical
15 5.73 18.84 2.34 0.84 Silt. very poorly
sorted. fine skewed
16 2.76 147.62 0.34 0.16 Fine sand. very well sorted. symmetrical
17 3.82 70.81 0.36 0.12 Very fine sand. well sorted. symmetrical
18 5.84 17.46 2.06 0.39 Silt. very poorly
sorted. symmetrical
116
Appendix B Outline proposal by Sunke Arkitekter
117
Appendix C Current patterns and sediment concentration
patterns and data disc with calculations
Contents of CD
I. Current patterns and sediment concentration patterns
of existing conditions and tested scenarios
II. Wind and water level data
III. In situ measurements with sediment transport
calculations
IV. Annual sedimentation calculations
The disc is located on the back of the thesis.