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Journal of Coastal Research SI 71 62–74 Coconut Creek, Florida 2014

__________________ DOI: 10.2112/SI71-008.1 received 12 February 2014; accepted in revision 18 August 2014. *Corresponding author: [email protected] © Coastal Education & Research Foundation 2014

Beach Erosion Driven by Natural and Human Activity at Isla del Carmen Barrier Island, Mexico Mireille Escudero†*, Rodolfo Silva†, and Edgar Mendoza†

ABSTRACT Escudero, M.; Silva, R., and Mendoza, E., 2014. Beach erosion driven by natural and human activity at Isla del Carmen Barrier Island, Mexico. In: Silva, R., and Strusińska-Correia, A. (eds.), Coastal Erosion and Management along Developing Coasts: Selected Cases. Journal of Coastal Research, Special Issue, No. 71, pp. 62–74. Coconut Creek (Florida), ISSN 0749-0208. The study site is located on the Gulf of Mexico and is part of the most valuable lagoon-estuarine system in Mexico, not only in ecological terms but also as an important economic and social site for the country. Although the current state of the environment is still reasonably healthy over much of its area, the natural equilibrium has been greatly affected by human activities, as in many of other coastal lagoons worldwide (e.g., infrastructure and building construction, installation of inappropriately designed defense structures, degradation of vegetation and an artificial lagoon opening). The consequence is a sediment deficit that, combined with the impact of tropical cyclones and the absence of an external sand supply, has led to persistent beach erosion. The analysis of the physical settings, historical shoreline changes and hydrodynamic patterns of the study site has helped to identify the critical elements affecting the current state of the beach. This paper proposes a shoreline protection scheme, based on the understanding of the current functioning of the site, in order to reduce local flooding and erosion risks, which affect the population and tourism, and to preserve the natural environmental services of the area. ADDITIONAL INDEX WORDS: Beach erosion, anthropic effects, numerical wave modeling, coastal hydrodynamic, longshore currents.

INTRODUCTION

A coastal lagoon is “a shallow coastal water body separated from the ocean by a barrier, connected at least intermittently to the ocean by one or more restricted inlets, and usually oriented shore-parallel” (Kjerfve, 1994); the most widely adopted definition in the literature (Anthony et al., 2009; Duck, 2012). Kjerfve (1994) classifies coastal lagoons into three groups according to the prevailing hydro-morphological conditions in the lagoon; “choked”, “restricted” and “leaky” coastal lagoons are terms which describe the single, two or many channels that allow exchange between the sea and the lagoon (as shown in Figure 1) (Knoppers, 1999; Mahapatro et al., 2013).

Figure 1. Types of coastal lagoons (a) choked lagoon, (b) restricted lagoon and (c) leaky lagoon (after Mahapatro et al., 2013).

Coastal lagoons border 13 % of the world’s continental coasts (Barnes, 1980) and are typically found on low-lying coastlines, of less than 5 m depth, and range in area from over 10,000 km2 down to less than a hectare (Bird, 1994). Coastal lagoons constitute highly productive ecosystems, rich in biodiversity and greatly valued by society (Anthony et al., 2009; Mahapatro et al., 2013). Their depositional morphology, evolution and dynamics are established according to the sediment delivered via waves, tides and currents within the lagoon (Bird, 1994). However, in many coastal lagoons considerable human intervention (uncontrolled demographic expansion, land-use change, shoreline hardening, dredging of the lagoon entrances, and so on) has adversely affected the internal sediment balance, inducing erosion during the passage of storm events (such as tropical cyclones or Cold-Fronts on the Gulf of Mexico and Atlantic Ocean coasts) (Anthony et al., 2009; Duck, 2012; Eitner, 1995).

Worldwide, several coastal lagoons are affected by such erosion problems in storm episodes; for example: Patos Lagoon, on the southern coast of Brazil (affected by uncontrolled land use and urban-industrial expansion) (Tagliani, 2007); Barataria Bay lagoon systems, Rhode Island salt ponds and Jamaica Bay, on the Atlantic and Gulf of Mexico coasts (with intense human development patterns and considerable social and ecological values) (Anthony et al., 2009); or Isles Dernieres, on the Gulf of Mexico (characterized by sediment deficiency and overwash processes associated with Cold-Fronts, tropical and extratropical driven storms) (Dingler et al., 1990). All these coastal areas are affected by natural and human phenomena, as is the study area of this paper, Isla del Carmen. According to Kjerfve (1994),

†Instituto de Ingeniería, Universidad Nacional Autónoma de México Mexico City, México www.cerf-jcr.org

www.JCRonline.org

Beach Erosion Driven by Natural and Human Activity at Isla del Carmen Barrier Island, Mexico 63 _________________________________________________________________________________________________

Journal of Coastal Research, Special Issue No. 71, 2014

contrasts between lagoons can be explained in terms of their geomorphological history and the effects of geological, climatic, hydrological and ecological factors (considering the effects of human activity).

The aim of this paper is to analyze the coastal processes that affect the evolution of the Isla del Carmen barrier island and identify the principal causes that have led to the current erosion of the beach, in order to propose potential shoreline protection measures. How the morphodynamics of the barrier island behaves at this site can be useful in understanding the evolution of other coastal lagoons affected by similar factors.

STUDY AREA

Isla del Carmen is a barrier island in the mouth of Laguna de Terminos, on the Gulf of Mexico, as shown in Figure 2. It is part of the largest and most environmentally valuable lagoon-estuarine system in Mexico. The study area is delimited by the two natural inlets of the lagoon, Carmen at the Southwest and Puerto Real at the Northeast. A small dune system (3-4 m high) extends along the 37 km of the island.

Figure 2. Location of the study area.

Ciudad del Carmen is the main city of the island; with an urban area around 2,700 ha and 170,000 inhabitants. Ciudad del Carmen has been an important hub of the nationalized Mexican petroleum industry since 1971 (PDU, 2009).

On the other hand, the ecological relevance of the island has been recognized through a number of national and international designations, including: Protected Natural Area of Flora and Fauna, Area of mangroves with biological relevance (CONABIO, 2009), UNESCO world heritage site. The beaches are important spawning sites for sea turtles, and the area has been catalogued as the most important coastal ecological unit in Mesoamerica due to the biodiversity of its wetlands (INE, 1997).

METHODS

For this study, data from previous studies, numerical modelling, collection of sediment samples and elevation data, and the analysis of aerial photography and satellite imagery were used in order to characterize the area. It is worth noting that numerical modelling was very important for this characterization in view of the lack of measured data for long time series for the site during storm conditions. This technique has been widely used worldwide when little data is available.

Wind and Wave Conditions A hindcast database providing 552,264 hourly sea states from

1948 to 2010 was produced from the application of the Hurac and WAM numerical models (Silva et al., 2007). The water depth at the point from where the wind and wave conditions were generated is 28 m, and this point is located 68 km off the coastline. The analysis is mainly focused on the extreme values, where a storm was defined as an event with a significant wave height exceeding 2 m (Gonzalez, 2014) for a minimum period of 12 h.

Sea Level

The data record from the tide gauge at Carmen mouth (http://www.mareografico.unam.mx), from 1957 to 2011, shows a diurnal mixed astronomical tide, characterized by registered mean and maximum high tide levels of 0.18 m and 0.92 m, respectively, and a mean low tide level of -0.244 m (in respect of the mean sea level). The difference between the mean high tide and mean low tide is 0.43 m (PDU, 2009). However, the most important sea level affectation in the area associated with extreme events is due to storm surge (Sedesol, 2011).

In this paper, storm surge is estimated as a function of the wind velocity and wind direction by means of the application of the MATO hydrodynamic model (Duran, 2010; Posada, 2007) in several cells situated at depths of 6.5–7.5 m. The model is applied under six uniform wind velocities (from 18 to 43 m/s) for each of the eight possible wind directions. The storm surge values considered were recorded when stability between the model results and the forcing parameters was reached. Figure 3a shows the model results for winds coming from the North as a function of the simulation time. The best fit of the storm surge values for each direction as a function of the wind velocity is shown in Figure 3b and the parabolic fit proposed for coefficients A and B as a function of the wind direction can be seen in Figure 3c. These figures and equations use simple methodology for estimating the storm surge only from wind characteristics (Duran, 2010).

Figure 3. Estimation of storm surge values as a function of wind velocity and direction (a) storm surge as a function of simulation time, (b) storm surge as a function of wind velocity and (c) fitting coefficients as a function of wind direction Ɵ.

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The estimated storm surge values at 6.5–7.5 m depths are used as input for the wave hydrodynamic model at 28 m depth, an approximation considered acceptable due to the uniform characteristics of the seabed configuration.

Although several rivers flow into the lagoon, in storm conditions the wind direction favors the entrance of sea water and the arrival of a lower river flow into the lagoon (PMOET, 2009). For that reason, the fluvial contribution to the sea level is considered negligible when compared to the storm surge values. Erosion history

The erosion history was studied through the analysis of aerial and satellite photography from 1985 to 2012. The information about data availability and coverage, in terms of shoreline extension, is included in Table 1.

Table 1. Available data of aerial photography and satellite imagery for Isla del Carmen Date Type of image Data Source Coverage of the shoreline (%) 1985 (Mar) Aerial photo INEGI 79.7 (PK 0-22.9; 28.5-33.5) 1990 (Mar) Aerial photo INEGI 91.4 (PK 0-12.0; 15.3-33.5) 1994 (Mar) Aerial photo INEGI 100 2002 (Jan) Aerial photo INEGI 100 2005 (Feb) Aerial photo INEGI 100 2008 (Feb) Aerial photo INEGI 100 2012 (Apr) Satellite digitalglobe 38.9 (PK 0-13.6) 2012 (Sept) Satellite digitalglobe 53.3 (PK 14.5-31.9) 2013 (May) Satellite digitalglobe 4.8 (PK 31.9-33.5) *INEGI: National Institute of Statistics and Geography.

The location of the beach profiles where coastline evolution

was analyzed and the most popular beaches on Isla del Carmen are shown in Figure 4 in red and blue, respectively. At the bottom of Figure 4 the line PK (Kilometer measured from west to east respect to the Point considered as the reference, PK 0), parallel to the X axis, is shown. This line is taken as a reference for the reader, to find places on the beach more easily and understand the dynamics of the island. PK 0 is taken in the X_UTM (15N WGS84) coordinate 621310.3 m.

The shoreline position is obtained every 500 meters along the coast, with the addition of profiles in the areas considered of interest and discarding those that have no comparative image.

Regarding the proxy for the shoreline, the coastline position was digitalized from the images, and possible errors due to sea level increase because of the astronomical tide were not considered.

Wave Hydrodynamics of the Coastal Zone

To study the wave hydrodynamics in the coastal zone of Isla del Carmen, the Refdif numerical model (Silva, 2006) was applied to propagate monochromatic wave conditions from a depth of 20 m to the coastline. The storm surge was considered as a uniform sea level increase in the entire calculation grid.

RESULTS

In this section, the results of the characterization of the study area are presented. These include the description of the coastal climate, with the definition of the representative wave, wind and

sea level conditions; topography and bathymetry; sediment characteristics; erosion history; human intervention; wave hydrodynamics of the coastal zone; and the circulation pattern in the lagoon.

Figure 4. Location of the most popular beaches and beach profiles analyzed on Isla del Carmen.

Coastal Climate The climate of the area is classified as Aw1, warm sub-humid

type (Villalobos-Zapata et al., 2010). Yañez-Arancibia&Day (2005) defines three main seasons in the year: “Rainy” season (from June to September), “Nortes” (or Cold, Winter fronts) (from October to February), and “Dry” season (from March to May). During the “Rainy” season tropical cyclones crossing the Atlantic Ocean usually occur; the “Nortes” have prevailing wind velocities often higher than 8 m/s from the northwest and north-northwest directions producing waves of over 2 m and up to 6.3 m.

From the analysis of the data base used, between three and eight cold-fronts a year hit the site (defined by a significant wave height of 2 m or more), lasting from a few hours to 6 days.

The tropical cyclones that took place in the periods from 1985 to 2012, where the evolution of shoreline position is later analyzed (in Erosion history), are included in Table 2.

Table 2. Presence of tropical cyclones, 1985 to 2012. Period Extreme events occurrence 1985-1990 No record of tropical cyclones Nortes and other less intense storms 1990-1994 No record of tropical cyclones Nortes and other less intense storms 1994-2002 Hurricane Opal and Roxanne, in October, 1995 Hurricane Mitch, in November, 1998 Hurricane Keith, in October, 2000 2002-2005 Hurricane Isidore, in September, 2002 2005-2008 Hurricane Stan, in October, 2005 2008-2012 Hurricane Richard, in October, 2010

The principal wind and wave parameters of those tropical cyclones, at 28 m depth, are presented in Table 3.

Hurricane Roxanne, in October, 1995, was the most damaging cyclone to hit the area. A maximum wind velocity of 22.3 m/s, a root-mean-square wave height of 3.5 m and a maximum significant wave height of 6.3 m, for almost 7 days, were



responsible for the breaching and overwash processes that affected 24 km2 of the coastal zone of Isla del Carmen (Palacio et al., 1999) (Figure 5).

Table 3. Wind and wave parameters of the tropical cyclones from1985to 2012. Tropical Duration Hrms Hsmax Tpmean Tpmax Dirmean VWmean VWmax DirWmean Cyclone (h) (m) (m) (s) (s) (°N) (m/s) (m/s) (°N) Opal 60 4.0 5.4 7.7 8.9 64 14.5 18.6 200 Roxanne 149 3.6 6.3 8.6 13.6 283 13.6 22.3 293 Mitch 30 3.1 4.9 6.8 8.5 337 12.7 19.0 305 Keith 81 2.0 3.7 5.5 7.5 337 8.3 14.8 319 Isidore 83 3.2 6.6 9.7 11.5 299 11.3 20.5 258 Stan 28 2.8 3.5 6.5 7.2 58 11.8 14.9 123 Dean 28 4.9 8.6 8.6 11.3 65 18.3 31.2 112 Richard 33 2.8 4.0 6.5 7.8 86 10.3 15.2 91

Figure 5. Impacts from Hurricane Roxanne in the coastal zone of Isla del Carmen, October, 1995 (after Palacio et al., 1999).

Wind and Wave Conditions The corresponding wind and wave parameters for the study

site were analyzed. Wind and waves roses (for normal and storm conditions) are presented in Figures 6, 7 and 8; and the joint probability of wave parameters (significant wave height, peak period and wave direction), in Figure 9. The results of the extreme value analysis of maximum wind velocity and significant wave height are shown in Figure 10, where specific values for several return periods of interest are extracted for analysis in the following sections, and included in Table 4.

The wind and wave roses show a predominant wind velocity of less than 5 m/s throughout the year, coming from the East and East-Northeast sectors. Regarding the wave climate, a dominance of calm conditions is observed with a significant wave height, Hs, of less than 0.5 m for the same prevailing wind directions; and a more intense wave height when the direction is North-Northwest.

Several studies report erosion in the Isla del Carmen coastal zone during “Rainy” and “Nortes” seasons, which is when storms take place (Palacio-Aponte et al., 2010; Torres et al., 2010). For that reason, the analysis focused on storm conditions.

In summer and autumn, the passage of tropical cyclones brings winds from the East and East-Northeast generally, with velocities of up to 35.7 m/s. In autumn, the highest waves (of 5 to 10.3 m) come more frequently from the North-Northwest; in

summer, the dominance of Northerly waves of between 6 and 8.6 m are evident.

Figure 6. Annual wind and wave roses including normal and storm conditions, at 28 m water depth (a) Wind rose and (b) Wave rose. *Note: wind direction indicates where the wind is going; and wave direction indicates where the wave comes from.

Figure 7. Wind roses for storm conditions, at 28 m water depth (a) Annual and (b)–(e) Seasonal. *Note: wind direction indicates where the wind is going; and wave direction indicates where the wave comes from.

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Figure 8. Wave roses for storm conditions at 28 m water depth (a) Annual and (b)-(e) Seasonal. *Note: wind direction indicates where the wind is going; and wave direction indicates where the wave comes from.

Figure 9. Joint probability of significant wave height Hs, peak period Tp and wave direction Dir (a) Joint probability Hs/Tp and (b) Joint probability Hs/Dir.

Figure 10. Extreme value analysis of annual maximum (a) Wind velocity and (b) Significant wave height (Gonzalez, 2014).

On the other hand, the joint probability analysis of wave height vs period and wave height vs wave direction show the prevalence of a peak wave period of between 5 and 7 s and between 10 and 12 s, with a significant wave height of 2 to 3 m, with a North-Northwest direction.

In Table 4, the results of the extreme value analysis for selected return periods are presented. Table 4. Significant wave height Hs and wind velocity Vw for given return period Tr. Tr Hs VW (years) (m) (m/s) 2 3.4 12.7 5 4.5 15.8 10 5.4 18.8 20 6.6 22.5 30 7.4 25.0 50 8.5 28.7 100 10.2 34.6 Sea Level

The estimated storm surge values are included in Table 5, for the selected return periods and most frequent wind directions. Table 5. Estimated storm surge values Ss for given return period Tr. Tr Hs VW ENE NE NNE N NNW NW WNW (years) (m) (m/s) 2 3.4 12.7 0.13 0.22 0.28 0.31 0.30 0.25 0.17 5 4.5 15.8 0.17 0.31 0.41 0.45 0.43 0.37 0.25 10 5.4 18.8 0.22 0.42 0.55 0.60 0.59 0.50 0.35 20 6.6 22.5 0.28 0.56 0.74 0.83 0.81 0.70 - 30 7.4 25.0 0.33 0.67 0.89 1.00 0.98 0.85 - 50 8.5 28.7 0.41 0.85 1.14 1.28 1.26 1.10 - 100 10.2 34.6 0.55 - - 1.80 1.79 - -

The importance of the Northerly winds to the sea level rise during the passage of storms is evident. Topography and Bathymetry

Figure 11 shows the configuration of the seabed in front of the coastal zone of Isla del Carmen and Laguna de Terminos, as well as the topography of the island.

The bathymetry in the coastal zone shows a plane surface with slopes of less than 3 %, slightly inclined from East to West (PDU, 2009) reaching a depth of 10 m at 13 km from the coastline. Within the lagoon, the bathymetry has rather uniform configuration, with a mean depth of between 2 and 4 m.

Regarding the topography, generally the island has an altitude of 1-3 m above the mean sea level and there is a considerable area lower than, or at, mean sea level. The maximum elevation of the beach profiles is defined by the altitude at the top of the dune, which is 3-4 m above the mean sea level across most of the island.

Sediment Characterization

The sediments in the area are mainly composed of sand, silt| and fine clays coming from marine, lagoon and fluvial sources.



Figure 11. Topography and bathymetry in the study area.

In the coastal zone, the sediments are mainly carbonated sand with a high volume of organic material and a considerable amount of biogenic material (remains of clams, snails and other molluscs that live in the area) distributed along the island, especially in the central zone. The eastern part of the coast of Isla del Carmen is dominated by calcareous sediments from the erosion of rocks, while around Carmen mouth the presence of finer material is due to the arrival of terrigenous sediments from the rivers into the lagoon (Figure 12).

Figure 12. Sediment origin (after Marquez, 2008).

Laguna de Terminos is located at the confluence of two major and highly differentiated physiographic regions, the Gulf Region and the Yucatan Peninsula; and, therefore, the continental

hydrological influences in the lagoon are different (Marquez, 2008). The central and western areas are dominated by calcareous sediments, while in the eastern zone muddy sediments abound as a result of the drainage of rivers in this area.

Figure 13 shows the mean size of sediment samples taken from surf, swash and dune zones along the beach in April 2012. The range of sizes obtained is a d50 of 0.171–0.9 mm; 0.159–0.85 mm; and 0.156–0.589 mm in surf, swash and dune zones, respectively. The presence of finer sand near to Ciudad del Carmen, of a coarse sediment from the erosion of rocks, calcareous organisms and biogenic materials in the central zone and of a coarse sediment from the erosion of rocks in the western zone of the island are seen.

Figure 13. Mean sediment size d50 of sediments along the beach of Isla del Carmen.

Analysis of the sediment flow from field measurements in

2006 (Marquez, 2008) shows similar behaviour at the two mouths in the dry season, but different behavior during the “Nortes” and rainy seasons.

In Puerto Real mouth (Figure 14b), water and sediment movements are normally directed to the lagoon, with a sediment flow of 6.3 t/s, 21 kg/s and 5.2 t/s in the “Nortes”, rainy and dry seasons of 2006, respectively (Marquez, 2008).

In Carmen mouth (Figure 14a), the flow goes in or out of the lagoon depending on the season; sediment accumulation takes place during “Nortes” season and sediment losses occur during rainy and dry seasons. A sediment flow of 20 t/s was measured in “Nortes” season, of 325 kg/s in rainy season and only of 16 kg/s in dry season of 2006 (Marquez, 2008). Erosion History

Analysis of aerial photographs and satellite images, from 1985 to 2012, shows that there has been erosion over a wide area of the beach. Figure 15 shows the coastline evolution from 1985 to 2012; and from 1990 to 2012, to analyze for the longest period available the stretches where there is no information of the coastline position in 1985 (see Table 1.).

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Figure 14. Predominant flows and sediment movements at the mouths of Laguna de Terminos (a) Carmen mouth and (b) Puerto Real mouth.

Figure 15. Evolution of the shoreline (a) 1985–2012 and (b) 1990-2012.

The analysis of the change rate of the coastline in the six intermediate periods, in Figure 16, shows clearly which periods had greater shoreline retreat, as well as allowing the analysis of the evolution in different sections.

The average shoreline change and the rate of shoreline change in some various segments in the different periods are presented in Table 6 and Table 7, respectively.

Table 6. Average of shoreline change in various segments along the coast of Isla del Carmen. PK Beach Average shoreline change (m) 1985 1990 1994 2002 2005 2008 1990 1994 2002 2005 2008 2012 6.2 -0.3 -11.3 -23.1 -17.4 12.2 -16.7 8.8 -22.2 -8.2 -15.1 3.0 -10.0 3.2 10.8 -24.5 20.7 -27.8 -20.9 -9.9 21.7 13 Club de Play a - - -34.7 -20.1 -15.7 -20.7 13.1 Club de Playa - - -43.8 -14.9 -13.0 -4.0 13.3 - - -50.8 -19.2 -27.0 -4.9 15.8 Tortugueros -88.3 19.3 61.6 -49.8 -85.8 75.7 18.3 5.1 12.4 -12.6 2.9 -6.7 -11.6 18.7 11.1 3.8 -5.5 -0.8 -14.7 -29.1 19.8 19.2 5.6 -30.6 -3.6 -7.6 -10.5 21 C.A.S.E.S -37.0 -20.6 11.6 -22.3 -13.6 -7.6 23.8 - 14.1 -31.2 -15.7 -3.8 -0.9 24.4 - -0.5 -27.9 -11.3 -0.9 9.4 28.6 4.3 -7.6 -19.3 -5.0 -2.4 -7.9 31.5 8.5 -9.5 -9.0 -5.8 -10.7 -0.7 33.4 Punta Real -6.8 15.6 -18.3 -5.3 2.4 -20.3 33.5 Punta Real -3.3 18.2 -15.0 2.8 -0.8 -32.1 *The values in red correspond to the maximum average erosion in each period. A positive value indicates accretion.

On the other hand, the coastline changes relative to the 1985

position, in Figure 17a, (or 1990, in Figure 17b) show the

sections of the beach which have registered most variation in shoreline position over the whole period. That analysis is useful in detecting more easily the critical sections, where a more detailed study can be carried out.

Figure 16. Rate of shoreline change from 1985 to 2012 (a) 1985–1990, (b) 1990–1994, (c) 1994–2002, (d) 2002–2005, (e) 2005–2008 and (f) 2008-2012.

From Figure 15, it can be seen that the sections of the beach that have experienced the most noticeable erosion over time are those around PK 6, 13.1 (Club de Playa Beach), 15.8 (Tortugueros Beach), 21 (Centro de Adiestramiento en Seguridad, Ecología y Sobreviviencia (C.ASES) Beach), 24, 28.5, 31, 33.4 (Punta Real Beach).

The analysis of the average shoreline change and rate of shoreline change in the different periods and segments shows the most critical zones: Club de Playa Beach, Tortugueros Beach, C.A.S.E.S Beach and Punta Real Beach. The greatest effects were in 2005-2008, in Club de Playa and Tortugueros Beaches; 1985-1990 and 2002-2005, in C.A.S.E.S Beach; and 2008-2012, in Punta Real Beach. The biggest shoreline changes, where erosion took place, were in PK 8.8, PK 10.8, PK 13 (Club



de Playa Beach), PK 15.8 (Tortugueros Beach); PK 21 (C.A.S.E.S Beach), PK 28.5, and PK 33.5 (Punta Real Beach).Therefore, the stretches of the beach for further analysis based on their erosion histories are: PK 6, PK 8.8, PK 10.8, PK 13 and PK 13.1 (Club de Playa Beach), PK 15.8 (Tortugueros Beach), PK 21 (C.A.S.E.S Beach), PK 24, PK 28.5, PK 31, PK 33.4 and PK 33.5 (Punta Real Beach). Table 7. Rate of shoreline change in various segments along the coast of Isla del Carmen

PK Beach Rate of shoreline change (m/yr) 1985 1990 1994 2002 2005 2008 1990 1994 2002 2005 2008 2012 6.2 -0.1 -2.8 -2.9 -5.8 4.1 -4.2 8.8 - 4.4 -2.1 -1.9 1.0 -3.3 0.8 10.8 -4.9 5.2 -3.5 -7.0 -3.3 5.4 13 Club de Play a - - -4.3 -6.7 -5.2 -5.2 13.1 Club de Playa - - -5.5 -5.0 -4.3 -1.0 13.3 - - -6.4 -6.4 -9.0 -1.2 15.8 Tortugueros -17.7 4.8 7.7 -16.6 -28.6 18.9 18.3 1.0 3.1 -1.6 1.0 -2.2 -2.9 18.7 2.2 1.0 -0.7 -0.3 -4.9 -7.3 19.8 3.8 1.4 -3.8 -1.2 -2.5 -2.6 21 C.A.S.E.S -7.4 -5.2 1.5 -7.4 -4.5 -1.9 23.8 - 3.5 -3.9 -5.2 - 1.3 -0.2 24.4 - -0.1 -3.5 -3.8 -0.3 2.4 28.6 0.9 -1.9 -2.4 -1.7 -0.8 -2.0 31.5 1.7 -2.4 -1.1 -1.9 -3.6 -0.2 33.4 Punta Real -1.4 3.9 -2.3 -1.8 0.8 -5.1 33.5 Punta Real -0.7 4.6 -1.9 0.9 -0.3 -8.0 *The values in red correspond to the maximum rate of erosion in each period. A positive value indicates accretion.

Figure 17. Shoreline changes (a) from 1985 and (b) from 1990.

The loss of material in Club de Playa Beach, C.A.S.E.S Beach

and Punta Real Beach coincides with the results of monitoring (reported in Marquez, 2008) of beach profiles in five periods of analysis, from August 2005 to June 2007 (in dry, Nortes and rainy seasons). Human Intervention

The anthropogenic interventions in Isla del Carmen are mainly the construction of protection structures, such as groins, gabions of rocks (along the coast and on the outer edge of the beach to protect the highway), breakwaters built with rocks and trunks parallel to the coast, jetties and geotextile bags; as well as infrastructure and buildings on the beach and dune system (a

highway and roads, esplanade, rocky walls, railings). The design of most of these protection structures is considered inadequate and some of them were destroyed by storms (as can be seen in Figure 18). Others were responsible for the increase in the erosion at adjoining sites (like the geotextile bags installed in Club de Playa Beach in 2009, whose position is still visible in the image of 2011, Figure 19). Moreover, the expansion and growth of Ciudad del Carmen, moving it closer to the beach year after year, the degradation of coastal dune vegetation, the continuous dredging projects in Playa Norte and, to a lesser extent, in Puerto Real mouth (justified in order to prevent sedimentation in the lagoon and thereby changes in the natural flow of currents between Laguna de Terminos and the Gulf of Mexico), have contributed to the alteration of the original equilibrium of the estuarine-lagoon system and the surrounding area.

Figure 18. Rockfill structures on Isla del Carmen Beach (a) Breakwater on Bahamitas Beach (after Bolongaro et al., 2007), (b) Rockfill wall in PK 27, to protect the highway and (c) Remains of breakwater on San Nicolasito Beach (Bolongaro et al., 2007).

In Figure 19, besides the lack of functionality of the

geotextiles installed in 2009 in Club de Playa Beach, the erosion that took place following the withdrawal of the rockfill (from 2005 to 2008) and the eroded state of the beach in 2012 can be seen.

Figure 19. Evolution of the coastline at Club de Playa Beach.

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The displacement of the highway landward (PK 30.6 - 31.3) from 1994 to 2002, due to the retreat of the coastline, can be seen in the satellite images from this period (Figure 20).

Figure 20. Displacement of the highway from 1994 to 2002.

Table 8 lists the main factors that have affected the natural

flow of sediments at the site from 1928. Figure 21 shows the defense structures found from the observation of the aerial photos and satellite images at the end of each period.

Table 8. Main anthropogenic interventions in Isla del Carmen from 1928 (Bolongaro, 2007; PDU, 2009) Year Anthropogenic activities 1928 Ciudad del Carmen has only 1580 houses 40’s Construction of jetties Construction of the highway that crosses the island (the height of the asphalt allows recovery from erosion in rainy season) 60’s Extension of jetty construction in Carmen mouth (radical transformation of Ciudad del Carmen) Expansion of Ciudad del Carmen 70’s Levelling of the highway (increase in the asphalt height, which impedes the exchange of sediment between the lagoon and the sea) Construction of dams in the main rivers flowing into the area, which retain the sediment The urban zone occupies twice the area of 1968 Start of the construction of the port in Ciudad del Carmen 80’s Construction of breakwaters and groins, with trunks and rocks 1982 Completion of port construction Sept 1988 Second levelling of the highway (increase in height and extension, which produces the block of sediment transport inside the system) 90’s Construction of several protection structures 2009 Installation of sand-filled geotextile bags as breakwaters, along 1000 m of the beach in Club de Playa Beach

In 2002, the spread of rocky material from the structures

installed along much of the beach is visible in Figure 21d. Nowadays, there are remains of rockfill and buried groins on practically the entire beach; they are particularly visible along the last two kilometers before Puerto Real mouth.

Erosion History, Human Intervention and Climate

Understanding the evolution of the coastline alongside the history of human interventions and the climate characteristics facilitates the identification of the causes of the current erosion problems of the study area.

From 1985 to 1990, the most significant erosion scenarios were linked to the location of badly designed defense structures (groins and rockfill breakwaters) along the beach, Figure 21a).

Figure 21. Defense structures on the coast of Isla del Carmen (a) in 1985, (b) in 1990, (c) in 1994, (d) in 2002, (e) in 2005, (f) in 2008 and (g) in 2012.

From 1990 to 1994, the beach quality was better, perhaps be

due to the response of the beach to the construction of a series of protection structures in the 90’s (mostly rocky breakwaters, Figure 21b). From 1994 to 2002, the construction of new defence structures within the morphodynamic unit (Bolongaro, 2007) blocked the arrival of sediment to the beach (groins at Sabancuy mouth and others along almost the whole coastal zone of Isla del Carmen); there is a general retreat of the coastline from PK 5.2 to the easthern part of the island. In addition, several tropical cyclones affected the island in this period (Hurricanes Opal, Roxanne, Mitch and Keith). One consequence of the erosion of this period was the displacement of the highway in PK 30.6-31.3 (close to the beach). From 2002 to 2005, there was an increase in the erosion processes in all the beaches of Isla del Carmen. Hurricane Isidore, in 2002, produced significant wave heights of up to 6.6 m over 83 h.

From 2005 to 2008, Club de Playa Beach (in PK 13.3) was significantly eroded after the removal of the rockfill there (Marquez, 2008), with a retreat of 27 m. Hurricanes Stan and Dean affected the area, especially Hurricane Dean which produced maximum waves of 8.6 m. From 2008 to 2012, accretion took place in several areas that had previously registered erosion, although there was still significant erosion. In this period, different protection structures were installed in an effort to solve the erosion problem on Club de Playa Beach,



such as geotextile bag breakwaters, but they did not produce the expected results (www.axisingenieria.com.mx).

Wave Hydrodynamics of Coastal Zone

Figures 22, 23 and 24 show the wave heights obtained with the model. Figure 22 shows simulation scenarios with return periods of 2 and 50 years, from Table 4, representative of prevalent “Norte” conditions and a category 1 hurricane, respectively. In Figure 23, several wave and sea level conditions are selected to simulate the mean conditions prevailing during Hurricane Roxanne and Mitch; and the maximum conditions during Hurricane Keith. In Figure 24, the results from the modeling of the root-mean-square wave height characteristic of a “Norte” event are presented. The PK reference in the figures facilitates the clear identification of the most affected areas during the passage of these natural phenomena.

Figure 22. Wave heights in the coastal zone of Isla del Carmen (a) Tr=2 years: N; H=3.4 m; T=10.5 s; storm surge=0.3 m, (b) Tr=2 years: NNW; H=3.4 m; T=10.5 s; storm surge=0.3 m, (c) Tr=2 years: NW; H=3.4 m; T=10.5 s; storm surge=0.2 m and (d) Tr=50 years: N; H=8.5 m; T=11 s; storm surge=1.3 m.

Table 9. PKs with the highest concentration of sea wave energy, for the selected scenarios with return periods of 2 and 50 years

Tr=2 years Tr=50 years N NNW NW N 6-8 0-4 0-3 10-15 14-18 6-10 14-20 16-18 20-30 18-22 23-26 20-26 32-33.5 24-33.5 27-30 27-30

From this analysis, it can be seen that the PKs previously identified for further study due to their erosion history are those stretches of beach most affected by the concentration of sea wave energy in most scenarios, especially between PK 20 and PK 30.

Table 10. PKs with the highest concentrations of sea wave energy, for the selected simulated scenarios of tropical cyclones and Nortes

Tropical cyclones Norte Roxanne Mitch Keith 0-4 0-4 0-6 1-6 6-8 16-22 18-24 8-10 12-15 24-26 26-29 16-24 20-22 28-32 30-33.5 26-33.5 28-32

Figure 23. Wave heights in the coastal zone of Isla del Carmen (a) Hurricane Roxanne (mean conditions): Dir=283º; H=3.6 m; T=8.6 s; storm surge=0.2 m, (b) Hurricane Mitch (mean conditions): Dir=337º; H=3.1 m; T=6.8 s; storm surge=0.2 m and (c) Hurricane Mitch (maximum conditions): Dir=332º; H=3.7 m; T=7.5 s; storm surge=0.3 m.

Figure 24. Wave heights in the coastal zone of Isla del Carmen for a Norte event: Dir=332º; H=2 m; T=8 s; storm surge=0.3 m.

The PKs with higher concentrations of the sea wave energy, for each

of the simulated scenarios in Figures 22, 23 and 24, are included in Tables 9 and 10.

72 Escudero et al. _________________________________________________________________________________________________


Circulation Patterns in the Lagoon In calm conditions, the dynamics associated with

anticyclonic conditions in the West of the Gulf of Mexico cause a net flow into the lagoon through Puerto Real mouth and an outflow through Carmen mouth, resulting in the formation of sand banks in Puerto Real mouth and keeping the natural channels at Carmen mouth stable (PDU, 2009). The flow currents inside the lagoon have a westward direction, as is the case for the coastal zone of Isla del Carmen.

In a “Norte” event, numerical modeling (Marquez, 2008) shows a net inflow through Carmen mouth, which reduces the flow entering through Puerto Real mouth, producing an anticyclonic rotation in the eastern part of the lagoon. This anticyclonic rotation moves eastwards and becomes smaller with the increase in the wind velocity. The currents in the lagoon go from West to East. When the intensity of the winds decreases, the predominant flow movement westward is restored (Marquez, 2008).

DIAGNOSTICS

The erosion problems in the coastal zone of Isla del Carmen are a result of the interruption or blocking of the natural sediment transport. The construction of the highway that crosses the island, in the 1940’s, with its subsequent maintenance, extension and raising of level, created a barrier that impedes the free flow of water coming from the adjacent wetlands and with it the sediments that balanced the natural coastal dynamics of the area each year (Bolongaro et al., 2007). The expansion of Ciudad del Carmen and the construction of buildings on the beach, the construction of rigid and inappropriate defense structures, the artificial widening of Puerto Real mouth, and the construction of dams and hydrological changes inland have further hindered sediment transport to the beach. In addition, constructions related to the port of Ciudad del Carmen have affected the original flow and sediment movement from the lagoon toward the Gulf. The emergency measures, apart from not solving the erosion problem, have polluted the sandy substrate with rocks and other unsuitable materials, increasing the currents and the concentration of wave energy in some areas. Another drawback of the blocking of the sediment flow from the lagoon to the sea is the sedimentation and acidification of the fluvio-lagoon system of Laguna de Terminos (Bolongaro et al., 2007).

Figure 25. State of the beach in 2012 and location of the sections which need to be stabilized.

Knowledge of the erosive trends of the beach in each region

and the current width of dry beach pinpoints the critical areas where action is needed. These are the regions PK 12.2 - 13.6 (Club de Playa Beach); PK 17.8 - 23 (C.A.S.E.S Beach); PK 31.5 - 33.5 (Punta Real Beach). In those areas, the highest concentration of wave energy takes place during the passage of the most frequent storms (Figures 22, 23, 24).

Although other areas are being eroded, such as the sections at PK 24.2-30.7 (close to the highway), equilibrium will probably return with the stabilization of the regions at PK 17.8-23 (C.A.S.E.S Beach) and PK 31.5-33.5 (Punta Real Beach). Figure 25 shows the situation of the defined critical stretches (in blue) and the appearance of some of these in April, 2012.

SHORELINE PROTECTION ALTERNATIVES The best alternative to protect the beaches would be the

retreat or removal of facilities, where possible. This would help to stabilize the beach-dune system and eliminate the reflective effect of the structures. However, it would also have serious negative economic and social consequences.

Other options include the adequate design of breakwaters to stabilize the stretches of beach that can be defined as erosion hot-spots, without interrupting the littoral transport of sediments. This measure must be followed by periodic beach nourishment, to obtain the desired width of dry beach. The sand for such nourishment could be obtained, in part, from the lagoon by means of a bypass procedure. This bypass operation would improve the water quality of the lagoon and at the same time reduce the costs of the nourishment. In the course of the stabilization of the beach, the removal of the rocks and other materials from the defense structures currently littering the beach is recommended. Moreover, the restoration, reconstruction or construction of a new dune (along with regulations regarding the use of the dunes, to ensure their preservation) would provide a reservoir of sand for the beach after storms.

DISCUSSION AND CONCLUSIONS

The erosion at Isla del Carmen is a serious threat to the ecological, economic and social wellbeing of the area. The importance of finding viable solutions to preserve its natural features, encourage tourism and protect the inhabitants of the island from the effects caused by natural phenomena is evidenced by the damage already done to the system.

However, the situation in Isla del Carmen is quite critical because of the conflicts of interests and lack of environmental education of the population: it seems that the only pressing issue is how to keep Ciudad del Carmen growing; ever closer to the beach. The consequences of the installation of buildings and infrastructure on the beach are not being considered at all. The same is true for the highway: the only concern is keeping it in perfect condition for communications: the fact that it interrupts the arrival of sediment to the beach is not considered; finally the lack of consciousness of the population in maintaining the dune in a sound state is rapidly degrading the entire ecosystem. In addition, the poor state of the beaches in Isla del Carmen is also due in part to the emergency protection measures adopted (some



of them by the inhabitants of the island) without previous studies.

The shoreline protection alternatives proposed in this study are the starting point for further, in-depth studies to obtain the detailed characteristics for the design of the protection structures to be installed, the quantification of the sand required for beach nourishment and the definition of operations for a bypass procedure for this. The adoption of these alternatives will help to stabilize the beach, facilitate the balance of sediments in the system, and guarantee the long term life of the beach.

ACKNOWLEDGMENTS

This publication is one of the results of the Regional Network Latin America of the global collaborative project ‘‘EXCEED – Excellence Center for Development Cooperation – Sustainable Water Management in Developing Countries’’ consisting of 35 universities and research centres from 18 countries on 4 continents. The authors would like to acknowledge the support of German Academic Exchange Service, DAAD, the Centro de Tecnologia e Geociências da Universidade Federal de Pernambuco, the Fundação de Amparo a Ciência e Tecnologia do Estado de Pernambuco-FACEPE and the Instituto de Ingeniería of the Universidad Nacional Autónoma de México for their participation in this EXCEED project.

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