An Alternative Solution to Erosion Problems at Punta Bete-Punta Maroma, Quintana Roo, Mexico: Conciliating Tourism and Nature

Transcription

1 An Alternative Solution to Erosion Problems at Punta Bete-Punta Maroma, Quintana Roo, Mexico: Conciliating Tourism and Nature Author(s): Itxaso Odériz, Edgar Mendoza, Cristina Leo, Gabriel Santoyo, Rodolfo Silva, Rubí Martínez, Ernesto Grey, and Raúl López Source: Journal of Coastal Research, 71(sp1): Published By: Coastal Education and Research Foundation DOI: URL: BioOne ( is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne s Terms of Use, available at Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research.

2 Journal of Coastal Research SI Coconut Creek, Florida 2014 An Alternative Solution to Erosion Problems at Punta Bete-Punta Maroma, Quintana Roo, Mexico: Conciliating Tourism and Nature Itxaso Odériz *, Edgar Mendoza, Cristina Leo, Gabriel Santoyo, Rodolfo Silva, Rubí Martínez, Ernesto Grey, and Raúl López Instituto de Ingeniería, Universidad Nacional Autónoma de México, Mexico City, México Huaribe-OHL Desarrollos Playa del Carmen, México Ecoturismo Tres Rios México Tecnoceano Cancún, México ABSTRACT Odériz, I.; Mendoza, E.; Leo, C.; Santoyo, G.; Silva, R.; Martínez, R.; Grey E., and López, R., An alternative solution to erosion problems at Punta Bete-Punta Maroma, Quintana Roo, Mexico: Conciliating tourism and nature. 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 Coconut Creek (Florida), ISSN This paper presents an erosion problem found between Punta Bete and Punta Maroma, on the Riviera Maya, Mexico. The case is worth studying as its features are quite complex: ecologically speaking, the area contains part of the Mesoamerican barrier reef, the second largest in the world, an important area of seagrass and large mangrove fields; on the other hand regarding economic aspects, the main activity is tourism, which is of vital importance to the region and nationally. These two opposing interests need to be conciliated in the adoption of a solution to problem of erosion on the coast as both are being degraded at the present. The chronic erosion reported in recent years is due to natural causes, such as the slenderizing of the coral barriers and to anthropogenic causes such as local constructions on the shoreline that interrupt the longitudinal sediment transport. This work describes the characterization of the zone and analyses of marine climate and beach sediment. The hydrodynamics have been numerically modelled with WAPO and COCO models and the morphological response of profiles with the XBeach model. Three possible solutions are proposed and numerically tested, each of which present strengths and weakness. The three alternatives are discussed in order to help decision making as some form of intervention is urgently required. ADDITIONAL INDEX WORDS: Beach erosion, Riviera Maya, touristic developments. INTRODUCTION Beaches and dunes provide storm protection and damage reduction against climatologic events, for both infrastructure and natural systems beyond the beach, (Kobayashi et al., 2009). For example, the damage found on the coast around Galveston, Texas, was found to be less where the dune was higher (Doran et al., 2009). This natural protection has also recreational, biological and economic benefits. These are some of the many reasons for preventing, or stopping, coastal erosion. This paper presents the case of the erosion of the beach between Punta Bete and Punta Maroma (henceforth referred to as PBPM). The area is located on the Mexican Caribbean, forming part of the Riviera Maya, in the state of Quintana Roo (Figure 1). The study area hosts a wide variety of activities with different characteristics as well as an ecologically invaluable environment. It is also an important tourist destination, so there are many conflicts between economic, social, ecological and political interests that increase the complexity of any possible solution to beach erosion. In the ecological part of the equation, the area contains a section of the Mesoamerican barrier reef, the second largest in the world and an important area of seagrass; inland there are broad mangrove fields; all of which make up a complex and delicate ecosystem. Economically, the zone has great touristic potential, according to INEGI (Mexican National Institute for Statistics and Geography) the touristic affluence in Quintana Roo is the highest in Mexico and tourism contributes DOI: /SI received 12 February 2014; accepted in revision 24 August *Corresponding author: itxaso.oderiz@gmail.com Coastal Education & Research Foundation 2014 over 70% of the gross domestic product of the State of Quintana Roo (Fay, 2013). Figure 1. Location of the study area, Punta Bete-Punta Maroma. PBPM is within Mexico s most vulnerable areas to hurricanes; 47 of these storms have been registered here from 1948 to 2007 (Silva et al., 2012). Recently, the occurrence of hurricanes e.g., Gilbert in 1988 and Wilma in 2005 ( for more details), caused severe damage to infrastructure, economic activities and social wellbeing. In recent years the area has suffered severe beach erosion which has increased the vulnerability of the infrastructure there.

3 76 Odériz et al. As a consequence, the tourist industry may decline and, in addition to the local economic repercussions, the attractive, positive image of Quitana Roo s beaches could affect tourism at national level. Any erosion in PBPM is considered to have a negative effect, so adequate coastal management is required and for this the features of the area need to be characterized in order to find a solution which can conciliate the social and economic interests with the environmental elements that made this site so important a touristic destination. This work, then, focuses on the characterization of the study area as the means to acquiring sensibility in order to propose alternative solutions within the aforementioned constraints, numerically test them, choose the optimal solution and issue coastal management recommendations at PMPB. CHARACTERIZATION OF THE STUDY AREA Littoral Cell: Land and Marine Environments The littoral cell is physically delimited by the Bete (PM) and Maroma (PM) tips, which are two natural headlands, Figure 1. Similar to many other littoral cells in the Mexican Caribbean, the PMPB coastal area is largely covered by vegetation (mainly mangrove) as can be seen in Figure 1. The area is located over an aquifer which discharges fresh water to the sea. There are few surface rivers in the area, but close to the central point between the two headlands there are three small (Figure 2). The marine zone can be divided in two areas: a) the bay, and b) the reef, as can be seen in Figure 2. Within these, ten types of environments can be distinguished: a shallow sandy area, seagrass, a landward coral reef, a seaward coral reef, a seaward transition zone, a sand channel, patches of Gorgonacea, a deep sandy area, a pronounced cliff and the continental shelf. In the reef section the reef crest is well defined and is approximately 4 km long, varying in distance from the shore from 250 to 800 m. Up to a point, it is the existence of the reef which is responsible for the formation of PM. In the 1990s a reduction in the coral reef mass was documented (De ath et al., 2012; Gadner et al., 2003; Hughes et al., 2003). This was due to disease, blanking and the growth of epibiont organisms (Lessios et al., 2001; Toller, 2001) As a result PM receeded. This observation coincides with the knowledge we have, that the crest height of a reef controls the amount of wave energy dissipation and that the distance between the reef-crest and the shore determines the vulnerability of the beach to morphological changes, (Ruiz de Alegria-Arzaburu et al., 2013). In this same study they found that a reef-crest degradation of 1 m can result in an increase in incoming wave energy of up to 10%. The bay section is the southern part of the study area and has shallow sandy areas with some small patches of seagrass. It is characterized by the absence of a coral reef, as coral is unable to grow here because of the superficial and subterranean fresh water discharges (Bacabes del Mar, 2013). At the southern limit of the study zone, PB, an incipient patch of coral reef can be found due to the absence of fresh water discharge spots. This reef gives shelter to the southern part of the study area and is, probably, responsible for the existence of Punta Bete. Geological evidence of the effect of the coral reef in the stability and regression of PM are the beach ridges found there that can be seen in the left panel of Figure 2. Figure 2. Marine elements and environment at PMPB. Present Sedimentology Characteristics The small bay formed between the two headlands shows almost no sediment interchange with neighbouring beaches, which lets us classify the bay as a closed littoral cell. The sand from the beach face is moved offshore in the form of cross-shore transport during storms; most of this material is usually deposited in the shallower part of the study area, but if the storm is intense enough, the sediment is carried offshore from near the cliff. In both cases, once the sand has been taken away from beach, it is almost impossible that it get back, as the mean wave climate is of quite low energy. Figure 3. Location and definition of the sand profile samples, coordinates UTM 16N. At the North, the beach is defined by mangrove, rocky and sandy areas and at the South the beach is sandy. The sediment of both areas is mainly of biogenic origin, formed by oolite, calcareous shells, and coralline fragments, (Carranza et al., 1996). These sources generate new sediment at very low rates, so the system has sediment deficit and thus the resilience of the beach is very low. To get an idea of the stability of the beach, seven sand profile samples (drying beach, swash zone and surf zone) were taken along the study area in the locations shown in Figure 3.

4 An Alternative Solution To Erosion Problems At Punta Bete-Punta Maroma, Quintana Roo, Mexico 77 To get an idea of the stability of the beach, seven sand profile samples (drying beach, swash zone and surf zone) were taken along the study area in the locations shown in Figure 3. Table 1 summarizes the main parameters of the sand samples, i.e. d 50, fine material portion (%f), roundness (R), sphericity (Sp), shape factor (Sh) and fall velocity (w). The mean measured density of all the samples was 2.2 g/cm3. In Table 1 shows the parameters of the samples: D stands for drying beach, Sw is swash zone and Su is surf zone. Table 1. Sediment relevant parameters. Site Sample d 50 %f R Sp Sh w (mm) (m/s) D MK Sw Su D TR2 Sw Su D TR1 Sw Su D K2 Sw Su KN Sw KV Sw D PM Sw Su Figure 4. Recent infrastructure constructed along the PMPB beach. Although a full topographic survey was not available, ten dune profiles from the southern part of the study area (Figure 4) were provided by Consultores en Gestión, Política y Planificación Ambiental. These profiles were recorded between April and May 2013 and are shown in Figure 5. The results of the sediment analysis show that at points K2 and MK the grain size is larger in the drying beach, which indicates profile instability and that the sand is being lost from these areas. The largest sediment sizes all along the study area were found in section TR1, probably because it is close to the river discharges and some terrigenous sediments can get there. This section, according to the grain size distribution along the profile seems to be reasonably stable. In all the other sample sections it was found that the grain size distribution along the beach profile decreases away from the shoreline, which indicates that these profiles are more stable than those at K2 and MK. Shoreline and Dune Profiles Unfortunately there is almost no historical data available for the study area; only some satellite imagery and the records of human actions in the last decade. A summary of the perturbations which took place in recent years is presented in Figure 4. The construction of groins and breakwaters and the placing of geotextile bags is visible in several sections of the coast; most of these structures were installed using touristic and recreational philosophy; disregarding the physical processes involved. In Figure 4 it can also be seen that the main consequence of the infrastructure selected is the interruption of the longshore sediment transport. This mechanism is highly undesirable as the natural capacity of the beach to distribute the sand and keep a sound state disappears. Figure 5. Dune profiles from the south of the study area. The beach in the areas located through the profiles P1, P2 and P10 has been preserved and native vegetation up to where the mangrove begins can still be found. However, the rest of the profiles were modified by the construction of a golf course and other infrastructure (P3, P5, P6, P7, P8 and P9) and only the foredune is still in a sound state. Profile P4 is in a relatively

5 78 Odériz et al. good state of conservation, as only the top of the dune has been flattened. The evolution of the shoreline in recent years (2006 to 2010) was assessed by comparing satellite photographs, Figure 6 shows detailed images for 3 zones. In A, PM, the erosion is evident, as mentioned earlier this is mainly due to the reduction of coral reef protection. The shoreline retreated here almost 10 m. The shoreline in the central section, B, has remained more or less in the same position, with only a small variation; some parts show erosion, others show accretion. Section C, PB is by far the most damaged as it has suffered from the naturally driven erosion (including the effects of Hurricane Dean in 2007) and also the effect of the badly planned infrastructure that blocks the longshore sediment transport. From 2006 to 2010 an average of 29 m of beach was lost. The methodology used, in order to define a feasible solution, comprised: the characterization of local hydrodynamics, the numerical modelling of wave and current fields and the numerical modelling of the beach response to storms (using available beach profiles) followed by the numerical testing of three possible alternative solutions and the selection of that with the best performance. Marine Climate Wave and wind data from 1948 to 2010 were taken from the Atlas of Wave Climate by Silva et al., (2007). This data are the result of a reanalysis based on the hybrid wave model WAM- HURAC, WAMDI-Group (1988) and Silva et al., (2002). Annual and seasonal statistical data were used to define the mean and extreme regimes; the results are shown in Figure 7. Figure 7. Mean period-significant wave height joint probability (a) and wave height and direction rose (b). Figure 6. Shoreline evolution, After the analysis of the sediments, shoreline evolution and the available profiles, it can be said that the present dynamics of the PMPB beach system are of low resilience as the beach and dune elements have been degraded by both natural and anthropogenic actions. In addition, as the headlands that govern the morphology are retreating, the shoreline in between is also eroding, a process worsened by the infrastructure that interrupts the longshore sediment transport. Therefore, taking into account that the tourism industry here is very important for the local and national economy, some form of engineering solution must be found. The main goals of any conservation action should be: to ensure the continuity of tourist activities, to offer security to visitors, to recover the resilience of the system and to preserve the valuable environmental elements described earlier. In Figure 7 it can be seen that the dominant waves comes from East and SE directions with significant wave height (H s ) lower than 1.5 m and a mean period (T m ) of 4-6 s. In winter, due to cold fronts, extreme events with directions NNE, NE and NEE, with Hs of 2-3 m and Tm of 6-8 s, occur frequently. In summer there are sporadic extreme events with higher energy: Hs of 4-13 m and Tm of 6-12 s with dominant incoming directions from E and ESE. Hurricanes have a strong negative impact on the stability of the beach, causing severe erosion. Three hurricanes had substantial effects: Gilbert in 1988 with a minimum central pressure of 925 mb and maximum sustained winds of 130 km/h when it was close to PBPM; Wilma, 2005, passed close to the zone with a central pressure of 930 mb and wind velocities of 120 km/h, in 2007 Dean was a weaker storm: 960 mb central pressure and 75 km/h wind velocity ( Regarding wind climate, the dominant directions are E and NEE, with a medium velocity of 5 m/s. In winter, the main directions are ENE and E with velocities of 10 m/s due to cold fronts. The highest wind velocities occur in summer, reaching 50 m/s. Astronomic tide time series recorded at Puerto Morelos, approximately 17 km north from the study area (see Figure 1) were taken as valid for the study area; the tide regime in PBPM is microtidal and semidiurnal with a mean range of 20 cm. The storm surge was obtained from synthetic data computed by Duran (2010) with the numerical model MATO (Posada et al., 2007), which solves the 2D Non Linear Shallow Water

6 An Alternative Solution To Erosion Problems At Punta Bete-Punta Maroma, Quintana Roo, Mexico 79 Equations (NLSWE). The estimated sea levels at the shoreline as a function of wind direction and velocity are shown in Figure 8. Figure 8. Sea levels at PMPB as a function of wind direction and velocity. Modelling of Wave Propagation and Currents To represent the hydrodynamic conditions in the study area two models were used: WAPO, Silva et al., (2005), a numerical wave propagation model based on the Modified Mild Slope Equation; and COCO, Mendoza et al., (2007), which computes wave induced currents solving the NLSWE. A total of 80 combinations of significant wave heights (1, 2 and 4 m), mean wave periods (7, 8, 10 and 12 s), wave directions (NEE, E, ESE and SE) and meteorological tides (0, 1 and 2 m) were modelled. With all these conditions, mean regime, mild storm, severe storm and hurricane scenarios were covered. cell, and another with high resolution (6 6 m), that only covered the southern part of the domain. This division was made as the southern area is exposed to larger waves (very few reefs exist here), so this is the area with worst erosion. Figure 9 shows an example of the results obtained with WAPO on the low resolution grid for Hs=2 m, T=8 s, incident direction NEE and meteorological tide of 2m. It can be seen, as was mentioned before, that the southern shoreline receives higher waves, up to 2.5 m high, while the area around PM is naturally protected by the reef barrier. This scenario corresponds to severe storm conditions but the general pattern is similar in all the modelled scenarios. These results are in agreement with previous works regarding the role of the coral reef as a natural defense (i.e. Alvarez-Filip, 2011; Gourlay and Colleter, 2005; Hearn, 1999; Lowe et al., 2005). Figure 10 shows an example of the results of wave propagation and wave induced currents on the high resolution grid for Hs=2 m, T=7 s, incident direction NEE and null meteorological tide which correspond to a mild storm; again the southern part receives waves of around 2.5 m and the wave induced currents reveal a pattern close to PB that clearly explain why the sediments are being lost to the south. Figure 10. Propagation of waves (a) and currents pattern (b) with high resolution mesh for Hs=2 m; Tp=7 s, dir=nee, tide=0 m. Figure 9. Propagation of waves, mesh with low resolution for H=2 m; T=8 s, dir=nee, tide=2 m. The hydrodynamics were estimated in two separate grids, one of low resolution (15 15 m), which covered the whole littoral Response of the Beach Profile The numerical model used to analyze the beach profile behavior under storm conditions was X-Beach, Roelvink et al., (2009). The only topographic data available was the modelled profiles presented in Figure 5. The main focus of this numerical exercise was to assess the outcome of the do nothing solution as well as verify the vulnerability of the actual state of the dunebeach system to mild (T1 and T2) and severe (T3, T4 and T5) storms and hurricanes (T6 and T7). Table 2 summarizes the

7 80 Odériz et al. modelled scenarios, where Mt stands for meteorological tide and t for simulation time. Table 2. Hydrodynamic parameters used in modeling Xbeach. Storm Hs (m) Tp (s) Dir Mt (m) t (h) T T2 2 8 ESE/E T3 2 8 ENE T4 2 8 ESEE T ESE/E T ENE T ESEE Figure 11 shows the results of X-Beach modeling, in which it can be seen that the profiles with a sound dune (profiles 1, 2, 4 and 10) can endure mild and severe storms with less damage and having the possibility of a natural recovery. In contrast, the profiles with the most damaged dunes are rapidly eroded. An important feature revealed by these results is that an important part of the sand taken by the storms is deposited in the nearshore area and only a small part is lost offshore; this means that it is available for artificial nourishment. Figure 11. Response of the profiles to storms. As a result of the characterization of the study area, the main problems detected in the study area are (1) Only a few of the original coastal dunes are conserved and in a sound state, which means that resilience is decreasing as the sand supply to the beaches during storms is limited or stopped. (2) Disregarding the causes, the reduction of the reef dimensions is increasing the wave energy that arrives onshore and thus the coastline is moving landward, looking for a new stable state. (3) The combination of density, sphericity and shape factor of the sediment results in easily transported sand; this, and the lack of sediment sources, means that the littoral cell has a sediment deficit. (4) The numerical results and the satellite imagery show the process of permanent erosion in the system. (5) Several structures, aligned perpendicular to the coast, i.e., groins, have been constructed along the beach interrupting the longshore sediment transport and thus worsening the sediment deficit in some areas. (6) The stability of the beaches is often threatened by storms and hurricanes. All the above factors lead us to describe the PBPM system as having low vulnerability, in a state of permanent erosion with a low possibility of auto-regeneration and in urgent need of intervention in order to preserve the natural value of the area and its social and economic interests. SHORELINE PROTECTION ALTERNATIVES The solutions suggested assume that regenerating and protecting the beach in certain segments can be the means to recovering the protection function of the dune-beach system in the entire littoral cell and thus preserving its attractiveness and, hence, its economic viability. It is desirable that the solution be a combination of rigid structures, intended to control wave energy, and soft actions (i.e. artificial nourishment), to control sediment transport. Submerged structures were chosen as the rigid elements as they have low visual impact, produce stilted wave breaking and reduce wave transmission, which helps water renewal in the protected area Three alternatives were proposed and numerically tested. For each alternative the same 80 wave conditions used in the characterization were modelled and the profile response was evaluated with X-beach model in the ten profiles shown in Figure 5. The description of each alternative and the numerical findings are presented below. It is important to remark here that all the alternatives include beach nourishment using sand taken from the nearshore area of the system. Alternative A has four medium to large submerged structures placed in critical positions, to recover the protective function of the reef barriers. A sketch of the proposed location of the structures is shown in Figure 12. All the barriers are crested at the mean sea level. Structures A1 and A2 of alternative A are intended to extend the length of the existing reef barriers, thus reducing the wave energy that reaches the shoreline. At the same time these structures may help to retain part of the sediment that travels offshore and cannot return the beach. In turn, structures A3 and A4 are also wave control structures which would reduce the wave energy that is reaching PM as a result of the reef slenderizing so that the new stable position of the coastline can

8 An Alternative Solution To Erosion Problems At Punta Bete-Punta Maroma, Quintana Roo, Mexico 81 be found as soon as possible thereby reducing the dry sand loss. The UTM coordinates of the tips of the structures, their lengths and crest heights are shown in Table 3. behind each barrier. This array of structures will not immediately benefit the whole littoral cell; only the most damaged sections. Anyway, the distribution of the sand from the nourishment will strengthen the beach in the long term as long as no severe storm or hurricane event occurs. The coordinates of the tips of the structures and their main dimensions are shown in Table 4. Figure 12. Location of the submerged structures in alternative A. Table 3. Location and dimensions of the structures in alternative A Structure East (m) North (m) Length (m) A A A A The numerical results for mean conditions show that the structures have little effect on wave propagation and wave induced currents, but as the wave conditions become more energetic (mild storm, severe storm and hurricane) the structures stop high waves reaching the coast. Some currents are found in the protected area, which help to distribute the material from the nourishment all along the littoral cell. In alternative B the existing structures placed to protect PM are eliminated, believing that the position and shape of the headland can be left to reach its stable state freely, while the protection and artificial stabilization can be focused in the more damaged sections of the cell. The reduced cost of this alternative is an additional reason for adopting this alternative. As can be seen in Figure 13, where the location of the structures is shown, structure B1 is similar to structure A1 in location, dimensions and performance. On the other hand, structures B2 to B5 are small submerged barriers intended to control wave energy but also help to control sediment transport. The numerical results of this alternative predict a local effect of structures B2 to B5 and the possible formation semi-tombolos Figure 13. Location of the submerged structures in alternative B. Table 4. Location and dimensions of the structures in alternative B Structure East (m) North (m) Lenght (m) B B B B B Alternative C has two groups of small submerged structures as shown in Figure 14. In this alternative the idea of replicating the function of the reefs has been abandoned for the sake of reducing overall costs. The effects of barriers C1 to C9 in controlling wave energy are purely local and their efficiency is higher for the mean regime and mild storms than for more energetic situations. On the contrary, they are better working as sediment traps when intense storms occur. Figure 15 shows an example of the wave propagation and the numerically produced wave induced current fields. The protected area offered by alternative C is smaller than in the other options, as is its impact across the whole littoral cell. However, this alternative has the advantage that it can be constructed in less time and with smaller equipment.

9 82 Odériz et al. The response of the beach profiles to the small barriers of alternative C was estimated with the X-Beach model for all the scenarios presented in Table 2. Figure 16 shows the results only for those profiles in front of which a barrier was placed, i.e. profiles P2, P3 P5 P7 and P8 of Figure 5. The numerical results show that in general with barriers placed, the erosion caused by the storms is less than without them. In agreement with the wave propagation results, the submerged structures lose protection efficiency as the intensity of the storm increases and, comparing the results in Figure 16 with those presented in Figure 5, for severe storms and hurricanes, the erosion is similar with and without the barriers. A great difference in the performance of alternative C is regarding sediment transport, that is, as the structures are closer to the shore, the possibility of semi-tombolo formation is greater. This means that the longshore sediment transport will be impeded and may be blocked, so the artificial nourishment may not distribute sediment along the entire shoreline, nevertheless it will stay in the most damaged areas and will not easily be removed. Figure 14. Location of the submerged structures in alternative C. Figure 15. Wave propagation (a) and wave induced currents (b), with alternative C barriers for Hs=2 m; Tp=8 s, dir=nee, tide=0 m. On the other hand, the maintenance costs needed to keep the barriers and the artificial nourishment working properly may be important in the event of a severe storm, perhaps even equaling the original investment in the case of a strong hurricane. This alternative seems to be most environmental friendly as it will not require any construction in the marine area nor close to the coral reefs. Table 5. Location and dimensions of the structures in alternative C Structure East (m) North (m) Length (m) C C C C C C C C C The retreat of the shoreline is less in profiles 2 and 8 with the structures of option C; on the other hand in profiles 5, 3 and 7 there are variations depending on the hydrodynamic conditions. With respect to the dune height, in profiles P3, P5 and P8 the effect of structure is not evident due to the wall at the edge of the grid, and in profile P2 (storm 1, storm 2), P3 (storm 2) and P7 (storms 4 and 6) the height is greater with the structure. There is a reduction in the percentage of area eroded. The shoreline retreat is very evident in P2 (storms 4, 5, 6 and 7) and P8 (storms 2, 3 y 4). Modelling shows that structures work, but dune regeneration would be recommendable to obtain a resilient profile and provide more protection for hydrodynamic conditions with longer return periods. Regardless the solution selected, as said before, artificial nourishment is recommended as well as the rigid barriers (preferably after constructing the barriers). To select the appropriate sand bank for this nourishment several factors were considered (CEM, 2006):

10 An Alternative Solution To Erosion Problems At Punta Bete-Punta Maroma, Quintana Roo, Mexico 83 Grain-size distribution: The mean diameter, d 50, of the fill sand should be larger than the original sand in order to ensure beach stability and reduce overfill. The optimum overfill ratio is 1.00 to Figure 16. Propagation of waves (a) and current patterns (b) with high resolution mesh for H=2 m; T=7 s, dir=ne-e, tide=0 m. Particle shape: The sphericity is better higher as this has greater fall velocities, higher roundness enhances beach stability and angular grains are more difficult to be suspended. Colour: As important touristic activities are developed in the area, the colour is important for the visual impact of the finished project. The sand found within the littoral cell in the nearshore area has the following characteristics: d 50 of mm, 1.5% of fine material, roundness of 0.867, sphericity of and a shape factor of 0.656; which considering the above mentioned recommendations, can be used for the artificial nourishment suggested. DISCUSSION AND CONCLUSIONS Conciliating the natural features and ecological services of the coastal area with medium or high density touristic infrastructure is difficult, and often seen as impossible. This is not the case in places like the coast of Quintana Roo, where it is precisely the natural richness of the area which has attracted touristic development. What was not properly considered during the planning and construction of the infrastructure is that Caribbean coasts are highly vulnerable because of their sand characteristics and availability, and that the intensity and energy of hydrodynamic conditions here causes them to be very destructive. As a result, there are many tourist developments which have fallen into a vicious circle in which the infrastructure degrades the natural elements (vegetation, dunes and beaches) leaving the infrastructure itself without natural protection, then the extreme meteorological events damage the infrastructure; as a response, more poorly planned infrastructure is constructed (e.g., groins and other barriers perpendicular to the shoreline) which again degrades the natural elements. This process occurs within the frame of larger natural cycles: atmospheric (el niño, la niña), marine (sea level rise) and ecological (reef slenderizing); which put the littoral systems in critical and chronic situations. Now, political, economic and social aspects make the do nothing or the remove all scenarios unviable, so a wellplanned engineering project must be carried out in order to let all those involved continue to prosper. The present paper suggests a solution combining rigid and soft measures as the means to recover the stability of all the elements in the littoral cell Punta Bete-Punta Maroma. The soft measure consists of artificial nourishment with sand taken from the nearshore area. This sand, as has been numerically demonstrated, was taken from the dry beach and placed underwater by storms so, from this point of view, the soft measure suggested can be viewed as a recovery of the original state of the beach-dune system. It is straight forward to think that this action alone may have a short life as the recurrent storms that occur in the Caribbean will take the sand back to the nearshore area. This is the main motivation for the addition of the rigid part of the proposed solution. Three alternatives for the rigid barriers were considered. Alternative A aims to reproduce the original condition of the littoral cell, in which the wave energy was controlled by the coral reefs, in order to stabilize the PB and PM headlands. Given that the position of the headlands determines the position of the shoreline of the whole littoral cell, a stable state can be reached

11 84 Odériz et al. in mid to long term (in the short term the stability will depend only on the soft measure). The construction of the large structures in this alternative will prevent the sediment going offshore but not to being transported underwater by mild storms. Alternative A presents greatest efficiency in controlling the wave energy of severe storms and hurricanes; this, the length of the barriers and the depth in which they will be placed (5-7 m), make this the most expensive option. Alternative B combines the reproduction of the reef action with local protection for the most damaged beach sections. The barriers suggested in this alternative will only stabilize the headland at PB, and a group of small structures will control the waves and work as sediment transport traps. The effects of this alternative will not affect the whole littoral cell; mainly the southern and central parts. The small barriers, as they are placed close to the shore, will stop the sediment in the beach being deposited underwater by storms. Alternative B was the most efficient for mild and severe storms. Alternative C focuses only on protecting the beach sections with worst erosion problems. This alternative has 9 short barriers, placed very close to the shore (2 m depth). The main objective of these structures is to dissipate wave energy and to produce protected areas where sand settlement is favoured and semi-tombolos are formed. This works well with the artificial nourishment as the barriers stop the sand moving to the nearshore, although longshore sediment transport is limited, meaning that the benefits will be local and not affect all of the littoral cell. This alternative shows better efficiency for a mean regime and mild storms and it is possible that the barriers and the soft measure may need maintenance and/or reconstruction after a strong hurricane. The costs of this maintenance are not high and the low initial investment of this alterative is cheaper than A or B. It is clear that there is no one, unique, solution and that the economic aspect is not the only one to be taken into account in making the decision. Instead, a coastal management plan is needed to determine which dune-beach performance and which of the engineering measures are desirable and compatible with future development and tourism. ACKNOWLEDGMENTS This publication is one of the results of the Latin American Regional Network 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. 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