DREDGING SUMMIT & EXPO ’18 PROCEEDINGS BEACH DEVELOPMENT AND PROTECTION OF RESORT COASTLINE USING GEOTEXTILE TUBES C. D. Timpson1 and T. C. Stephens2 ABSTRACT Shore erosion is a serious problem that is present in many countries with coastal regions. Mexico is not an exception with coasts on the Pacific Ocean, the Gulf of Mexico, and the Caribbean Sea suffering from erosion. Climate change has played an important role in this matter, increasing the strength of storms that hit the Mexican coasts every year. These vulnerable coasts need to be protected to avoid beach retreat and property damage. Knowing how to design suitable coastal protection is important to help recover and stabilize a beach. This paper will present the wave modeling and analysis to design and evaluate performance of structures incorporating geotextile tubes in protecting and stabilizing a 500 meter stretch of Caribbean beach at a resort in the state of Quintana Roo, Mexico. Also, after 9 years of observation of what has happened on the protected beach, this paper will detail how it has made it possible to better understand erosion protection projects more fully for this area and to incorporate the lessons learned to design projects for adjacent properties including beach renourishment dredging. Keywords: Dredging, geotextile tubes, erosion, wave attack, shoreline. INTRODUCTION The coasts of Quintana Roo, in Mexico have been experiencing increasing erosion, especially in the past 10 years, as storms have been more frequent and stronger than they have been historically. In October 2005, Hurricane Wilma (Category 5 on the Saffir-Simpson Hurricane Scale) passed over the northern part of the state, affecting the beaches of Cozumel, Playa del Carmen and Cancun. All beaches were severely damaged and eroded with many hotels affected by damage to their structure, installations and furniture. This study is about a property where there was a hotel called Capitan Laffite, at coordinates 20º 40’30.7935” N, 087º o1’17.9711” W (fig. 1). The hotel lost around 20 m of beach and its’ foundations were submerged in the sea. As recovery would be too expensive, the land was sold to a developer, who planned to build a new hotel; the Gran Vela, Riviera Maya. This developer invested in hydrographic studies for a project to recover and stabilize the beach to the original extent of the seaward property line for the following reasons: The hotels in the Riviera Maya and Quintana Roo live from tourism and need wide white sand beaches for people from all over the world who come to see and enjoy them. Seasonal storms develop medium to high waves with enough energy to make sand move and erode beaches. In winter, cold fronts generate waves from the Northeast that last up to a week in duration. In the summer, winds from the Southeast and East generate waves that for weeks. All this movement displaces the sand in front of the hotels, resulting in the narrowing of the beaches and which exposes the hotel facilities, especially public areas such as pools, restaurants, and bars. Protecting the beaches from erosion helps the hotels to maintain the hotels their facilities in good condition. Mexican law states that the “Maritime Federal Zone” in 20 m from the High Sea Level, measured inland. Therefore, if the beach is eroded, the maritime Federal Zone moves, resulting in the hotel property becoming part of the Federal Zone. Then the property limits are adjusted and the hotel loses. It is important for developers to keep the width of the beach so that there is at least 20 m before the High Sea Level. 1 Engineer, TenCate Geosynthetics, 365 South Holland Drive, Pendergrass, Georgia 30567, USA, T:706-206-9683, Email: [email protected]. 2 Director, TenCate Geosynthetics, 365 South Holland Drive, Pendergrass, Georgis 30567, USA, T:404-660-2317, Email: [email protected]. 1 DREDGING SUMMIT & EXPO ’18 PROCEEDINGS Mexico Yucatan Quintana Roo Punta Bete Figure 1. Location of Study In the case of Gran Vela Hotel, bathymetric and topographic surveys were performed, and a project was initiated to protect, recover and stabilize the shoreline of 500 m of beach front. The project planned five 70 m breakwaters, parallel to the shoreline, with 30 m separation between the first four and 130 m between the last breakwater. In addition, a 600 m line of sand filled geotextile tubes were set parallel to the property line and covered with sand to form the core of a sand dune. The dune was vegetated with the objective being to catch sand moved by the wind and to fix the dune with roots of the plants. 2 DREDGING SUMMIT & EXPO ’18 PROCEEDINGS The project was finished in 2008 and has been working well. The beach has been widened. However, no salient due to sand accretion has been clearly formed, so the question is now being, “Was the design the best option?”. The beach has been widening and narrowing a little with the different local wave conditions, so that it has now reached equilibrium. The sea bottom is rocky and no sand banks are found close to the project site. Sand moves only alongshore specifically from South to North. Neighboring beaches are still being eroded every year but the Gran Vela Hotel beach is wide and pleasant for the tourist to use. CHARACTERISTICS OF THE STUDY AREA The study area is vulnerable to hurricanes and it is necessary to consider that year after year tropical storms hit the area. The Gran Vela Hotel location has the typical characteristics of Quintana Roo coasts; flat and low. The average elevation at the dune is about 2 m above sea level, rising slowly inland. The beach slope is 20:1 and the sea bottom is rocky continental platform with sandy areas of between 5 cm and 2 m. Small reef heads may be present on the sea floor. The sea bottom gradually deepens slowly up to about 50 or 55 m offshore where a ledge appears and the bottom drops to 400 m deep. The average tidal range between the Mean High Water and Mean Low Water is about 30 cm. Predominant waves and winds are from Southeast and East, with winds from the Northeast in winter due to cold fronts. Several field studies were carried out such as a topography survey, a bathymetric survey, current measurements, and exploration dives with SCUBA equipment to look for marine sand banks. Figure 2 provides details of the field work being conducted. Figure 2. Examples of Field Work Work was carried out using computer programs to determine wave patterns. The surveys provided the following information: Topography and bathymetry Currents mainly travel from Southwest to Northeast No nearby sand banks of importance for beach nourishment Good movement of sand parallel to the beach from South to North for most of the year with some episodes in the opposite direction in winter due to the cold fronts that last between 2 and 7 days From the mathematical model Wave Watch III, information based on 9 years of measurements made by NOAA at a deep-water points North of Cozumel Island with coordinates 21º n, 86.5º W, and using program STWAVE (Figure 3). Table 1 was obtained (http://polar.ncep.noaa.gov/waves/index2.shtml). 3 DREDGING SUMMIT & EXPO ’18 PROCEEDINGS Figure 3. Deep Water Wave Characteristics Table 1. Characteristic of Waves in Deep Waters Wave propagation from the numerical simulation provided wave heights and periods in front of the property at a distance of 100m offshore according to Table 2 and Figure 4. Table 2. Shallow Water Wave Characteristics 4 DREDGING SUMMIT & EXPO ’18 PROCEEDINGS Figure 4. Wave Directions, Wave Period, and Wave Height SHORELINE PROTECTION ALTERNATIVES The following possible solutions for shoreline protection were taken into consideration when developing the mitigation strategy for the beach at the Gran Vela Hotel. Beach Nourishment: This option represents a solution with a low-impact on the coastal ecosystem and tourism. However, it requires a sediment supply source (e.g. a big sand bank close to the beach). It is obvious that it would be necessary to continue this nourishment, since sand will continue to be mobilized under storm wave conditions and go downdrift or move out to sea. For this reason, a structure retaining the sand would be preferable. Jetty: The local ecological authority does not allow the building of jetties or any other structure that stops the long shore sediment transport. Therefore, this alternative was not considered. Breakwaters parallel to shore: Breakwaters parallel to the shoreline were the only option that could be authorized, so this was the solution that was adopted. Additionally, an artificial dune made of the sand filled geotextile tube and aligned in front of the hotel is perfect for vegetation, protecting the sand from erosion caused by wind and providing a barrier in case of extraordinary storm waves. The geotextile tube line should be heavy enough to withstand wave impact and should have an anti-scouring apron to prevent the geotextile tube from rotating forward. This solution selected is the construction of several breakwaters parallel to the shoreline. This system would help dissipate wave energy, and capture sand. No side effects would impact neighboring properties. There would be no interruption of longshore sediment transport. The quantity, size, and separation between breakwaters would be determined by formulas to obtain the optimum results for wave energy reduction. MODELING OF WAVE CONDITIONS IN FRONT OF THE PROTECTED COAST Modeling of different wave conditions was performed using Genesis software (Gravens et al 1991) with four of the breakwater structures. The wave conditions modeled a) Height = 0.7 m, Time = 6.6 s and Direction = 259º, b) Height = 0.7 m, Time = 6.6 s, and Direction = 281º, c) Height = 0.7 m, Time = 6.6 s, and Direction = 238º, and d) Height = 2.45 m, Time = 6.6 s, and Direction = 333º.