EXPERIMENTAL STUDY ON GEOTEXTILE TUBE APPLICATIONS AS SUBMERGED BREAKWATERS FOR BEACH PROTECTION IN YUCATAN, MEXICO

Abstract

The Yucatan coast, with a wide continental shelf and low sediment sources, is subject to a microtidal regime and low energy, short waves (Hs < 1 m, T=4-6s) with two main incident wave directions (from the E and NE) that generate a dominant westward longshore sediment transport. Approximately 60 of the 245 km extent of the Yucatan coastline have seen significant urban, industrial and touristic developments that have led to coastal erosion of 1 m/year over the last 10 years. In order to mitigate this trend, many structures (most of them emerged groins) have been constructed, but the majority of these efforts have been unsuccessful. From 2005 to 2007 geotextile tubes were empirically used as submerged breakwaters (GT-SBW) at several sites on the Yucatan coast, many of them placed between 10 and 30 m seaward from the shoreline. It was found that, in some cases, the beach was stabilized after placing the structure; but in other places the results were far from those expected. (Fig. 1). Considering the lack of knowledge in the design criteria and in the hydrodynamic performance of GT-SBW, this study analyses the results obtained through experimental tests using different model scales and geometries. The motivation for this study is to better understand the wave-structure interaction in terms of the effectiveness of the structure in minimizing the wave transmission coefficient (Kt). The main goal of this paper is to define optimal design criteria for the GT-SBW. The experimental tests were conducted in a wave flume (37x0.8x1.20 m) at the Coastal Engineering Laboratory of the Engineering Institute at UNAM, using two scales (1:5 and 1:10). The wave generation system is of a piston-type vertical paddle from HR Wallingford. Throughout statistical analysis of hindcast data, the mean wave regime was obtained in order to use representative data of the Yucatan coast. A total of 12 capacitance wave gauges were placed along the flume (before and after the structure) to record water surface elevations and to compute the transmission (Kt) and reflection (Kr) coefficients; Mansard and Funke's (1980) separation method was used. During the experimental tests it was observed that the hydrodynamic behaviour of these structures is quite similar to that of low-crested breakwaters with very low permeability and that when the elevation of the breakwater crown is slightly below the mean water level, the waves break violently, thus dissipating wave energy. On the contrary, if the waves fail to break they are only steepened and the transmission coefficient can be even greater than one. From the data analysis it was found that the optimal freeboard should not exceed twice the design wave height. The experimental results and an empirical formula are shown in (Fig. 2).