A study has been conducted to develop safe swimming conditions along the North-West coast of Egypt. Investigations have been made for the perched beach approaches. Both numerical models and actual scale field data have been employed in this study. Attention has also been given to the empirical models available in literature for computing the transmitted wave energies and lessons learned from the application of protection structures in Egypt and overseas.
An actual scale model of a perched beach is designed and constructed to provide safe swimming conditions using submerged breakwater and two jetties. The project area is constructed along the west boundary of the Alexandria governorate of Egypt. The latter area has long suffered from rip currents as large as 0.7m/sec and a limited safe swimming strip of less than 40m during the prevailing wave conditions in the summer season. Several design alternatives have been studied using a numerical model, namely known as Surface water Modeling System (SMS), adopting the actual wave rose of El-Dekhila port and bathymetric survey of the project area. Field measurements were carried out in May 2009 before construction of the perched beach, considered as the baseline condition. The model has been calibrated and validated against the collected and measured field data. Based on all the collected data and information, the numerical model has been applied to investigate different alternatives of the proposed structure including submergence ratio, crest level, the effects of the gap size (if used) and permeability of the structure on the wave height, radiation stresses, current velocities, sediment transport rates and shoreline changes.
It has been found that using the guidelines prepared by the Shore Protection Authority (2002) a safe swimming zone, acceptable flushing condition and minimum flooding of the shoreline due to wave damping effect of the perched beach, especially in case of Low Crested Structures (LCS), could be provided. However, some negative impacts have been noticed both numerically and in field, including some shoreline erosion in the down drift zone and possible trapping of floating debris. The latter impacts can be mitigated by the use of some openings to allow the flow of water and sediments from the up drift to the down drift side to minimize possible erosion in the down drift. It has also been found that the maximum length of the impacted beaches is about 3 times the length of the jetties on both sides. The latter length could be lessened by the aid of lower jetties and/or openings in the jetties. The residual possible impact could be alleviated through a routine sand nourishment program of the shoreline at a frequency that can be decided on a case-by-case basis.
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