Seawall-Waves Interaction and Impact on Sediment Morphology

A study was conducted to investigate hydrodynamic and sediment transport mechanisms induced by the interaction of seawalls or breakwaters and the incident wave field. Use has been made of field data of two case studies and the numerical predictions of Genesis. The main case study examines the seabottom morphology and cross shore profiles in front of Rosetta seawalls, constructed in 1990, on the Nile Delta coastline to decelerate coastal erosion of the headland after the operation of the Aswan high dam in 1965. The analysis of the field data indicated lowering of the cross shore beach profiles, an increase of scour depth and an increase of the longshore sediment transport rate in front of the seawalls followed by downdrift accretion. These results are inline with the analysis of the second case study conducted to investigate the bathymetric changes, 1998–2004, at the Véran seawall located at the Gulf of Lions at the French Mediterranean coast. The modifications of sediment transport rates are attributed to the generated short-crested wave pattern in front of the seawalls and the modifications of long shore current. Further, the present analysis indicates that a significant component of sediment scour, in front of the seawall, is manifested as an offshore sediment transport current. Thereafter this offshore current is deflected alongshore contributing to sediment accretion down drift the seawall. Implications of these results to the design steps for coastal zone management and optimization of breakwater design, to minimize sediment accretion at entrances of small craft- harbors, are discussed.