Coastal Protection Techniques for Storm Damage: Experimental and Numerical Study on Rock Seawall in Swash Zone to Reduce Wave Overtopping and Overwash of Sand Beach

Hydraulic model tests were conducted in a wave flume to reduce storm induced damage to backshore areas. Because no such methods were available to design structures to reduce the storm damage, four test series: sand beach with a berm, sand beach with a dune, a rock seawall and a sand-covered rock seawall with 14 tests and 140 runs were conducted in the small-scale experiment to compare the effectiveness in reducing storm wave overtopping and sand overwash. Using almost same incident irregular waves, water level was raised to increase the wave overtopping and sand overwash. It was found that the dune was effective to reduce wave overtopping compared to the berm without the dune. However, when the still water level (SWL) was increased, the narrow dune was easily eroded. The stone seawall eliminated the erosion problem of the dune; however, the buried seawall combined the aesthetics of the dune and the robustness of the stone seawall. The stone seawall buried inside the dune functioned like the dune initially and like the seawall after the sand on and inside the porous seawall was eroded by overtopping waves. Both stone seawall and buried stone seawall were deformed but reduced wave overtopping. The existing numerical model, CSHORE is extended to predict sand transport on and inside the porous structure in the swash zone. The hydraulic model tests and the numerical model are compared to evaluate the effectiveness of coastal storm protection techniques.