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Seawalls are the most common form of coastal defence, they are the physical barriers between land and sea. They are parallel to the beach used to prevent overtopping and flooding due to storm waves. They can be constructed in the front or back of the shoreline. However, the front face of a seawall is subjected to high energy wave action, which can induce the problem of toe scour. Scours in front of the breakwaters and the seawalls have been investigated heavily in the past, but it still remains a concern for coastal engineers. In those previous studies, the sea bottom was considered as flat or having a very mild slope. These may not be valid for steep slope beach in front of a structure. The seawall stands in runup region in this study. The seawall and the beach profile interaction was investigated by experimentally. The beach slopes were 1/5 and 1/10 in the experiments.

Experiments were carried out in a horizontal channel with a free jump and jumps with jets introduced at various angles. Particles within the jumps were traced using a high-speed video camera and the images were processed using a computer to determine particle velocities within the jumps. A simple numerical model using Strip Integrals across the flow is used to predict the kinematic characteristics of such jumps. Numerical and experimental results agree satisfactorily. The results show that even a relatively small momentum flux of the impinging jet can profoundly influence the jump in terms of the velocities within the jump and its surface profile. Plunging breakers occurring on a beach as the wave breaks may also be represented as a hydraulic jump with a weak jet impinging at its toe. This study shows that the impinging jet can increase energy dissipation within a plunging breaker.

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