Field and physical model investigation of seawall toe scour and associated wave flow processes

Abstract

In the UK one third of seawall failures are attributed to scour, placing the communities living behind these structures at risk of flooding or erosion. Despite this, field evidence of scour remains anecdotal. Our current understanding is adapted from deepwater breakwater studies and small scale seawall model tests which do not adequately represent shallow water sediment processes. Two medium scale model tests have been conducted, providing guidance for only a limited range of conditions. However a link between the key hydraulic processes and the resulting sediment transport has yet to be developed. As part of a larger research project, beach level data was collected from the toe of a vertical seawall in Blackpool over a 3 year period, and over a 1 month period at a 1:2 sloping seawall in Southbourne. These results were combined with model experiments conducted at medium scale using irregular waves on a sand beach. To clarify the toe scour processes and the interaction of breaking waves with vertical seawalls, a hydraulic flow study was conducted at small scale. The research demonstrated a causal relationship between wave breaking and the risk of toe scour at seawalls. Waves impacting at the wall generated high velocity downrush flows, peaking at 0.6 m.s"1 in a 1:40 scale model and producing an intense toe vortex. By combining knowledge of the near wall wave breaker type, flow pattern and sediment response, improved methods to predict and mitigate scour were demonstrated. The deepest scour occurred when the wave conditions and toe water depth were optimal for waves to impact onto the seawall {d, /Lm~ 0.018) and generate high flow velocities. Flows velocities were reduced under spilling and pulsating wave conditions. The duration of exposure to the most critical condition determined the scour risk, though the depth of maximum scour always remained less than incident wave height. Further model tests and field experiment are required to extend this research to a wider range of beach and wave conditions.

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