Large-scale experiments on wave downfall pressures

Abstract

Many exposed vertical or steep-fronted coastal structures experience large horizontal impact pressures generated by breaking waves. Breaking and non-breaking waves can however also generate a large uprush of water at the structure, in some cases reaching heights of 70 m and more. This uprush is often carried over the structure, leading to overtopping. It has only recently been shown in small-scale model tests that the downfalling water mass can also generate significant vertical impact loadings on the deck of a breakwater. Within an ongoing research project, large-scale measurements of wave impact and downfall generated pressures on vertical and steeply-faced seawalls and breakwaters were conducted in the Large Wave Channel (GWK) at the Coastal Research Centre (FZK) in Hanover, Germany. The downfall pressures were found to consist of very short pressure peaks (durations down to 0·5 ms) of up to 220 kPa magnitude (corresponding to 12 ρgHi). The highest downfall pressures occurred for near-breaking waves; non-breaking and breaking waves generated smaller pressures of 20–70 kPa (corresponding to 2–6 ρgHi). The magnitude of the observed downfall pressures is in the range of horizontal wave impact pressures and suggests that this type of loading, for which no guidance exists, should be considered in the design of coastal structures.