ICE - Institution of Civil Engineers - Virtual Library

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 ρgH _i). 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 ρgH _i). 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.

This paper presents the findings of a new coastal processes study at Slapton Sands, south Devon, UK. This work was commissioned as part of a major new study to assess the likely future barrier beach stability. The study followed a major beach erosion event in January 2001, which caused a collapse of the main road that runs along the top of the shingle barrier beach. This barrier beach has been the subject of many previous studies, which are concisely summarised in the literature review section. The paper then presents new data and analyses of the coastal processes over both geological and contemporary time-scales. The paper concludes that the cause of the 2001 erosion event was a combination of prolonged storm attack and differential long-shore transport, causing cut-back of both the cross-shore profile and the mean shoreline position. It was further found that overtopping is likely to occur about once in five years but that a major breach event is unlikely to occur within the next 30 to 50 years. Thereafter, with no intervention measures and expected rates of sea level rise, it is anticipated that the barrier beach will eventually be breached, forming tidal inlets.

This paper describes a physical model investigation conducted at the UK Coastal Research Facility into the shoreline response to a detached breakwater scheme. The effect of random unidirectional waves on beach morphology is examined and the influence of wave transmission and choice of sediment and shoreline evolution is also investigated. Little difference is observed in the evolved shoreline between monochromatic and random wave tests. However, a significant difference in morphology occurs below the waterline because of the different wave–current field and accompanying velocity moments. The plan shape is also affected by the choice of material for the mobile bed, and wave transmission through the breakwater. Measurements and empirical predictions of beach evolution behind the breakwaters are compared, and the latter found to be only partially successful. It is recommended that plan shape models continue to be used to predict long-term shoreline change but incorporating a wave refraction/diffraction model. This should be combined with a coastal area numerical model, to investigate beach response during individual storms and recovery after storms. Further research into influence of tides and tidal currents is proposed as well as development of numerical models, which can predict short and long wave interaction and non-linear transformation of multidirectional waves in shallow waters. The application of such models to mixed beaches also requires further research.