Laboratory Tests of Scour at a Seawall

A set of medium-scale laboratory tests of wave-induced scour at seawalls has been performed in a flume at HR Wallingford. The methodology is presented along with test conditions and summarized results. The scour depth at the toe of the seawall is highly dependent on the form of wave breaking onto the structure. Sea states where waves plunge directly onto the wall generate jets of water that may penetrate to the seabed and cause a local scour hole immediately adjacent to the seawall. This is a different scouring mechanism to that observed in deeper water and is also absent when the seawall is well within the surf zone and most of the large waves have broken before they reach the seawall. Theoretical limitations are discussed

Suspension by regular and groupy waves over bedforms in a large wave flume (SISTEX99)

Suspended sand concentrations and bedforms under waves were measured in the controlled environment of a large wave flume. Three suspension conditions are discussed here; those occurring with regular (monochromatic) waves of height 0.55m over anorbital ripples, regular waves 1.0m high over orbital bedforms, and repeating wave groups (with a significant wave height of 0.6m) also over orbital-scale features. In all cases the wave-to-wave variability in suspended load was high (∼30%). Patterns of suspension were dependent on the bedform type and on instrument location relative to the bedform. Regular waves suspended an order of magnitude more sediment than groupy waves with a similar significant wave height illustrating,the importance of sequences of high waves in pumping-up sediment concentration into the water column

Failure mechanisms of shingle barrier beaches

Physical model tests were undertaken at HR Wallingford to address gaps in the knowledge of the failure process of shingle barrier beaches. During these tests, numerous factors were considered such as sediment characteristics, the type of waves (storm or swell), and beach crest geometry (height, width, back slope). This technical note summarises the test procedures, the experimental set-up, the test conditions and the data obtainedFloodsit

Large-scale experiments on wave downfall pressures

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 rho gH(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 rho gH(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