Erosive and accretive coastal profile response

Abstract

Project SANDS To advance the mobile bed scaling laws and to test new instruments, identical tests are carried out on different scale levels in three hydraulic facilities; Hannover GWK Flume, Barcelona Flume and Delft Scheldt Flume. Data obtained in Hannover and Barcelona are not yet available. Focus is on the Delft data; on beach profile development and on distorted models. The initial bed slopes were fixed, respectively 1/20, 1/15 and 1/10. Two wave time series are run consecutively, the erosive (storm) and the accretive (moderate) wave condition. The bed profiles exposed to an accretive wave condition show a shoreward migration of the breaker bar, caused by a dominant seaward transport by undertow. The profiles subject to an erosive condition show an offshore migration, caused by a dominant landward transport by wave asymmetry. No equilibrium profile is reached. The length between the breaker bar and the coastline should be very large, to be able to reduce the wave height to such an extent, that the seaward migration stops. Such a profile is not likely to occur. Profiles exposed to an accretive condition, did not reach an equilibrium either. Distorted models are physical models in which the horizontal length scale and the vertical length scale are different. Distorted hydraulic models are used because of limitations on the available space in the flume, or because of a lack of control over the modelling materials and conditions. Erosion and deposition volumes are analysed to find the morphological time scale factor ( ). The morphological time scale factor of test 1:15 is 2-2.5 and for test 1:10 the average time scale factor is 8. By means of the time scale factor, erosion and deposition volumes of the erosive tests 1:10 and 1:15 are re-calculated and correspond fairly well to the original erosion and deposition volumes of test 1:20. However, the bed profiles of test 1:10 and 1:15 translated to the reference test do not correspond with the reference test. The distortion scale should be as small as possible, to prevent the generation of scale effects. Project VOP The Dutch coast is maintained by sand nourishment. This study focusses on shoreface nourishment. At this moment, no hydrodynamic measurements or data on sediment transports are available. A low and a high nourishment design are implemented in the Scheldt flume. Test results will enhance knowledge of efficiency of shoreface nourishment designs by linking process-based measurements with bed profile development. The bed profiles with the nourishment designs and the reference profile are exposed to two wave conditions, similar to the SANDS experiments. The low Nourishment Design 1, leads to a relative increase of sand volume, 20% for the accretive condition and 40% for the erosive condition in the coastal zone. The high Nourishment Design 2, results in the largest relative increase of sand volume, 60% for both wave conditions. The presence of a shoreface nourishment significantly affects the wave height, the wave-induced return flow, the wave asymmetry and the sediment transport, whereas it does not have a clear relation with the sediment concentration. The presence of a shoreface nourishment has a combined relative effect. On one hand, the shoreward sediment transport is reduced due to the decreased wave-induced return flow. On the other hand, the landward sediment transport is increased because of an increased wave asymmetry.