The influence of set-up currents on sediment movement behind detached breakwaters

The authors performed a series of physical model experiments in the UK Coastal Research Facility (CRF) at HR Wallingford in 1998. These tests formed part of a wider programme of physical model studies at a scale of 1:28 of the Elmer breakwater scheme in West Sussex. The authors lowered the crest levels of the original breakwaters in order to investigate the hydrodynamic and morphodynamic effect of using submerged rubble mound breakwaters, instead of emergent ones, to provide coastal protection in a macro-tidal region. Lowering the crest levels would have reduced the raw material and construction costs of the scheme. The visual intrusion would have been diminished and the inshore water would have been be more regularly flushed, reducing the likelihood of water quality problems developing at the beach. Although lowering the crest height increases the transmitted wave energy above certain water levels, it can be determined using traditional theory. However the impact on beach evolution of the 'set-up' currents generated behind breakwaters when the water level is near crest level is not well understood. Their magnitude can not currently be predicted easily. The results from the series of CRF experiments reported herein demonstrate the nature and magnitude of these set-up currents. The authors present an equation to determine the 'set-up' in 2D models and outline a volume continuity method to relate 2D 'set-up' to the magnitude of the 3D set up currents.