Experimental investigation on wave set up and nearshore velocity field in presence of a BDS

An extensive full scale laboratory investigation on a 2D physical model of a Beach Drainage System (BDS) was performed at the GWK in Hannover, Germany. The experiments aimed at improving the knowledge of the hydrodynamic and morphodynamic behaviour of a drained beach in order to allow the definition of the main design criteria. The present paper deals with the nearshore hydrodynamic aspects of a beach equipped with BDS. The adopted BDS consisted by 4 pipes installed below the swash zone and at a variable distance from the shoreline. Several drain configurations were considered under three wave energy conditions, paying attention to water table oscillations, wave set up, undertow currents and high-order velocity moments influenced by the system start up. The results show that the drains lead to a reduction in the water table level and, as a consequence, a lowering of wave set up on the beach. This effect induced a decrease in undertow currents and high-order velocity moments in the surf and swash zones. The observed flow characteristics can be used for energetic-based transport models able to predict erosional/accretionary trends in the nearshore morphodynamic changes under the influence of a BDS. The analyses yield useful hydrodynamic information on the general efficiency of a BDS in different wave conditions.