The influence of a Beach Drainage System on wave reflection and surf beat processes

Beach Drainage Systems (BDS) have been installed at many sites around the world as a soft-engineering method able to stabilize the shoreline. The present work pays attention to hydrodynamic aspects concerning wave reflection and surf beat oscillations influenced by an artificial element acting in the nearshore such as a BDS. Full scale experiments on a BDS were carried out at the GWK flume in Hannover, in order to obtain useful information for defining correct design guidelines for such a system. Experiments were performed with different configurations, having one or more drains active. The BDS consisted of 4 PVC pipes located below the beach and at variable distance from the shoreline. The wave reflection on the beach was evaluated through the nearshore wave gauges using two different methods and the pressure transducers located inside the beach near the drains were used to evaluate surf beat oscillations. The main results show an increasing trend of the reflection coefficient during each test due to the progressive beach saturation and a stabilization of the reflection process induced by the lowering of the water table inside the beach due to the drainage. Moreover the presence of the BDS tends to back shift the wave frequencies towards the frequency range of long period infragravity waves. The presence of a larger infragravity energy induced by the drainage tends to enhance the onshore sediment flux. The analyses give useful hydrodynamic information on what influences the morphodynamic behaviour of a beach equipped with BDS