Field Measurements of Sediment Dynamics in Front of a Seawall

Abstract

A field experiment has been carried out to examine the effects of seawalls on hydrodynamic and sediment dynamic processes on sandy beaches. Pressure transducers, electromagnetic current meters and optical backscatter sensors were deployed directly in front of a seawall at Teignmouth, South Devon (U.K.) in June 1995. Similar instruments were deployed simultaneously on the adjacent natural beach. Reflection coefficients were in the range 0.7 to 1.0 at the wall and around 0.2 on the natural beach for incident wave frequencies (0.125-0.36 Hz). Reflection coefficients at lower frequencies (0.04-0.125 Hz), were close to unity at the wall, whilst on the beach the reflection coefficient increased with decreasing frequency, reaching 0.9 at the low frequency spectral peak. Both sediment suspension and transport were altered significantly by the presence of the wall. Mean suspended sediment concentrations were found to be up to three times larger in front of the wall than on the natural beach. This increase was attributed to the increase in wave reflection. The largest differences occurred when the waves were largest, and the water was shallow. A net onshore sediment transport by incident waves was observed on the natural beach. In front of the wall, this net oscillatory transport was considerably reduced. The longshore current in front of the wall was stronger than that observed on the natural beach. Combined with the increase in suspended sediment, this enhanced longshore current resulted in a longshore sediment transport rate which was an order of magnitude greater in front of the wall than on the natural beach.