Mechanisms for beach erosion during storms

Abstract

Simultaneous time-series measurements of waves, currents, and suspended sediment concentrations (SSCs) were obtained from the surf zone of a high energy, macrotidal, dissipative beach (Llangennith, Gower, South Wales, U.K.) during “storm” and “calm” conditions. A collocated pressure transducer (PT), electromagnetic current meter (EMCM) and optical backscatter sensor (OBS) were used to measure waves, bi-directional currents and SSCs respectively. Incident wave heights were found to be saturated in the inner surf zone. As the incident waves decayed shorewards, low frequency (infragravity) oscillations in water level and current velocity grew. During the storm, the inner surf zone was dominated by strong (± 1m s −1), low frequency (≈0.01Hz), cross-shore fluid motions which accounted for up to 80% of the total spectral energy. Suspension events associated with the infragravity motions reached peak concentrations of over 70 g l −1, 0.04 m above the bed, and persisted for periods of 30–40 s. Co-spectra between the SSC and cross-shore velocity time-series were computed and used to examine the frequency dependence of the near-bed cross-shore (suspended) sediment transport rate, which was seen to be composed of mainly onshore transport due to asymmetric flows at incident wave frequencies, and predominantly offshore transport coupled with infragravity oscillations in the cross-shore current velocity. A mean (steady) transport component was also measured in association with the undertow (directed offshore). The combined effect of the infragravity band and mean offshore transport components was responsible for the erosion of the beach during the storm