Observations of morphological change and sediment transport on a steep gravel beach

摘要

Observations of morphological change and swash sediment loads, concurrent with measurements of water level, waves and currents, were obtained during a spring-to-spring tidal cycle on a steep macrotidal gravel beach in southwest England. The wave conditions experienced during the field experiment were relatively calm and the significant inshore wave height and period averaged 0.4 m and 6 s, respectively. Incident-frequency water motions dominated the hydrodynamics, and the phasing between water surface elevation and cross-shore and alongshore currents was inconsistent with standing edge wave motion. Morphological change was directly related to maxima in shoreline elevation and four modes of berm adjustment identified: regression and rollover in response to falling and rising water levels, respectively; and removal and reformation during and after storms, respectively. Swash processes transported substantial amounts of sediment at rates comparable to sand beaches and the net transport under modal wave conditions was estimated to be 1 kg per swash onshore per unit metre beach width. Steep hydrodynamic gradients induced by the abrupt slope breaks of the step and berm are one of the main causes of the large morphological variability characteristic of gravel beaches.