Laboratory investigation of coastal beach erosion processes under storm waves of slowly varying height

Abstract

Storm-induced large coastal waves are a predominant driver for coastal erosion problems, but the physical processes of storm erosion have not been fully understood due to some technical difficulty in simulating storm waves with timely varying heights in the laboratory and collecting storm beach profiles in the field. In this study, novel series of experiments are proposed to study sandy beach processes under coastal storm conditions in a laboratory wave flume of 60 m long, 3 m wide and 1.5 m high. Two water depths of h = 0.5 m and h = 0.7 m were used to investigate beach response under low and high tidal levels. Two initially uniform plane beach slopes of tanβ=1/15 for dissipation beach and tanβ=1/5 for reflection beach were used to study beach profile changes. Additionally, intermediate beach tanβ=1/10 was also experimentally investigated under regular waves in another wave flume of 30 m long, 0.6 m wide and 1 m high. Non-intrusive instrumentation of high-resolution cameras was used to record time-series photo images on beach profile evolution in spatial resolution of 0.2 cm and a temporal resolution of 0.1 s. Image analyzing techniques were also developed to transform the recorded photo images to digital data on both beach and water-surface profiles. In analyzing the collected data, it is found that the variation of storm wave height results in changes of wave breaking point and also sediment transport direction before and after the wave breaking point. Sediment transport direction was found to move onshore initially under small storm waves, and then a sand-bar was generated and a certain portion of sediment also moved offshore under large waves, but such monotonic relationship between sediment transport and wave forcing could transform as morphology feedback and phase lags between wave orbital fluid velocity and sediment concentration. The wave energy flux at the breaking point is shown to be suitable parameter in predicting beach mass change or storm erosion rate, while the dimensionless sediment settling velocity parameter Ω′ may not be a suitable criterion for predicting beach erosion or accretion under storm waves.