The physical processes of sandy beach evolution under storm and non-storm wave conditions simulated in wave flume

Large waves and high water levels generated by coastal storms are predominant drivers for coastal beach erosion. This experimental study is undertaken to investigate the physical processes of beach profile evolution under storm waves by simulating step varying storm waves and variable water levels in a wave flume of 30 m long, 0.6 m wide and 1 m high and mild beach slope tanθ=1/10. The storm waves were simulated by following a conceptual storm hydrograph of three stages: at the first stage of 60 min both wave height and water level were gradually increased to peak storm wave height and water level, at the second stage of 60 min both maximum wave height and water level were kept for constant, and at the last stage of 60 min storm wave heights were gradually decreased to non-storm waves and water levels. Non-intrusive high-resolution cameras were used to record time-series of free surface water elevation, wave breaking point position, sandbar migration and location, and beach elevation profile in a spatial resolution of 1.4 mm and a temporal resolution of 0.1 s. In analyzing the collected experimental data, the physical processes of beach evolution under both non-storm and storm waves were found significantly different: under non-storm waves, the movement of sandbar always was found to be in the seaward direction, and the beach profile was eroded and accreted before and after the sandbar, respectively, but under storm waves, the sandbar location was found to migrate in either the landward or seaward direction, the beach profile was slowly accreted before the sandbar and eroded after the sandbar when the storm waves were gradually decreased to the non-storm waves. The surf zone is the main change area in beach morphology modification, and the sandbar is the important indicator for distinguishing the cross-shore sediment exchange. Under both non-storm and storm waves, large waves, high water levels, and especially great gradient of wave energy flux due to changes in wave height, period and water depth are the main factors controlling the formation and migration of sandbar, and resulting in beach erosion and accretion before and after the wave breaking point, respectively.