Berm accretion resulting from inner surf and Swash zone processes

Abstract

Swash zone morphology is important for coastal management, flood protection and touristic use of beaches. Starting from different post-storm beach profiles, this study investigated berm accretion during wave energy reduction after storms (beach recovery). The large-scale wave flume experiments featured measurements of beach profile evolution, water surface elevation, outer flow velocities and suspended sediment concentration. The measurements were combined with simulation of onshore wave propagation, sediment advection under turbulent bores in the swash zone and Energetics-type sediment transport to explain the processes causing berm accretion. When comparing low (dimensionless fall velocity $\Omega =\frac{H_{rms}}{T_p\ast w_s}=\{0.72,1.05\}$) to high ($\Omega =1.44$) energy recovery waves, an onshore shift in breaking caused more wave asymmetry-related onshore transport through the inner surf zone. This resulted in 2–3 times as much recovered berm volume under the low energy recovery waves. When comparing the two low energy recovery wave conditions ($\Omega =\{0.72,1.05\}$), differences in breaking at the shoreline (potentially related to profile shape inherited from the storm) influenced bore-related onshore advection of sediment to the berm crest. This caused twice as much vertical berm accretion under one condition ($\Omega =0.72$) in contrast to twice as much horizontal berm accretion (shoreline recovery) under the other condition ($\Omega =1.05$).