Experimental study of bore-driven swash-swash interactions on an impermeable rough beach slope

Abstract

Detailed hydrodynamics within the swash-swash interactions are studied based on experimental measurements. Two bores are generated using a double dam-break mechanism and interact on a 1:10 impermeable sand-rough beach. Two series of experiments are conducted. The first series is carried out to explore the overall features of swash-swash interaction. The water levels in the reservoirs and opening times between the two gates of the dam break mechanism are varied to generate a large number of different swash-swash interactions. Experimental flow depth data is measured using acoustic displacement sensors. The results are combined with estimated times of flow reversal, obtained from a model based on the nonlinear shallow water equation, to determine the type of swash-swash interaction. Two types of interactions are identified: wave capture interaction, when the interaction with the second bore occurs during the uprush of the first bore, and weak wave-backwash interactions when interaction happens during the backwash of the first bore. The second series of experiments investigates the detailed hydrodynamics of these two types of swash-swash interaction by using a combined particle image velocimetry and laser induced fluorescent system, which enables comparisons to be made between wave capture interaction, weak wave-backwash interaction and the single swash event. The results of the first series of experiments show that the relative strength of the bores at the initial shoreline and time between their arrivals determines the initial type of interaction, however the type as well as the intensity of the interaction may vary throughout the swash-zone for the same swash event. The results of the second series of experiment show that during wave capture and weak wave-backwash interactions, the fluid of the first bore is advected upwards, then mixed with the fluid of the second bore by highly turbulent vortices generated at the front of the second bore because of the relative velocity differences between the two bores. Discussions of the results obtained in the experimental study highlights that swash-swash interaction results in large shoreward local fluid accelerations and strong turbulence intensities in particular with a large vertical fluctuating component. In addition, the velocity gradients indicate that the bed shear stress also increases during interaction. All these factors indicate that swash-swash interactions have the potential to enhance shoreward sediment transport in the swash zone.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology