Intra-swash hydrodynamics and sediment flux for dambreak swash on coarse-grained beaches

Abstract

The paper reports on dambreak-type swash experiments in which intra-swash hydrodynamics and sediment flux are measured for swash on a coarse sand beach and a gravel beach. Flow velocity and depth are measured using PIV and LIF respectively; the intra-swash sediment flux is measured using sediment traps. Comparison of measured hydrodynamics with the immobile, permeable bed experiments of Kikkert et al. (2013) indicate that bed mobility impacts on the swash hydrodynamics, reducing the maximum run-up by approximately 8% for both beaches, compared to the maximum run-up on the corresponding immobile beach. The measured intra swash sediment flux at a given location is characterised by high flux at the moment of bore arrival, followed by rapid decay during uprush, becoming zero at some time before flow reversal. For the gravel beach, the backwash sediment flux is negligibly small, while for the sand beach the backwash flux increases slowly as the flow accelerates down the beach, and peaks at about the time of maximum backwash velocity. Intraswash sediment flux calculated using the Meyer-Peter and Müller bed load transport formula, with measured hydrodynamics as input and bed shear stress estimated using both the Swart and Colebrook formulae, are within a factor 2 of the measured intra-swash flux. The agreement between the calculated and measured flux is better for the sand beach than for the gravel beach, and better for uprush than for backwash. For the sand beach there is good agreement between calculated and measured total uprush and total backwash sediment volumes. The agreement is less good for the gravel beach, for which calculated and measured uprush volumes show a similar trend but the calculated backwash volumes over-estimate the (negligible) volumes observed in the experiments. Field experiments investigating swash hydrodynamics and sediment dynamics have developed substantially over the last two decades in terms of the sophistication of the instruments deployed and the degree to which detailed processes are captured by the measurements. Regarding sediment flux, sediment trapping has been used to measure total uprush and total backwash transport volumes (Hughes et al., 1997;Masselink and Hughes, 1998;Austin & Masselink, 2006;Masselink et al., 2009) and high-resolution bed elevation measurements across the swash zone have been used to obtain net sediment transport volumes for individual swash events (e.g. Blenkinsopp et al., 2011). These measurements are extremely valuable in terms of quantifying sediment fluxes and morphological change for swash events in the field. However, they do not provide a complete picture since they reveal little (in the case of traps) or nothing (in the case of bed elevation measurements) of the sediment flux during a swash event. In principle, measurements of intra-swash sediment flux can be obtained from co-located measurements of velocities and concentrations, but obtaining these measurements sufficiently accurately over the complete water column to give total sediment flux...

Entrained air in bore-driven swash on an impermeable rough slope

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Bank Erosion Processes in Regulated Navigable Rivers

“…C w is the wave celerity taken as the velocity of the bore ( C b , Hansen & Svendsen, 1987), estimated by Svendsen et al (1978) as with d c being the water depth at the bore crest and d t the water depth at the trough. The bed shear stress is computed with a Chézy‐type formula (Equation 14, Jonsson, 1966), with f b a friction coefficient for oscillatory flows computed with Equation 15, the Colebrook formula (Briganti et al, 2018; O'Donoghue et al, 2016). where Re = u w h / ν is the Reynolds number for instantaneous depth and velocity, with ν being the water kinematic viscosity (m 2 /s).…”

Field measurements of shear stress and friction in the surf zone

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