Swash zone morphodynamic modelling including sediment entrained by bore-generated turbulence

Abstract

In this paper we introduce a mathematical model of sediment entrainment due to bore-generated turbulence in a shallow water context. In this model, the entrainment is assumed to be proportional to the energy decay rate across a bore on a mobile bed. The energy decay rate across a bore on a mobile bed is derived analytically. This model is incorporated into the one dimensional morphodynamic model developed by [1], which includes bed-and suspended load transport. This results in new shock conditions, which allow for sediment entrainment at a shock. With it we investigate the effects of sediment entrainment due to bore-generated turbulence on beachface evolution under a single swash event driven by a solitary wave. The simulation results imply that sediment entrainment by bore turbulence at the incoming bore dominates over sediment mobilisation by bed shear stress. In contrast, the backwash bore is dominated by bed shear stress related processes. The morphodynamic impact of bore turbulence on this swash event is primarily erosion of the sea bed seaward of the initial shoreline. Any sediment remaining in the water column seaward of this point is then available to be transported by subsequent events. It is shown that the bed step is primarily a bed load related feature, with sediment entrained as suspended load counteracting the bed step growth.

Vertical dependences in swash-zone flows and sand transport

“…It is widely accepted that bore-related turbulence is an essential driver for sediment transport in the swash zone (e.g. Butt et al, 2004;Alsina et al, 2009;Zhu and Dodd, 2020). This is again affirmed in the present study, which shows that the limiter for the k − ω model, introduced by Wilcox (2006) in this situation leads to underpredicted sediment concentrations, and as such can be presumed to underpredict turbulent kinetic energies as well.…”

“…Because of this, one can assume that the model performs reasonably well in the uprush, but has more difficulty in the backwash. Given the observations of Alsina et al (2009) and Zhu and Dodd (2020), this suggests that the model accurately captures the suspended transport due to wave-breaking related turbulence and sediment advection, which is more important in the uprush, but understimates the shear-related transport in the backwash. However, the different results from the four different configurations show that this balance is delicate and can be difficult to achieve.…”

“…These models can give a lot of insight into important processes driving morphodynamics (see e.g. Reniers et al, 2013;Incelli et al, 2016;Zhu and Dodd, 2020). However, they have difficulty reproducing intraswash sediment dynamics, in particular suspended sediment transport (Ruffini et al, 2020;Mancini et al, 2021).…”

“…For instance, Alsina et al (2009) modelled turbulence using a modified version of the model by Deigaard et al (1986) and found the contribution of bore-generated turbulence essential to achieve accurate cross-shore suspended sediment transport compared with measurements. Zhu and Dodd (2020) used a different approach by estimating the turbulent kinetic energy from data instead. They found that bore-generated turbulence dominated the sediment entrainment, being more important than entrainment from pure shear.…”

A laboratory study of the effect of varying beach slopes on bore-driven swash hydrodynamics

“…When the waves reach the shoreline, the beach area is alternately covered and uncovered by water. The area between the maximum runup and rundown of waves is known as the swash zone, where multiple dynamic processes occur (Zhu and Dodd, 2020). The flow in this area presents high speed, high transient, violent aeration, and non-uniform characteristics, which play an important role in wave structure interaction, sediment transport, and morphological dynamics (Zhang and Liu, 2008).…”