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 Zhu and Dodd (2015), 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.
Highlights
The swash zone is a very dynamic region in which the beachface is repeatedly submerged and dried, and in which considerable sediment is transported, as both bed and suspended load
Sediment suspended by bore turbulence has been included in some modelling approaches [5, 7], typically using the Nonlinear Shallow Water Equations (NSWEs), in which bores can be naturally simulated as shocks [9]
In order to investigate the effect of sediment entrainment by bore turbulence, we examine a swash event driven by a solitary wave in which an incoming bore forms when the solitary wave approaches a sloping beach and a backwash bore forms when the flow recedes
Summary
The swash zone is a very dynamic region in which the beachface is repeatedly submerged and dried, and in which considerable sediment is transported, as both bed and suspended load. Bore turbulence has been recognised as being important for sediment suspension [4, 5, 6, 3, 7] Both field and laboratory measurements show that high suspended sediment concentrations (hereinafter SSC) are found associated with 10 the passing of bore fronts [3, 8]. Sediment suspended by bore turbulence has been included in some modelling approaches [5, 7], typically using the Nonlinear Shallow Water Equations (NSWEs), in which bores can be naturally simulated as shocks [9]. In the NSWE-based cross-shore suspended sediment transport model developed by [5], 15 the sediment suspension term was related to energy dissipation due to both bottom friction and wave breaking. The energy dissipation due to wave breaking is further separated from that due to bed friction. The suspension 20 and settling of sediment are linked to a bed change equation to allow morphological change. [7] extend the approach of [5] by considering a transport equation for turbulence, and by considering the vertical distribution of suspended sediment
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