Numerical modelling of the flow and bed evolution of a single bore-driven swash event on a coarse sand beach

Abstract

This paper examines the numerical prediction of the sediment transport and bed evolution for a single swash event on a coarse sediment beach. In these conditions bed load is the dominant mode of sediment transport. Laboratory experiments of a single bore-driven swash event are simulated numerically using a fully-coupled model that solves the system of Non Linear Shallow Water Equations and the Exner sediment conservation formula. The analysis focuses on two aspects: the optimal choice of parameters for the Meyer-Peter and Müller sediment transport formula and the model used for computing the shear stress. The methods tested for the bed shear stress are the momentum integral method and the Chezy formulation in which the friction factor is computed using two different formulae. Infiltration into the beach and its effects on the shear stress and sediment transport are also modelled. Results show that the basic Meyer-Peter and Müller sediment transport formula provides good results in the run-up. On the other hand, the sediment transport in the early stage of the backwash is overestimated. A reduction of the sediment mobility constant in the formula in the backwash marginally improves the results. However, the causes of the overestimation of the sediment transport at the early stage of the backwash is the overestimation of the shear stress, while at later stages there are several contributions that are identified, i.e. modelling of the sediment transport and infiltration. It is also suggested that the Meyer-Peter and Müller sediment transport formula might not capture the complexity of the processes involved during the backwash. The comparison of the methods for the estimate of the bed shear stress show that comparable results can be obtained using the momentum integral method and the Chezy formulation with time and space varying friction factor. The resulting bed evolution is also described. In the predicted final profile, deposition is found in the upper part of the beach and erosion in the lower part. A bed step is formed just below the position of the initial shoreline. This feature is determined by the onset of an hydraulic jump during the backwash.

Numerical modelling of intra-wave sediment transport on sandy beaches using a non-hydrostatic, wave-resolving model

“…Relatively simple unsteady Bottom Boundary Layer (BBL) models, such as the momentum integral method (Sumer et al 1987) used also in NLSWE solvers (e.g., Briganti et al 2011), could be considered. However, the results in terms of τ b are comparable with simpler formulations, such as the one considered in this study (see e.g., Briganti et al 2018). Also, phase differences could be significant and more complex BBL models should be used (e.g., Rijnsdorp et al 2017).…”

“…The Meyer-Peter and Müller (1948) formula is considered appropriate for the swash zone according to previous studies (see Chardón-Maldonado et al 2016 among others) and variations of the formula have been tested, for example in Postacchini et al (2012) for sand and Briganti et al (2018) for coarse sand. When compared with the original Meyer-Peter and Müller (1948) formula, the Postacchini et al (2012) formulation showed very similar results in terms of net bed changes (see Briganti et al 2016).…”

High-resolution, large-scale laboratory measurements of a sandy beach and dynamic cobble berm revetment

“…High quality field and numerical investigations are providing new insights into a wide variety of coastal processes and coastal protection solutions 1 , 2 . However, numerical modelling approaches are not yet capable of accurately reproducing all coastal hydro and morphodynamic phenomena, and the difficulties involved in capturing field data in the desired wave, tide and wind conditions mean that controlled laboratory wave flume experiments remain extremely valuable.…”

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).…”