Surface-groundwater flow numerical model for barrier beach with exfiltration incorporated bottom boundary layer model

Abstract

A surface-groundwater flow model is developed for the swash flow on a barrier beach. The non-linear shallow water equations are used to simulate the surface flow. Laplace's equation is used to describe the groundwater flow and is solved using the Boundary Integral Equation Method to provide potential heads and normal potential derivatives at and within the boundaries of the barrier. An exfiltration incorporated bottom boundary layer sub-model is used to obtain bed shear stress. The groundwater model is verified against the numerical test results in Kazemzadeh-Parsi and Daneshmand (2012) for the groundwater flow through a rectangular dam. The coupled surface-groundwater model is validated against the prototype-scale BARDEX II experimental results (Turner et al., 2016). The steady-state groundwater flow comparisons show excellent agreement in phreatic surfaces. The comparisons of groundwater flow under the action of waves show reasonably good agreement with experimental results in phreatic surfaces. The simulated time averaged pore velocities for the runs with and without waves are in satisfactory agreement with experimental results in general, and certain discrepancies are observed near the beach side. The bed shear stress variation due to exfiltration is investigated by incorporating the modified Highlights • A coupled surface-groundwater flow numerical model is developed and validated against the BARDEX II experimental results.• Seepage under a moving bore shows alternate exfiltration and infiltration before and after the bore front respectively.• The effects of exfiltration on the bottom boundary layer evolution and bed shear stress are investigated.

A Richards’ equation-based model for wave-resolving simulation of variably-saturated beach groundwater flow dynamics

“…Thereby, a circulation is established with infiltration in the upper part of the swash zone and exfiltration in the lower part. This confirms the observations of Turner et al [9], supported by Perera et al [27], which show that wave action is powerful enough to rule the groundwater dynamics in this area (see also Sous et al [10] for similar observations on the field). As noted experimentally by Turner et al [9] and simulated by the present model, the cases A4 and A6 show greater seaward hydraulic gradients than the case A2, see the tightened equipotential lines in Fig.…”

“…This is observed both in numerical and experimental results but the numerical models tends to overestimate the vertical component compared to the experiments. The Darcy-based numerical model from Perera et al [27] led to the same discrepancy. They suggest that the experimental velocity component is not as strong because of localized unsaturated conditions which decrease the permeability near the beach surface.…”

“…in the case of "wave" conditions, the same initial condition can be used [27] but it has been found that the Richards-based model does not provide good results after one run of 300 s because it is very sensitive to the initial unsaturated state. Thereby, the initial state of the BARDEX II time periods was reproduced numerically by launching successively 11 runs of 300 s. The final numerical solution of one run was kept to initialize the next one.…”

“…The two other processes involved in sediment transport modification by in/exfiltration flows through the beach face are, on one hand, the modification of the boundary layer shear stress (the "ventilated bed" effect [13,14,15,16]) and, on the other hand, the change of the relative weight of sediment. Significant research efforts have been engaged to estimate and model the net effect of these processes on sediment dynamics [17,18,19,20,21,22,23,24,25,26,27].…”

“…Similar assumptions of purely horizontal flows were made in the coupled approach using Boussinesq model for wave propagation and Darcy-Forchheimer within the porous medium [22,32]. Finer vertically-resolving model have been proposed based on the Darcy-related 2D Laplace law to model groundwater dynamics forced by tides and waves [33,19] and further refined to describe in/exfiltration bed exchanges [27]. The groundwater circulation cell highlighted by coupled VOF-RANS model [24] has been later confirmed by laboratory [28] and field [10] experiments.…”