Revisiting Surface-Subsurface Exchange at Intertidal Zone with a Coupled 2D Hydrodynamic and 3D Variably-Saturated Groundwater Model

Zhi Li,Ben R Hodges

doi:10.3390/w13070902

Abstract

A new high-performance numerical model (Frehg) is developed to simulate water flow in shallow coastal wetlands. Frehg solves the 2D depth-integrated, hydrostatic, Navier–Stokes equations (i.e., shallow-water equations) in the surface domain and the 3D variably-saturated Richards equation in the subsurface domain. The two domains are asynchronously coupled to model surface-subsurface exchange. The Frehg model is applied to evaluate model sensitivity to a variety of simplifications that are commonly adopted for shallow wetland models, especially the use of the diffusive wave approximation in place of the traditional Saint-Venant equations for surface flow. The results suggest that a dynamic model for momentum is preferred over diffusive wave model for shallow coastal wetlands and marshes because the latter fails to capture flow unsteadiness. Under the combined effects of evaporation and wetting/drying, using diffusive wave model leads to discrepancies in modeled surface-subsurface exchange flux in the intertidal zone where strong exchange processes occur. It indicates shallow wetland models should be built with (i) dynamic surface flow equations that capture the timing of inundation, (ii) complex topographic features that render accurate spatial extent of inundation, and (iii) variably-saturated subsurface flow solver that is capable of modeling moisture change in the subsurface due to evaporation and infiltration.

Highlights

Fine Resolution Environmental Hydrodynamic and Groundwater (Frehg) is exercised over a coarse-resolution model of the Trinity River Delta (Texas, USA) in Section 4.4 to examine the combined effects of evaporation, tide-/wind-induced wetting/drying and surface-subsurface exchange in a large-scale simulation
Section 2), these results indicate that the pressure gradient and bottom friction are wellbalanced and the inertia terms are relatively unimportant for this rainfall-runoff case
A coupled surface-subsurface flow model (Frehg) is developed to understand the effects of various model simplifications that are often adopted in shallow marsh simulations

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Coastal aquifer models are often built in the two-dimensional (2D) vertical (x-z) plane, neglecting the influence of complex topography in the x-y plane (e.g., [4,5,6,7]). The DW approximation neglects the inertia terms in conservation of momentum, which is reasonable for flow varying over large spatial and long time scales [13,14]. Topography of coastal marshes has variability over a wide range of spatial scales [15], which makes it difficult to prove that large scales are dominant. Blade [16] noted that the DW method could be slow at fine grid resolution because of more restrictive stability criteria. Similar findings are reported in Neal et al [17]

Methods

Results

Conclusion