Subsurface Flow and Moisture Dynamics in Response to Swash Motions: Effects of Beach Hydraulic Conductivity and Capillarity

Abstract

A combined field and numerical study was conducted to investigate dynamics of subsurface flow and moisture response to waves in the swash zone of a sandy beach located on Cape Henlopen, DE. A density‐dependent variably saturated flow model MARUN was used to simulate subsurface flow beneath the swash zone. Values of hydraulic conductivity (K) and characteristic pore size (α, a capillary fringe property) were varied to evaluate their effects on subsurface flow and moisture dynamics in response to swash motions in beach aquifers. The site‐specific modeling results were validated against spatiotemporal measurements of moisture and pore pressure in the beach. Sensitivity analyses indicated that the hydraulic conductivity and capillary fringe thickness of the beach greatly influenced groundwater flow pathways and associated transit times in the swash zone. A higher value of K enhanced swash‐induced seawater infiltration into the beach, thereby resulting in a faster expansion of a wedge of high moisture content induced by swash cycles, and a flatter water table mound beneath the swash zone. In contrast, a thicker capillary fringe retained higher moisture content near the beach surface, and thus, significantly reduced the available pore space for infiltration of seawater. This attenuated wave effects on pore water flow in the unsaturated zone of the beach. Also, a thicker capillary fringe enhanced horizontal flow driven by the larger‐scale hydraulic gradient caused by tides.

Groundwater Flow and Moisture Dynamics in the Swash Zone: Effects of Heterogeneous Hydraulic Conductivity and Capillarity

“…In this paper, two types of heterogeneous K fields are considered: without and with heterogeneity in capillarity, labeled as heterogeneous K and heterogeneous K and α , respectively. The same homogeneous system simulated in Geng et al (2017) is also presented herein for comparison.…”

“…The time step was automatically adjusted by the model to ensure convergence criteria was met (Courant number < 1). This numerical scheme has been studied and validated in earlier studies of groundwater dynamics in homogeneous sediments in the swash zone (Geng et al, 2017). The parameter values used for the simulations are reported in Table 1.…”

“…The swash zone is an important region on the beach that controls intertidal benthic community structure and biodiversity (Gray et al, 2014; McArdle & McLachlan, 1992). Previous studies have revealed that complex groundwater mixing occurs in the swash zone due to high‐frequency inundation and seawater infiltration by individual waves (Cartwright et al, 2002; Geng et al, 2014, 2017; Geng & Boufadel, 2015; Heiss et al, 2014; Malott et al, 2017; Xin et al, 2010). Swash and tidal action are a primary control on nearshore biogeochemical processes and submarine discharge of various chemical species (e.g., nutrients, carbon, arsenic, and trace elements) into coastal waters (Boehm et al, 2006; Brown & Boehm, 2016; Kim et al, 2017; Lässig, 2019; Mohapatra et al, 2011; Praveena et al, 2012; Rakhimbekova et al, 2018; Waska, 2019).…”

Numerical Study of Solute Transport in Heterogeneous Beach Aquifers Subjected to Tides

“…Tides are an important oceanic forcing, which recirculate groundwater and seawater at the shallow part of coastal aquifers, subsequently forming a so‐called upper saline plume, underneath which terrestrial groundwater discharges near the low tide mark (Boufadel, ; Heiss & Michael, ; Li, Boufadel, & Weaver, ; Mango et al, ; Robinson, Gibbes, et al, ). Compared to tides, waves have been characterized as high‐frequency oscillations whose breaking causes very complicated energy dissipation along beach surface; they, thus, cause high‐frequency seawater‐groundwater recirculation along the swash and surf zones of coastal beaches with large spatial and temporal variations (Bakhtyar et al, ; Geng, Heiss, et al, ; Geng & Boufadel, ; Heiss et al, ; Longuet‐Higgins & Smith, ; Xin et al, ). Evaporation could be also an important driver, altering coastal groundwater flow and causing solute (e.g., salt) accumulation near the beach surface (Geng, Boufadel, & Jackson, ; Geng & Boufadel, ).…”

Biological Cohesion as the Architect of Bed Movement Under Wave Action

“…Under the wave trough the bed rebounds, and the pore pressure reduces (Figure 3a and 3b, right). Seepage flow induces an upward drag on the grains due to the pressure gradient, passing through the pores where the hydraulic conveyance of the bed is reduced by EPS (Figure 3c, right) (Geng et al., 2017; Volk et al., 2016). EPS bridges would be further destroyed due to the relative displacement.…”