Coupled surface‐subsurface hydrologic measurements reveal infiltration, recharge, and discharge dynamics across the swash zone of a sandy beach

Abstract

Swash‐groundwater interactions affect the biogeochemistry of beach aquifers and the transport of solutes and sediment across the beachface. Improved understanding of the complex, coupled dynamics of surface and subsurface flow processes in the swash zone is required to better estimate chemical fluxes to the sea and predict the morphological evolution of beaches. Simultaneous high‐frequency measurements of saturation, water table elevation, and the cross‐shore locations of runup and the boundary between the saturated and unsaturated beachface (surface saturation boundary) were collected on a sandy beach to link groundwater flow dynamics with swash zone forcing. Saturation and lysimeter measurements showed the dynamic response of subsurface saturation to swash events and permitted estimation of infiltration rates. Surface and subsurface observations revealed a decoupling of the surface saturation boundary and the intersection between the water table and the beachface. Surface measurements alone were insufficient to delineate the infiltration and discharge zones, which moved independently of the surface saturation boundary. Results show for the first time the motion and areal extent of infiltration and recharge zones, and constrain the maximum size of the subaerial discharge zone over swash and tidal time scales. The width of the infiltration zone was controlled by swash processes, and subaerial discharge was controlled primarily by tidal processes. These dynamics reveal the tightly coupled nature of surface and subsurface processes over multiple time scales, with implications for sediment transport and fluid and solute fluxes through the hydrologically and biogeochemically active intertidal zone of sandy beaches.

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

“…The simulation results are compared to the field data measured in Heiss et al (2015). Then swash-induced pore pressure transmission, spatial and temporal moisture variation, subsurface flow pathways, seawater infiltration, and particle transport are quantified.…”

“…Measurements were conducted between 5 and 16 Hz depending on the sensors for 8 h starting from the rising tide to the following ebb tide. Further details of the field measurements can be found in Heiss et al (2015).…”

“…The saturated hydraulic conductivity was estimated at 2.4 3 10 24 m/s based on the grain size analysis using the Kozeny-Carman equation (Bobo et al, 2012). The height of the capillary fringe was estimated at 20 cm based on the field observations (Heiss et al, 2015). The estimation value used in the MARUN model allowed close matching of measurements of pore pressure and moisture in the field (discussed below).…”

“…However, the mechanisms of pore water flow through the capillary fringe and corresponding vertical and horizontal pressure transmission have not been investigated. Heiss et al (2014Heiss et al ( , 2015 investigated moisture dynamics beneath the swash zone and infiltration across the unsaturated beachface at two sandy beaches at Cape Henlopen, DE, USA. It was found that water content in the unsaturated region of the swash zone responded to wave runup and swash infiltration, with a rapid rise in water content followed by a slower decline.…”

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

“…Apart from this study and a few others including laboratory experiments [Boufadel et al, 2007;Heiss et al, 2015;Malott et al, 2016;Sous et al, 2016;Turner et al, 2016;Malott et al, 2017], investigations of wave-induced water exchange and groundwater dynamics have mostly been based on numerical simulations due to the challenge of measuring wave-induced groundwater flows in the field, including how to decouple wave and tide effects. Simulation of wave hydrodynamics coupled with densitydependent groundwater flow is computationally challenging but has been attempted by Bakhtyar et al [2012] and Geng et al [2014], who solved the Reynolds-Averaged Navier-Stokes (RANs) equations to simulate wave motion across a permeable beach surface.…”