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Abstract The interactions between the atmosphere, ocean, and beach in the swash zone are dynamic, influencing water flux and solute exchange across the land‐sea interface. This study employs groundwater simulations to examine the combined effects of waves and evaporation on subsurface flow and salinity dynamics in a shallow beach environment. Our simulations reveal that wave motion generates a saline plume beneath the swash zone, where evaporation induces hypersalinity near the sand surface. This leads to the formation of a hypersaline plume beneath the swash zone during periods of wave recession, which extends vertically downward to a maximum depth of 30 cm, driven by the resulting vertical density gradients. This hypersaline plume moves approximately 2 m landward to the top of the swash zone and down the beachface due to wave‐induced seawater infiltration and is subsequently diluted by the surrounding saline groundwater. Furthermore, swash motion increases near‐surface moisture, leading to an elevated evaporation rate, with dynamic fluctuations in both moisture and evaporation rate due to high‐frequency surface inundation caused by individual waves. Notably, the highest evaporation rates on the swash zone surface do not always correspond to the greatest elevations of salt concentration within the swash zone. This is because optimal moisture is also required—neither too low to impede evaporation nor too high to dilute accumulated salt near the surface. These insights are crucial for enhancing our understanding of coastal groundwater flow, biogeochemical conditions, and the subsequent nutrient cycling and contaminant transport in coastal zones.
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This content will become publicly available on March 1, 2026
Groundwater Hydrodynamic Oscillations From Swash With Transparent Sand (GHOSTS)
Abstract Interactions between surface flows and groundwater in beaches can influence erosion and accretion, wave overtopping, groundwater levels and salinization, and transport of nutrients and pollutants. Laboratory experiments using transparent crushed quartz and optically matched mineral oil as proxies for sand and water allow the degree of saturation to be computed at pore‐scale (0.7 mm resolution) enabling detailed investigations of the wave runup driven infiltration into a beach in a wave flume for a range of slopes and flow boundary conditions. The evolution of the wetting front resulting from wave runup on an initially unsaturated beach is described in detail, including the formation of an infiltration wedge in the subsurface of the swash zone and the wave‐driven rise in fluid elevation inside the beach. The elevation of the runup for each event is found to be related closely to the saturation of the beach face, reaching an equilibrium state once the subsurface in the swash zone reaches capacity. The back wall boundary condition in the flume has a significant role in how subsurface flows increase saturation within the beach, especially with boundary head elevations greater than the initial phreatic surface. The results of these novel experimental observations are used to develop dimensionless relationships between the surface wave runup and the subsurface saturation rates. To improve monitoring and interpretation of future coastal groundwater studies, three distinct cross‐shore regimes are defined for assessing change in subsurface fluid elevation in the beach.
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- Award ID(s):
- 1933010
- PAR ID:
- 10586414
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 130
- Issue:
- 3
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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