Beach groundwater

Beach morphodynamics not only depends on water flow over the beach surface, but also on subsurface water flow. Surface flow and subsurface flow are related through infiltration and exfiltration. The groundwater table fluctuates at different timescales: the timescale of swash (wave uprush and backwash), the timescale of alternating high and low tidal waters and the seasonal time scale of storm surges and fresh water runoff. The water flow through the beach surface influences the net sediment transport up or down the beach, resulting in accretion or erosion (see Swash zone dynamics). The water circulation associated with the fluctuating groundwater table influences the mixing of seawater and fresh water, with an impact on subsurface seawater intrusion (see Submarine groundwater discharge). This article introduces some principles of beach groundwater dynamics.

Subsurface zones

In this article we consider beach soils which are homogeneous over a depth range of at least the tidal range and the swash excursion. Three zones in the beach subsurface soil can be distinguished:

1. the unsaturated upper beach (also called vadose zone), where the pores (sediment grain interstices) contain air as well as water. The vadose zone extends from the top of the beach surface to the capillary fringe. Water infiltrating in the vadose zone has a pressure head less than atmospheric pressure, and is retained by a combination of adhesion and capillary action.
2. the intermediate capillary fringe (tension-saturated zone), in which groundwater seeps up from the water table by capillary action, completely filling the pores. The thickness of the capillary fringe can be up to about one meter in very fine sediment and not exceed a few centimeters in very coarse sediment.
3. the saturated lower beach (phreatic zone), the zone below the water table where the pressure head equals the atmospheric pressure and where grain interstices are completely filled with water. The pore space, which is a fraction of the grainsize, largely determines the hydraulic conductivity of the soil. Pore water can contain a small fraction of air which strongly influences the transmission depth of pore water pressure fluctuations (see Wave-induced soil liquefaction).

Groundwater response at the tidal time scale

Fig 1. Beach groundwater dynamics at tidal time scale. The vertical scale is strongly exaggerated. Redrawn after Zheng et al. (2024).

Figure 1 shows the groundwater response of a sloping beach on the tidal time scale. On this time scale, sea level can be considered as rising and falling with a nearly horizontal flat surface. The water table rises faster during high tide (rising sea level) than it falls during low tide (falling sea level). Water infiltrates the dry beach face more easily than it drains from the sand matrix, unless the beach consists of very porous coarse material. The tidal mean water table is therefore higher than the mean sea level. Most seawater infiltrates near the high tide mark and exfiltrates near the low tide mark, resulting in a net groundwater circulation from the upper to the lower tidal zone.

Groundwater response at the wave time scale

Fig 2. Beach groundwater dynamics at wave time scale. The vertical scale is strongly exaggerated. Redrawn after Zheng et al. (2024).

Fig. 3. Seven Mile Beach, NSW, Australia, a dissipative beach. The picture shows the shiny seepage beach face after swash rundown. Photo credit: Dr Hannah Power.

Figure 2 illustrates seawater infiltration into the unsaturated beach. Here, the water table and the capillary fringe are lagging behind the wave uprush, a situation that typically occurs during flood. A wetting front forms beneath the uprushing swash lens, resulting in downward infiltration through the unsaturated zone. During backwash, the water table intersects with the beach surface above the retreating backwash lens, forming a seepage face with a shiny appearance where groundwater exfiltrates (Fig. 3). The exit level (intersection of the water table with the beach surface) gradually declines and finally drops below the capillary fringe.

Similarly as with the tide, seawater infiltrates the beach more readily during the uprush stage than it drains during the backwash stage. The resulting elevation of the water table is further enhanced by wave set-up. There is also a net groundwater circulation from the upper to the lower tidal zone in this case, as seawater infiltration is greatest in the upper swash zone, while most exfiltration occurs near the backwash limit – the point where the next incoming wave collapses onto the beach. Beach sand permeability and water saturation greatly influence the water exchange rate across the beach face. In the case of a coarse sandy beach, the unsaturated zone is rapidly filled by the infiltrating seawater. In fine sediment, on the other hand, the wedge-shaped wetting front is kept close to the beach surface for most of the uprush and does not penetrate far into the saturated zone. The wetting front declines during backwash, but this decline is much slower than the backwash retreat (Fig. 4).

Fig. 4. Left: Beach of Belongil (Australia, Google Earth image), which consists of well sorted sand with grainsize $d_{50}$=0.28 mm; the beach slope ≈ 1/50 . Right: Horizontal variation of the groundwater table intersection with the beach face (the exit level, red line) in response to swash excursions (blue line) measured on the beach of Belongil. The average deep water significant wave height and peak period during the experiment were 1.5 m and 12.2 s respectively and the tidal range was 1.3 m. The groundwater infiltration follows the swash uprush onto the unsaturated beach zone, but exfiltration is much slower than the backwash, decoupling the exit point from the instantaneous shoreline. The largest (infragravity) swash events therefore dominate the groundwater dynamics. Figure redrawn from Cartwright et al. (2006).

Related articles

Swash

Swash zone dynamics

Submarine groundwater discharge

Groundwater management in low-lying coastal zones

Beach drainage

Wave-induced soil liquefaction

References

The main author of this article is Job Dronkers Please note that others may also have edited the contents of this article. Citation: Job Dronkers (2025): Beach groundwater. Available from http://www.coastalwiki.org/wiki/Beach_groundwater [accessed on 19-03-2025] For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Dronkers J