Beach drainage or ‘beach face dewatering’ involves the localized lowering of the water table beneath and parallel to the beach face. This has been demonstrated to cause accretion of sand above the installed drainage system. Sand is in continual movement on a wet beach face due to wave and tidal action in the ‘swash’ zone. Under specific conditions, beach drainage systems can halt beach erosion and promote sand accretion by adjusting the dynamic equilibrium that exists on sand beaches.

The accretion or erosion of a beach is influenced by a number of hydrodynamic forces in a beach surf zone. The effects and interaction of these sediment transport mechanisms have been studied since the 1940’s. It is well understood that lowering the water table in granular soils improves their stability and eliminates the tendency for them to move (ie, ‘well-pointing’). A number of theories have been proposed to explain the empirical evidence for sand deposition from beach drainage (ie, backwash reduction, seepage reduction, liquefaction reduction) and these continue to be studied around the world.

It is evident that lowering the water table in the beach face eliminates buoyancy factors and reduces the lubricating effect between the grains, restoring the frictional characteristics of the sand. In addition, the percolation of 'swash water' into the beach means less backwash energy, which encourages suspended sand to settle out on the beach face.

Cross-section. Water table lowered by beach drain (not to scale)

A useful side effect of the system is that the collected seawater is very pure because of the sand filtration effect. It may be discharged back to sea but can also be used to oxygenate stagnant inland lagoons or used as feed for heat pumps, desalination plants, land-based aquaculture, aquariums or seawater swimming pools.

This is achieved by installing a drainage system in the beach that lowers the beach face water table, intercepting the flow of swash, tidal and inland ground water. Collection pipes are buried in the beach parallel to the coastline to create an unsaturated zone beneath the beach face. This unsaturated zone is achieved by draining the seawater away by gravity to a collector sump and pumping station. The sump and buried pumping station can be located at the back of the beach, where they are not readily visible. A typical pumping station might consist of two submersible electric pumps located in a buried concrete chamber. The only visible feature of the system may be the pump station control panel that regulates and monitors the pumps, sends data and receives control signals.

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