Swash

Swash, or forewash in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken.

The swash action also plays an important role as one of the instantaneous processes in wider coastal morphodynamics.

Which type of swash motion prevails is dependent on the wave conditions and the beach morphology and this can be predicted by calculating the surf similarity parameter

Infiltration (hydrology) and sediment transport by swash motion are important factors that govern the gradient of the beachface.

[4] The beachface is the planar, relatively steep section of the beach profile that is subject to swash processes (Figure 2).

The beachface is in dynamic equilibrium with swash action when the amount of sediment transport by uprush and backwash are equal.

The berm protects the backbeach and coastal dunes from waves but erosion can occur under high energy conditions such as storms.

The beach steps generally comprise the coarsest material and the height can vary from several centimetres to over a metre.

Beach steps form where the backwash interacts with the oncoming incident wave and generate vortex.

Currently there are two theories that provide an adequate explanation for the formation of the rhythmic beach cusps: standing edge waves and self-organization.

It is relatively recent that the computational resources and sediment transport formulations became available to show that the stable and rhythmic morphological features can be produced by such feedback systems.

[4] The beach cusp spacing, based on the self-organization model, is proportional to the horizontal extent of the swash motion S using the equation

The cross-shore sediment exchange, between the subaerial and sub-aqueous zones of the beach, is primarily provided by the swash motion.

[4] The onshore and offshore sediment transport by swash thus plays a significant role in accretion and erosion of the beach.

The uprush, which is mainly dominated by bore turbulence, especially on steep beaches, generally suspend sediments to transport.

The flow velocity increases towards the end of the backwash causing more bed-generated turbulence, which results in sediment transport near the bed.

The direction of the net sediment transport (onshore or offshore) is largely governed by the beachface gradient.

Many other factors, including human activities and climate change, can also influence the morphodynamics in the swash zone.

Construction of sea walls has been a common tool to protect developed property, such as roads and buildings, from coastal erosion and recession.

Building a seawall can raise the water table, increase wave reflection and intensify turbulence against the wall.

[8] Boulder ramparts (also known as revetments or riprap) and tetrapods are less reflective than impermeable sea walls, as waves are expected to break across the materials to produce swash and backwash that do not cause erosion.

Rocky debris is sometimes placed in front of a sea wall in the attempt to reduce the wave impact, as well as to allow the eroded beach to recover.

[9] Understanding the sediment transport system in the swash zone is also vital for beach nourishment projects.

[9] Understanding and prediction of the sediment movements, both in the swash and surf zone, is vital for the nourishment project to succeed.

The coastal management at Black Rock, on the north-east coast of Phillip Bay, Australia, provides a good example of a structural response to beach erosion which resulted in morphological changes in the swash zone.

This resulted in depletion of the beach in front of the sea wall, which was damaged by repeated storms in winter time.

The project had failed to take the seasonal patterns of longshore drift into account and had underestimated the amount of sand to nourish with, especially on the southern part of the beach.

For wave run-up measurements, for example, Guza and Thornton (1981, 1982) used an 80m long dual-resistance wire stretched across the beach profile and held about 3 cm above the sand by non-conducting supports.

Holman and Sallenger (1985) conducted run-up investigation by taking videos of the swash to digitise the positions of the waterline over time.

However, the gaps in understanding still remain in swash research including turbulence, sheet flow, bedload sediment transport and hydrodynamics on ultra-dissipative beaches.