Swash

Did You Know?

• The direction of the uprush varies with prevailing wind, while the backwash is always perpendicular to the coastline.
• Uprush generally has higher velocity and shorter duration than backwash.

Swash morphology

The swash zone lies between the backbeach and surf zone, where intense erosion occurs during storms, and is alternately wet and dry. Infiltration above the water table and exfiltration below it occur between the swash flow and the beach groundwater. Key morphological features include the beachface, berm, beach step, and beach cusps. The beachface is a planar, relatively steep section extending from the berm to low tide level, in dynamic equilibrium when uprush and backwash sediment transport are equal.

A flatter beachface leads to net onshore transport, while a steeper one results in net offshore transport. The equilibrium gradient depends on sediment size, permeability, fall velocity, wave height, and wave period, and cannot be isolated from surf zone processes. The berm is the planar part where sediment accumulates at the landward limit of swash motion, protecting the backbeach and dunes but eroding under high-energy conditions. Berms are more defined on gravel beaches, and their height is governed by the maximum elevation of sediment transport during uprush, predicted by the equation of Takeda and Sunamura.

Sediment transport

Swash motion provides the primary cross-shore sediment exchange between subaerial and sub-aqueous zones, with transport rates much higher than in the surf zone and suspended sediment concentrations exceeding 100 kg/m³ near the bed. The uprush, dominated by bore turbulence especially on steep beaches, suspends sediments with greatest flow velocities and concentrations at its start, then dissipates turbulence toward the end. The backwash is dominated by sheet flow and bedload transport, with flow velocity increasing toward its end and generating bed turbulence. Net sediment transport direction is largely governed by beachface gradient. Longshore drift occurs due to beach cusp morphology or oblique waves, and when no slack-water phase exists during backwash, sediments can remain suspended and move offshore.

Management

The swash zone is highly dynamic and susceptible to human activities, with at least 100 million people living within one meter of mean sea level. Understanding swash processes is vital for coastal communities facing erosion and storm surge, but these processes cannot be considered in isolation from surf zone processes, human activities, and climate change. Seawalls are commonly built to protect property, but they often fail to retain the beach; impermeable structures can raise the water table, increase wave reflection, and intensify turbulence, leading to adjacent beach erosion or structural failure.

Boulder ramparts, revetments, riprap, and tetrapods are less reflective, as waves break across the materials to produce swash and backwash that do not cause erosion. Rocky debris placed in front of seawalls can reduce wave impact and allow beach recovery. Understanding sediment transport in the swash zone is also essential for beach nourishment projects, as swash distributes added sand; past failures have occurred due to inadequate understanding.

Research

Research and field measurements in the swash zone are challenging due to its shallow, aerated environment with rapid unsteady flows, though accessibility allows high-resolution data. Irregularity of data has impeded analysis and theory-observation comparisons. Many studies involved engineering structures to establish design criteria against overtopping. Since the 1990s, researchers such as Hughes M.G., Masselink J., and Puleo J.A. have advanced understanding of swash hydrodynamics, including turbulence, flow velocities, groundwater interaction, and sediment transport. Gaps remain in understanding turbulence, sheet flow, bedload transport, and hydrodynamics on ultra-dissipative beaches.