overwash

Overwash is a coastal geomorphology term that describes the flow of water and sediment from the ocean over the crest of a beach or dune and onto the landward side. It typically occurs during storms, high tides, or hurricane-driven surge when waves, runup, and surge overtopp coastal barriers, transporting sand, shell, and debris landward.

Mechanisms and features: Overwash arises when wave runup and surge exceed the height of dunes or beach barriers, causing overtopping. The landward transport of sediment creates washover deposits or fans on the backshore, and can widen or breach dunes. Repeated overwash can alter barrier island morphology, reconnect backbarrier basins, and modify sediment budgets. Overwash can occur without complete dune breach, but breaches are common in intense events.

Impacts: Overwash erodes dunes and can compromise coastal defenses, increasing vulnerability to flooding during storms. It reshapes the shoreline and barrier dynamics, sometimes creating new backshore habitats while degrading dune-dependent ecosystems. It also moves nutrients and organic material landward, influencing local ecology. In inhabited coastlines, overwash events pose hazards to infrastructure and communities, though they are also part of natural barrier island evolution.

Significance: In geology and geomorphology, overwash deposits serve as records of past storm activity and coastal change. For coastal management, understanding overwash informs dune restoration, beach nourishment, and sea-level rise adaptation strategies, helping to balance resilience with natural barrier dynamics.