Coastal Processes FAQ - What is overwash?

Overwash is a term used to describe the landward transport of beach sediments across a dune area. Large coastal storms and their associated high winds, waves, and tides can result in overwash of the beach and dune system. During storm conditions, elevated storm tides and high waves may erode beaches and dunes, and the eroded sand can be carried landward by surging water. The sand and water may wash over or break through the dunes, and spill out onto the landward side of the barrier dune. This deposit is usually fan-shaped and therefore is known as an overwash (or washover) fan.

Low-lying areas such as a break in the dune system are particularly vulnerable to overwash. Along developed shoreline areas, the breaks in dune systems are typically found at street ends or pedestrian dune cross-over pathways. When large storm waves and high tides breach the dunes, overwash occurs as sand is transported through the open dune area and onto the street.

In natural beach areas, the overwash of storm waves carries sand from the ocean side of the barrier through the dune to a back dune area. Vegetation usually grows up on the overwash fan, and new dune growth is initiated. This process of landward movement of beach sand is considered vital to the survival of the barrier beach system.

As sea level rises, the sandy beach retreats (moves in a landward direction). In order for a barrier beach to maintain its size and elevation, the back side of a barrier must also move landward. This is accomplished when sand from the front (ocean) side of a barrier is transported to the back (bay) side. In Delaware, this usually occurs by growth of flood tidal shoals or by the process of overwash.