Coastal Change Hazards: Hurricanes and Extreme Storms

Storm-Induced Coastal Change

Overwash

When storm-induced waves exceed the height of the dune, sand is transported over top of the dune and deposited inland. This process, known as overwash, causes a significant change in the landscape of the island.  During the overwash process, portions of the dune are often completely eroded, and sand is deposited inland in large layers called overwash fans (Figure 1). These fans can bury the first floors of homes, cover roads, fill ponds, and cover coastal vegetation (Figures 2 and 3). Depending on the severity of the storm and the width of the island, these overwash fans can extend the entire width of the island. When sand is transported from the seaward to the landward side of the island, the island migrates landward in a process known as barrier island rollover (Figure 4).

Figure 1. During Hurricane Ivan (2004) many beaches in the Panhandle of Florida, such as this location on Santa Rosa Island, FL, were overwashed by large waves and surge. Sand was eroded from the frontal dune, transported landward, and deposited in overwash fans on the sound-side of the island. [larger version]	Figure 2. During Hurricane Isabel (2003), large amounts of sand were removed from the beach and transported inland. These overwash deposits covered parts of Highway 12 along the Outer Banks of North Carolina with as much as meter of sand. [larger version]
Figure 3. During Hurricane Ivan (2004) overwash deposits buried roads, cars, and mailboxes along Pensacola Beach, FL. [larger version]	Figure 4. Dramatic shoreline erosion and large overwash deposits along Dauphin Island during Hurricane Katrina (2005) demonstrate classic barrier island rollover. [larger version]

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