Science Center Objects

Hurricanes and other extreme storms generate storm surge and large waves, eroding the beach and dune system and reshaping the coastal landscape.

During the most extreme events, changes can occur across the width of an entire barrier island. Six types of coastal change observed along the coastlines of the United States are: beach erosion, dune erosion, overwash, inundation and island breaching, marsh erosion, and coastal cliff erosion.

Beach Erosion

beach elevation plotted at cross-shore distance

Figure 1. Beach erosion in Currituck, North Caolina, resulting from Hurricane Isabels's 2003 landfall. The elevation of the beach near the shoreline decreased ~1.5 m and the shoreline retreated ~15 m. (Public domain.)

houses with waves right at their bases

Figure 2. Hurricane Dennis battered the Outer Banks of North Carolina for many days during its 1999 landfall. Large waves caused considerable beach erosion, putting some of the oceanfront homes in this small community in jeopardy of collapse. (Public domain.)

Beach erosion occurs when waves and currents remove sand from the beach system. The loss of sand causes the beach to become narrower and lower in elevation (Figure 1). Storm waves carry the sand offshore, depositing and storing the sediment in large sandbars. In weeks and months following the storm, the sand is returned to the beach by calm-weather waves. Because many states rely on beach-related tourism as a major source of revenue, beach erosion has become a serious concern for coastal counties throughout the United States. Beach erosion also threatens coastal properties and infrastructure, such as roads, homes, and businesses. A series of storms can cause significant retreat of the shoreline, leaving coastal property more vulnerable to future storms (Figure 2).

Dune Erosion

graph of elevation vs cross-shore distance

Figure 3. Dune erosion in Rodanthe, North Carolina resulting from Hurricane Isabel's 2003 landfall. The elevation of the dune decreased by almost a meter. 

(Public domain.)

broken staircase and sand dune

Figure 4. High storm surge and wave runup during Hurricane Isabel (2003) caused widespread dune erosion in Nags Head, North Carolina. 

(Public domain.)

Dune erosion occurs when storm surge elevates waves higher on the beach, allowing them to attack and erode the coastal dune. As sand is removed from the dune, the front face becomes very steep, or scarped. The overall volume and elevation of the dune is reduced due to the erosion process (Figure 3). Dune erosion makes properties behind the dune more vulnerable to future storms and the potential for overwash (Figure 4). The eroded sediment is carried offshore and stored in large sandbars. The sand dune can recover over a period of years, gaining in elevation and volume through deposits of wind-blown sand. Because dunes rebuild over a long period, many coastal counties choose to build temporary dunes following a large storm.

Overwash

elevation plotted against cross-shore distance

Figure 5. During Hurricane Ivan (2004) many beaches in the Panhandle of Florida, such as this location on Santa Rosa Island, Floria, 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. (Public domain.)

sand covers parts of Highway 12

Figure 6. 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. (Public domain.)

mailbox buried in sand

Figure 7. During Hurricane Ivan (2004) overwash deposits buried roads, cars, and mailboxes along Pensacola Beach, Florida. (Public domain.)

Dauphin Island overwash

Figure 8. Dramatic shoreline erosion and large overwash deposits along Dauphin Island during Hurricane Katrina (2005) demonstrate classic barrier island rollover. (Public domain.)

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 5). These fans can bury the first floors of homes, cover roads, fill ponds, and cover coastal vegetation (Figures 6 and 7). 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 8).

Inundation and Island Breaching

elevation plotted agains cross-shore distance

Figure 9. Low elevations along Dauphin Island, Alabama, made portions of this island susceptible to inundation during Hurricane Ivan (2004). (Public domain.)

Inundation occurs when the beach system, or the sandy profile located between the most seaward (primary) dune and the shoreline, is completely submerged under the rising storm surge. Inundation may occur over large stretches of the coasts, with surge covering the beach system along an entire island, or it may be localized to a small region of the coastline. Inundation is most likely to occur on an island that is naturally low in elevation (Fig. 9), although it may also be triggered by the severe erosion and overwash of the coastal dune system. During the period of inundation, currents and waves carry large volumes of sand from the seaward to the landward side of the island. When water levels subside, currents may reverse direction carrying sediment from sounds and bays back to the open ocean. These strong currents may carve a channel in the island, causing the island to be bisected in a process known as island breaching (Fig. 10). Breaches can vary in width from several kilometers to tens of meters (Fig. 11). Smaller breaches often fill with sand in the months following the storm, while larger breaches may become permanent inlets.

Lidar topography of Pine Beach, Alabama

Figure 10. Lidar topography of Pine Beach, Alabama, collected in November 1998 (top) and on September 19, 2004 (bottom), three days following the landfall of Hurricane Ivan to the east in Gulf Shores, AL. Large waves and surge during the storm inundated the beach system leading to barrier island beaching. (Public domain.)

Island breaching on Dauphin Island, Alabama

Figure 11. Island breaching on Dauphin Island, Alabama, during Hurricane Ivan in 2004 (middle panel) was intermittent, extending over 1.35 km along the island. One year later, during Hurricane Katrina's 2005 landfall, the breach widened and became continuous for over 2 km. (Public domain.)

Marsh Erosion

photograph of undeveloped Hackberry Beach shows widespread flooding

Hurricane Rita caused extensive erosion of marshes in western Lousiana. A photograph of undeveloped Hackberry Beach shows widespread flooding of the marshes four days after the storm's September 24, 2005 landfall. (Public domain.)

Marsh erosion occurs along wetlands coastlines that are exposed to the open ocean and wide bays. Storm-induced waves and currents erode the muddy wetlands soil, causing these fragile coastlines to erode significantly, often transforming land area into open water. One recent example of marsh erosion is the loss of over a hundred square miles of land during Hurricanes Katrina and Rita in 2005 (Barras and Johnson, 2006).

For more information about marsh erosion, visit the website of the Wetland and Aquatic Research Center.

Barras, J. and J.B. Johnston, 2006. USGS reports latest land-water changes for southeastern Louisiana, USGS Fact Sheet.