Sandy beaches and barrier islands along the northern Gulf of Mexico are highly vulnerable to beach and dune erosion as Hurricane Isaac makes landfall this week, according to a new U.S. Geological Survey assessment. The projections also show which coastal areas may see storm-surge topping sand dunes and beaches.
Probabilities for coastal change have been calculated for parts of the Louisiana, Mississippi, and Alabama coastlines and for the Florida Panhandle.
On Saturday, the USGS released an assessment of expected coastal change in Florida, but the change in the storm’s path has reduced the concern for erosion of west central Florida beaches, shifting focus primarily to sandy beaches and barrier islands from Florida’s Panhandle to Louisiana.
Beaches and dunes can serve as a first line of defense for residents on coasts exposed to these types of storms. During tropical storms and hurricanes, elevated water levels and waves can lead to dramatic coastal change. According to the latest USGS research, during even the weakest hurricanes, wave height and storm surge combine to increase water levels along the Gulf Coast shoreline by 14 and a half feet higher than their normal levels.
"We can now combine more accurate coastal data with powerful new scientific models to provide the very best predictions of hurricane surge, waves, and erosion," said USGS Director Marcia McNutt. "USGS scientists used the latest NOAA forecasts to ensure that the people of the Gulf Coast are not surprised by the impacts of Isaac."
Using a USGS-developed model, scientists incorporated measurements of beach elevation and wave and surge forecasts from the National Oceanic and Atmospheric Administration to determine the probabilities of collision, overwash and inundation during the coming storm.
Collision is when waves attack the base of dunes and cause dune-front erosion. Overwash occurs when waves and storm surge overtop dunes and transport sand landward. Inundation, the most extreme of the three, occurs when increased water levels completely submerge beaches and dunes.
"Earlier this year, USGS released a report detailing hurricane induced erosion hazards for category 1-5 storms. The predictions released this week are based on the same methods, but tailored to waves and storm surge specific to Hurricane Isaac," said Hilary Stockdon, USGS research oceanographer. "As the storm moves across the Gulf, we use real-time storm and wave forecasts from NOAA to determine where beach erosion and inundation are most likely to occur."
These predictions, available on the USGS Hurricanes and Extreme Storms website, show the probable impacts of Isaac at each location along the coast. This assessment may be updated if there are significant changes in the storm’s projected track or strength. Based on the current assessment, the percentages of each type of coastal change were estimated for each of the following states:
- Beach and dune erosion is very likely for 66 percent of Louisiana's sandy beaches
- 29 percent of the sandy beaches in this area are very likely to experience overwash.
- 18 percent of the Louisiana's barrier islands are expected to be inundated; however, the majority of this inundation will occur only on Louisiana's Chandeleur Islands.
- 100 percent of the Chandeleur Islands are very likely to experience overwash and 83 percent of these islands are very likely to be inundated. This beach system was devastated during Hurricane Katrina, making it more vulnerable to-low intensity storms.
- Beach and dune erosion is very likely for 89 percent of Mississippi's sandy beaches and barrier islands.
- 20 percent of the sandy beaches are very likely to experience overwash.
- Inundation of the beach system is not expected.
- Beach and dune erosion is very likely for 52 percent of Alabama's sandy beaches and barrier islands.
- 11 percent of the sandy beaches are very likely to experience overwash.
- 3 percent of the sandy beaches are very likely to be inundated
- Beach and dune erosion is very likely for 32 percent of the Panhandle's sandy beaches
- 2 percent of the sandy beaches are very likely to experience overwash.
- Inundation of the beach system is not expected.
After the worst of the storm passes, USGS may send teams to the field to measure the impact of Hurricane Isaac on the coastline.
Aerial photography and elevation surveys of post-storm beach conditions are used to document the impacts of hurricane waves and currents on the beach. Information obtained from the surveys allows scientists to discern the degree of changes to beaches and coastal environments and determine how much the land has eroded and where new inlets have cut through. The newly acquired data will also be used to make more accurate predictive models of future coastal impacts from severe storms and identify areas vulnerable to extreme coastal change.
The areal photos can visually demonstrate the severe impacts of hurricane surge on beaches. This photo shows the effects that hurricane waves and surge have had on Dauphin Island, Alabama from 2004 -2008.
In May, the USGS released a report assessing probability of coastal change for the Gulf Coast when facing hurricanes of different strengths. The report included an interactive map that allows users to focus on different parts of the Gulf Coast shoreline to view how the probability of erosion, caused by waves and storm surge, will vary depending on hurricane intensity.