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Hurricane Gustav
Initial Assessment of Potential Coastal-Change Impacts
Posted: 08/30/2008 9pm
Note: The initial (pre-storm) assessment of coastal-change impacts is done by comparing pre-storm dune/berm elevations to maximum surge elevations obtained from model simulations of hypothetical storm scenarios.
The potential for inundation and associated coastal change during Hurricane Gustav was assessed for five focus areas located within the August 30, 2008 cone of uncertainty for hurricane landfall: Galveston Bay, Chenier Plain (southwest coast of LA), Isles Dernieres, Chandeleur Islands, and Dauphin Island.
Location map for five areas of focus. Click on any of the five areas to see the potential inundation map for that area. |
Predicted track of Hurricane Gustav from National Hurricane Center Aug 30, 2008, 1100EST update. [larger version] |
Hurricane landfall and associated elevated water levels, waves, and currents can lead to severe coastal change through erosion and re-deposition. The potential for coastal change due to hurricanes is predicted using a USGS-developed storm-impact scale that compares predicted elevations of hurricane-induced water levels to known elevations of coastal topography to define four coastal change regimes. These regimes describe the dominant interactions between beach morphology and storm processes and the resulting modes of coastal change along beaches that serve as the "first line of defense" for many coasts exposed to hurricanes. The most extreme coastal change regime is associated with inundation, where the elevation of modeled storm surge exceeds the elevation of the primary dune or beach berm. Under these conditions the beach system is completely submerged. Typically, larger magnitudes of dune erosion, shoreline retreat, and beach erosion will occur during inundation. On barrier islands, inundation allows strong currents to cross the island, potentially creating new breaches.
The potential inundation, I, of the beach system was defined by calculating the difference between the elevations of SLOSH-modeled storm surge (Maximum of the Maximum Envelope of Water values at the shoreline) and lidar-derived dune/berm crests. Positive values (reds) indicate that water levels are predicted to be higher than the dune crests and that the beach is more likely to be inundated by storm surge during the direct landfall of a hurricane. Negative values (blues) signify areas where water levels are predicted to be lower than the dune crest. These estimates assume landfall at mean astronomical tide and do not include the effects of wave setup. Landfall at high tide and significant wave setup would increase the potential for inundation. Additionally, the maps represent the vulnerability of the beach system as it was during the time of the pre-storm lidar survey. Changes to beach morphology between the survey date and the date of hurricane landfall can affect the predicted potential inundation.
When interpreting the potential inundation maps for each region, it is important to remember that they describe only one scenario (inundation) that is relevant to coastal change due to the approaching hurricane. Coastal change due to other scenarios (collision and overtopping) can be extreme even if there is not inundation. Additionally, the map represents a worst-case scenario with respect to storm surge levels. This analysis assumed hurricane landfall immediately to the west of a specific location; consequently, the presented inundation potential is localized and will not occur along the entire stretch of coast for a single storm landfall.
Galveston Bay, TX
The beach and dunes near Galveston are variable in elevation and physical description. The city of Galveston, to the south of the larger inlet on the map, is armored by a 16 km-long seawall and other structures, offering protection from both inundation and coastal change. These structures were built after the 1900 hurricane that inundated the once low-lying city and killed thousands of people. Elsewhere along the coast, the elevations are substantially lower. The level of inundation and coastal change in the area near Galveston will be variable if Gustav comes ashore near this location as a Category 2 hurricane or higher.
Initial assessment of inundation potential (I, storm surge minus dune elevation) for the barrier island beaches near Galveston Bay, TX for the direct landfall of Category 1-5 hurricanes. Reds indicate areas where NOAA SLOSH-modeled surge exceeds the elevation of the primary dune, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicate that the expected surge elevation is lower than the dune crest and that the potential for severe coastal change is lower than in areas likely to inundate. (Dune/berm crests measured from a September 2005 lidar survey.) [larger version] |
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Western Louisiana Chenier Plain
The Chenier Plain consists of low beaches and dunes backed by a broad coastal marsh and grasslands. The very low elevations along this coast offer little protection for hurricane surge and waves and the Chenier Plain beach system and coastal infrastructure may be completely inundated by storm surge. During Hurricane Rita (2005) entire communities that were located well inland from the eroding beach were wiped out, not due to erosion or other coastal change, but due to direct attack by waves and surge that moved inland over the low beach and dunes. If Gustav makes landfall nearby as a Category 1 hurricane or higher, nearly all of the Chenier Plain is expected to experience inundation and extensive coastal change.
Initial assessment of inundation potential (I, storm surge minus dune elevation) for the Chenier Plain of southwest Louisiana for the direct landfall of Category 1-5 hurricanes. Reds indicate areas where NOAA SLOSH-modeled surge exceeds the elevation of the primary dune, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicate that the expected surge elevation is lower than the dune crest and that the potential for severe coastal change is lower than in areas likely to inundate. (Dune/berm crests measured from a September 2005 lidar survey.) [larger version] |
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Isles Dernieres, LA
The Isles Dernieres consist of narrow barrier islands that have long been a site of concern for Louisiana due to severe and chronic erosion that was diminishing the islands. If the islands were to disappear, the mainland would be exposed to larger waves and storm surge. In the late 1990's, most of the islands were restored to be both higher and wider. Prior to restoration, the islands rose only 1-1.5 m high; after restoration, some were rebuilt to 2.4 m, an elevation sufficient to contain a Category 1 surge (area classified as blue on the map). If Gustav makes landfall nearby as a Category 3 hurricane or higher, nearly all of the Isles Dernieres are expected to experience inundation and extensive coastal changes.
Initial assessment of inundation potential (I, storm surge minus dune elevation) for the Isles Dernieres, LA for the direct landfall of Category 1-5 hurricanes. Reds indicate areas where NOAA SLOSH-modeled surge exceeds the elevation of the primary dune, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicate that the expected surge elevation is lower than the dune crest and that the potential for severe coastal change is lower than in areas likely to inundate. (Dune/berm crests measured from a 2006 lidar survey.) [larger version] |
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Chandeleur Islands, LA
The Chandeleur Islands are extremely low barriers consisting of a mixture beach, low dunes, and marsh. Dunes in this chain were destroyed by severe erosion during 2005's Hurricane Katrina. The islands were completely inundated and nearly all the sand was removed from the beaches and dunes, leaving just fragments of the back-barrier marshes. Some recovery of low beaches has occurred as some island marshes continued to erode; but only modest growth of dunes has occurred, leaving this island chain with very low elevations. If Gustav comes ashore near this location as a Category 1 hurricane or higher, inundation and extreme coastal change are expected over the entire area.
Initial assessment of inundation potential (I, storm surge minus dune elevation) for the Chandeleur Islands, LA for the direct landfall of Category 1-5 hurricanes. Reds indicate areas where NOAA SLOSH-modeled surge exceeds the elevation of the primary dune, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicate that the expected surge elevation is lower than the dune crest and that the potential for severe coastal change is lower than in areas likely to inundate. (Dune/berm crests measured from a June 2008 lidar survey.) [larger version] |
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Dauphin Island, AL
The western half of Dauphin Island has beach elevations barely reaching over 2 m elevation. The eastern half of the island generally has higher dunes but with considerable spatial variability. During Hurricane Katrina (2005) parts of the western end of the island were inundated by storm surge and a new inlet was cut. In the center of the island, extreme overwash occurred, transporting sand across the island into the Mississippi Sound. This was a classic example of the process of barrier island migration. If Gustav comes ashore near this location as a Category 2 hurricane or higher, inundation and extreme coastal change are expected on western Dauphin Island. Inundation is expected in some locations of eastern Dauphin Island for Category 3, and total inundation of the beach system is expected only during Category 5 storms.
Initial assessment of inundation potential (I, storm surge minus dune elevation) for Dauphin Island, AL for the direct landfall of Category 1-5 hurricanes. Reds indicate areas where NOAA SLOSH-modeled surge exceeds the elevation of the primary dune, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicate that the expected surge elevation is lower than the dune crest and that the potential for severe coastal change is lower than in areas likely to inundate. (Dune/berm crests measured from a June 2008 lidar survey.) [larger version] |
Disclaimer: This experimental product is based on research results of the USGS National Assessment of Coastal Change Hazards Project and is intended to indicate the potential for coastal change due to inundation caused by storm surge only. This product is based on an analysis that simplifies the problem to include some of the most important aspects (estimated dune elevations and predicted surge levels). This product does not directly consider potential property damage or the impacts of high wind speeds and heavy rainfall. This product applies to open coast environments and does not consider potential coastal change along bays, passes, or inland lakes. The actual changes that occur during extreme storms are complex functions of a number of processes and variables including ocean waves, currents, and tides. The public should not base evacuation decisions on this product. Citizens should always heed the evacuation advice of local emergency management authorities.
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U.S. Department of the Interior |
U.S. Geological Survey
