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Hurricane Ike
Initial Assessment of Potential Coastal-Change Impacts
Posted: 09/10/2008 5:30pm
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 Ike was assessed for five focus areas located within the region expected to be affected by hurricane landfall: South Padre Island, TX; Matagorda Island, TX; Galveston, TX; Chenier Plain (coast of southwest LA); and Isles Dernieres, LA.
Location map for five focus areas. Click on any of the five areas to see the potential inundation map for that area. Data also available in zipped KML format (1 MB), viewable with Google Earth. |
Predicted track of Hurricane Ike from the National Hurricane Center Sep 10, 2008, 2100 GMT 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.
Assessment Approach
The potential inundation of the beach system was defined by calculating the difference between the elevations of model-derived storm surge and lidar-derived dune/berm crests. The storm surge model (SLOSH) was used to simulate the surge resulting from numerous hypothetical hurricane landfall scenarios. The envelope representing the maximum, simulated surge elevations was used to represent a worst-case surge value at each location where potential inundation was evaluated. Positive inundation values indicate that water levels are predicted to be higher than the dune/berm crests and that the beach is likely to become inundated by storm surge during the direct landfall of a hurricane. Negative inundation values indicate areas where water levels are predicted to be lower than the dune/berm crests and are not likely to become inundated. We present an estimate of the inundation potential for Category 1-5 hurricanes, based on predicted maximum surge elevations for each hurricane intensity category. These predictions are presented as five parallel color strips that are aligned with the coastline. Red colors indicate high inundation potential and blue colors indicate low inundation potential.
When interpreting the potential inundation maps for each region, it is important to remember that they describe only one coastal change 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. Furthermore, the inundation 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. Furthermore, 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.
South Padre Island, TX
South Padre Island, TX, is a narrow barrier island with dune heights that rise as high as 10 m. The south end of the island contains extensive housing and other infrastructure that is built on top of the dunes, while the north end of the island is not developed. A characteristic feature of the north end of the island is an extensive dune field that is broken by cuts that can allow surge to flow across the island. If a Category 2 or stronger hurricane makes landfall nearby, inundation is expected at the location of the cuts between the dunes and dune erosion could be severe if surge levels raise water levels sufficiently high to allow wave attack. Only a Category 5 hurricane would be expected to inundate the higher dunes.
Pre-storm assessment of inundation potential (I, storm surge minus dune elevation) for South Padre Island. Reds indicate areas where maximum surge elevations obtained from model simulations of hypothetical storm scenarios exceed the elevations of the primary dunes, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicate that the expected surge elevations are lower than the dune crests and that the potential for severe coastal change is lower than in areas likely to inundate. Gray areas indicate missing data. Dune elevations based on October 2005 lidar survey data. [larger version] |
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Matagorda Island, TX
Matagorda Island, TX, is a narrow barrier island with dune heights that barely rise above 2 m. It is the site of a state park and, otherwise, is not extensively developed. There is extensive vegetation behind the dunes that offers some resistance to wave and surge attack. If a Category 3 or stronger hurricane makes landfall nearby, Matagorda Island is expected to experience inundation and extensive coastal changes. The region with highest vulnerability is at the northeastern end of the island.
Pre-storm assessment of inundation potential (I, storm surge minus dune elevation) for Matagorda Island, TX. Reds indicate areas where maximum surge elevations obtained from model simulations of hypothetical storm scenarios exceed the elevations of the primary dunes, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicate that the expected surge elevations are lower than the dune crests and that the potential for severe coastal change is lower than in areas likely to inundate. Gray areas indicate missing data. Dune elevations based on October 2005 lidar survey data. [larger version] |
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Galveston, TX
Galveston, TX, is situated on a narrow barrier island. 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, the elevations are substantially lower. If a category 2 or stronger hurricane makes landfall nearby, the potential for inundation and type of coastal change in the area near Galveston will be quite variable.
Pre-storm assessment of inundation potential (I, storm surge minus dune elevation) for Galveston, TX. Reds indicate areas where maximum surge elevations obtained from model simulations of hypothetical storm scenarios exceed the elevations of the primary dunes, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicate that the expected surge elevations are lower than the dune/berm crests and that the potential for severe coastal change is lower than in areas likely to inundate. Gray areas indicate missing data. The large dot marks the location of the oblique photograph. Dune elevations based on September, 2005 lidar survey data. [larger version] |
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Chenier Plain, LA
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Oblique aerial photograph of the Chenier Plain coastline of western Louisiana. Photo taken Sept. 28, 2005, after landfall of Hurricane Rita. [larger version]
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The Chenier Plain of western Louisiana consists of low beaches backed by a broad coastal marsh and grasslands. The very low elevations along this coast offer little protection from 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 neighborhoods 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 berms. If a Category 1 or stronger hurricane makes landfall nearby, nearly all of the Chenier Plain is expected to experience inundation and extensive coastal change.
Pre-storm assessment of inundation potential (I, storm surge minus dune elevation) for the southwest Chenier Plain of LA. Reds indicate areas where maximum surge elevations obtained from model simulations of hypothetical storm scenarios exceed the elevations of the primary dunes/berms, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicate that the expected surge elevations are lower than the dune/berm crests and that the potential for severe coastal change is lower than in areas likely to inundate. Gray areas indicate missing data. The large dot marks the location of the oblique photograph. Dune elevations based on September, 2005 lidar survey data. [larger version] |
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Isles Dernieres, LA
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Oblique aerial photograph of the Isles Dernieres. Photo taken Sept. 30, 2005, after landfall of Hurricane Rita. [larger version]
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The Isles Dernieres, LA, consist of very 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 a Category 3 or stronger hurricane makes landfall nearby nearly all of the Isles Dernieres are expected to experience inundation and extensive coastal changes.
Pre-storm assessment of inundation potential (I, storm surge minus dune elevation) for the Isles Dernieres, LA. Reds indicate areas where maximum surge elevations obtained from model simulations of hypothetical storm scenarios exceed the elevations of the primary dunes, suggesting greater potential for inundation of the beach system and for severe coastal changes. Blues indicate that the expected surge elevations are lower than the dune crests and that the potential for severe coastal change is lower than in areas likely to inundate. Gray areas indicate missing data. The large dot marks the location of the oblique photograph. Dune elevations based on December, 2006 lidar survey data. [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 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 rain. 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
