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Busy Hurricane Season for Coastal & Marine's Extreme Storm Impact Project

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Hatteras Island, NC after Hurricane Dennis
Hatteras Island, NC, on September 8, 1999, looking south toward Cape Hatteras after Hurricane Dennis. Road was wiped out by overwash and is being re-sited further inland. Note new position for Hatteras Lighthouse (arrow), recently relocated away from the eroding shoreline.
In the past few weeks, Hurricanes Dennis and Floyd have wreaked havoc along the U.S. East Coast. In terms of peak wind speed, Dennis was a marginal hurricane that deteriorated into a tropical storm before coming ashore in North Carolina. However, its duration was remarkable, sitting off the northern Outer Banks for nearly a week and generating large waves that pounded the coast. In fact, at the Army Research Pier at Duck, NC, wave height was the third highest in the 20 years of records, reaching 6.3 m at a buoy in 20 m of water. Sea level (including tide and storm surge) was also near record levels. Preliminary calculations, by Hilary Stockdon (St. Pete), of the elevation of wave run-up on the beach (that is, the highest reach of the waves) was among the highest .05% on record. These kinds of calculations will be used to understand the distribution and magnitude of dune retreat, a major impact during the storm. Along the northern Outer Banks, dunes are prominent in places and afford the first line of defense to storm erosion and wave overwash.

To measure the amount of coastal change, USGS and its partners at NASA and NOAA conducted an airborne laser altimetry survey of the impacted areas, to south of Cape Hatteras at Ocracoke Inlet. Prior to the storm, the laser altimeter had been mounted on a four-engine NASA P-3 for another project. The P-3 conducted the survey flying out of Wallops Island, VA. These data are being compared to a pre-storm survey of the same area acquired in 1998. We should have initial estimates of magnitudes of dune erosion in a few weeks.

To help interpret and understand the laser data, Dennis Krohn, Karen Morgan, and Dana Wiese of St. Pete acquired oblique aerial video/photos of the impacted areas. Comparisons of before and after photos will soon be available on the St. Pete extreme storms Web site. Groundtruth surveys between Oregon Inlet and Cape Hatteras (by Abby Sallenger, Jeff List, Karen Morgan, and Hilary Stockdon) showed reaches (kilometers long) of significant impact to the protective coastal dunes. However, between these heavily impacted areas were areas (also kilometers long) of virtually no impact. These “hot” and “cold” spots of coastal change are similar to what Jeff List has observed with respect to shoreline changes along beaches farther to the north.

Whereas Dennis was notable because of duration, Floyd at one time was one of the strongest storms to threaten the East Coast this century. Prior to impacting the Bahamas, Floyd was packing sustained winds of 155 mph, only 1 mph short of being classified Category 5. The rules of thumb for Category 5 storms are "catastrophic damage" and storm surge in excess of 6 m. As Floyd approached the East Coast, the largest evacuation in U.S. peacetime history was ordered-more than 3 million people from South Florida, Georgia, and the Carolinas to and including the Virginia coast were ordered from their homes. Fortunately, Floyd's winds decreased in magnitude. At landfall near Cape Fear, NC, it had sustained winds of 110 mph, generating a storm surge of about 3 m. The maximum surge occurred near high tide, contributing significantly to extensive overwash, dune retreat and damage to homes on barrier islands such as Oak and Topsail Islands. As was done for Dennis, these changes were measured using airborne laser altimetry and oblique video/photography.

One of the largest impacts of Floyd was the extreme rainfall and associated fluvial flooding which has reached record proportions. Through the Interdivisional Storm Response conference calls, which serve to coordinate activities between Divisions, Jerry Ryan, the NC WRD District Chief, learned of our plans to acquire oblique video/photography of the impacted coast. He requested our assistance to document the extreme flooding along the Tar River. Hence, after the coastal surveys, we diverted the plane inland to pick up Jerry and with Dennis Krohn, Russell Peterson, and Dana Wiese, the extremely hard-hit areas were documented with GPS-located video. Examples of these images can be viewed at the USGS Center for Integration of Natural Disaster Information (CINDI) Web site.

Related Web Sites
Hurricane & Extreme Storm Impact Studies
U.S. Geological Survey (USGS)
Aerial Photographs of Flooding in North Carolina: Damage Mounts Days After Floyd's Winds Have Blown North
USGS Center for Integration of Natural Disaster Information (CINDI)

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