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Mitigation Assessment Team Report: Hurricane Fran in North Carolina (FEMA 290)

On September 5, 1996, Hurricane Fran made landfall near Cape Fear, North Carolina. Coastal areas experienced significant erosion and scour. Erosion caused by Hurricane Fran was exacerbated by the previous dune erosion caused by Hurricane Bertha, which had made landfall in the same area only 2 months earlier.

Observations

Erosion of up to 4 to 6 vertical feet beneath oceanfront homes was measured. Localized scour measured at vertical foundation members generally reached 1 to 1.5 times the diameter or width of the member. This erosion and scour, added to the average erosion rate of 1 to 2 feet a year, left many oceanfront homes unable to withstand the loads experienced. The loss of supporting sand left many short pilings either completely exposed or embedded less than 2 feet.

The MAT observed very little damage in some areas, where velocity flows, wave action, and severe erosion occurred. The successful performance of buildings in these areas demonstrates the value of compliance with National Flood Insurance Program (NFIP) requirements.

Although the MAT noted that breakaway walls generally performed as intended, three design and construction errors were observed that are worth noting:

Recommendations

Piling Embedment for Structural Support. It is critical that coastal foundations be designed to survive the anticipated amount of erosion and scour. Erosion and scour combine to impact coastal piling foundations in three distinct ways.

First, in the absence of cross-bracing, the loss of soil adjacent to a thin vertical foundation member results in a longer unsupported length.

Second, the loss of soil adjacent to pilings leaves less soil to counteract lateral loads applied to the pilings by the structure.

You can download FEMA 290 from the FEMA Library, or order a copy from the FEMA Distribution Center. To order this or other publications please call 1-800-480-2520 or fax 1-240-599-0525 Monday through Friday 8 a.m. – 5 p.m. EST. You may also email your request to FEMA-Publications-Warehouse@dhs.gov. Please provide the title, item number, short number, and quantity of each publication, along with your name, address, zip code, and daytime telephone number.

Last Modified: Wednesday, 23-Feb-2011 13:18:12 EST