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 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 one to 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:
- Breakaway wall panels installed immediately adjacent to and seaward of cross-bracing. When the panels broke away, they were pushed against the cross-bracing by flood waters. The resulting force on the vertical surface generated loads far in excess of the design strength of the cross-bracing.
- Utilities installed on, through, or adjacent to breakaway wall panels.
- Sheathing installed on the exterior of breakaway wall panels.
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 may obtain a copy of this publication by contacting FEMA's Distribution Center at 1-800-480-2520, and requesting publication number FEMA 290.
Last Modified: Tuesday, 18-Apr-2006 14:36:49 EDT