FLOOD10133

• County: Statewide
• Community: State of North Carolina
• Basin: Statewide
• Title: Two Months of Flooding in Eastern North Carolina, September-October 1999: Hydrologic, Water-Quality, and Geologic Effects of Hurricanes Dennis, Floyd and Irene
• Date: 1/1/2000
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

Hurricane Dennis approached the
North Carolina coast on August 29 as
a Category 2 hurricane, and eventu-
ally made landfall south of Cape
Hatteras on September 4 as a tropical
storm. Although winds from Hurri-
cane Dennis were not particularly
strong, the duration of the effects of
the storm on North Carolina's shore-
line was remarkable. Hurricane
Dennis meandered about 150 miles
otTthe coast of the northern Outer
Banks for nearly a week, generating
large waves that pounded the coast.
In fact, at a buoy located in 65 feet of
water near the U,S. Army Corps of
Engineers Research Pier at Duck
(fig. 23), the maximum wave height
during Hurricane Dennis was
20.5 feet, which was the third highest
wave height documented at the site in
more than 20 years of record. Sea
level, which includes the effects of
both astronomical tides and storm
surge, was also near record heights at
the Research Pier during the pro-
longed storm. Storm tides were 3 to 5
feet above normal along much of the
North Carolina coast on both August
30 and again on September 4 (Beven,
2000). According to preliminary cal-
culations, the elevation of wave
runup on the beach at Duck (that is,
the highest reach of the waves) was
among the highest on record. These
kinds of calculations are being used
to understand the distribution and
magnitude of dune retreat, a major
etfect of the storm on the Outer
Banks. Along the northern Outer
Banks, from near the Virginia-North
Carolina State line to south of Cape
Hatteras at Ocracoke Inlet, dunes are
prominent geologic features and
afford the first line of defense against
storm erosion and wave overwash.

In order to measure the amount of
shoreline change resulting from Hur-
ricane Dennis, the USGS, in cooper-
ation with National Aeronautics and
Space Administration (NASA) and
National Oceanic and Atmospheric
Administration (NOAA), conducted
an airborne laser altimetry survey of
the northern Outer Banks using

18'

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dunes between Cape Hatteras and
Oregon Inlet was highly variable
along the coast. In some areas,
dunes retreated more than 100 feet,
whereas in other areas the dunes
were stable,
Overtopping and overwash
occurred at some locations along the
Outer Banks, seriously affecting

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Figure 23. Locations at which shoreline change and overwash were
documented.

NASA's Airborne Topographic Map-
per (ATM) on September 8, immedi-
ately following the storm. These data
were compared to a survey com-
pleted I year prior to the storm. Ini-
tial analyses indicate that erosion of

Two ~1onths of Flooding in Ea,tem North Carolina. September-October 1999

N,C. Highway 12 (figs. 24, 25,
and 26). Before Hurricane Dennis,
the dunes at a location north of
Buxton (fig, 23) were nearly 20 feet
high (fig. 26), Replacement dunes
were constructed by using bulldozers