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

However, several streams in the Chowan River Basin experienced 50- to
greater than 500-yearflood flows (table 3; fig. 7). The previously recorded
maximum water levels were exceeded at Potecasi Creek (site 4) and
Ahoskie Creek (site 5) in North Carolina, as well as on the Nottoway (site
I) and Blackwater (site 2) Rivers in Virginia near the North Carolina-
Virginia State line (fig. 7). The previously recorded maximum water level
on the Cashie River (site 6) was exceeded by 7 feet during Hurricane Floyd,
and the flood recurrence interval was greater than 500 years.
The high rainfall amounts in southeast North Carolina (table 1) had a
dramatic elfeet on the Waccamaw River (site 44; fig, 7), where streamflow
has been recorded for 60 years (table 3). The maximum streamflow
recorded following Hurricane Floyd was more than 2.5 times greater than
the highest streamflow ever recorded at the site (table 3), and the flood-
flow recurrence interval was greater than 500 years, The previous highest
streamflow occurred as a result of Hurricane Fran in 1996. The maximum
streamflow in the Lumber River at Boardman (site 45) was approximately
equal 10 the highest previously recorded flow (in 1945) at the site, which
has 70 years of record. The highest previously recorded water level for the
New River (site 34) was established in 1955 as a result of Hurricane lone
(fig. 5; table 3). However, the maximum water level for the New River
resulting from Hurricane Floyd rainfall exceeded that from Hurricane lone
by more than 5 feet (table 3), and peak flow resulting from Hurricane Floyd
was almost double the 1955 peak flow,

Data from ADCP discharge measurement at Neuse River at Kinston, N.C. Blue-green
lines sho\\' velocity direction and magnitude (scale at left) along the boat pdth (shiptrack,
red line). Total measured discharge was 27,300 ft3/s, and total length of measurement
was 4,310 feet

FLOOD RECURRENCE
INTERVALS

A statistical technique called
frequency analysis is used to esti-
mate the probability of occurrence
of a flood peak having a given
magnitude, The recurrence interyal
(sometimes called the retum "'
period) of a peak flow is the protl"a-
bility that the flow will be equaled or
exceeded in any given year. For W
example, there isa 1 in 100 (or one
percent) chance that a streamflow
of at least 45,500 ft3/s will occur
during any year on the Tar River at
Tamoro (sile 14, table 3; fig, 7).
Thus, a peak flow of 45,500 ft3/s at
site 14 is said to have a 100-year
recurrence interval, or to be the
100-year flood. This is not to say J'
that a flow of 45,500 W/s will occur
only once during the next 1 00
years, but rather that there is a l)n
100 chance that a flow of 45,506
ft3/s will be equaled or exceeded
during any given year. Moreover,
from a statistical point of view, the
fact that a 1 DO-year flood occurs
one year does not affect the proba- .
bility of such a flood occurring the '
following year.
The standard procedures
(Hydrology Subcommittee of the
Interagency Advisory Committee
on Water Data, 1982) used to
compute flood recurrence intervals
from data collected at a stream-
gaging site are based on a number
of assumptions, including the
following:

Distribution of the logarithms
of the annual peak flows can
be approximated by the Pear- W
son Type-III distribution;

Annual peak flows are
independent;

Two Months of Flooding in Eastern North Carolina. Septembcr-Octobcr 1999