FLOOD07843

• County: Statewide
• Basin: Statewide
• Title: Nationwide Summary of US Geological Survey Regional Regression Equations for Estimating Magnitude and Frequency of Floods or Ungaged Ssites 1993
• Date: 1/1/1993
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

the application of each State's equation and the
weighted frequency curve.

Table 1. Frequency curves for Sucamoochee River at
uvingston, Alabama.

Recurrence Computed Computed Weighted
Peek Q In Peek Q In frequency
InterVel Mlaaleelppl Alabama eatlmatea
(yeers) (,,"/a) (,,"/a) (,,"/a)
2 16.000 9.680 13,000
5 27,900 15.700 22,100
10 36.100 20,300 28,600
25 47.400 27,100 37.800
50 58.200 32,600 46.100
100 63.800 38,700 52.000
500 85,700 55,200 71.300

The weighted frequency curve was obtained by using
the following equation:

Qw(T)

286 320
= 606 QAL (T) + 606 QMS (T)

where

QAL(T), QMS(T) = computed T-year peak dis-
charge, in cubic feet per second, using the Alabama and
Mississippi regression equations, respectively;
Qw(T) = weighted T-year peak discharge, in
cubic feet per second.
If the estimating equation for one state is used for
the entire basin, the difference between T-year can be

significant, depending on which state equation is used.
For example, the 100-year flood discharge for the
Sucarnoochee River would be about 64,000 cubic feet
per second if the basin was entirely within Mississippi
but only about 39,000 cubic feet per second if the basin
was entirely within Alabama.

Summary

The use of gaging station data, such as watershed
and climatic characteristics and station frequency
curves, in the testing of Nfl' indicated that the curves
used to compute the extrapolated 500-year peak dis-
charges in NFF confonned to log-Pearson TYpe III dis-
tributions. When compared to observed station 500-
year peak discharge estimates, the extrapolated peak
discharges agreed closely, differing by an average of
0.04 percent. Comparison of skew coefficients from the
at-site frequency curves, computed by NFF with gener-
alized skew coefficients, indicated only minor differ-
ences. Manual procedures for computing frequency
curves across State boundaries and their limitations
were described. The testing process indicated that the
extrapolation procedure for the 500-year flood was rea-
sonable and gave estimates similar to those based on
station data and regional equations developed by
regression analysis.

TESTING AND VAUDATION OF TECHNIQUES 19