FLOOD07468

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: The Design of Encroachments on Floodplains Using Risk Analysis
• Date: 10/1/1980
• Prepared For: USDOT
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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6.2 Hydrologic and Hydraulic Data Analysis

6.2.1 Flood Frequency

Gaging data is usually not available at a crossing. Gaging data at stations
upstream and downstream of the crossing can be adjusted to obtain data
for the crossing. This data is then input to a flood frequency distribution
function to obtain probabilities for various discharges. The Log Pearson Type
III distribution is a recommended distribution. Procedures for applying the
Log Pearson Type III distribution are described in WRC Bulletin 17A (19).

For ungaged watersheds (usually smaller watersheds), a designer may have to
rely on empirical methods such as the USGS Regional Analyses, or the Utah
State method described in Report Nos. FHWA-RD-77-158 and 159 (20).

The USGS regional analyses typically include regression equations for
various flood frequencies ranging from 2.33-year (the annual peak discharge)
to lOO-year. The analyses in several States include equations for 200 and
500-year frequencies. The Utah State method resulted from a national effort
for FHWA. The method divides the nation into 24 hydrophysiographic zcnes as
illustrated in figure 6.1. The method includes regression equations which are
summarized in table 6.2, relating the 10-year flood to watershed parameters.
The regression equations are in the form:

010 = C Ael Re2 DHe3

Where: C, el, e2 and e3 are the regression
coefficient and exponents
A = Watershed area
R = isoerodent factor which is related
to the annual maximum 30-minute rainfall
intensity
DH = Elevation difference between the main
channel at its most distant boundary
and the drainage structure site

Other frequency floods are then related to the 10-year flood by the equation
below:

where:

b
Ot = (a) 010

a = 0.46921 and b = 1.00243 for t =
a = 1.45962 and b = 1.02342 for t =
a = 1.64380 and b = 1.02918 for t =

2.33 yrs.
50 yrs.
100 yrs.

Application of the Utah State method should be limited to watersheds smaller
than 50 square miles in the U.S. The Utah State method also includes an equation
for the "probable maximum runoff peak" which is:
2
o (max) = 10(3.92+.812(10g A)-.0325(10g A) )
P

where: A = drainage area in square miles

Use of this equation should also be limited to small watersheds (less than 50-100
square miles). 0 (max) is not related to a specific frequency flood and can be
p
viewed as an order of magnitude indication for very large floods (say 0200 to Q500)
which should be included in a risk analysis.

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