FLOOD08379

• County: Statewide
• Community: State
• Stream Name: All
• Basin: Statewide
• Title: Procedures for Determining Peak Flows in Colorado
• Date: 3/1/1984
• Prepared For: State of Colorado
• Prepared By: USDA Soil Conservation Service
• Floodplain - Doc Type: Educational/Technical/Reference Information

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SECTION 7

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ESTIMATION OF RUNOFF FROM SNOWMELT

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This section supplements figure S-IB as a more detailed method for analyzing
flooding from snowmelt conditions. Methods of snowmelt forecasting for
irrigation and similar purposes, are described in the Snow Survey Handbook
of the Soil Conservation Service.

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Details of the thermodynamics of snowmelt are omitted from this section
because of their limited value in the method presented here. Some standard
references are:

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Clyde, George
231, August 1931.

D. - Snow-melting characteristics. Technical
Utah Agricultural Experiment Station, Logan,

Bulletin
Utah.

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Light, Phillip - Analysis of high rates of snow-melting. Pages 195-205,
Transactions of the American Geophysical Union, 1941.

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Wilson, W. T. - An outline of the thermodynamics of snowmelt. Pages
182-195, Transactions of the American Geophysical Union, 1941.

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Significance of Snowmelt Floods. Bankfull capacities in csm (cfs per square
mile) are normally greater for small watersheds than for large ones. Since
snowmelt rates are relatively low in csm there may be flooding on large
watersheds when streams on small watersheds are flowing less than bankfull.

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Water Resource specialists acquainted with an area will know the relative
importance of snowmelt as a source of flooding in that area. In doubtful
cases the data normally gathered by interview for a historical flood series
will usually define the character of flood flows. In other instances, the
runoff records will show the importance of snowmelt flooding. It is seldom
necessary to make detailed hydrologic investigations into the matter.

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Regional Analysis Method. This method is for estimating the magnitudes and
frequencies of peak discharges or runoff volumes for ungaged watersheds by
use of relationships for nearby gaged watersheds. The method in its
simplest form is as follows:

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A. Select nearby gaged watersheds that are climatically and physically
similar to the ungaged watershed. These watersheds and nearby areas
like them comprise the region that gives the method its name.

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B. Construct frequency lines for peak discharges or runoff volumes of
the gaged watersheds.

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C. Plot peak discharges or runoff volumes for selected frequencies
(only the 2- and 100-year frequencies if the frequency lines are
straight) of each gaged watershed against its drainage area size,
using log paper for the plotting and making straight-line
relationships for each frequency.

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