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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Slope interpolation tables and adjustments for water areas in the
watershed are found in Appendix E of TR-55.

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C. Figures S-lB and S-lC charts of peak discharge versus drainage area,
and mean annual precipitation. Figure S-lB applies to predomi-
nately snowmelt streams in the mountain areas of Colorado. See
Section 7 to help make this determination.

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D. Figure S-2, a chart for making areal adjustments to point rainfall
values. This chart is applicable to all of Colorado, for drainage
areas greater than 10 square miles. It is not appropriate for the
design of Earth Dams and their spillways. For this purpose refer to
Chapter 21 of NEH-4.

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E. Figure S-3, a chart of hydrologic soil cover complex and associated
curve numbers for range and forest lands in Colorado.

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F. Figure S-4, hydrologic soil cover complex and associated curve
numbers for croplands.

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G. Figure S-5, a chart for adjusting CNs for various antecedent mois-
ture conditions. Antecedent moisture condition II should be used
for design purposes unless permission to modify is obtained from the
State Conservation Engineer. Such permission will be based on
rainfall and streamgage evidence that supports AMC and CNs signifi-
cantly different from those reflected in Figures S-4 and S-5 for AMC
II. Modified AMC conditions for an area or region may be determined
as follows:

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1. Select one or more representative watersheds that have at least
10 years of streamgage record. Make a frequency analysis of
annual maximum discharge in accordance with Chapter 18, NEH-4.

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2. Determine basic Tc, CN (for AMC-II), and precipitation-frequency
data for the selected watersheds, and compute theoretical dis-
charges using figure S-l. Compare theoretical discharges with
streamgage values in step 1 for the same frequencies.

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3. Revise CNs established in step 2 until theorotical discharges
are similar to values in step 1.

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4. Use final revised CNs of step 3 along with initial CNs of step 2
in figure S-3 to determine modified AMC condition appropriate
for the area. This AMC condition may be used for other ungaged
watersheds in the area that are hydrologically and meteorologi-
cally similar.

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H. Figure S-6 nomograph to determine time of concentration.

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I. Figure S-7, rainfall frequency maps for Colorado.

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J. Table S-l, time of travel and Tc by stream hydraulics.

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