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 6

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PEAK FLOW DETERMINATIONS

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Procedures for determining peak flows from rainfall or snowmelt are included
in this section. A more detailed discussion of snowmelt runoff is given in
Sect ion 7.

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Peak discharge methods descrihed in Technical Release 55 have been supple-
mented herein because of climatic differences between Eastern and Western
Colorado. Peak flow determinations should be made using charts and figures
found in TR-55 and supplemental charts and figures referred to in this
section (Section 6). Certain charts found in TR-55 have been repeated in
this section for more efficient use of the handbook.

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Hydrologic procedures and charts for Eastern Colorado will apply East of the
8,000 ft. elevation in the upper reaches of the South Platte and Arkansas
River Basins. Hydrologic procedures and charts for Western Colorado will
apply West of this designated boundary.

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Figure S-lB applies to the mountain areas, including either western or
eastern slope streams that historically flood from snowmelt conditions,
regardless of elevation.

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The following figures and tables will be considered a supplement to Techni-
cal Release No. 55 "Urban Hydrology for Small Watersheds", and are to be
used in conjunction with procedures described therein:

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A. Figure S-l, a chart of peak discharge CSM/inch versus time of
concentration (Tc) for Type IIA storm distribution in Eastern
Colorado and Type II storm distribution in Western Colorado.

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B. Figure S-lA charts of peak discharges versus drainage area, CN, and
slope. These charas apply to undeveloped watersheds with a shape
factor of L = 209a .6 (See TR-55, pg. E-5). For cases differ-
ent than this refer to TR-55 Appendix E for adjustment procedures.

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The average slope can be determined as follows:

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1. Determine watershed area (DA) and height (Z) which is the
elevation difference from high to low point in the watershed.

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2. Select three contours located at 25, 50 and 75 percent of the
watershed height (Z).

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3. Measure the length of each contour LC25, LC50,
LC75.

4.

Solve for average watershed slope by:

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S = 0.25 Z (LC25 + LC50 + LC75)
DA

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Note: Units DA

square ft., Z = ft., LC

ft.

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7