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 1

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GENERAL

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Hydrology is concerned with the origin, distribution, and properties of
water. The designer is primarily interested in peak runoff for a selected
design frequency. The peak runoff is the maximum instantaneous discharge of
a flood.

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It is common in Colorado for flooding to occur in the spring from snowmelt
and in the summer from rainfall. The type of flooding most critical to an
area depends primarily on the elevation and location of the watershed. High
elevation watersheds are more susceptible to flooding from snowmelt than
rainfall. Foothill streams flood from either condition and occasionally
from a combination of the two. Low elevation watersheds generally produce
flooding from rainfall.

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The methods of runoff prediction are simplified for general situations. It
would be almost impossible and too time consuming to apply all runoff para-
meters in a drainage basin. To make the methods usable, only a few signifi-
cant parameters are used~

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Limitations

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If streamflow data are available for the area being studied, these data
should be used in preference to the procedures in this handbooK. Streamflow
data from other but similar areas should be used to check the reasonableness
of results obtained by the handbook procedures.

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More detailed analysis using the Soil Conservation Service National Engi-
neering Handbook Section 4 and Technical Release 20 should be used to
estimate peak rates of runoff for drainage areas greater than 25 square
miles or for special situations.

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Factors Affecting Surface Runoff

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Precipitation, Whether it occurs as rain or snow, is the potential source of
water that may run off the surface of watersheds. The soil and vegetative
cover on a watershed has a major effect on the amount of precipitation that
runs off. Mechanical treatment on a watershed, along with its topography
and shape, affect the rate at which water runs off. Soil, vegetative cover
and conservation practices are reflected as hydrologic soil cover complex
numbers in runoff determinations. The time factor is reflected as a storm
distribution type which varies with rainfall intensity. Travel time is
expressed in terms of time of concentration and time to peak. All of these
factors affect the shape and peak of a flood hydro graph.

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