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 5

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TIME OF CONCENTRATION

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Time of concentration is the time it takes for runoff to travel from the
hydraulically most distant part of the watershed to the point of reference.
It is a basic element involved in determining timing and magnitude of a
flood hydrograph. Methods discussed in this handbook include:

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1. Upland or overland flow;

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2. Modified curve number;

3. Channel flow;

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4. Hydrograph analysis;

5. Nomograph

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Chapter 3 of TR-55 gives detailed explanations of the first four methods.
Table S-l is intended as an aid in using the channel flow method, which is
the preferred procedure for projects of significant size and cost.

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Peak discharge charts, figure S-lA, and figure D-2 (TR-55), do not require
Tc values directly. The modified curve number method for determining Tc has
been used to translate Tc into terms of CN, watershed slope and drainage
area for average shaped natural watersheds. Lag time values from the
modified Curve Number Method must be adjusted for watersheds that are no
longer in their natural condition. Chapter 3 of TR-55 gives details of
these necessary modifications.

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The nomograph, figure S-6 is a simplified solution of Tc requ1r1ng only
watershed length and height. Its use is self explanatory. It is recom-
mended that this nomograph be an alternative method for drainage areas less
than 1,000 acres.

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6

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