FLOOD06938

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Storm Drainage and Urban Region Flood Control Planning
• Date: 10/1/1974
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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RATIO OF PEAK FLOW UNDER STATED CONDITIONS
TO THAT UNDER EXISTING CONDITIONS

Moderate Intense
Watershed Existing land Use Urbanization Urbanization
1.7 mi.2 1.0 1.75 2.20
15 mi.2 1.0 1.2 1.5
(without channel
improvement)
15 mi. 2 1.3 1.6 1.95
(with channel
improvement)
These two examples indicate that the amount of peak flow increase for
urbanization is a function of the intensity of urbanization and of the.
exceedence interval of the event of interest. The intensity of urbanization
relates to the amount of the watershed that is covered by impervious areas
and the amount of the ~Iatershed that is storm se~le,red. The relative effect

of urbani zati 011 for the 1 ess frequent events tends to dampen out for the

following reasons: (a) for the increasingly rare events the watershed

under natural conditions would be more saturated; (b) the channels would

be more filled or overflowing; and (c) the flow velocities would be greater,

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and therefore conversion to more efficient man-made systems has incrementally
less impact. In other words,' under natural conditions when the watershed is
saturated, it is in effect quite impervious.

Analytical Techniques
The general state-of-the-art in developing information for the sizing
of the initial drainage systems (sewer systems) is the simple concept
termed "the rational formula." The rational formula is based on the assump-
tion that the amount of flOl'1 contributing to the peak runoff is directly
proportional to the intensity of rainfall for a duration that is equal

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