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 2

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PRECIPITATION

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The highest rates of runoff (CSM - cubic feet per second per square mile)
are caused by intense rainfall on small watersheds.

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The intensity of rainfall varies considerably during a storm period. Three
typical 24-hour storm types, Type I, Type II and Type IIA, were developed
from U. S. National Oceanic and Atmospheric Administration (Weather Bureau)
data. They are associated with climatic regions. Type I intensities are
less than Type II and Type II intensities are less than Type IIA. Type I
storms are typical for maritime climates with wet winters and dry summers
which are characteristic of the coastal side of the Sierra Nevada and
Cascade Mountains in California, Oregon, Washington, in Hawaii and Alaska.
Type II and IIA are typical of the more intense storms that occur over the
rest of the United States, Puerto Rico and the Virgin Islands.

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Rainstorm intensities and amounts are also significantly different between
the mountainous areas of western Colorado and the plains of Eastern
Colorado.

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Summer rainstorms that cause flooding on the plains are generally associated
with frontal systems involving cold air from the Northwest and warm moist
air from the Gulf of Mexico. The storms vary in duration from a few hours
to several days depending on the extent of the storm and its speed. One or
more storm cells of high rainfall intensities generally exist and the path
of these cells often determine the location of flooding.

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The mountains of Central and Western Colorado are a barrier to the movement
of warm moist air from the south. Because of this, the predominant source
of moisture to areas West of the Continental Divide is the Pacific Ocean.
Summer thunderstorms occur relatively frequent but without the intensities
or amounts experienced in the plains of Eastern Colorado.

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Areal Adjustment of Rainfall

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Rainfall-frequency maps show point rainfall amounts and can be used to
determine the average precipitation in drainage areas up to ten (10) square
miles. However, larger areas should be modified for the areal extent of the
storm. Adjustments are necessary to convert point precipitation values to
average precipitation over the watershed area. Rainfall adjustments for
drainage areas greater than ten (10) square miles should be made according
to Figure S-2.

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2

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