FLOOD06564

• County: Boulder
• Community: Boulder
• Stream Name: Goose, Wonderland, Elmer's Twomile, 2-mile, 4-mile
• Basin: South Platte
• Title: Major Drainageway Planning Volume 1, Part B
• Date: 4/1/1969
• Prepared For: Boulder County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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A unit hydrograph is defined as the hydrograph of one inch of direct
runoff from the tributary area resulting from a unit storm. A unit
storm is a rainfall of such duration that the period of surface runoff
is not appreciably less for any rain of shorter duration. The unit
hydrograph thus represents the integrated effects of factors such as
tributary area, shape, street pattern, channel capacities, and stream
and land slopes,

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To apply the unit hydrograph, the effective precipitation depth for
the "unit storm" periods are multipl led by the ordinates of the unit
hydrograph and added to obtain a design storm runoff.

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The basic premise of the unit hydrograph is that individual hydrographs
resulting from the successive increments of rainfall excess that occur
throughout a storm period will be proportional in discharge throughout
their length, and that when properly arranged with respect to time, the
ordinates of the individual unitgraphs can be added to give ordinates
representing the total storm discharge. The hydrograph of total storm
discharge is obtained by summing the ordinates of the individual
hyd rographs.

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The derivation and appl ication of the unit hydrograph are based on
the following assumptions:

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I. The rainfall intensity is constant during the storm that produces
the unit hydrograph,

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2. The rainfall is uniformly distributed throughout the whole area
or the drainage basin. The small basins in North Boulder make
this a va) id assumption.

3. The base or time duration of the design runoff due to an effective
rainfall of unit duration is constant.

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4. The ordinates of the design runoff with a common base time are
directly proportional to the total amount of direct runoff repre-
sented by each hydrograph.

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5, The effects of all physical characteristics of a given drainage
basin, including shape, slope, detention, infiltration, drainage
pattern, channel storage, etc., are reflected in the shape of
the unit hydrograph for that basin,

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Equations

There are two basic equations used in defining the 1 imits of the
synthetic hydrograph, The first equation defines the lag time of the
basin in terms of time to peak, tp' which, for the Colorado Urban
Hydrograph Procedure, is defined as the time from the center of the
unit storm duration to the peak of the unit hydrograph. This study
used a ten-minute unit storm duration.

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