FLOOD00850

• Designation Number: 31
• County: Arapahoe
• Community: Unincorporated Arapahoe County
• Title: Major Drainageway Planning - Little Dry Creek, Report, Volume I
• Date: 2/1/1974
• Designation Date: 6/1/1974
• Floodplain - Doc Type: Floodplain Report/Masterplan

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1. Estimate precipitation duration and depth for each frequency from
rainfall records in the area.
2. Arrange the total rainfall amount into a typical rainfall
pattern for the area.
3. Estimate the surface runoff deducting watershed retention and
losses from total precipitation.
4. Determine hydrograph shape using watershed characteristics.

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A similar procedure was used to prepare the spillway design hydrographs

for the darns, except that the probable maximum precipitation was taken

from Technical Paper No. 38, U.S. Weather Bureau, 1969. This publication

gives the probable maximum precipitation for a 6-hour duration over a 10-

square mile area in the region to be 22 inches.

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Following completion of the Phase A part of the Little Dry Creek basin

study, additional preliminary design work was done on Englewood Darn by

the firm of Woodward-Clevenger & Associates, Inc., under contract with

the Urban Drainage District. Their preliminary design work was reviewed

as a part of the work done for this report. The spillway design hydro-

graph that they developed is used herein. The flood volume is about the

same as we calculated and reported in our Phase A Little Dry Creek study,

but the peak flow at Englewood Dam was 40 percent greater than our Phase

A calculation.

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Precipitation

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Basic rainfall data for the Little Dry Creek basin was taken from Figures
7-1 through 7-8 of the Drainage Criteria Manual. These figures show
that the depth of given frequencies of rainfall events varies substantially
in different parts of the basin. Accordingly, the basin was divided
into ei~ sub-basins and rainfall depths were calculated for each sub-
basin. Table 2, which follows, shows the drainage basin sub-basins together
with the rainfall depths for the 10, 25 and 100-year rainfall events.

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