FLOOD08251

• County: Arapahoe
• County: Douglas
• Community: Englewood, Cherry Hills Village, Greenwood Village
• Stream Name: Little Dry Creek
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 2/1/1974
• Prepared For: Arapahoe County / Douglas County / Englwewood / Cherry Hills Village
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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tively higher peak rate of discharge that develops more quickly than would
a more gently sloping watershed. Peak flow rates from upstream watershed
areas may even be reduced as they pass through long narrow gently sloping water-
sheds because of the effect of natural channel and flood plain storage capacity.

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The unit hydrograph method was used to develop flood hydrographs for the

8 sub-basins of Little Dry Creek basin. The unit hydrograph method

used is the Colorado Urban Hydrograph Procedure as described in the

Drainage Criteria Manual. As in most such methods of unit hydrograph

analysis, it was first necessary to determine the area, stream length,

location of the centroid, and average slope of each sub-basin drainage

area. These physical characteristics of each sub-basin plus two empirical

coefficients were used to develop the unit hydrographs. The two em-

pirical coefficients, Ct and Cpo are coefficients of time to unit hydro-

graph peak and peak rate of runoff, respectively. These coefficients

are given in the Drainage Criteria Manual for a 60 to 20 percent range

of watershed imperviousness, and it was necessary to extrapolate somewhat

to obtain equivalent coefficients for 10 percent imperviousness, the pre-

sent condition of the Little Dry Creek Basin. The time units of the unit

hydro graph are the same as the time units of the excess precipitation.

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The foregoing hydrographic analysis was required to determine the volume

of flood runoff from excess rainfall depth. In addition, it was necessary

to determine the peak rate of discharge for a given flood volume, This

required that the sub-basin characteristics. including the two empirical

coefficients, and the rainfall data were used in a computer program

to obtain a total of 420 hydro graphs for the 8 sub-basins at 37 hydrolo-

gical points. The location of the hydrologic points is shown on Chart 2,

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Several 100-year frequency flood hydro graphs were also computed using

both the Drainage Criteria Manual method and the independent method

used by the Soil Conservation Service, United States Department of Agri-

culture, for rural areas. It was found that the Drainage Criteria Manual

method and the Soil Conservation Service method produced nearly equi-

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