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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H Y D R 0 LOG Y

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Flood hydro graphs were prepared at 37 hydrologic points in the

Little Dry Creek Basin as shown on Chart 2. Hydrographs were

computed at each point for 10, 25 and 100-year frequency floods, for

both existing and future conditions of basin development under two

conditions; (1) with Englewood Dam alone, and (2) with Englewood Dam

plus five proposed flood detention dams. In addition, spillway design

flood hydro graphs were developed for the preliminary design of Englewood

Dam and the five proposed flood detention dams. Spillway design flood

hydrographs were derived from estimates of the probable maximum precipi-

tation, or PMP, that can occur in the area. Such precipitation amounts

are many times greater than the 100-year frequency precipitation amount,

but their use is considered necessary for dam spillway design where possi-

ble dam failure could result in loss of life or very heavy property damage.

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Floods

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There are neither stream flow records nor precipitation records available

in the Little Dry Creek basin that can be used directly to estimate the

frequency of either major floods or major rainfall events. Therefore,

it was necessary to prepare estimated rainfall events and flood hydrographs

indirectly using records and data available in the surrounding area.

Major floods in the area occur as a result of heavy rainfall events. Be-

cause of the relatively low elevation of the basin, major floods do not

result from snow melt nor is snow melt runoff considered to increase

rainfall floods significantly.

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Procedure

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The Little Dry Creek basin hydro graphs were prepared using primarily

the data and methods presented in the Urban Storm Drainage Criteria

Manual, Volume I, Denver Regional Council of Governments, 1969, which is

referred to herein as the Drainage Criteria Manual. The general sequence

followed in preparing the 10, 25, and 100-year flood hydrographs in the

basin was:

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