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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None of the waterways in the basin has a significant base flow. In May

1972, after the spring snowmelt, the base flow in Little Dry Creek at

the South Platte River was about three cubic feet per second (cfs), while

Greenwood Gulch at University Boulevard ran about one cfs. A discharge of

0.63 cfs was measured at Blackmer Dam. The flows were progressively

smaller upstream.

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Englewood Dam

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Englewood Dam is the largest hydraulic structure in the basin. Built in

1936 to engineering standards somewhat less stringent than today's, the

dam is apparently little changed since its construction. Water has not

yet been known to rise close to the spillway crest.* An ungated 36-inch

outlet conduit drains water from the reservoir. This outlet pipe is

small, and if the reservoir were full to the spillway level, it would

take six to seven days to empty the reservoir. Calculations show

that under existing watershed conditions, the lOO-year storm runoff would

not raise the reservoir level to the existing spillway crest; however,

when the watershed becomes fully urbanized above the dam the 100-year

storm runoff would raise the level to about l.3 feet above the present

spillway crest.

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Under today's dam design criteria, a dam situated above an urban area

should be safe for a probable maximum precipitation, PMP, flood inflow.

The PMP rainfall is estimated to be 22 inches in six hours in the Denver

metropolitan area. For Englewood Dam, by one method of calculation, such a

flood gives a peak inflow of 38,500 cfs. The existing dam would be over-

topped and undoubtedly would fail under the current PMP flood criteria.

Therefore, improvements are urgently required.

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It is understood that Federal W.P.A. funds were used to build Englewood

Dam, but the Federal Government does not have title to it. Title is now

held in private ownership. The owners desire to be relieved of possible

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* Within 14 feet in August, 1956. Ripple & Howe Consulting Engineers,
"Status of the Existing Englewood Flood Control Dam," 1956. About the
same level in May 1973.

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