FLOOD09590

• County: Arapahoe
• Community: Aurora
• Stream Name: Cherry Creek Lake
• Basin: South Platte
• Title: Hydrologic Improvement Assessment Cherry Creek Lake
• Date: 9/1/1993
• Prepared For: State of Colorado
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Project

Also, in the Cherry Creek Reservoir Regulation Manual, paragraph 10-02 it
is stated: ".. . it is imperative that the pool be drawn down to at least the
half-full condition prior to the occurrence of the spillway design flood. A
release rate to a maximum of 5,000 cfs from the Cherry Creek outlet works will
ensure that no floodwaters are stored in the upper half of the flood control zone
for more than 5 days, This will allow the design spillway flood assuming a
starting pool of half full."

It was assumed that there was no discharge through the outlet works in the
determination of the existing threshold flood or when formulating the array of
alternatives. The reason for making this assumption was that it facilitated the
development of the large array of alte~natives. If the discharge through the
outlet works were included in the model, the peak pool elevation for the PMF.
with outflow confined to the outlet works and spillway, would be 0.8 foot lower
and the threshold flood would be increased to 66 percent of the PMF. If this
study proceeds to the feasibility stage, routings will be updated to include flow
through the outlet works for all alternatives analyzed.

AREA CAPACITY AND SPILLWAY RATING CURVES

The area capacity curve used for Cherry Creek Reservoir was developed from

1975 surveys of the lake. The spillway rating curve for the existing spillway

was taken from "Design Memorandum No. CC-ll, Spillway Performance Review, South

Platte River, Cherry Creek Lake, Colorado." dated January 1985,

FREEBOARD

With a starting flood pool half-full, the minimum freeboard requirement for

the Cherry Creek embankment is 5 feet above the maximum still water pool

elevation. In order to verify the adequacy of the 5 feet minimum freeboard

requirement, a wind wave analysis was performed. This analysis was based on the

all-directional probability and duration data from ANSI (A58.1-1982). A worst

case condition was assumed in the analysis in which the design winds occur at the

time of the peak pool elevation resulting from the inflow design flood, At the

face of the embankment, the total wave ~unup and setup for the significant wave

are 4.0, 4.4. and 4.6 feet for winds with recurrence intervals of 25. 50, and 100

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