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

channel and spillway. As shown on table 16, this is not a low cost alternative;

therefore, significant cost savings cannot be argued as a trade,off against the

negative impacts just discussed.

Three of the spillway alternati"es involve new concrete spillways that
discharge in~o Cherry Creek. Compared with the chute spillway in the embankment
and the side channel spillway in the left abutment, the left abutment channel
spillway has the lowest cost and has the most acceptable performance reliability
of the new spillway alternatives. The first two new spillway alternatives are,
therefore, eliminated from further consideration in favor of the left abutment
channel spillway. At $31 million ~his has the lowest cost of all other
alternatives that would handle the full PMF according to Corps of Engineers
criteria. The left abutment channel spillway with a 9-foot dam raise would have
no significant adverse environmental impacts and would reduce discharges into
Sand and Toll Gate Creeks for essentially all Cherry Creek Dam inflow events.
A major disadvantage of a new spillway that discharges into Cherry Creek is
demonstrated in figure 2. The existin~ dam controls flood damage downstream on
Cherry Creek up to the "threshold flood" event of 63 percent of the PMF. The new
spillway alternative would begin discharging into Cherry Creek at 13 percent of
the PMF. The downstream flood damages would exceed a catastrophic $l billion
which compares with no damage for this event with the existing project.

The alternative of hardening the downstream face of the existing Cherry

Creek Dam would improve the downstream damage characteristics over both the left

abutment channel spillway and the existing project. Figure 2 shO>1s no downstream

releases into Cherry Creek until 70 percent of the PMF. Downstream releases

increase -rapidly with increasing flood magnitude and damages exceed $2 billion

at 100 percent of the PMF although the dam is protected from overtopping and

failure. According to figure 3, the al~ernative of hardening the dam face would

not significantly increase flood damages downstream on Toll Gate and Sand Creeks.

The major problems with this alternative are cost and performance reliability.

This alternative is essentially a 2-mile -long spillway structure, constructed on

embankment material, that would convey overflows 100 feet down a steep slope.

A potential for catastrophic dam failure would remain and high levels of

maintenance and replacement would be required. Furthermore, because this is one

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