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

DAMAGE INCREASE OVER EXISTING CONDITION
Some of the alternative dam safety mod.ifiC:~:ltions would produce an increased
threat of loss of life and increased flood damages downstream for flood events
that are now handled safely by the present project (events less than 63 percent
of the PMF). The Risk - Damage Curves on figuI'iE' 2 show th.at a new left abutment
spillway emptying into Cherry Creek would prod\.l.ce substan1:ial downstrE~am damage
starting at 13 percent of the PMF; whereas, the existin,g project produces no
downstream damages until about 63 percent of the PMF. Alt:hough the spillway plan
should handle 100 percent of the PMF underexist:ing developed conditions without
threat of dam failure, outflow events from the spillway would produce over $1
billion in downstream Cherry Creek damage starting at about 40 percent of the
PMF. The plans involving new spillways would increase ChE!rry Creek damages for
events less than the threshold flood unless 1:he spill""y crests were set at
5644.5.

Figure 3 compares damages incurred on Cherry Creek versus Sa.nd and Toll Gate

Creeks. Figure 3A, which is a blowup of the Sand and Toll Gate Creek damage

curves presented in Figure 3, compares the flood damages produced on Sand and

Toll Gate Creeks by outflows from the Gherry Creek Dam Spillway for the existing

project and for the various modification ,alternatives. Widening the existing

spillway (Plan ID) would increase Sand Creek and Toll Gate Greek damages over the

full range of spillway operation. On the other hand, Castlewood Dam (Plan SA)

or a new left abutment spillway (Plan 4E) would reduce overflo;ls into Sand and

Toll Gate Creeks for essentially all Cherry CrE!ek inflow events. Hardening the

dam face (Plan 6) would not alter the existing overflow

into Sand and Toll Gate Creeks until about 70 percent of t:he PMF, and produce a

slight increase at 100 percent of the l'MF. RaIsing the dam (Plan 7) would also

not alter the existing overflow into Sand and Toll Gate Greeks until about 70

percent of the Cherry Creek PMF inflow, but would increase overflows into Sand

and Toll Gate Creeks for events beyond 70 percent of the PMF, the increase

depending on the amount of dam raise.

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