PROJ02027

• Contract/PO #: FS0044FX
• Contractor Name: Larimer County, Board of County Commissioners
• Contract Type: Miscellaneous
• Water District: 0
• Loan Projects - Doc Type: Feasibility Study

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Alternative Flood Control Solutions

Three alternative flood control solutions were investigated. These were: peak flow diversion
(1980 recommended study, GAl), complete storage, and a combination of storage and diversion
of residual flow.

Peak flow diversion ~ ~

The 1980 GAl Study recommended diverting the peak: 100 year flood flow of Dry Creek,
estimated at that time to be 2,900 cfs from a location just upstream of Will ox Lane south and
west into the Cache La Poudre River. The cost of the peak flow diversion was estimated (in 1980
dollars) to be $2,171,000. URS Engineers investigated the peak flow diversion alternative based
on the updated hydrologic modeling results. The result of that investigation shows that for the
higher estimated peak flow of some 5,000 cubic feet per second, the size and cost of a structure
needed to convey flow across Willox Lane and the Union Pacific Railroad would be prohibitive.
Given the higher current estimated peak flow, capturing the flood flow and directing it into a
diversion structure would be an issue. A third concern involves legal issues which might arise
from directing 5,000 cfs of stormwater flow from Dry Creek into the Cache La Poudre River
some 2.5 miles upstream ofthe location where it would otherwise enter the floodplain of the
river. For these reasons, peak flow diversion was determined not to be a viable alternative.

Complete Flood Stora!!e ~ I);

A 100 year flood would generate about 2,200 acre feet of runoff at the Larimer-Weld Canal. If
this water could be stored in a reservoir and released slowly, a Dry Creek flood could be averted.
No potential dam sites with sufficient volume to store the entire flood could be identified in the
area immediately upstream of the Larimer - Weld Canal. Some storage could be constructed
about 2 miles upstream, but runoff generated downstream. of the storage site would still require
diversion or other means of disposition of floodwaters at the lower end of the basin. This
alternative was rejected due to the lack of viable sites for storage of floodwaters.

Combination of Flood Stora!!e and Diversion ~ ~

A combination of partial storage and downstream diversion of residual flow was considered. An
analysis of Douglas Reservoir showed that sufficient additional capacity to store a 100 year flood
from the upstream basin could be made available by construction of a segmented fuseplug type
spillway. This spillway would allow the reservoir to effectively store the 100 year flood from the
watershed upstream of it, while releasing flow in excess of the estimated 100 year rate of flow.
It was hoped that this approach would significantly reduce peak flows arriving at the Larimer-
Weld Canal. With improvement of the Douglas Reservoir spillway, Peak flows at the Larimer-
Weld Canal were not significantly reduced, although the flood volume was considerably
reduced. It was concluded that additional storage would have to be constructed between Douglas
Reservoir and the Larimer-Weld Canal.

Five hundred cubic feet per second was considered to be reasonable as the design residual
discharge downstream of the additional storage sites, since the peak 100 year runoff originating

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