WSP07965

• File Number: 8446
• Description: Cache La Poudre Project
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 7/1/1983
• Title: Cache La Poudre Project Study and Related Correspondence
• Water Supply Pro - Doc Type: Report/Study

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The amount of rainfall and snow melt in the lower basin is small in
relation to the total water supply and is accounted for in this ~tudy as being
available to meet future agricultural uses directly.

Discussions with U.S. Geological Survey staff indicate that ground-
water flow into the lower basin is minimal and, therefore, is not included as
a potential future source of water supply in this study.

There is the possibility of additional water being available to the
Cache la poudre River Basin in the future from additional runoff from forest
management plans [4]. This has been estimated to be about 2,600 acre-feet per
year for that portion of the national forest lying within the basin. Since
this additional yield is speculative and minor in comparison to the total
basin yield, it was not considered as part of the future water supply during
this study.

In summary, the average annual future developable water supply at the
mouth of the canyon for a potential Cache la Poudre Project would consist of
271,000 acre-feet of native flows and 37,000 acre-feet of transbasin imports.
In addition, the total annual average water supply for the Cache la Poudre
service area would include 115,000 acre-feet of water from the combined
Colorado-Big Thompson Project and Windy-Gap Project, an increasing amount of
return flows from Fort Collins and an increasing amount of water from the Big
Thompson River.

C. INFLOlf DESIGN FLOODS

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The inflow design floods for potential dams considered as part of this
study were determined from an envelope curve of flood peak versus drainage
area which is shown in Figure 111-7. The curve was determined from the record
of all measured major flood peaks along the front range and piedmont in
Colorado and some inflow design flood values developed for existing reservoirs
in the Cache la poudre area.

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By inspecting the information in Figure III-7, an. envelope curve with a
Creager's C 'of 150 was chosen for preliminary planning of all potential dam-
sites except for the New Seaman Dam and inflow design flood peaks which were
estimated based on the area above the potential damsite. (The inflow design
flood for New Seaman Dam was developed during recent studies by International
Engineering Company [28]). The resulting values were used in sizing and
estimating costs of spillways for the potential dams. The difference in the
cost of the spillways designed to carry different peak flood discharges
(obtained by using different values of C) was determined to be minimal when
considering the accuracy of designs and cost estimates at the level of detail
being employed in this study.

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The inflow design floods for the major reservoirs considered in the study
are as follows:

III-5

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