FLOODC00270

• County: Jefferson
• County: Arapahoe
• Title: Chatfield Reallocation Study: Storage Use Patterns 2/1/2003
• Date: 2/1/2003
• Prepared For: Jefferson County / Arapahoe County
• Prepared By: US Army Corps of Engineers/CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The potential reallocated storage pools specified in the Work Plan (Colorado Department of
Natural Resources Colorado Water Conservation Board, December 2001, see Appendix A
herein) for analysis were: 2,900 acre-feet (5,434 ft. MSL), 4,500 acre-feet (5,435 ft. MSL), 7,700
acre-feet (5,437 ft. MSL) and 20,600 (5,444 ft. MSL) acre-feet. These values were pre-selected
by the Corps based on previous reservoir routing studies. In the course of this investigation,
considerable effort was expended in determining whether a larger reallocated storage pool should
be investigated. After additional calculations, meetings with the Study Advisory Group and
meetings with the Omaha District, U.S. Army Corps of Engineers, and the CWCB it was
determined to use the four storage pools specified in the Work Plan for this investigation.

The results from simulation analysis of the 4,500 acre storage pool were, for practical purposes,
indistinguishable from the results obtained for the 7,700 acre-feet pool. Therefore, only the
results from the simulation studies for the 2,900, 7,700 and 20,600 acre-feet pools are reported
herein.

It is important to note that the maximum reallocated storage volume of20,600 acre-feet may be
increased or decreased when the final proposed Chatfield operating rules are determined using
the Corps HEC-5 reservoir routing model, not yet completed. The analysis described in this
report to determine the effects ofthe maximum reallocated storage pool on reservoir fluctuation
and the efficiency of storage use could be repeated for the revised maximum storage pool. For
purposes of this investigation, these reallocated storage pools were placed on top of Denver
Water's operating pool, which has a normal level of 5,432 MSL (27,428 acre-feet).

Sixteen potential users of the storage pool were identified and are listed in the report. These
potential users fall into four general categories:

I. Municipal water suppliers,

2. Entities requiring augmentation water, primarily for replacing out of priority
depletions from irrigation,

3. Entities concerned with maintaining minimum instream flows in the South Platte
River, and

4. Municipal and industrial (M&I) and conjunctive use water users.

Available water rights for storage in Chatfield Reservoir storage were inventoried from
II entities. For purposes of this investigation, specific water rights, which were judged to be
representative of the municipal water supply, augmentation water, instream flow and M&I and
conjunctive use user categories, were used to estimate potential inflow to Chatfield Reservoir for
each of these categories.

Five demand scenarios were developed for purposes of specifying target release schedules:

1. Municipal water supply. The demand scenario and target release schedules used
in this investigation were based on data provided by Denver Water where the
primary use would be for drought contingency storage for municipal water
supply.

P:\Data\GEN\CWCB\Chatfield\ReportI2.02\RevisedChatReportl 03\Report2-03 .doc

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