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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

SECTION 2

TARGET REALLOCATED STORAGE POOLS

The first step in evaluating the proposed operation for reallocated storage in Chatfield Reservoir
was to develop feasible target storage pools. Three pools sizes were developed:

I. 5,434 mean sea level (MSL) elevation or 2,900 acre-feet. At this level, the
existing recreational features can be mitigated without relocation of structures;

2. 5,437 MSL or 7,700 acre-feet. At this level, expenditures will be required to keep
the existing recreational features operational;

3. 5,444 MSL or 20,600 acre-feet. This is a previously determined level that may
accommodate an acceptable adjustment in the flood control pool of Chatfield
Reservoir.

The original scope of work for the present project included a potential reallocated storage pool of
4,500 acre-feet. Based on the similarity of the results of the 4,500 acre-feet pool with the 7,700
acre-feet pool, simulation results for the 4,500 acre-feet pool are not presented in this report.

In the course of the present investigation, consideration was given to analyzing a reallocated
storage pool larger than the 20,600 acre-feet. In May 2002, a number of discussions were held
with the Colorado Water Conservation Board (CWCB) and the Omaha District, U.S. Army
Corps of Engineers concerning the evaluation of Chatfield Reservoir operational rules for the
design flood event. Based on this evaluation of Chatfield Reservoir operational rules, it appears
that the 20,600 acre-feet of reallocated storage is a reasonable maximum reallocated storage
volume for purposes of this investigation. This maximum reallocated storage volume may be
increased or decreased when the final proposed Chatfield operating rules are determined using
the HEC-5 reservoir routing model. These operating rules will include Chatfield, Bear Creek
and Cherry Creek Reservoirs operating as a system.

If the new HEC-5 runs by the Omaha District, U.S. Army Corps of Engineers for investigating
Chatfield Reservoir, Bear Creek Reservoir and Cherry Creek Reservoir system operations
indicate potential for reallocating more than 20,600 acre-feet of storage in Chatfield Reservoir,
the analysis described in this report for determining the effects of reallocating Chatfield
Reservoir storage could be repeated for a larger reallocated storage pool.

For purpose of this investigation, these reallocated storage pools were placed on top of Denver
Water's operations pool, which has a normal maximum operating level of 5,432 MSL
(27 ,428 acre-feet).

P:\Data\GEN\CWCB\Chatfield\Report 12.02\RevisedChatReport 1 03\Report2-03.doc

7