WSP04310

• File Number: 8449.600
• Description: Chatfield Reservoir
• State: CO
• Basin: South Platte
• Date: 1/1/1984
• Author: CWCB
• Title: Division of Parks Temporary Storage in Chatfield - Background Data
• Water Supply Pro - Doc Type: Data

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MROED-HE
SUBJECT;

(1 Jul 82)
Evaluation
Colorado

29 September 1982
of Potential Water Supply Storage at Chatfield Reservoir in

(~) Condition 3. The design assumptions for this condition were based
on stor~n$ a June standard project flood on top of a 25-year pool (20,000 acre-
teet alreadY ~n the flood pool) and 24,000 acre-feet of added storage due to a
10-da1 shutdown and stepped release operation. In the reevaluation of this condition
the antecedent pool ,level was dropped to conservation level so that the level of
protection is comparable to that discussed in the beginning of this report. A
10-year, 10-day volume was selected as the inflow during the shutdown period and
stepped release operation. The results of this condition are also summarized in'the
following table.

Table 1

Summary of Design Storage
Requirements for Conditions 1-3

Condition 1

DesiF;n Reevaluation
Spring Runoff into Storage (AF) 12,300 0
SPF Rain Flood (May) (AF) 130,000 130,000
Inflow During Shutdown & Stepped Release (AF) 73,000 21.600
Total 215,300 151,600
Condition 2
500-year Spring Runoff Into Storage (AF) 107,500 134,000
Coincident Rain Flood (AF) 28,000 4,000
Inflow During Shutdown & Stepped Release (AF) 79;000 25.400
Total 214,500 163,400
Condition 3
Initial Flood Storage (AF) 20,000 0
SFP Rain Flood (June) (AF) 160,000 160,000
Inflow During Shutdown & Stepped Release (AF) 24,000 22,000
Total 204,000 182,000

(4) When the results of' the reevaluation are compared it becomes evident
that under the new assumptions Condition 3 becomes the critical factor in determin-
ing flood control storage requirements. On the basis of this analysis we can say
that 33,000 acre-feet of excess flood storage has been provided in Chatfield. It
is important to reiterate that this is only so if the original design release
criteria is used. Experience has shown that releases less than 5000 cfs can
create problems downstream; Furthermore, no consideration was given to coincident
releases from Cherry Creek and/or Bear Creek when the Chatfield design release
rate was selected. Therefore, any decision that needs to be made on storage
availability in either Chatfield or Bear Creek will necessarily have to wait
until our Denver reservoir system studies have been completed.

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