Analytical Framework for Evaluating Channel Maintenance Flows in Colorado

• File Number: 8461.100
• Description: Adaptive Management Workgroup
• State: CO
• Basin: South Platte
• Water Division: 1
• Author: Charles F. Leaf
• Title: Analytical Framework for Evaluating Channel Maintenance Flows in Colorado
• Water Supply Pro - Doc Type: Report/Study

An Analytical Framework for Evaluating Channel Maintenance Flows in Colorado
AlpinelSubalpine
, Upper Basln
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Proposed ?
Reservoir B ?
Monlane
. ? Lower Basin
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UPPER LOWER •
BASIN BASIN l
Point B Point A • /
Drainage Ares 40.4 40.4 MI.z ;?• •
Bankfull Disch. 165 C.F.S.
Avg. Annual PPT. 22 In.
Channel Classification C3
Figure 2. Example Watershed.
1,000 tons/year at point A in order to maintain the
channel after the storage reservoir is constructed
upstream.
A channel stability and watershed evaluation of
the lower basin has determined that the critical reach
at point A is near its geomorphic threshold due to a
significant introduced sediment component. Hence,
the bedload sediment rating curve developed for this
reach during the baseline study was used as the basis
for determining a minimum post-project channel flow
regime.
A stream gage has been operated at point A since
1924. [The stream flow record used in this example is
from Goose Creek, just upstream from Cheesman
Dam and Reservoir. The bedload rating curve was
taken from Gordon (1995).1 It was assumed that the
59-year record period between 1924 and 1982 was suf-
ficiently long to develop operating criteria for the pro-
ject such that a minimum long-range average 1,000
tons/year of bedload transport capacity would be
maintained through the reach at point A. The stream-
flow record was analyzed as follows:
(1) The number of days that average daily flows
equalled or exceeded bankfull discharge (165 c.f.s.)
was determined for each year of record.
(2) The cumulative bedload transport capacity in
tons for these days was calculated using the baseline
bedload sediment rating curve (Figure 3), from Gor-
don (1995, p. 74).
(3) The duration and frequency of flows greater
than or equal to bankfull discharge (QBF) necessary
to transport the minimum required 1,000 tons/year of
bedload sediment was determined. These calculations
are summarized in Table 2.
Results
The number of days that average daily flows
equalled or exceeded bankfull discharge during the
1924 to 1982 record period is summarized in column
2. It is interesting that during 34 of the 59 years,
average daily flows did not reach bankfull discharge.
(The consecutive occurrence of these low-flow years
varied from one to as much as six years.) The average
number of days each year that mean daily tlows
equalled or exceeded bankfull was 6.9 days. The
range varied from 0 to 46 days.
The transport capacity for all mean daily flows
equal to or greater than bankfull discharge is summa-
rized in column 3. This column was calculated by
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 871 JAWRA