Lower South Platte River Study

• Contractor Name: Colorado Water Conservation Board
• Loan Projects - Doc Type: Feasibility Study

Lower South Platte River Study
Colorado Water Conservation Board
' December 2001
The upstream face of the main dam is protected by riprap consisting of broken
' concrete. The outlet works consist of two 4.5-foot-wide and 5-foot-high rectangular-
shaped concrete conduits cut into bedrock. This work, which was begun in 1992,
was completed in 1995.
'
Spillway
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' The spillway on the reservoir is located north of the Main Dam 4. The spillway
consists of an unlined earth channel with a minimum crest elevation of 3,709.5 feet,
which is approximately 2.5 feet above the normal water level. The spillway crest is
the gravel-surface county road. The spillway cross section is a trough-shaped cross
section 1,600 feet wide and 5.5 feet deep.
'
Currently, a flood storage and wave freeboard height of 5.5 feet is provided above
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the spillway level, which provides approximately 8,900 acre-feet of uncontrolled
' flood surcharge storage. Additional surcharge storage of 3,700 acre-feet is available
between the normal water level and the spillway crest (2.5 feet).
' There are several concerns related to the existing dam and potential reservoir
enlargement as identified in the Wheeler-Kumar report. These are paraphrased
' below:
' • A parapet wall may be needed on Dams 2 and 3 to provide sufficient freeboard
for wave runup
• The existing spillway can safely pass the flood associated with a 75 percent PMP
' event. A larger spillway will be needed if the dam is raised because a raised dam
will require a 100 percent PMP design
• The inlet canal (Harmony Ditch) would need to be modified for any reservoir
' enlargement
• Raising of the existing outlet works tower platform and lengthening of the outlet
' conduit would be required for any reservoir enlargement
• Seepage through the embankments is a current dam safety concern and will be a
critical concern for design and construction of any enlargement
• Seepage through the foundation also will be a critical design consideration. A
deep impervious core or slurry cutoff wall likely will be required to control
foundation seepage and prevent piping failure of a raised dam
' • The upstream slopes of Dams 2 and 3 are overly steep and should be flattened to
increase the stability of these existing dams
' • Dam 1 does not meet accepted stability standards and should be rehabilitated to a
condition equivalent to the project features
GEI Consultants, Inc. 4-9 01002 Dec 2001 Lower South Platte River Study