PROJ00221

• Contract/PO #: FS0017X
• Contractor Name: Ouray, City of
• Contract Type: Loan
• Water District: 0
• County: Ouray
• Loan Projects - Doc Type: Feasibility Study

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B.

Project Hydrology

A debris flow analysis was performed by Mussetter Engineering Inc. in 1997 and is
appended to this report. The Mussetter study presents a 100-year peak clearwater flow
for Skyrocket Creek of 540-cfs (based on COE information) and peak flow with debris of
1080-cfs. Total volume of debris from the 100-year event was estimated at 839,500
cubic feet. The Mussetter study utilized a HEC-2 model to determine depth and velocity
of flows at the diversion structure for 1080 cfs. Velocities were estimated at 20 to 24 feet
per second (fps) with depth of flows estimated at 5.1 to 7.3 feet. These numbers were
used for the feasibility study as they have been previously accepted by the COE and are
acceptable to CWCB (discussions with Larry Lang).

The man-made diversion channel was estimated at IS-feet deep and 10-feet wide. An
estimate of this channel capacity is 1900-cfs using the Manning equation with channel
slope of 2-percent and roughness coefficient of 0.04. Actual channel capacity during a
debris flow may be impeded by blockage if logs or large boulders become lodged in the
channel.

c.

Field Investigations

On August 16, 1989 Buckhorn Geotech visited the Skyrocket Creek diversion wall with
staff members from City of Ouray. Buckhorn Geotech again visited the site in 1989 with
Larry Lang from CWCB. The conclusion of the site observations at that time was that
the existing diversion wall of earth, rocks, rotting timbers and broken and frayed cables
was in need of replacement. Photographs from these visits are attached in the appendix.

Topographic mapping of the diversion structure location was performed by Monadnock
Mineral Services in conjunction with the preliminary cost estimated for the diversion
structure as prepared by Ayres Associates in 1997. Resultant mapping is presented in
that document as appended to this study.

Buckhorn Geotech was asked to evaluate the rock quality and soundness for the COE in
2000 to verify the feasibility of tying the proposed diversion wall into surrounding rock
material. Field observations and tests were conducted to determine strength parameters
for the formational rock at the diversion structure location.

D.

Alternative Evaluation

The two primary alternatives were considered for the Skyrocket Creek drainage:
1. The no action alternative.
2. The reconstruction of the diversion wall.

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