PROJ00492

• Contract/PO #: C153364
• Contractor Name: Beck, R. W. and Associates
• Water District: 0
• County: Garfield
• Bill Number: SB 81-439
• Loan Projects - Doc Type: Feasibility Study

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dam site was based upon USGS quadrangle maps. The 100-year flood flow at
Parachute of 35,000 cfs is based on the USGS flood frequency curve developed
for the Cameo stream gage (Fig. II-4) adjusted on a drainage area basis.
Flood profiles for a flow of 15,000 cfs were also calculated. This flow
represents a common spring runoff flow, having a return period of 1.3 years.

The approximate existing water surface profiles (without the Proj-
ect) were cal<mlated on the above basis for 35,000 and 15,000 cfs and are
shown in Figs. II-9 and II-10, respectively, for the river reach near Para-
chute. The e:dsting river stage at the Parachute Bridge for the 100-year
flood is approximately EI 5073 or 3 feet above the proposed normal maximum
reservoir level. Approximately 3,000 feet upstream, the stage increases to
EI 5080 and 10,000 feet downstream it decreases to EI 5052. The proposed
normal maximllD pool level (El 5070) intersects the 100-year water surface line
approximately 1,000 feet downstream of the bridge. The water level at the
bridge for 15,000 cfs is approximately EI 5069 and the proposed normal pool
level intersects the water surface flow line approximately 500 feet upstream
of the bri dge .

Flood profiles for post-Project conditions at the same two flows,
35,000 and 15,000 cfs, are also shown in Figs. II-9 and II-10. Three profiles
are compared based on reservoir water levels at the dam of EI 5070, 5065, and
5058. For thE, lower two water levels a greater portion of the post-Project
water surface profile approximates the water surface profile under existing
conditions. In the areas of approximation, not only river stages but other
hydraulic parameters such as velocity and river top-widtb are of s:j.mllar mag-
nitude. In areas with similar hydraulic characteristics, the sediment trans-
port capacity of the river wder post-Project conditions would also be similar.

The backwater effect of the proposed reservoir during flood flows
becomes more sIgnificant as the reservoir is held at a higher level. However,
it can be seen fran the profiles that flood stage aggravation under existing
channel conditions is not significant even with the reservoir held at EI 5070
if the Project boundary is set at EI 5075 or above and no sedimentation of the
existing channl~l were to take place.