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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3. Sediment Transport Capacity

As discussed above wder existing channel conditions, the backwater
effect of the reservoir is not severe. If, however, water levels at the dam
were held significantly above EI 5058 during periods of large sediment inflow
to the reservoir, the backwater effect would gradually increase and with time
could significantly aggravate flood stages due to the formation of a deltaic
deposit just downstream of the Parachute Bridge. If the river channel becomes
filled in by the delta, flood stages could rise above EI 5075.

The backwater effect of the reservoir decreases the sediment trans-
(5)
port capacity of the river. The relati ve sediment trans port capacities
for the 35,000 cfs and 15,000 cfs nows are compared in Table II-8 for varying
degrees of the backwater effect. The com par ati ve trans port is calculated on
(1)
the basis of the Meyer-Peter, Muller equations and for a median sediment
size of one millimeter. In addition to the sediment load (pounds per second)
decreasing with increasing backwater effect, the sediment particle size being
(7)
transported also decreases. These combined effects cause the formation
of a delta as the sediment load is dropped. The position of the delta with
respect to the bridge can be controlled by lowering the pool elevation and
thus moving the area of significant backwater effect farther downstream. The
possi bility of flood-stage aggravation to a level higher than that of the
Project boundary would be limited and probably avoided altogether by causing
the delta to form a sufficient distance downstream from the bridge. By draw-
ing the reservoir down to EI 5058 at the dam during the passage of the 100-
year flood, existing sediment transport capacity can be closely approximated
as far downstream as 6,000 feet from the Parachute Bridge and hence avoid
formation of a significant delta in the upper reservoir area.

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The above analysis is based on approximate procedures and esti-

mates. It is doubtful whether more detailed study would give substantially

different results. More detailed study should be conducted during advanced

feasi bility studies. However, to confirm the results of these studies, the

advanced studies should be based on surveyed river cross-sections and actual

measurements of the volume of bed load and suspended sediments transported by

the Colorado River for various flow conditions.

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