WSP09076

• File Number: 8443.300
• Description: Narrows Project - Legislation
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 8/1/1966
• Author: US DoI
• Title: Report on the Narrows Unit Colorado
• Water Supply Pro - Doc Type: Report/Study

" .., r .;
...UJ..

Plan of Development

coefficient of permeability, it is expected that an appreciable seepage
loss would occur through the sand into the ancestral channel. Overlying
the alluvial sand, on the right side of the present channel, is a layer
of terrace silt of varying thickness.

At the left abutment and beneath the river channel, the shale is
at shallow depths, but has undergone weathering effects to considerable
depth. Where the impervious embankment, spillway, and the outlet
conduits are on Pierre shale, excavation should be made to unweathered
rock. Because of swelling tendencies when subjected to saturation,
a sprayed protective coating or mortar would be required on excavated
shale surfaces for the conduits and spillway structure. In addition,
there will be some rebound as the load is removed by excavation, and
both design and construction practices should take this into considera-
tion.

The Pierre shale formation slopes down f'rom the river channel
toward the right end of the dam to such depth that it is not feasible
to seal the impervious embankment to bedrock. Probable water losses
through the sand and gravel overlaying the shale along the right abut-
ment cannot be fully controlled; but relief wells and toe drains, as
included in the dam design, would provide structural safety and limit
downstream seepage to tolerable quantities.

Dam and Reservoir

The dam shown on the feasibility design drawing, exhibit 3,
would be a zoned, rolled earthfill structure with a height above
streambed of 146 feet, a crest width of 30 feet, and a length of
22,120 feet at crest elevation 4453.0. A cutoff trench backfilled
with impervious material would extend under the left abutment and
across the river channel. The upstream face of the dam would be
protected by a 3-foot layer of rock riprap placed on an 18-inch gravel
blanket. The downstream face would be covered with 12 inches of
topsoil and seeded to grass. Total volume of embankment in the dam
would be about 8,840,000 cubic yards.

An uncontrolled concrete spillway discharging into a chute and
stilling basin would be located in the left abutment. A 30-foot-wide
concrete bridge would be provided across the spillway for relocated
State Highway 144. The spillway, with a crest elevation of 4428.5,
the top of the flood control pOOl, was designed with a capacity of

16