PROJ00519

• Contract/PO #: C153597
• Contractor Name: Windsor Reservoir and Canal Company
• Contract Type: Loan
• Water District: 3
• County: Larimer
• Bill Number: HB 91-1006
• Loan Projects - Doc Type: Feasibility Study

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

Woodward.CIyde Consultants

spillway "A", located about 1100 feet left (south) of the outlet works, and spillway "B",
located about 1600 feet (~ollowing the crest of the dam) right (north) of the outlet works.
They have a combined capacity of only about 15% of the PMP. To upgrade the
spillways of Douglas Dam, three options were considered:

1) Spillway A would be enlarged and excavated to a depth of 7.5 feet (from the dam
crest elevation) so that it would be able to pass the 100-year flood by itself.
Spillway B would be shallower so that it would be dry during the 100-year flood,
but long enough so that both spillways would be able to pass 75% of the PMP.
One objective of this design is to keep access to the outlet works control house
during the 100-year flood.

2) The roles of spillways A and B as presented in option 1 were switched for this
option.

3) Spillway A would be backfilled to the crest of the dam and spillway B enlarged
so that it would safely pass 75% of the PMP.

The third option was selected over the two other ones for the following reasons:

· It is simpler to design.

· It appears to be less expensive.

· During the 100-year flood, the water flowing over the spillway would be
discharged to Dry Creek.

· The outlet works control house would be accessible at all times.

The new proposed spillway will be located on the right (northwest) side of the
embankment. It will have a crest length of 642 feet with an invert elevation of 5207.3
feet. It will be 7.5 feet lower than the crest of the dam (El. 5214.3 feet) and it will have
5:1 (horizontal to vertical) side slopes for vehicle access.

22539GIDQUGLAS.DAM 12.17-90IREPORTS3

26