C150252 Feasibility Study

• Contract/PO #: C150252
• Contractor Name: Henrylyn Irrigation District
• Contract Type: Loan
• Water District: 0
• County: Weld
• Loan Projects - Doc Type: Feasibility Study

The dam crest will be graded at a 1.0% slope toward the reservoir and 4 inches of roadbase placed
on the dam crest for erosion protection.
3. Construction Alternative 2- Complete Rehabilitation
This alternative considers a complete rehabilitation of the dam including breaching and removing
and replacing the outlet works, applying riprap bedding and riprap to the upstream face of the dam
from the dam crest to the upstream toe, and installing toe drains along the downstream berm or
downstream toe from station 1+00 to station 40+00. A summary of the components considered is
as follows.
a. Breach the dam at a 1.5 to 1 slope and remove the existing outlet works.
b. Replace the existing outlet conduit with two 48 inch diameter ductile iron pipes
encased in a minimum of 12 inches of concrete.
c. Gate operators will be hydraulicly operated and will be in stalled above the normal
water 1°evel in a gate control house. Gate stems will extend from the gates to the
operators and will be supported on an 18 inch by 24 inch deep concrete grade beam.
The reservoir gage rod will be supported on the grade beam.
d. T'he inlet structure will be concrete and will support the trash rack and two 48 inch
by 48 inch control gates.
e. The outlet Structure will be a USBR Type 8 baffled energy dissipater.
f. The upstream face of the dam will be stripped of the existing riprap and 8 inches of
bedding will be placed on the subgrade and 24 inches of riprap (d So = 12 inches)
placed on the bedding.
g. The dam crest will be graded at a 1.0% slope toward the reservoir and 4 inches of
roadbase placed on the dam crest for erosion protection.
Prospect Reservoir Dam
1. No Action Alternative
One course of action considered was to not do no major repair or rehabilitation of the dam and outlet
works. The results of this alternative would not necessarily result in no cost to HID. The result of
not repairing the outlet and controlling the seepage in the dam will ultimately result in major
restrictions on the reservoir storage. This reservoir is currently restricted to GH 35.5 feet which is
a loss of 1.5 feet of storage or 576 acre-feet. The restriction is due to the steep downstream slope
and the low factor of safety against a slope £ailure. We would expect additional restrictions if the
outlet is allowed to deteriorate without repair. The repair of the outlet will make no change to the
current restriction however which will require modification to the dam embankment.
Restrictions due to the outlet conditions would expected in less than five years. Irregardless of the
potential restrictions, however, both conditions can cause failure of the dam and the risk and cost of
the results of a failure would financially be catastrophic.
Horsecreek & Prospect Reservoirs - 06.014
Hettrylyn Irrigation District Page 9