C153804 Feasibility Study

• Contract/PO #: C153804
• Contractor Name: Left Hand Ditch Company
• Contract Type: Loan
• Water District: 5
• County: Boulder
• Bill Number: HB 95-1155
• Loan Projects - Doc Type: Feasibility Study

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accomplished by placement of additional fill material on the dams or by construction of parapet

walls along the upstream crests of the dams. Fill material will have to meet embankment

specifications and will be subject to the SEO rules regarding crest width which will be narrowed by

placing additional fill. The parapet wall can be a simple conventional concrete retaining wall which

will provide additional surcharge storage witho\1t narrowing the dam crests. A detailed discussion

of these elements is included in subsequent sections. For reasons explained in Chapter V, Section

C.3, placement of fill for raising the dams is !lot a feasible option at this site. The parapet wall

concept is considered in the new spillway alternatives.

The most significant narrowing of the new spillway occurs when the flowline crest is
constructed at an elevation lower than the existing spillway and a fuse plug embankment is
constructed across the new spillway. The fuse plug will cause the existing spillway to be used
initially. When the surcharge storage is great enClugh the fuse plug would overtop, erode away, and
open up the full capacity of the new spillway. With the fuse plug eroded out of the spillway channel
the capacity can be very large because of the greatly increased head. For reasons explained in
Chapter V, Section C.4, a fuse plug spillway c<Jnstructed on an RCC spillway floor located below
the normal high water line of the reservoir is not feasible and is not included for further
consideration for this project.

Based on the preceding discussions, Plan C is reduced to the consideration of the required
size of an RCC spillway to be constructed over the north dam of Left Hand Yalley Reservoir. The
following is a brieflist of the major components of the preliminary design of an Plan C spillway as
shown in Figures IY-2 and IV-3.

. An upstream crest section at elevation 5,3 51 will be constructed of two layers of roller
compacted concrete each one foot thick to form the floor of the spillway. A cutoff wall
consisting of three layers of RCC, nine feet wide and one foot thick will be placed in an
excavated trench at the upstream end of the RCC floor. The layers of RCC used to form