C153857 Feasibility Study

• Contract/PO #: C153857
• Contractor Name: Western Mutual Ditch Company
• Contract Type: Loan
• Water District: 2
• County: Weld
• Bill Number: SPL
• Loan Projects - Doc Type: Feasibility Study

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Western Mutual Diversion Repairs and Improvements
eWeB Feasibility Study
May 6, 1998 (Revised December 29, 1998)

The Company's engineer provided technical engineering assistance in the evaluation of the above
alternatives, The critical check replacement design issue was providing the ability to check the
river up to historical levels to restore physical diversion capacity, while also providing a low flow
bypass to be used if repairs need to be made to the main diversion dam and/or head gate. This
required that water flow over the top of the check structure during high flows and be prevented
from flowing in the south channel during low flows, maximizing the ability to divert at the ditch
headgate, Another consideration was the protection of the island located between the north
channel and south channel. Without erosion protection the island might erode away sufficiently to
cause the river to take a different course,

Island Revetment Alternatives Evaluated

In order to prevent further erosion of the island formed between the north and south channels of
the South Platte River, the Ditch Company seeks to construct an erosion barrier, or revetment, on
the eastern end of the island from the south end of the Company's diversion dam, to the north end
of the proposed new check structure. Two materials were evaluated for this construction, concrete
rubble riprap and rock riprap, Both would be built on a granular bedding layer proposed to be 3-
inch minus crushed recycled concrete. Because the size required for the revetment, an average
stone size (dso) of 8 inches, is not very large (recycled concrete is not available in very large sizes
with desirable shape characteristics), either material would be technically feasible. A cost
difference of $10 per ton ($17-20 per ton for rock riprap vs. $7-10 per ton for recycled concrete)
makes the recycled concrete aggregate more economically feasible. Estimated construction costs
for the island revetment are presented in Table 3,

Table 3

Summary of Estimated Island Revetment Costs
Western Mutual Ditch - Diversion Repairs and Improvements

Island Revetment - Recycled Concrete Rubble Riprap
Description Quantity Unit Unit Costs Item Costs
Low Estimate High Estimate Low Estimate High Estimate
Riprap 2,200 Tons $7,00 $10.00 $15,400,00 $22,000,00
Bedding 900 Tons $11 ,00 $ 13. 00 $9,900,00 $11,700,00
Total $25,300.00 $33,700.00

(Continued)

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