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sizes of spillways for several heights of dams were used. Cost studies were limited to dam heights no greater than those fixed by the maximum allowable reservoir water surface elevation.
(a) Tallwter Computations.— A tailwater curve was computed for the Republican River below the dam site for both the present and improved channel conditions. The values of Manning's coefficient of roughness "n" used for these computations are indicated below:
Flow Present Improved
Channel 0.030 0.025
Overbank 0.075 0.045
These values are based on measurements made from the 1935 flood, which had a discharge of 200,000 second-feet.
11. Economic and Other Considerations.- Comparative cost estimates were made involving various schemes in which the location of the spillway with respect to the dam, the length of the spillway, the type of spillway, and the method of operation of the spillway gates together with the resulting height of dam were compared on the basis of the total cost of the dam. The location of the spillway with respect to the earth dam was given consideration in an effort to determine a location that would be the most economical and suitable on the basis of the geological characteristics of the foundation material. Gated and ungated spillways were studied in combination with gravity and slab and buttress overflew crests, as was also the effect of the method of operating the gates to control the spillway discharge.
The method of connecting the concrete spillway structure to the earth dam was given special consideration, and the following designs were investigated:
(1) A counterforted wall separating the earth dam from the spillway structure.
(2) Gravity abutment sections at both sides of the spillway structure, which would permit the embankment to be constructed to a low elevation at the spillway structure, increasing in height to the dam crest elevation at the outer end of the abutment sections.
(3) Slab and buttress abutment sections on both sides of the spillway structure designed for increasing heights of embankment as in the gravity abutment section design. This was the scheme adopted in the final design.
Consideration was also given to schemes combining the river outlets with the spillway in place of the construction of a separate river outlet works.
In each of the schemes investigated, full consideration was given to the safety of the project, the geological limitations of the site, and the accepted structural and hydraulic design standards.
12. River Diversion Plan. - Studies made with a 10-year frequency flood having a peak flow of 51,000 second-feet and a volume of 61,000 acre-feet, indicated that a flood of this magnitude could not economically be diverted through a diversion tunnel or conduit. Therefore, it was decided to leave a gap in the~dam embankment for the river flow while the major portion of the embankment was being constructed. The gap had 4 to 1 side slopes and sufficient width to permit the passage of the 10-year flood without flooding the railroad or interfering with construction activities. Closure of the gap and diversion through the spillway was not permitted until the entire embankment, except that portion used for temporary diversion, had been completed to elevation 2760, and construction of the spillway had advanced to such a stage that it could be used for diversion.