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CHAPTER III--DESIGN

9. Hydraulic Requirements. - In accordance with flood control regulations of the Flood Control Act dated December 22, 1944, and field working agreement between the Bureau of Reclamation and the Corps of Engineers dated October 26, 1954, capacities for dead storage, irrigation storage, and flood control storage were established for Trenton Dam and Reservoir. Discharge capacities of regulating structures were specified and allocations in the reservoir were made as follows:

Water surface Storage,

Purpose elevation acre-feet

Flood control 2752.0 to 2773.0 133,800

(initially)

Irrigation storage 2710.0 to 2752.0 116,100

(normal water (initially)

surface)

Dead storage Streambed to 2710.0 4,100

(invert of canal
outlet)

Total storage to elevation 2773.0 (top of gates in closed
position) 254,000

Surcharge above elevation 2773.0 107,670

Total reservoir capacity at maximum water surface
elevation 2785.0 361,670

The limiting water surface elevation of 2797.0 that would cause no property damage was determined for the town of Stratton, located about 8 miles upstream from the dam. However, some measures would be necessary for protecting the water supply wells and sewage treatment plant against pollution or damage when this water surface is reached. A maximum allowable reservoir water surface elevation of 2785.0 was established by allowing 12 feet below the limiting elevation for backwater conditions and aggradation of the stream channel.

An irrigation outlet was required for supplying 300 second-feet of water to the Meeker Irrigation Canal with canal water surface at elevation 2705.0. A river regulation outlet was required with a discharge capacity of 1,000 second-feet at reservoir water surface elevation 2720.0. Economic and physical feasibility studies were made to determine whether these requirements could best be met with one or two outlet structures. There will be no demand for discharge into the canal during the nonirrigation season, but discharges for river regulation may be required at any time.

For flood control, a spillway was required having a discharge capacity of 10,000 second-feet, the safe channel capacity of the river, with the reservoir level at normal water-surface elevation 2752.0. The spillway structure was to be capable of regulating the discharge to a value not exceeding the safe channel capacity until the reservoir water surface is above elevation 2773.0, the top of the flood control pool. An elevation of 2710.00 was required for the sill of the outlet works in order to maintain the dead storage at that elevation.

10. Flood Routing. - A final flood routing using a flood hydrograph provided by the hydrologlsts and based on a spillway discharge curve determined by a model test gave a maximum reservoir water surface elevation of 2785.6, a surcharge of 113,420 acre-feet, and a total reservoir capacity of 370,000 acre-feet. (See fig. 7.)

In making flood routing studies, an inflow hydrograph having a peak discharge of 295,000 second-feet and a 3-day volume of 396,000 acre-feet was routed through the reservoir. At the start of the routing, one-half of the flood control storage was considered as filled. Outflow was limited to the safe channel capacity of 10,000 second-feet until the flood control storage space was filled. For the study, various schemes and

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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) Taituater Computotions.— 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. Rive r D ive r s ion PI an. - 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.

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