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Figure 6.--Railroad and highway relocation around Swanson Lake (the reservoir formed by Trenton Dam). (Sheet 1 of 2.) From drawing 328-D-999.
determine the most economical route. A route through Trenton, Nebr. , was favored by the Bureau of Reclamation. Subsequently, the Bureau of Public Roads in conjunction with the State of Nebraska, recommended a route north of the relocated railroad and additional studies were made of the "all northside" route (fig. 6). This route eliminated two underpass structures, some objectionable grades, and the traffic hazard of passing the highway through the city limits of Trenton. Also, the State of Nebraska designed the relocated highway in conformance with modern standards at a cost exceeding that of a replacement in kind. All of these additional costs were to be borne by the State. An inventory, including photographs of the existing highway, was made for record purposes and to aid in negotiations .
The Bureau's estimates were based on an estimated grade placed on a centerline profile. Structure locations were selected. Waterway openings were computed from drainage areas using the Talbot formula with C = 0.9. Lengths of culverts were estimated by using standard fill sections at structure sites, and bridge lengths were estimated from profiles. Unit costs were then applied to excavation, structures, and other items.
(c) County Road.— Nine miles of county road south of Swanson Reservoir were relocated. This relocation work involved the preparation of design data, including preliminary alinement and profile cross sections, and drainage areas.
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:
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 hydrologists 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