CHAPTER IV--DESIGN OF SPILLWAY AND OUTLET WORKS

A. Preliminary Studies and Basic Data

34. Selection of Design Flood. In selecting a spillway design flood, flood design studies were made and flood potentialities were analyzed by several methods. These included the unit hydrograph method of analysis, and a study of envelope curves of experienced floods. For the latter study, a probability curve for flood peaks and volume were developed by the California method from estimated values at Enders Dam site obtained from a 14-year record at stations upstream and downstream from the dam site. Data on floods determined by several methods and the recommended design flood are indicated below:

The recommended design flood was used as a basis for spillway design. The peak flow of this flood represents a peak runoff of 247 second-feet per square mile, and the volume represents a runoff of 7.15 inches on the contributing drainage area (820 square miles) in 3-1/2 days, or 382 acre-feet per square mile.

The 200-year frequency flood having a peak inflow of 14, 200 second-feet with a 3-day volume of 29,800 acre-feet was selected as a reservoir design flood for determining the flood control storage space.

35. Diversion Studies.

For determining the probable diversion flood during construction, peak flow frequency and flood volume frequency curves, both at Enders Dam site from 1929 to 1942 inclusive, were considered. From these curves the following data were obtained:

These flood flows were expected to be limited to the period from April 1 through September. Mean daily flows during the nonflood season were seldom expected to exceed 150 second-feet.

As a result of these studies, plans were made to divert the river in two stages during the construction period, as follows:

(1) Through the natural river channel.

(2) Through an opening provided at the base of the inlet structure of the outlet works during the drier seasons when the major flood danger had passed and when the entire dam embankment had been completed except the closure section.

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Fig. I

Fig. I

The diversion through the outlet works could be accomplished prior to the installation of the regulating valves, either before or after the 84-inch outlet pipe had been installed. (During construction, the diversion was accomplished prior to installation of the outlet pipe.) Discharge capacities of the temporary 7.5- by 11-foot blockout provided at streambed level and at the entrance of the outlet works conduit are indicated below:

36. Flood Routing Studies. - For flood routing studies, the spillway gates were considered closed until the reservoir water surface reaches the top of the gates (elevation 3127). Smaller flood flows would be handled in temporary storage in the flood control pool and released by the uncontrolled crest. For determining the spillway design, the flood routing was started with the initial reservoir level at elevation 3116.2. A head of 3.9 feet was assumed for an inflow of 200 second-feet at the start of the flood. The following criteria were assumed:

(1) When the reservoir water surface elevation reaches 3127.0, the gates are to be opened at a rate so that the outflow does not exceed the inflow and a water surface elevation of 3127 is maintained.

(2) After the gates are 100 percent open, the reservoir water surface will rise until the peak of the flood has passed. When the water surface elevation has receded to 3127, the gates are to be gradually closed and finally completely closed in order to maintain a water surface elevation of 3127.

(3) Release of flood control storage is through the uncontrolled notch and through the outlet works or partial opening of the spillway gates.

A spillway design flood routing with the gates operated in accordance with the above conditions and with automatic controls is indicated in figure 13. With an initial reservoir level at elevation 3116.2, the reservoir surface will reach the top of the gates after 15 hours and 8 minutes. The inflow at that time will be 82,200 second-feet. All gates will be open after about 21 hours at which time the reservoir will have risen to elevation 3129.5 and the maximum outflow will be about 198,000 second-feet.

A spillway design flood routing under the same conditions but with the gates manually operated is shown on figure 14. The gates were assumed open until the reservoir reaches elevation 3127 and fully opened when the reservoir reaches elevation 3128.8, at which time the maximum outflow will be 194,000 second-feet.

37. Tailwater Studies. - Three tailwater curves were considered for use in the design work. These curves were computed for discharges between 48,000 and 200,000 second-feet. A curve was computed with values of n equal to 0.03 and 0.04 for the channel and the overbank flood plain, respectively. This curve was used for model tests and for the final design of the outlet works and spillway stilling basins and was believed to represent very closely the actual conditions.

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