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Figure 40.-- Piezometer terminal well with sump for 60 tips.

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CHAPTER IV --Design--SPILLWAY AND OUTLET WORKS

A. Spillway and Outlet Works Design Criteria

22. Design Flood Studies. - (a) Historical Floods.-- The maximum flood of record at Palisades Dam site was recorded in 1918 at the nearby Heise gaging station. This flood had a maximum mean daily discharge of 51, 500 second-feet and a 30-day volume of 2, 206, 000 acre-feet.

The maximum known historical flood occurred in 1894. There was a record of gage heights of this flood maintained near Idaho Falls, where the drainage area is somewhat larger than at Palisades. It was necessary to estimate the peak discharge. It is believed this flood produced a peak of about 75,000 second-feet and a maximum 30-day volume of 3,600,000 acre-feet near Idaho Falls. A slight reduction for the flows into Palisades Reservoir would have prevailed.

A maximum discharge of about 60,000 second-feet occurred May 19, 1927, partially as a result of washing out of a landslide on the Gros Ventre River.

(b) Development of the Snowmelt Flood.--The maximum probable snowmelt flood was developed by using correlations of accumulative mean daily temperatures in excess of 32 F. at Moran, Wyo., and accumulated runoff records for the 1909 and 1918 floods at Lyon and Heise, Idaho, respectively. The maximum probable 1-, 5-, 10-, 15- and 30-day temperatures above 32° F. at Moran, Wyo., were derived from probability curves covering 29 years of temperature record and applied to the temperature runoff correlation of the 1909 and 1918 events. The relation of these temperatures to the 1918 correlation proved to be more critical and was used to derive the maximum probable snowmelt flood. This maximum probable snowmelt flood when 20,000 second-feet of base flow was added resulted in a maximum mean daily flow of 83,000 second-feet and a maximum 30-day volume of 3, 200, 000 acre-feet.

(c) Rainfall Analysis.-- It is possible that some runoff from rainfall might occur during the maximum probable snowmelt flood. To determine the rain flood, a frequency of precipitation at four stations in the drainage area, namely Afton, Bedford, Jackson and Moran, was obtained. This frequency of precipitation indicated the possibility of 2.10 inches of precipitation. No precipitation stations in the basin are above elevation 6800, mean sea level datum. It was believed that any precipitation above elevation 7800 would probably occur as snow, so for the design rain flood it was assumed that only 2,500 square miles of drainage area would contribute. A runoff factor of 40 percent was applied to the 2. 10 inches of precipitation. Based on the above assumptions an approximate hydrograph was drawn. Lack of data at the time the study was prepared (1945) precluded a more elaborate analysis.

The design rain flood hydrograph was added to the design snow hydrograph beginning on the 12th day. A composite hydrograph resulted in an inflow design flood having a peak discharge of 105,000 second-feet and a 30-day volume of 3, 320,000 acre-feet.

(d) Selection of the Inflow Design Flood.--The inflow design flood studies considered records of historical floods, results of historical floods, results of probability studies, and theoretical analyses by accepted flood prediction methods. Results of these studies are as follows:

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The recommended design flood hydrograph used as the basis of spillway design is shown on figure 42. This flood was derived by combining the design snowmelt runoff hydrograph and the design rain hydrograph. The Palisades Reservoir was assumed full to the top of the joint-use (conservation and flood control) storage pool at the beginning of the 30-day flood event with a leading flow of 37.000 second-feet passing through the outlets.

(e) Frequency Studies.-- To evaluate diversion requirements and reservoir operation during construction, the probable 5-, 10-, 25-, and 50-year flood hydrographs were prepared using the following values taken from the probability curves included in the flood studies:

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23. Reservoir Operations. - (a) Reservoir Storage Allocations.--The Palisades project is a multipurpose development învolving irrigation, power, flood control, recreation, and fish and wildlife benefits. The project will provide supplementary irrigation water for about 650,000 acres in the Upper Snake River valley, will give flood protection for several thousand acres of irrigated land on the Snake River plain above Idaho Falls, and will furnish hydroelectric power to serve irrigation pumping loads, municipalities, rural cooperatives, and others. Of a total of approximately 1,400,000 acrefeet of reservoir storage, 200,000 acre-feet will be inactive storage to provide the minimum power operating head, and 1,200,000 acre-feet will be joint-use storage to serve irrigation, flood control, and power generation uses.

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(b) Reservoir Operation Plan.--The entire active storage space in the Palisades Reservoir is to be used jointly for irrigation and flood control purposes. The production of power at the Palisades Powerplant will be entirely incidental to the operation of the reservoir for irrigation and flood control. The Palisades Reservoir will of necessity be operated in conjunction with other reservoirs along the Snake River, and the production of power will be possible throughout the year because of the passing of certain amounts of water to fill prior storage rights downstream.

The Palisades Reservoir is to be operated concurrently with the Jackson Lake Reservoir upstream for regulating flood runoffs. For the combined reservoirs, not less than 75 percent of the total required flood control space must be made available in Palisades Reservoir.

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