26. Preliminary Studies and Designs. - (a) 1935 Design.--Preliminary design studies for a development at the Palisades Dam site were initiated in 1935. The development was then designated the "Grand Valley Dam" and contemplated a reservoir with a flow line at elevation 5620, to store an assumed 1,400,000 acre-feet of active irrigation and flood control storage. A layout of the scheme is shown on figure 45.

The spillway indicated for this design was placed on the left abutment and consisted of a side channel controlled with five 72- by 14-foot drum gates leading to an open channel chute with stilling basin. The spillway had a capacity of 70,000 second-feet with reservoir head at elevation 5620.

Diversion was to be accomplished through a 32-foot-diameter concrete and steellined tunnel, controlled by two 19-foot butterfly valves located in the tunnel in a chamber beneath the dam axis. After construction, the tunnel was to be converted to an outlet works by installing needle valve controls and a stilling basin at its lower end. The outlet capacity was 18,000 second-feet at minimum reservoir water surface elevation 5455 and 31,700 second-feet at water surface elevation 5620. Power penstock takeoffs from the tunnel were contemplated to serve a three-unit 50, 000-kilowatt powerplant.

(b) 1939 Design.-- The preliminary design of the Palisades Dam development was restudied in 1939 to correct certain deficiencies inherent in the earlier scheme. Principal revisions were an enlargement of the outlet works and power penstock system and a reduction in the spillway size. The revised scheme is shown on figure 46.

The 1935 scheme was considered unsatisfactory because of the single tunnel to serve both the outlet valves and the powerplant, since the high velocities and possibilities of water hammer under maximum discharge conditions would affect efficient production of power. Further, it was considered that both the outlet works and spillway could be utilized to release flows resulting from the inflow design flood, thus reducing the required capacity of the spillway.

The revised scheme considered two 27-foot-diameter diversion tunnels, one of which would be converted to an irrigation outlet and the other to a combined flood control outlet and power penstock. Each tunnel was provided with two 14- by 28-foot emergency closure gates at the upstream end and a 20-foot-diameter steel pipe leading to regulating gates at the downstream end. Flows from the irrigation outlet tunnel were to be regulated by three 96-inch ring seal gates and two 78-inch tube valves. Supplementary flows from the power penstock tunnel were to be provided by a bypass controlled with three 96-inch ring seal gates. The capacity of these outlets was approximately 20,000 second-feet at minimum reservoir elevation 5455 and approximately 34, 000 second-feet at normal reservoir elevation 5620.

The spillway for the revised scheme consisted of an open channel chute placed in the left abutment adjacent to the dam. The spillway was provided with three 14- by 50-foot fixed-wheel control gates and terminated with a 150-foot-wide stilling basin. The spillway was designed for a maximum capacity of 50,000 second-feet at normal reservoir water surface elevation 5620. The stilling basin walls were provided with sufficient freeboard to accommodate a tailwater of 80,000 second-feet in the downstream channel, resulting from a combined flow through the spillway and outlet works. Deep gates were selected for the spillway control to permit releases for flood control at lower reservoir levels, in the event the outlets were inoperative.

The powerplant installation for this scheme contemplated a generation capacity of 30,000 kilowatts at a minimum power head of 104 feet. The scheme was incorporated in the December 9, 1941, report and findings of the Secretary of the Interior which was presented to President and the Congress for authorization. This report is recorded in House Document No. 457 of the 7th Congress, First Session.

(c) 1947 Revisions.--Although the Palisades project was initially authorized in 1941, further work was held in abatement until after World War II. A restudy of the inflow design flood was made in 1945, resulting in a revision of the inflow design flood hydrograph, increasing the flood peak from 90,000 to 105,000 second-feet and increas ing the 30-day flood volume from 3, 000, 000 to 3, 320,000 acre-feet. During the interim after authorization, a resurvey of the reservoir site was made, indicating lesser

reservoir storage capacities than formerly assumed. Damsite foundation exploration better delineated the foundation conditions for the contemplated structures. Flood control considerations further indicated a desirability for a study of alternative outlet works capacities, to provide discharge capabilities of either 20,000 or 30,000 second-feet at minimum reservoir water surface. In consequence of the above, a re study of the 1939 scheme was undertaken in 1947, resulting in the alternate designs shown on figure 47.

The revised schemes essentially adopted the arrangement employed in the 1939 study, except that a tunnel spillway was substituted for the open channel design considered previously. This change was deemed desirable after a study of the foundation geology at the left abutment disclosed better conditions for the tunnel scheme, since it was considered that deep open channel cuts on steep slopes might not prove stable. Also, snowslide hazards from the high, steep slope above the spillway favored the closed tunnel scheme. Further a decision to handle all flood outflows initially through the outlet works and to consider the spillway only as an auxiliary discharge device permitted omitting the spillway stilling basin, thus placing the tunnel in economic competition with the open channel.

The normal reservoir water surface was kept at elevation 5620. Both alternatives considered an outlet works and power penstock arrangement similar to that utilized in the 1939 design. A comparison of the alternative schemes is tabulated below: 1947 alternative, 1947 alternative, 20,000 second-feet 30,000 second-feet outlet capacity outlet capacity

1939 scheme