(1) The slope of the floor between the gate structure and beginning of the vertical curve was selected with the flattest grade which would support supercritical flow and produce a slightly accelerating velocity for all flow conditions. A 2 to 1 grade of the lower tangent leading to the stilling basin was selected as the steepest which would insure stability against sliding for the walls and floor. A 200-foot-radius curve connecting the upper and lower slopes was selected to provide an easy transition of flow and to minimize the tendency for high-velocity flows to spring away from the chute floor.

(2) The divergence of the channel was made uniform from the gate structure to the stilling basin. The flare from 361 feet at the gate structure to 400 feet at the stilling basin in a length of 297 feet, resulted in a flare angle between the chute walls and spillway centerline of 3.750. This value was checked with the permissible angle of flare from other spillway designs, from model tests of accepted designs, and by the emperical formula1,

1/From "Characteristics of Supercritical Flow at an Abrupt Open Channel Enlargement", by Dr. B. V. Bhoota--Thesis, University of Iowa, 1942.

The accepted angle of flare was less than the computed permissible for all flows up to the maximum of 200,000 second-feet. This was later verified by model tests.

Friction losses and depths of flow in the chute were computed by using a roughness coefficient, n, of 0.014 in Manning's formula.

46. Stilling Basin. The spillway stilling basin was designed to provide hydraulic-jump energy dissipation for all discharges up to a maximum of 200,000 second-feet. For a uniform flow distribution at the maximum discharge and allowing a maximum flow of 500 second-feet per foot of width, a width of 400 feet was established for the spillway.

The hydraulic jump characteristics were determined from the theory of conjugate stages of flow, derived from the force-momentum equation. For computing the theoretical depth of jump, the following equation was used:

in which d1 and d2 are the conjugate depths before and after the jump, respectively, and V1 is the velocity of supercritical flow before the jump.

The energy available for producing the jump was determined by using a coefficient, n, of 0.014 in Manning's formula. Initially, a length of 120 feet was determined. As a result of model tests, it was determined that acceptable performance could be obtained by raising the basin floor 5 feet and shortening it 5 feet. This change resulted in a reduction in the cost of the stilling basin. The following basin dimensions and flow characteristics for a discharge of 200,000 second-feet were adopted:

Under maximum discharge conditions, the entire jump will not be contained within the basin. However, the use of dentated sills at the end of the shortened basin should be instrumental in holding the jump on the stilling basin apron and in deflecting high-velocity flows from the bottom upward. As a result, undesirable scour in the downstream areas will be minimized. Riprap material was provided in an area about 75 feet long adjacent to the downstream end of the stilling basin, to protect the foundation against scouring.

47. Model Studies. - Hydraulic model tests of the spillway and outlet works were conducted concurrently with specifications designs. After specifications No. 1410 for the construction of Enders Dam were issued, the hydraulic model studies were completed and additional tailwater studies were made. As a result, better discharge characteristics through the spillway gate structure were obtained and the assumed tailwater elevations were reduced. This permitted several important modifications to be made in the original design. These modifications included the following:

(1) A reduction in the assumed maximum water surface elevation from 3130.7 to 3129.5.

(2) A reduction in height of dam, spillway gate structure walls, and piers equal to 1.5 feet. (Crest of dam was lowered from elevation 3139.0 to elevation 3137.5.)

(3) A reduction of 11 feet in the maximum assumed tailwater elevation.

(4) Changes in the spillway which included: lowering of the level of the stilling basin apron by 5 feet; a reduction in height of the stilling basin walls; a reduction in length of the concrete portion of the spillway structures by 10 feet; and a change in the dentated sills. By lowering the stilling basin apron, better foundation conditions were obtained, and the length of the sheet piling cutoff at the end of the stilling basin floor was reduced.

A substantial reduction in the spillway cost was realized through these design modifications. A report of model tests for the spillway and outlet works is given in the Bureau's Hydraulic Laboratory Report No. Hyd. 252, December 1948.

2. Structural Design

48. Design Stresses, Loadings, Etc. - Loadings, stresses, and other data used in the design of various structures of the spillway are indicated in appendix D. Other loadings and data applicable to a specific structure of the spillway are indicated in the design section for that structure or by an appropriate appendix reference.

49. Inlet Walls. Flow is guided into the spillway gate openings by left and right vertical-faced walls. The left wall is divided into four curved sections, each provided with two counterforts extending radially from the face wall, and one straight section with three counterforts extending normally from the face wall. Each curved section is 31. 4 feet long, and the straight section is 43 feet long. The wall height is about 52. 5 feet. The right wall is divided into two curved and two straight sections, each provided

with counterforts extending radially or normally from the face walls. The lengths of the straight and curved wall sections are 31.4 feet and 28 feet, respectively. The height of the right inlet wall is 50 feet. Both the right and left inlet walls have a toe width of 12 feet and an overall footing width of 37 feet. These walls were designed and reinforced in accordance with standard Bureau practice. A tabulation of loading conditions and other data used in wall design are given as appendix E.

50, Inlet Floor Slab. To protect against erosion and provide a percolationresistant medium upstream from the gate structure, the inlet channel floor and sides were lined with a concrete slab 15 inches thick. This lined area is about 95.5 by 270 feet. For convenience of concrete placement and to minimize the effects of temperature and shrinkage stresses, the slab is divided by metal-sealed contraction joints into blocks about 30 feet square. These contraction joints were also placed between the floor slab and right and left inlet walls, and between the floor slab and gate structure. These seals tend to increase the watertightness of the slab and materially reduce percolation under the slab. It was assumed that the downward weight of water on the inlet slab will be greater than the uplift below the slab, since the slow change of reservoir levels should permit unbalanced pressures under the slab to become equalized with reservoir pressures. A 6-foot-deep cutoff at the upstream edge of the floor slab prevents leaching of foundation materials into the riprap blanket which was placed immediately upstream. Reinforcement bars were placed in the floor slab and cutoff to resist temperature and shrinkage stresses only. This reinforcement consisted of 5/8-inch round bars placed at 12-inch centers both ways in the top face of the floor slab and in each face of the cutoff. 51. Gate Structure. The spillway gate structure includes a crest slab with attached piers, left and right abutment walls, hoist deck, compressor and battery room structure, and highway bridge. The spillway gate structure has an overall length, including the abutment wall footings, of 411 feet, and a length upstream and downstream of 69.5 feet. A 3- by 6-foot cutoff at the upstream edge of the crest slab serves as a percolation barrier and as an anchor against sliding for the structure. The crest has an overall width of 361 feet from face to face of the abutment walls, and is 69.5 feet long in an upstream and downstream direction. This width includes six 50-foot-wide gate openings, four 8-foot-wide intermediate piers, two 9.5-foot-wide center piers, and a 10-foot-wide uncontrolled crest. To improve their hydraulic characteristics, all piers have an elliptically rounded upstream nose and a gradually tapering downstream tail. The piers support the 19.5-foot-wide hoist deck, the 26-foot-wide highway bridge, and by means of attached brackets near the downstream end support radial gate arm bearings. The two center piers support the 14.5-foot-long compressor and battery room structure which spans the uncontrolled crest.

The pier and crest units were designed to resist stresses induced by (1) the weight of the hoist deck, compressor and battery house, and highway bridge; (2) reservoir water loads on the piers and on the gates; and (3) live and impact loads resulting from highway bridge traffic. The piers and crest are reinforced to resist moments and shear resulting from various loading conditions. Where no moment reinforcement was Indicated, the exposed surfaces of the structure are reinforced with a minimum of 3/4inch bars at 12-inch centers in order to control temperature cracking.

The abutment walls are of the counterfort type, with the toe serving as a portion of an overflow weir. The top surface of the toe is shaped to conform with the profile of the weir. The bases for these walls have an overall width of 50 feet and extend to elevation 3088,75. The length of the toe-crest combination is 25 feet. The cutoff wall at the upstream edge of the crest is extended under the base slab to the back side of the wall footing. The walls are 49.5 feet high above the base and are notched to accommodate the hoist deck and highway bridge. Backfill is placed behind the walls to elevation 3137.5.

The hoist and operating deck, crossing the spillway channel along the upstream side of the gate structure and supporting the gate hoist mechanism, is a reinforced concrete beam and deck slab 369 feet long and 19.5 feet wide. The deck consists of six simply supported spans 68 feet long and a center span 19.5 long. Each 58-foot span is made up of two parallel girders, each 2,5 feet wide, 5 feet deep, and spaced at 15.5foot centers, and a 10-mch-thick deck slab. An 18-inch overhang cantilevers from the

downstream girder. Five 12-inch-wide and 3.8-foot-deep diaphragm walls are placed between the girders for lateral stiffness at points of greatest concentrated load. Two of these walls are located near the end of each span and one near the center. The girders are reinforced with longitudinal bars and U-shaped stirrups to withstand moment and shear stresses resulting from dead load, hoist loads, and assumed live loads. The diaphragm beams and the deck slab are each reinforced with bars in both faces. The overhanging cantilever is reinforced with stirrup bars which extend into the adjacent girder. The hoist and operating deck was designed to support the hoisting machinery with loadings resulting during hoisting operations, plus an assumed live load on the deck, and wheel loads of an 18-ton traveling crane.

The compressor and battery room structure, about 14.5 feet wide, 40.5 feet long and 10 feet high, is of reinforced concrete construction. Shelves on the center piers provide support for the structure. A portion of the roof of the structure serves as a connecting span of the hoist deck. The compressor and battery room was designed as a rigid frame and is reinforced to resist stresses induced by temperature, shrinkage, dead loads, batteries, and live loads from compressor machinery and an 18-ton traveling crane. The floor slab is 12 inches thick, and the roof slab ranges in thickness from 4 to 12 inches. The roof beams under the traveling crane track rails are 3 feet wide and 21 inches thick. The end walls of this structure are reinforced with 5/8-inch, bars placed at 12-inch centers. The side walls and roof slab are reinforced to resist moment stresses, and the floor slab is reinforced with two layers of reinforcement to resist moments. In parts of the structure where moment reinforcement was not indicated, reinforcement was provided to control temperature and shrinkage cracking.

(a) Loadings, Foundation Pressures, and Sliding Factors. --Since stability and stresses of the structure change with different loading conditions, analyses were made of several loading conditions. The selected design was based on the most critical loading conditions. For the design of the piers, abutment walls, and crest structures of the spillway gates, the following loading conditions were investigated:

(1) No water in reservoir; gates open.

(2) No water in reservoir; gates closed.

(3) Water in reservoir to top of gates, elevation 3127.0; gates closed.

(4) Water in reservoir to maximum water surface, elevation 3129.5; gates open.

(5) Water in reservoir to elevation 3127.0; one gate open and adjacent gate closed.

For each of the above conditions, a loading of dry backfill behind the abutment was assumed. Resultant foundation pressures under the gate structures were computed by assuming a parabolic foundation pressure distribution. These computations were based on Technical Memorandum No. 596, "Foundation Pressure Distribution on Symmetrically Loaded Slabs. For these computations, the following foundation and concrete properties were assumed:

Modulus of elasticity of foundation, p.s.i.
Modulus of elasticity of concrete slab, p.s.i.
Ratio of modulus of elasticity of concrete to
modulus of elasticity of foundation

26,700 4,000,000

150

The arrangement and dimensions of the gate structure components were selected that the maximum foundation pressure and sliding factor would not exceed 4 tons and 0.45, respectively. The actual computed values are indicated below:

In the design of the hoist deck, compressor and battery house the following loadings were assumed:

52. Highway Bridge. --A highway bridge, 369 feet long and 26 feet wide, crosses the spillway gate structure at the downstream side of the structure. It is divided into six 58-foot-long spans and one 21-foot-long span. These spans are simply supported on the piers and abutment walls of the gate structure. Each 58-foot span consists of four parallel girders, each 21 inches wide, about 4 feet deep, and spaced at 6.75-foot centers, and a 7-inch minimum thickness of deck slab cast integrally between the girders. The deck slab extends 24 inches beyond the upstream and downstream girders, and a 12- by 12-inch curb is placed at each overhang. The deck slab surface cross section has a 3-inch parabolic crown, and each span is cambered 1-1/4 inches. Diaphragm beams 12 inches wide and 26 inches deep are placed between the girders and at the ends of the span. Each bridge span rests on fixed rocker-bearing plates on one end and on 8-inch-diameter rollers at the other end. The 21-foot center span has a 14-inch minimum thickness deck slab and is simply supported by the two central piers of the gate structure. A view of the bridge is shown in figure 21.

The highway bridge was designed in accordance with the American Association of State Highway Officials specifications for two lanes of wheel traffic, and for trailertruck loadings of H20-S16-44 plus 30 percent increase in live load stresses to allow for impact and vibration effect. The following loading assumptions were used in the design: