PALISADES REGULATING SLIDE GATE

RECOMMENDED DESIGN 7'-6" x 9'-0" DOWNSTREAM SEAL GATE

COEFFICIENT OF DISCHARGE VERSUS GATE OPENING-FREE DISCHARGE
Data from 1:19 Scale Model

Figure 68.-- Recommended design for 7.5- by 9.0-foot regulating slide gate--Coefficient of discharge versus gate opening. HYD-387-20.

(1) Trashrack losses. The head loss assumed through the trashracks was based on experiments conducted in the Munich Hydraulic Institute. 6/ The head loss is expressed as follows:

For 5/8-inch-wide bars spaced 6 inches on centers, e approximates 0.90, resulting in the following equation:

Flow net plots based on electric analogy tray studies (fig. 69) indicated that for full reservoir only that rack area below elevation 5533, approximately, will be effective in producing flow through the trashracks. For the minimum reservoir level, all of the rack area is considered effective in producing flow through the racks. On the basis of the net area through the racks the following velocities are indicated:

Since the waterway downstream from the rack bar installation converges rapidly, the velocity assumed in the trashrack loss equation was the net velocity indicated above.

(2) and (3) Entrance and transition losses. The outlet tunnel and power penstock intake waterways converge from the plane of the trashbars to a 39. 4- by 22. 4-foot entrance in a distance of 31. 5 feet; then contract as a rectangular bellmouth to a 27.8- by 19.7foot rectangular throat section in a distance of 23 feet; and finally change to the 26-footdiameter shaft section through a trapezoidal to circular transition in a distance of

6/"Hydraulic Laboratory Practice," edited by John R. Freeman, published in 1929 by the American Society of Mechanical Engineers, pp. 461-470.

36.2 feet. The head loss in this entrance reach was assumed at 0.1 hy, where hy is the velocity head measured in the 26-foot-diameter tunnel.

(4) Bend losses. The inclined entrance of each tunnel connects to the horizontal leg through a 45° vertical bend. Immediately downstream from this curve, the tunnel alinement turns through a 15° horizontal bend. The bend losses through these bends were computed from the equation h = 0.25hy, where hy is the bend loss in

terms of the velocity head hy in the tunnel, and ▲ is the central angle of the bend. For the 450 and 150 bends, the bend losses were computed to be 0. 18 h、 and 0.1 hy, respectively.

(5) Friction losses. Friction losses were based on the Manning formula, with a friction coefficient, n, assumed at 0.013 to represent the friction loss both in the concrete-lined reach and in the asphalt-coated steel-lined reach. On this basis, the friction loss coefficient was computed to be:

or, for the 26-foot-diameter tunnel with an assumed n of 0.013,

The respective lengths of the outlet tunnel and power penstock tunnel, from their entrances to the points of measurement established for determining the manifold coefficients, are 1, 350 feet and 1, 410 feet, approximately. On this basis, the assumed friction loss coefficient, hf, was computed to be 0. 55 hy for the outlet tunnel and 0.57 h, for the power penstock tunnel.

38. Stilling Basin Design. - Releases from the outlet control gates and valves are discharged into the outlet works stilling basin, where the fl w energy is dissipated by turbulence induced by hydraulic jump action and by impact and diffusion generated by chute blocks and baffle piers. The stilling basin is arranged so that satisfactory stilling action is accomplished either when uniform releases are made from all the controls, or when varied and unsymmetrical releases are made through a patterned sequence of opera

tion.

Flows will issue from the gates and valves in an essentially horizontal direction. The maximum head which can produce flow through the gates is approximately 235 feet. This head will produce a jet trajectory whose parabolic path is defined by the equation x2 -y = 4H where x and y are respectively the abccissa and ordinate of the trajectory, and H is the available head measured to the center of the gate or valve opening. To provide support for the issuing jets and thereby minimize the formation of subatmospheric pressures on the floor of the chute, and to consolidate the jet flowing from the hollow-jet valves, the profile of the floor was made flatter than the jet trajectory indicated above. The floor x2 x2 profile, -y = shown in figure 66 is based on the relation -y = with K chosen 1078' 4KH' at 1.15 to result in a floor profile somewhat flatter than the theoretical jet path. The parabolic floor of the chute thus drops 33. 5 feet in a horizontal distance of 190 feet.

For the discharge of 47,000 second-feet through the outlet gates and valves under maximum reservoir head, assuming uniform distribution of flow across the approximately 150 feet of basin width, a theoretical depth of 44 feet from the tailwater surface to the basin floor was needed. On the basis that a satisfactory jump condition will still be obtained if the depth is reduced about 15 percent, the apron level of the basin was placed at elevation 5345. This basin provides a tailwater depth of about 38 feet, which is 85 percent of the theoretical. The apron floor length, established at 160 feet, is approximately 3.6 times the tailwater depth theoretically required.

As noted in sections 36 and 37, the designs were based on the assumption that operation of the outlet gates with each gate either fully open or fully closed may be desirable,

with intermediate flows being released through the hollow-jet valves; it was further assumed that releases will be made through the controls served from the outlet tunnel to their full capacities before the power penstock bypasses are opened. It was therefore required that the stilling basin provide satisfactory stilling action for any combination of unsymmetrical releases.

Model tests 5/ indicated that the basin would perform satisfactorily for symmetrical releases, but that dividing walls and chute and floor blocks were necessary to provide proper energy dissipation for unsymmetrical releases. Because of the variable spacing of the gates and valves and the consequent variations in the spacing of the dividing walls, and because of the differences in release capacities of the different controls, uneven distributions of flow in the basin will occur. Table 1 shows the dissipation characteristics in the various compartmented portions of the basin resulting from different releases. Although deficient tailwater depths are indicated where the flow concentrations are large, the turbulences induced by the chute blocks and baffle piers and flow diffusion in that length of basin beyond the end of the dividing walls permit a satisfactory dissipation of flow for most conditions of discharge. If part open gate releases cannot be tolerated, satisfactory hydraulic jump performance can be achieved by resorting to the operation sequences described in the hydraulic model test report. 5/

2. Structural Design of Outlet Works

39. General. The reinforced concrete designs for the outlet works and power penstock structure components were based on the same provisions and assumptions adopted for the spillway structure, discussed in section 32a. Specific assumed loadings and design criteria for the various outlet works component structures are further discussed in subsequent sections.

40. Trashrack Structure. Since foundation conditions precluded the selection of a vertical intake tower or of a vertically placed trashrack structure, an inclined structure placed on the abutment slope was adopted. Also, since it was considered that raking of the trashracks might be required, an intake with flat-surfaced racks was chosen in preference to a semicircular trashrack structure.

The supporting members of the trashrack structure, where covered with the rack bars, were designed for a loading based on a 20-foot differential hydraulic head occurring between the outside and inside of the racked surface. This assumption imposed a load of approximately 1, 250 pounds per square foot on the racked surface. The loadings from the trashrack panels are carried laterally to the abutment walls and to intermediate inclined reinforced concrete beams and base piers. Three sets of base piers and beams are provided, dividing the rack surface width into four spans, each of about 13-foot length. The base piers are about 20 feet high and are tied into the base slab of the structure. The inclined beams are keyed to the tops of the base piers and are supported above the piers by lateral arches spanning the intake structure width. The inclined beams rest on the arches at the quarter points and at midspan of the arch. The lateral arches, spaced at about 18.5-foot centers, are fixed-ended onto 5-foot-thick abutment slabs which are cast directly onto the abutment rock foundation. The skewbacks of the arches are tied monolithically to the slab to provide a fixed-end support, and the abutment slabs are anchored to the foundation with 15-foot-deep anchor bars, to minimize rotation of the slab and to prevent slab movement induced by rib shortening of the arches. The piers, beams, and arches are streamlined with thin trailing edges to minimize hydraulic head losses. The trashracked deck at elevation 5535 is supported by a lateral vertical bent whose sloping legs rest on brackets cast onto the abutment slabs, and by girder beams which tie into the abutment slabs.

For the upper portion of the intake structure a relatively light beam-bent system for supporting the trashrack guides is provided. These members were designed to carry the loads which could be imposed during installation of the racks or from a loaded trashrake moving along the guide beams. The inclined beams are supported on eight lateral

5/op. cit., p. 117.