(about $284,000 more than scheme B-1) and because of the inadequacy of the foundation for an unlined spillway. Velocities would range from about 7 feet per second at the entrance of the approach channel to about 18 feet per second at the crest. Scheme B-2 would cost about $760,000 more than scheme B-1, but would have some advantages because the sill of the radial gates was at the top of the conservation storage space. Among the advantages are:

(1) Foundation pressures would be more favorable.

(2) More stable and safer gate structure would be possible.

(3) No water would be against the gates when the reservoir is at or below the top of the conservation storage. The gates would remain open at all times except during floods.

(4) Gates would not have to be designed for ice pressures, and ice preventive equipment would not be necessary.

(5) Maintenance costs would be less.

After various phases of schemes B-1 and B-2 were considered, scheme B-1 was adopted for specification designs because the greater expenditure necessary for scheme B-2 was not justified.

Plans were originally made to operate the spillway gates by hydraulic means. Studies indicated that this method of operation was not rapid enough for this relatively small reservoir. As a result, the more rapid electric operation was adopted for the final design. Automatic controls are provided for the operation of the gates when a reservoir water surface elevation of 3127 is reached. At this elevation, two gates will start to open. All gates will be fully open when the reservoir water surface elevation reaches elevation 3129.5.

41. Description of Spillway.

The spillway is a concrete structure and consists basically of an inlet channel, crest and gate strucutre, chute, stilling basin, and outlet channel (fig. 18).

(a) Approach or Inlet Channel.-- The approach or inlet channel is excavated to elevation 3090 and approaches the gate structure on a 800-foot-radius curve bearing left out of the reservoir into the spillway. The right side of the channel has a 2 to 1 side slope, and the left side is extended to the excavation for the outlet works approach channel which is excavated to elevation 3040. The bottom of the spillway approach channel is approximately 460 feet wide and 350 feet long. To protect against erosion caused by the entrance velocities of the water, dumped riprap was placed 2 feet thick across the bottom and up the side slopes of the right bank for a distance of 50 feet upstream from the edge of a concrete inlet slab. This slab is 15 inches thick and extends 95.5 feet upstream from the upstream edge of the gate structure. On the upstream edge of this slab, a 6-footdeep cutoff is provided. Right and left inlet training walls are provided at the spillway entrance. The left wall extends from the gate structure on the 40-foot-radius curve through 1800 to the upstream face of the dam and then on a tangent to the dam crest. The right wall curves similarly but through 900 and then extends on a tangent to the right abutment. The concrete inlet slab warps around the toe of the left wall to direct the flow and prevent the formation of undesirable drawdown along the face of the wall for flows exceeding 160,000 second-feet.

(b) Crest and Gate Structure. -- The crest and gate structure (fig. 19) is divided by keyed construction joints into seven component parts; namely, two side wall abutments, four intermediate crest and pier sections, and one center pier and crest section which includes the uncontrolled weir crest. Six 50-foot-wide by 30-foot-high radial gates with their control mechanisms are supported by the gate structure piers. The gate sills are placed at elevation 3097. The uncontrolled weir located between the two piers near the centerline of the spillway has a crest length of 10 feet and an elevation of 3112.3. A hoist and operating deck which carries the gate control mechanism is situated along the upstream side of the gate structure. It is a concrete beam and deck slab bridge 369

feet long and 19.5 feet wide, consisting of six independent simply supported spans 58 feet long bridging the spillway gate portion of the crest, and one 14.5-foot section spanning the uncontrolled weir crest portion of the spillway structure. The deck is at the level of the crest of the dam.

A highway bridge, whose centerline is on the dam axis, is also supported by gate structure piers. This is a two-lane concrete beam and deck slab bridge, 369 feet long and 26 feet wide. The bridge spans are supported in the same manner and placed at the same level as the hoist and operating bridge. A compressor and battery room structure is located above the uncontrolled crest section and immediately below the hoist deck. This structure is supported on shelves provided on the center piers so that a portion of its roof slab forms the center span of the hoist deck. Access to the compressor and battery room from the hoist deck and highway bridge is through a removable hatch cover.

(c) Chute. The spillway chute is a concrete-lined channel extending from the spillway gate structure to the stilling basin. It has an overall length of 282 feet and a width ranging from 361 feet at the upstream end to 400 feet at the stilling basin. Side walls for the chute consist of both cantilever and counterfort types and range in height from 23 to 49.5 feet. The floor thickness ranges from 18 inches at the upper end to 4-1/2 feet at the junction of the chute with the stilling basin floor. Both the floor and side walls are divided into convenient sized floor slabs and wall panels to facilitate concrete placement and control cracking because of shrinkage, differential settlement, and abrupt variations in foundation profiles. The floor slabs are provided with bell-and-spigot type joints which prevent misalinement and minimize the flow of water through the joints.

To relieve uplift pressure which may develop because of an accummulation of subsurface seepage and leakage of water through joints in the floor, two independent systems of drains embedded in graded sand and gravel are provided (fig. 20). An upstream system is placed under the floor of the upper portion of the chute, and a downstream system is placed under the floor of the stilling basin and the lower portion of the chute. Flows from the upstream system discharge into a 12-inch transverse collecting drain around and through the side walls into the chute. Flows from the downstream system discharge through chute blocks at the upper end of the stilling basin.

A filter blanket of graded sand and gravel with a thickness of about 18 inches was placed under the entire chute of the stilling basin. This filter blanket insulates the foundation against deep frost penetration, prevents leaking of fine subgrade materials into and through the drainage system, and provides a pervious zone between the floor slab and foundation which prevents the accumulation of uplift pressures under the slab.

(d) Stilling Basin.-- The stilling basin is a rectangular concrete channel, 115 feet long and 400 feet wide, and is provided with counterforted side walls. These walls are 46 feet high; and each wall is divided into three sections, two 37.5 feet long and one 40 feet long. Wing walls extend to the right and left of the downstream end of the 40-foot section of the right and left side walls, respectively. The left wing wall extends to join the right wing wall of the outlet works stilling basin. The right wing wall extends into the dam abutment. The stilling basin floor is a level apron with the top surface at elevation 3016.00; its thickness ranges from 4.5 feet at the upstream end to 2 feet at the downstream end. Chute blocks, each 5 feet wide, 6 feet high and 12 feet long, are placed at 5-foot centers at the upper end of the basin, and 12-foot-high dentated sills are placed at the lower end of the basin. A 3- by 10-foot cutoff wall extends laterally across the downstream end of the stilling basin. Also, interlocking sheet metal piling has been provided under the cutoff trench and along the heels of the stilling basin wall footings. The chute blocks, dentated sills, and cutoff wall were cast integrally with the stilling basin floor.

A spillway outlet channel approximately 400 feet wide is provided at the downstream end of the stilling basin for leading spillway discharges into Frenchman Creek. The channel is unlined except for the first 75 feet downstream from the basin where the bottom and the side slopes are protected by a 3-foot blanket of dumped riprap.

1. Hydraulic Design

42. General. The spillway has a designed capacity of 200,000 second-feet at maximum reservoir surface elevation 3129.5. Flow through the spillway is controlled by a 10-foot-wide ungated notch, and six 50-foot-wide by 30-foot-high radial gates which are hoist-operated and controlled by pushbuttons. Automatic gate operation is provided when the reservoir rises above elevation 3127.0. The controlled notch has a designed capacity of 2, 000 second-feet with reservoir surface elevation 3127.00.

43. Gated Crest. The shape of the gate crest upstream from the apex was selected to conform with the under nappe of a jet flowing over a sharp-crested weir inclined 3 to 1 downstream and under a 32. 8-foot head. The shape downstream from the apex was selected to conform with a thin jet flowing horizontally under a 30-foot head. X2 This shape was computed from the equation Y = in which H is the 30-foot head above 4H' the sill of the gate. A 50.37-foot-radius vertical curve was used to blend the parabolic profile into the shape of the spillway chute floor.

44. Uncontrolled Crest. The shape of the uncontrolled spillway crest approximates the under nappe of a jet flowing over a vertical sharp-crested weir under a head of about 17 feet. The ogee shape was defined by approximating the profile upstream from the apex as a circular curve and the profile downstream as a parabola. The apex is 4 feet downstream from the upstream vertical face. The section of the profile upstream from the apex has a radius of 5. 48 feet, and the downstream profile is defined by the X2 parabola Y =_ The continued profile was blended into the spillway chute floor by a 30

sloping tangent (S= 0.6273) and a 50.37-foot-radius curve.

45. Chute. The profile and divergence of the spillway chute, extending from the gate structure to the stilling basin, were selected to comply with the following hydraulic requirements: