About this time it was decided to limit the study to dams with hydraulic heights of not less than 200 feet and to consider no more than three dams to develop the entire 700 feet of head being considered. The reasoning here was based on some preliminary studies which indicated that although the mass of a series of low dams would itself be less costly than the mass of a smaller number of higher dams that would develop approximately the same total head, the cost of the appurtenant structures, such as spillways, diversion tunnels, and outlet works, which often constitutes a major portion of the cost of a dam on a river this size, seldom is appreciably less and occasionally is greater for a low dam than for a high dam at the same site. The unit cost of generating facilities also is somewhat greater for small plants than for large plants. For a series of small dams, duplicate housing, access facilities, materials areas, and switchyards would possibly also have to be provided. It was also realized that by moving the uppermost dam to successive locations downstream, thereby reducing the total number of dams in a particular plan, other benefits aside from possible savings in costs would be gained. For example, if it was decided to retain the same volume of active storage, which would result in a shallower active pool, the average increase in power head (and power benefits) would be greater than the head gained by moving the damsite.

These considerations and the lack of adequate damsites on the river as it leaves Wyoming eliminated the necessity for considering damsites in Wyoming. This decision was further strengthened by observation of the nature of the terrain in Wyoming. Here the slope of the river is only about 3 feet per mile and a 200-foot dam in Utah near the Wyoming-Utah border would be capable of impounding about 1 year's runoff of the river. The upper portion of the section of the river being considered, therefore, appeared to be an ideal site for a fluctuating or regulating reservoir. Additional dams and powerplants constructed downstream could operate essentially as constant-head afterbays.

From a map study of this section of the river, beginning at the downstream reach of the upper end of the Dinosaur National Monument, the most promising damsites between Dinosaur National Monument and the Utah-Wyoming boundary were inventoried. The criterion here was simply to select those sites with the smallest cross sections which might be feasible of development. The first obvious site was the Swallow Canyon site some 6 miles upstream from the monument. Proceeding upstream, nine more sites were selected for study, the uppermost being the original Flaming Gorge site in Utah just below the mouth of the Henrys Fork River near the Utah-Wyoming boundary.

Because of the configuration of the river, it was also possible to investigate two different sites which would have developed appreciable amounts of head with comparatively short lengths of tunnels.

It was next decided to consider the possibility of constructing one large dam to develop the entire head available, as well as to provide maximum regulating storage. The first site available, moving upstream from the monument, was the Swallow Canyon site which lay about 30 feet higher in elevation than the monument boundary. This site, however, did not have abutments at its narrow section high enough to contain a 670-foot dam without extensive diking, and it was therefore eliminated from further study. However, the possibility of constructing only one dam of lesser height at one of the upstream sites was not eliminated.

By this time the following decisions were made:

(1) That the studies had to be limited to a section of the Green River about 114 miles long having a difference in elevation of 670 feet.

(2) That several plans involving nine dam sites and two tunnel sites could be considered with not more than three dams having hydraulic heights of not less than 200 feet each being considered for any one plan.

(3) That the uppermost reservoir would be a stream-regulating reservoir because of the relatively gentle slope and large reservoir basin available in the upper end of the reach being considered.

During the process of isolating the better possibilities, it was found that one of two choices for development of a particular section could be discarded simply by comparing

costs if the benefits for the least costly plan were obviously the greater. Usually, however, it was necessary to make complete benefit and cost analysis to differentiate between plans. It was finally concluded from the results of all of the studies, including benefit and cost analysis, that a dam at the present site of construction, backing water to Green River, Wyo., was the best single site available for a water-regulating and power-producing unit. A reservoir here with its stream-regulating ability would also enhance the power potentialities of any downstream development by permitting the lower reservoir to operate for power purposes at near maximum head.

7. General Information for Flaming Gorge Unit. The Flaming Gorge unit, which is the subject of this publication, consists of a dam, powerplant, and reservoir located on the Green River in northeastern Utah about 6 miles south of the Wyoming-Utah border and about 20 miles west of the Colorado-Utah border (fig. 3). The Flaming Gorge unit was the second feature of the vast Colorado River Storage project to be started, being preceded by the Glen Canyon unit in northern Arizona.

The reservoir formed by the dam is some 91 miles long, having a surface area of 42, 000 acres and extending to within approximately 5 miles of Green River, Wyo. The widest section of the 3, 789, 000-acre-foot reservoir is immediately south of the WyomingUtah border. At this point the reservoir is approximately 9 miles wide, but it narrows to a few hundred feet to enter the Flaming Gorge. From the Flaming Gorge to the dam, the reservoir meanders some 27 miles through deep, rugged, steep-walled and very colorful Red Canyon averaging 1, 500 feet or more in depth.

The dam is a thin-arch concrete structure having a structural height of 502 feet above the lowest point of the foundation, a crest length of 1, 285 feet, and a concrete volume of 986, 600 cubic yards. The crest of the dam, which is 6047 feet above sea level, accommodates a two-lane highway which is the only practical access to the Dutch John community.

Floodwaters will be passed through a 675-foot length of tunnel-type spillway extending through the left abutment. The concrete-lined tunnel has a maximum capacity of 28, 800 second-feet and varies in diameter from 26-1/2 feet at the upstream portal to 18 feet at its downstream portal. The spillway intake structure is controlled by two 16.75- by 34-foot hydraulically operated fixed-wheel gates.

The outlet works for the dam are comprised of two 72-inch steel pipes through the dam, reducing to 66 inches at the toe of the dam and continuing downstream to a valve structure on the left riverbank where the discharge is directed into the river channel downstream from the powerplant tailrace. Each outlet is controlled by a 66-inch hydraulically operated ring-follower gate at the downstream toe of the dam and a 66-inch hydraulically operated hollow-jet valve at the valve structure at the downstream end of the outlet pipe. Maximum capacity of the river outlets is 4, 000 second-feet.

Three 10-foot-diameter penstock pipes located near the center of the dam convey water to the three turbines. These penstocks each have an 8.27- by 15.82-foot hydraulically operated fixed-wheel gate at the upstream end, which is enclosed in a trashrack structure.

The powerplant is located at the downstream toe of the dam, normal to the river channel. It is a reinforced concrete structure 220 feet long, 100 feet wide, and 115 feet high above foundation. The powerhouse has a reinforced concrete substructure from foundation to generator floor and a superstructure of steel rigid-frame bents and 10-inch concrete walls. The roof is built of lightweight precast concrete slabs with 3-inch-thick insulation and four-ply built-up roofing.

The powerplant houses three 36, 000-kilowatt generators each driven by a 50, 000horsepower Francis-type turbine. The turbines are designed to operate at 240 revolutions per minute and at a 365-foot head. However, they are also designed to operate effectively within an effective head range of 260 to 440 feet.

8. Cost Summary. The following tabulation summarizes the total estimated cost of the Flaming Gorge Dam, Powerplant, Switchyard, and appurtenant features as of June 30, 1965. A detailed cost statement by features is included in appendix A.

Figure 3.--Flaming Gorge Dam and Reservoir location map.

CHAPTER II. SITE INVESTIGATIONS AND GEOLOGY

A. Site Investigations

9. Site Investigations for Flaming Gorge Unit. The preliminary exploration on Flaming Gorge damsite was conducted during the fall of 1949 when six diamond core drill holes were completed. Information obtained from studies of these holes and cores indicated that the general quality of the rock at the damsite was excellent. Data from these holes were not sufficient, however, for final design purposes and a greater amount of detail data was required. Owing to the existence of faults, joint systems, and shale seams in the rock at the damsite, it was necessary to carry out a thorough and rather extensive geological investigation and map a program to obtain sufficient detailed geological information about the foundation deficiencies so that an adequate design could be prepared for the dam and appurtenant structures.

From July 1956 through May 1958, a total of 72 additional diamond drill core holes were drilled at the dam site, on the diversion tunnel alinement, on the spillway tunnel alinement, in the switchyard, and at the domestic water-pumping plant site. Total footage drilled was 1, 087 feet in 1949, and 6, 457 feet in the years 1956 through 1958. These holes were both angle and vertical holes. All holes were carefully logged and the cores were recovered. Data and cores obtained from these holes were forwarded to the Denver office for consideration by the Chief Engineer and subsequent preparation of design. The location of these holes and logs showing the type and character of the material encountered, together with notes of the percolation test for each hole, are attached to the final geology report.1/ Seismic studies were also made for the purpose of correlating with the elastic property tests.

Two horizontal drifts were excavated in 1956, one in each abutment. Drift 1 went 24.7 feet into the right abutment along a 3.5-foot shale bed. Drift 2 went 15.5 feet into the left abutment along a soft sandstone layer and followed some prominent joints. Logs of these drifts are attached to a preliminary geological report. 2/ Except for jointing, both drifts encountered rock of good quality.

Suc

Percolation tests were made in bedrock in all drill holes at 10-foot intervals. cessive pressures of 25, 75, and 150 pounds per square inch were used in these tests for periods of 5 minutes each. There was a wide range of losses from 0 to 18 gallons per minute (the capacity of the pump), but in practically all the tests it was possible to obtain 150 pounds per square inch of pressure which indicated that the joints were fairly tight. Generally speaking, the most solid rock with the least jointing and attending water loss was in the river channel under the downstream toe of the dam. The greatest losses were high in the abutments where the joints were more open.

The faults, joints, slip planes, and shear zones in the rock in the vicinity of the damsite were not extraordinary or uncommon for this type of terrain; however, they represented a weakness in the rock that required careful consideration and attention. Some of the problems that were anticipated because of this rock weakness were seepage, overbreak or extra excavation to reach sound rock during excavation, and pinning of certain areas to prevent possible movement. The overbreak problem was evidenced during keyway excavation operations and resulted in considerable overrun in order to reach sound bedrock, and this extra excavation eventually had to be filled with concrete during concrete placing operations. The seepage problem was expected and has been relatively minor. Special consideration for this problem included extension of the main cutoff grout curtain deeper into both abutments. As anticipated, the seepage problem became more evident after reservoir storage was initiated in 1962 and seepage water began to show up on both abutments in small but increasing amounts as the reservoir head increased. Pinning of rock areas with anchor bars on both abutments was expected and was performed as required during construction operations in order to assure stability of certain rock areas and to prevent possible rockfalls on the powerplant. 1/Wongwai, G. T.I., "Geology Summary of Flaming Gorge Dam, Flaming Gorge Unit-Utah-Wyoming--Colorado River Storage Project, Bureau of Reclamation, 1963

(unpublished).

2/"Preliminary Geological Report of Flaming Gorge Dam and Reservoir Site, " G-109, Bureau of Reclamation, Region 4, Salt Lake City, Utah, January 1957 (unpublished).