PART II- DESIGN CHAPTER IV. Design --INTRODUCTION A. General 22. Preliminary Designs. The original designs and estimates for Ashley Dam (now Flaming Gorge Dam) were prepared in the Bureau's regional office. The unit was designed as a gravity-type concrete structure with the powerplant at the toe of the dam and a tunneltype spillway through the canyon wall. Data on the designs and estimates are contained in the referenced report. 1/ A description and estimate is also included in House Document No. 364, "Colorado River Storage Project," dated April 6, 1954. A joint field inspection of the site was made in October, 1955, by representatives of the Office of Chief Engineer (the Denver office) and the regional office. As a result of this inspection, it was decided that reconnaissance-type designs and estimates be prepared for alternate-type structures in an effort to arrive at the most economical designs. Reconnaissance-type estimates were prepared for the following three layouts: (1) A low earth dam with reservoir level at elevation 5838, to be used in conjunction with a second earth dam upstream at Horseshoe Canyon damsite, which would have a reservoir level at elevation 6050. (2) A high earth dam with reservoir level at elevation 6050. (3) A concrete arch dam with reservoir level at elevation 6050. Engineers' cost estimates for the above structures were transmitted to the regional office. Further consideration of the low earth dams at the Ashley and Horseshoe Canyon sites was rejected by the regional office due to excessive costs. The alternative of either a high earth dam or a concrete arch dam remained, since either the earth or concrete arch structure was lower in cost than the concrete gravity dam estimated by the regional office. 23. Decision for a Concrete Dam. The above estimates indicated the cost for the high earth and concrete arch dams to be comparable, with a slight advantage for the earth dam. It was decided that more detailed estimates would be required to determine which structure should be specified. The more detailed estimates indicated costs to be virtually identical. By memorandum of April 16, 1956, 2/ it was recommended that the concrete arch-type structure be selected. This recommendation was approved by the Chief Engineer and concurred in by the Regional Director. It was noted in the recommendation that the concrete arch-type structure was selected because of various small advantages in construction and operation. 24. Description of Features as Constructed. The final design for the dam provided for a variable-center arch structure (fig. 15). The height of the dam above the lowest foundation is 502 feet with a base thickness of 130 feet at the plane of centers. The ratio of base thickness to height is 0.26 which classifies the dam as a thin arch structure according to Bureau of Reclamation standards. These standards classify structures in the thin arch category whose thickness to height ratio is 0.30 or less. The dam has a crest length of 1, 285 feet with a radius for the axis of 650 feet. The crest accommodates a 27-footwide roadway and a 3-foot-wide downstream sidewalk. A tunnel-type spillway structure is located through the left abutment. The tunnel varies in diameter from 26.5 to 18 feet, with a maximum capacity of 28, 800 second-feet. The spillway intake structure is located adjacent to the axis of the dam and is controlled by two 16.75- by 34-foot fixed-wheel gates. 1/"Colorado River Storage Project and Participating Projects--Upper Colorado River Basin," Appendix B--Designs and Estimates, September 1950. 2/Internal memorandum "Selection of type of dam at Ashley site-- Flaming Gorge Unit-Colorado River storage project," from Chief Designing Engineer to Chief Engineer, April 16, 1956 (unpublished). |