The Medina dam near San Antonio, Texas, presented probably the extreme of favorable conditions for simple and inexpensive diversion. The dam is 166 ft. high across a canyon whose level limestone floor was some 500 ft. wide. The depressions in the limestone floor were but slight. On each side was an earth bench, say 5 ft. to 20 ft. deep at the foot of the cliff; this material could easily be excavated by scrapers while the masonry plant was being erected. The dam was erected to conserve storm waters, the ordinary flow being so small that it could be easily diverted by means of a sand-bag dam. A flood at any time during the progress of the work could not have done more than cause a few days' delay. There was no deep pit to be filled up, in fact, no large quantity of loose material that could be moved by a flood. The cost of handling the water, though it has not been publicly stated, must have been merely nominal.
In the case of the Roosevelt dam, Type 3, the river was about 250 ft. wide between nearly vertical canyon walls, average depth (through sand and gravel) to bed rock about 30 ft., maximum depth to lowest point of completed foundation 38 ft.; the stream carried considerable silt which would assist in making tight an earth dam. The floods though occurring in fairly well defined seasons were in winter enormous, and at all times very sudden. Hydro-electric power was generated by the Government and sold to the contractor at 1/2 cent per h.p.-hour. For pumping purposes hydraulic ejectors were used as plenty of water was available from the adjacent power canal at a head of 220 ft., thus the pumping was as simple and cheap as it could possibly be. A large part of the finer material in the excavation was handled by hydraulic excavators. A feature of the permanent project was a tunnel (of 110 sq. ft. cross-section) at river level through the canyon wall around one end of the dam; this, of course, was to be utilized to its capacity to carry the river during construction. As the work was started after several years of low or very moderate flow, the possible or even average flood conditions were not appreciated at anything like their true value, so it was contemplated to construct a flume across the pit in order to supplement the capacity of the tunnel. Soon after this was started a flood of 130,000 c.f.s. occurred, raising the river 30 ft. in fifteen hours. Before it was possible to resume work it had been amply demonstrated that it would be foolish to attempt to build and maintain a flume in such a stream, so it was decided to proceed with the tunnel alone. Temporary dams were constructed above and below the pit; the capacity of the tunnel was about 1300 c.f.s. and work was prosecuted whenever the flow of the river did not exceed this amount. The work was under water many times but each step made the succeeding one easier and finally the masonry reached such a height that troubles were over. The river was rather narrow and there was not nearly so much work below water as on the dams discussed above. The temporary dams could readily be rebuilt of spoil from the pit and from quarries on the hillsides. A permanent flume was a practical impossibility for any feasible flume would have required rebuilding several times, would have been expensive, objectionably in the way of masonry construction and would have added very little to the amount of working time on the masonry. The excavation of the tunnel cost about $21,000; if constructed for temporary diversion purposes alone, there would have been an additional, though not heavy, charge for permanently closing it.
The contractors' expenditures for starting flume, building and maintaining temporary dams, excavating washed-in material, damages except loss of time; in fact, all expense chargeable to floods and diversion of water, were about as follows:
Cofferdam, equipment (cost, erection and repairs)...........
Cofferdam, materials and power..................................
Cofferdam, repairs on account of floods....................
Re-excavating washed in material and other expense due to floods....................
At the Arrowrock dam (see Fig. 54) now being constructed in Idaho by the U.S. Reclamation Service the river is about 200 ft. wide between rock walls. As the dam is to be about 350 ft. high the distance from heel to toe of masonry will be somewhat more than 200 ft. Borings indicate that the bottom of the foundation will be generally 65 ft. to 70 ft. below river level with a maximum of 90 ft. The average maximum floods are 15,000 c.f.s. to 18,000 c.f.s. and, under the circumstances, it was the obvious course to provide a channel with sufficient capacity to carry them. Thus the tunnel, which is 487 ft. long, was made 30 ft. in width X 25 ft. in height, to top of arch. The bottom and sides are lined with concrete so that a maximum velocity of 30 ft. per second is expected, corresponding to a discharge of about 20,000 c.f.s. With such a depth of foundation, and 225,000 cu. yd. of excavation below river level, a procedure such as at Roosevelt, where only the low flow was diverted, would have been absolutely futile. The working season between floods would have been too short to make the necessary excavation and lay any masonry, to say nothing of the expense of repeating the process.
As to cost of diversion works, the eleventh annual report of the U. S. Reclamation Service gives the following figures for work up to June 30, 1912: Tunnel excavation and lining; concrete bell mouths at inlet and outlet; and some work on the timber crib cofferdams above and below the pit, though they are not complete; cost $227,490.
At the Shoshone (Wyoming) dam it was 75 ft. between canyon walls, and about 85 ft. below river level to the lowest point of foundations. Here a small tunnel 10 ft. square in cross-section and 500 ft. long was driven; 900 ft. of 9 X 13 flume carried the water to the tunnel and 400 ft. of similar flume returned it to the river below. The temporary diverting dam was a crib structure 430 ft. long and 17 ft. maximum height,1 filled with earth and rock and covered with planking. The diverting works were planned to carry 2000 c.f.s. The flood season occurs from May to August when discharges of 12,000 to 17,000 c.f.s. may be expected.