The large flume was in commission about a year and a half before the first stone was laid, and nearly three years altogether. It was abandoned at a low stage of the river and the flow turned through four 48 in. outlet pipes through the dam. A subsequent flood just topped the dam by a few inches at about 40 ft. higher elevation.
The scheme of handling the water was, of course, determined by the depth and amount of excavation, the expense and lost time if the pit had been filled, the length of time that the pit was required to be open and the fact that a flume within reason could be expected to carry the maximum flood.
The conditions at the new Croton dam were similar but much more pronounced. A very similar scheme was also adopted, namely, that of maintaining on one side of the valley a channel designed to carry the entire flow. The pit was larger and deeper and open for a much longer time. On the north side of the valley was excavated a channel, principally in rock, 1100 ft. long and 125 ft. wide, with its bottom 5 ft. above the original river bed. The natural rock formed the north side of the channel; the south side, next to the pit, was formed of a masonry wall extending 300 ft. each way from the center line of the dam. (See Plate IV, Figs. A and B.) also Fig. 17. This wall was about 35 ft. high, 13 ft. thick at the base and 3 ft. at the top; except where crossing the foundation trench it was backed by an earth embankment. The inlet and outlet was formed by curved, earth embankment wing walls extending from the ends of the masonry wall, and with tops 10 ft. wide and 5 ft. higher in elevation'. Through the center of these embankments was a core of 3 in. tongued and grooved sheet piling, and the toe of their slopes on the channel side was protected from erosion by crib work faced with sheeting which extended down several feet below the bottom of the channel. The entire length of masonry and earth wall on the south side of the channel was 1600 ft. Above the dam site are 360 sq. miles of watershed, but the effect of several storage reservoirs, and the daily draft of 300,000,000 gal. to 375,000,000 gal. to supply the City of New York, was such that for much of the time this channel carried but from 1 ft. to 3 ft. of water. During freshets it rose to a depth of 15 ft, to 19 ft. and upon two occasions to such a height that it poured over the masonry wall into the pit, though luckily at a time when the work was so far advanced that little damage or delay resulted.
Fig. 17. New Croton dam showing temporary stream diversion.
The cost of the entire works for diversion and handling of the river during the construction of the dam was approximately as follows:
Excavating new channel.....
Diverting the river.....
Damage due to floods......
Building and afterward closing relief openings through dam..
Another similar situation exists at the Elephant Butte dam now under construction in New Mexico by the U. S. Reclamation Service. (See Plate III, Fig. D, and Plate IV, Fig. D, also Fig. 55.) At this point the Rio Grande is about 500 ft. wide, much of the bed rock is 70 ft. below river level and the material to be excavated is loose sand. The run-off of the river varies between 50,000 acre ft-and 2,000,000 acre ft. annually, with a mean of about 860,000 acre ft. High water may occur at almost any season of the year. The depth of the pit, the quantity of loose material to be excavated, 350,000 cu. yd., and the certainty that even a slight overflow of the work would result in filling the pit with the loose material, determined the scheme of diversion.
A flume 1050 ft. long; floor and land side of concrete, land side batter 1/4 to 1, floor 461/2 ft. wide; river side of timber with batter of 1/2 to 1, was built along one side of the river. The capacity of the flume is 20,000 second feet, or sufficient to carry all floods except such as might occur once in eighteen or twenty years. Where the flume crosses the dam, the foundation was prepared and the permanent masonry put in up to the floor of the flume, a maximum depth of 58 ft.
The Eleventh Annual Report of the U. S. Reclamation Service gives the following costs of the diversion works to June 30, 1912, at which time the final closures of the cofferdams had not been made; also there remained some work to be done on the flume at the crossing of the dam.
Preliminary for excavation and construction of cofferdams:
Flume intake and outlet.....
Excavation and flume, construction of flume, flume intake and outlet and cofferdams...............
Construction of flume, woodwork......
Concreting flume sections........
Concreting dam sections of flume.......
For the construction of the Croton Falls dam the diversion channel started from a point 600 ft. above the dam, the total length was 1400 ft., and the capacity was 1000 million gallons per twenty-four hours or about 1550 c.f.s.
For a length of 800 ft., the channel was a timber flume 24 ft. wide X 8 ft. 2 in. deep, supported across the excavation. (See Plate VII, Figs. C and D.) The contract price for the diversion works was $90,000.
At the Cross River (Plate VI, Fig. D) and Kensico dams (Plate X, Figs. A and C) the amount of water to be handled must have been comparatively insignificant, as in the former case it was carried across the masonry in a couple of pipes, and in the latter case through a small flume. The Kensico dam is being built in order to store water from the Catskill Aqueduct. The watershed tributary to the reservoir is insignificant.
At the Cataract dam, N. S. W., built 1902 to 1907, the water seems to have been handled through 48-in. pipes, except that one rather exceptional flood topped the dam when it was up at elevation about 60. The diversion must have been simple and not large in quantity as the cost is stated as only $19,000.