Page:Popular Science Monthly Volume 61.djvu/272

266 at six inches in depth per month from the surface of the lake will amount to about 1,070 cu. ft. per second if the traffic through the canal should amount to 10,000,000 tons per annum in ships of ordinary size. This draft per second is the sum of 406 cu. ft. per second for lockage, 207 for evaporation, 250 for leakage at the lock gates and 200 for power and other purposes, making a total of 1,063, which has been taken as 1,070 cu. ft. per second. The amount of storage in Lake Bohio between the elevations of 85 and 82 ft. above sea-level, as designed, is sufficient to supply the needs of that traffic in excess of the smallest recorded low-water flow of the Chagres River during the dry season of a low rainfall year. The lowest monthly average flow of the Chagres on record at Bohio is 600 cu. ft. per second for March, 1891, and for the purposes of this computation that minimum flow has been supposed to continue for three months. This includes a sensible margin of safety. In not even the driest year therefore can it be reasonably expected that the summit level of the canal would fall below the elevation of 82 ft. until the total traffic of the canal carried in ships of the present ordinary size shall exceed 10,000,000 tons. If the average size of ships continues to increase, as will probably be the case, less water in proportion to tonnage will be required for the purposes of lockage. This follows from the fact that with a given tonnage the greater the capacity of the ships the less the number required, and consequently the less will be the number of lockages made.

On the other hand it can be shown that with a depth of five feet of water on the crest of the Gigante Spillway the discharge of that weir 2,000 ft. long will be at the rate of 78,260 cu. ft. per second. If the flood water of the Chagres should flow into Lake Bohio until the head of water on the crest of the Gigante weir rises to 7.5 ft. the rate of discharge over that weir would be 140,000 cu. ft. per second, which, as already shown, exceeds at least by a little the highest flood rate on record. The operation of Lake Bohio as a flood controller or regulator is therefore exceedingly simple. The flood waters of the Chagres would pour into the lake and immediately begin to flow over the Gigante weir, and continue to do so at an increasing rate as the flood continues. The discharge of the weir is augmented by the increasing flood and decreases only after the passage of the crest of the flood wave. No flood even as great as the greatest supposable flood on record can increase the elevation of the lake more than 92 to 92.5 ft. above sea-level, and it will only be at long intervals of time that floods will raise that elevation more than about 90 ft. above sea level. The control is automatic and unfailingly certain. It prevents absolutely any damage from the highest supposable floods of the Chagres, and reserves in Lake Bohio all that is required for the purposes of the canal and for wastage by evaporation through the lowest rainfall season. The floods