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Page:Popular Science Monthly Volume 61.djvu/271

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show that the maximum discharge in 1879 might have been at the rate of 136,000 cu. ft. per second. As a matter of fact the total channel section in that year was less than it is at the present time. Hence if it be assumed that a flood of 140,000 cu. ft. per second must be controlled an error on the safe side will be committed. Other great floods of which there are reliable records are as follows:

1885—Height at Bohio 33.8 ft. above low water.
1888—Height at Bohio 34.7 ft. above low water.
1890—Height at Bohio 32.1 ft. above low water.
1893—Height at Bohio 2S.5 ft. above low water.

The maximum measured rate of the 1890 flood was 74,998 cu. ft. per second, and that of 1893, 48,975 cu. ft. per second. It is clear therefore that a flood flow of 75,000 cu. ft. per second is very rare, and that a flood of 140,000 cu. ft. per second exceeds that of which we have any record for practically forty years.

It is obvious that the dam, as designed by the commission, is of such a character that no water must be permitted to flow over its crest, or even in immediate proximity to the downstream embankment. Indeed it is not intended by the Commission that there shall be any waste way or discharge anywhere near the dam. At a point about three miles southwest of the site of the dam at Bohio is a low saddle or notch in the hills near the headwaters of a small stream called the Gigante River. The elevation of this saddle or notch is such that a solid masonry weir with a crest 2,000 ft. long may readily be constructed with its foundation on bed rock without deep excavation. This structure is called the Gigante Spillway, and all surplus flood waters from the Chagres would flow over it. The waters discharged would flow down to and through some large marshes, one called Pena Blanca and another Agua Clara before rejoining the Chagres. Inasmuch as the canal line runs just easterly of those marshes it would be necessary to protect it with the levees or embankments to which allusion has already been made. These embankments are neither much extended nor very costly for such a project. The protection of the canal would be further aided by a short artificial channel between the two marshes, Pena Blanca and Agua Clara, for which provision is made in the estimates of the commission. After the surplus waters from the Gigante Spillway pass these marshes they again enter the Chagres River or flow over the low, half-submerged country along its borders, and thence through its mouth to the sea near the town of Chagres about six miles northwest of Gatun.

The masonry crest of the Gigante Spillway would be placed at an elevation of 85 ft. above the sea, identically the same as that which may be called the normal summit level of the canal. It is estimated that the total uses of water in the canal added to the loss by evaporation taken