REPORT OF BOARD OF CONSULTING ENGINEERS, PANAMA CANAL.


severe floods, like either of those of November, 1885 or 1888, in close succession have never been observed in such a connection, not to cite the phenomenal flood of 1879.
 If a dam be constructed at Gamboa with an elevation of its top at 180 feet above mean sea level, or 130 feet above the river bed, and if the highest flow line of that reservoir be taken at 170 feet, the area included within that flow line will be 29.47 square miles. With the minimum depth of water of 40 feet provided for the sea-level canal, the minimum wetted cross section would have an area of over 8,000 square feet, so that if 15,000 cubic feet of flood water per second from the Chagres be permitted to enter the canal prism at Gamboa, the resulting current, if the entire quantity admitted flows in one direction, will be but one and one-fourth miles per hour, a negligible quantity so far as its effects upon navigation are concerned; but the plans for a sea-level canal contemplate a provision which would permit the discharge through the canal prism and regulating sluices near the tidal lock on the Pacific side of approximately one-third of this Gamboa discharge, and to that extent, at least, dividing the flow between the two oceans and consequently reducing the current velocity. For the purposes of estimation inconnection with this problem of flood control the Board has therefore assumed that the controlling sluices to be provided in the Gamboa dam may admit the flood waters of the Chagres to the canal prism at the uniform maximum rate of 15,000 cubic feet per second.
 If a flood should occur with a discharge equal to that estimated for 1879, viz, 65,000 cubic feet per second at Gamboa for a period of forty-eight hours, and if a uniform outflow of 15,000 cubic feet per second be permitted during the same time, there would be accumulated in Gamboa Lake 8,640,000,000 cubic feet of water, which is that portion of the volume of the lake included between water surfaces at elevations of 159 feet and 170 feet above sea level. Furthermore, a uniform outflow from the lake at the rate of 15,000 cubic feet per second would discharge the entire maximum average 48-hour flow of the 1879 flood in 8.7 days. It is seen, therefore, that there would be no practical difficulty in depressing the surface of the water in Gamboa Lake between two severe floods sufficiently toreceive the entire maximum average 48-hour flow of such a phenomenal flood as that of 1879.
 The capacity of flood control provided in sucha lake as that under consideration is further illustrated by the fact that its volume between water surfaces at 108 and 170 feet above mean sea level is sufficient to take the aggregate discharge of three times the maximum average 48-hour flow of the 1879 flood without any water escaping through the regulating sluices of the dam; or the volume between elevations 128 feet and 170 feet will hold three times the flow of such a flood if a uniform discharge of 15,000 cubic feet per second be permitted concurrently through the regulating sluices.
 These computations demonstrate conclusively that the controlling capacity of the Gamboa Lake as proposed by the Board is ample for all the exigencies of flood flow which can ever occur in the Chagres River without any other regulating or controlling aid, especially when it is observed that the highest mean monthly discharge for the rainy months of any year since 1890 (for 1892) is a little less than 5,300 cubic feet per second. There would only be required a simple grade of supervision, under which the water surface would always be depressed immediately after any flood low enough to receive any subsequent sudden flood flow which might possibly occur. This grade of supervision requires no special estimation or provision of future events, but is quite within the ordinary administration of this feature of canal maintenance and operation.
 The elevation of water surface assumed at 170 feet is sufficient to permit the use of an open channel between the Chagres watershed and the headwaters of the Gatun River for the discharge of surplus flood waters in that direction, should it ever be required. The controlling capacity of the Gamboa Lake, however, is so complete and satisfactory that the Board does not believe that it will ever become desirable to construct this open channel across the divide between the Chagres and Gatun watersheds.
 The Gamboa Lake affords complete regulation and control of the Chagres River above Gamboa. It has already been stated that there are small streams now discharging into the Chagres River below Gamboa which must be taken care of during the construction of the canal. Ample provision has been made for the control of these smaller streams, either by utilizing the


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