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divided that small vessels may use short sections, or several small vessels pass through the whole lock together. Assuming the traffic equal to the maximum capacity of the locks, and utilizing records of experience with the Sault Ste. Marie Canal, the Board of Consulting Engineers estimated the traffic at 80,000,000 register tons per year, as against 30,000,000 tons for the “Soo" Canal, and an actual maximum at that time of 15,500,000 tons for the Suez.* The amount of water required for lockage was found by the designing engineers to be about 2618 cubic feet per second, which means about one lockage in each direction per hour, but the assumed maximum traffic will not be reached for many years.
Adding the total losses from all causes gives a total of 4183 cubic feet per second, applicable during the dry months, when evaporation is the greatest. The question now arises: Where is this rather enormous quantity of water coming from? The input into Gatun Lake comes from rainfall directly on the lake, which is absent in the dry season, however, and from the flow of Chagres River and of minor streams. The data desired for this purpose pertains to the driest period that may be reasonably expected, and the best way to predict it is from records of the flow of the Chagres in past years. The records of the New French Panama Canal Company furnish much reliable information, while that obtained from the old company is fragmentary and incomplete.
The Suez Canal passed 5373 vessels in 1912, about 12 per day, with a total tonnage of 28,008,945 the largest year of record. The tonnage in 1913 was less.
The driest consecutive four months in the available records of 19 years showed a flow, on an average, of 1190 cubic feet per second into the lake. Unfortunately, a 19-year period is hardly sufficient to determine the future probable minimum, and the average of 1190 cubic feet which occurred in 1908, the year the computations were made, was followed in 1912 by an average flow for four months of less than 900 cubic feet per second, or about 25 per cent less. This will not affect the problem adversely, because of the liberal allowances made in determining losses and the possibility of using an oil-fired steam plant in place of water power.
It is apparent that the 900 cubic feet per second supplied to Gatun Lake will not provide the 4183 cubic feet per second to be consumed. The balance, or 3283 cubic feet per second, will be obtained by filling Gatun Lake to a level of 87 feet above the sea (the gates and copings are 92 feet) before the end of the rainy season, and then, during the succeeding dry season, drawing the lake down gradually to a level of 80 feet above sea level, if need be. The storage capacity the lake between these two levels, at an average area of about 159 square miles, will supply this amount of water with a slight margin. The problem is identical, in many respects, with that involved in the great impounding reservoirs of modern city water works, such as those of Boston and New York, where storage tides over the dry season.
It may be noted that this drawing off of the upper 5 feet of the lake explains one reason why the depth of
channel through the Culebra Cut was made 45 feet at normal lake level. The water level in the cut is the same as in the lake, and when the lake falls to 80 feet, the channel in the cut will have 40 feet depth of water.
At this point it becomes clear that one of the greatest responsibilities of the canal operating force will be the conservation of the water. The operator must be thoroughly versed in problems of rainfall and hydrology, and should begin the dry season with a full lake, and he must be careful not to be caught by an unexpectedly early or unusually dry season; he must each year be prepared for the worst. No apprehension need be felt that the water supply will give out, however, if reasonable care is taken. Should increased storage capacity for water be required to meet new conditions of the distant future, it may be obtained by building a reservoir on the upper Chagres, with a dam at Alhajuela, where some of the flood waters of the Chagres, which now waste over the spillway, may be stored until needed in the dry season. It was here that the French proposed building a reservoir for supplying the highest level of their canal through a tunnel.
It is seen that Gatun Lake can be kept full, but the designing engineer was required to determine that it could be filled initially. An examination of the records of flow of the Chagres for all available years left no doubt that the water in the rainy season in excess of all losses was more than sufficient to fill the lake in two successive years. The driest rainy season of record, 1911-1912, afforded an average flow of 6556 cubic feet per second, which would have filled Gatun Lake in about 400 days, or two rainy seasons, making deductions for reduced losses on account of there being no lockages, no hydraulic power plant in operation, and less leakage, evaporation, and seepage, due to reduced lake area and head of water. Two rainy seasons were actually taken to fill the lake, although in fact the rate of filling depended more on the contingencies of construction work than on the amount of water available.
THE GATUN DAM
The Gatun Dam, which made Gatun Lake possible, is the key to the American Panama Canal scheme. (See plan No. 2.) The lock-level canal might have been built with a dam at a different location, 9 miles upstream at Bohio adopted by the French in their final scheme and selected by the first Isthmian Commission on the lock canal alternative; but the area of the lake would have been very much less, with a consequent loss of opportunity to navigate in wide, unrestricted channels, and a great loss in storage capacity. The dam at Bohio could have been built of masonry on a rock foundation, for which the French made considerable excavation. A masonry dam on rock foundation was not possible at Gatun, because the rock is too far below the surface. It was only after advice had been obtained of some of the ablest engineering talent in the world, familiar with similar problems elsewhere, that an earth dam at Gatun was decided on. This decision was probably the most momentous one in connection with the canal construction. Elaborate investigations were made of the character of the underlying material through test pits and innumerable borings. It was found that the top layer consisted of fine sand intermixed with a large proportion of clay, which extended to a maximum depth, at one point, of practically 80 feet. Below this, for a distance of 100 feet or more, is a thick deposit of impervious blue clay, containing a little sand with a quantity of shells interspersed. Below the clay, and directly overlying the bed rock, is a miscellaneous layer of variable thickness up to 20 feet, consisting of boulders and gravel consolidated with finely divided clays and silts.
Several important factors enter into the design of this dam and the determination of its dimensions. The dam itself must be impervious to water, or, on finer analysis, it would be more accurate to say the seepage must be a minimum. If a well, extending below the ordinary level of the ground water, and without tapping subterranean water channels, is pumped, the ground water in the surrounding territory will flow towards the well and its level will gradually fall and assume a curve joining the surface of the water in the well with the normal ground-water level some distance away. The slope of this curve depends upon the character of the material and the amount of friction which it exerts against the flow. Deeper pumping will lower the curve and extend it farther back. To maintain a fixed level of water in the well will require a fixed rate of pumping, equal to the seepage through the ground, so long as no rain falls on the area affected by the well. The conditions at the Gatun Dam are similar, with the ground-water level in the valley below