The material to be excavated in this division is sand, gravel, clay, and in the “divide” cut rock, which will be utilised in the construction of the breakwater at Brito, in pitching the canal slopes, and in concrete for the locks, culverts, weirs, and the dam across the Rio Grande. The location of the canal in this division is the same as that proposed by the engineer Menocal on his return from Nicaragua in 1880. The prism, however, has been increased, the number of locks reduced, and their location changed. The enlargement of the terminal section is also a new feature.
The canal enters Lake Nicaragua, an inland sea, 40 miles wide, and over 90 miles long, which forms its summit level, and with the Chontales Mountains on the left, the route is continued to Fort San Carlos at the outlet of the lake into the Rio San Juan. Throughout this distance of 56½ miles, 28 feet of water can be carried to within 2400 feet of the mouth of the Lajas on the west shore of the lake, and within eight miles of Fort San Carlos on the south-eastern shore. In the former distance some dredging and rock excavation under water will be necessary, and in the latter, dredging in soft mud to an average depth of 3½ feet. From Fort San Carlos the route proceeds 64 miles down the San Juan river, which, with the exception of the 28 miles from the lake to Toro Rapids, has a depth varying from 28 feet to 130 feet, to the dam thrown across the river at Ochoa just below the mouth of the Rio San Carlos. Throughout this stretch of river, the only work to be done is dredging in mud and gravel, and some rock excavation under water to an average depth of four feet along a distance of 24 miles, below Fort San Carlos, and light excavation above water on some points in the lower river in order to flatten the bends.
The dam just mentioned is located between two steep, rocky hills, at a point where the river is 1133 feet wide between the banks, with an average depth of 6·6 feet. Its length on the crest will be 1255 feet, its height 52 feet, the depth of foundations 20 feet below present water level, and it is to be constructed entirely of concrete, with timber-lined crest, front, and apron, and rip-rap protected back, forming a monolith wedged between rock abutments. This dam will back the water of the river the entire distance to Fort Carlos and into the lake, maintaining the water of the latter at the proposed level of 110 feet, and will convert the upper San Juan into an extension of the lake, with a fall of ¾ inch per mile.
The valley referred to, flooded by the back water from the dam, affords an excellent basin at the entrance of the canal, free from the influence of the river current, and the latter forms a natural, ready-made canal, 3300´ long, needing only slight excavation on the points of two or three spurs for rectifying the channel. From the head of this valley, a canal 1·82 miles long extends across a broken country of moderate elevation, intersecting one deep narrow ravine, debouching towards the San Juan, across which a short embankment will be necessary, and enters the valley of the river San Francisco. This river San Francisco flows east, north-east, and east, approximately parallel to the San Juan, and separated from it by a range of hills to a point about nine miles (in straight line) from the dam, then, receiving a considerable tributary (the Cano de los Chanchos) from the north-east, turns abruptly to the south-east and south, and enters the San Juan. Its valley thus forms an irregular flattened Y, with its foot or stem resting on the San Juan, one arm extending westerly to within a short distance of the dam, and the other easterly in the direction of Greytown.
Across the stem of this Y, just below the junction of the two arms, will be built an embankment 6500 feet long on the crest, and having a maximum height of 51 feet. This embankment will retain the water of the San Francisco and its tributaries, flooding the whole upper valley (the arms of the Y) to a depth of from 30 to 50 feet, and forming a large lake at the same level as the river above the dam—in other words, a continuation of the summit level.
Proceeding from the end of the short canal already described, the main canal passes down the westerly arm of this broad, deep, crescent-shaped basin, past the embankment, then up the easterly arm to the western foot of the divide between the San Francisco and the San Juanillo, 12·55 miles from the dam, and within 19·48 miles of Greytown. Here the eastern division of the canal is entered, beginning at the Saltos de Elvira, whence it proceeds nearly due east, through the broad, flat upper valley of the Arroyo de las Cascadas, cutting a spur here and there to the “divide,” less than one mile from the Saltos, and 280 feet above the sea. Then curving gradually to the south-east, across the little plain at the summit, it cuts a steep, narrow spur, enters the valley of the Deseado, a stream flowing into the San Juanillo, follows its bed a short distance, then crosses to the left bank, and reaches the site of the upper lock of the eastern flight, 14,200 feet from the Saltos. The average cut for this distance is 149 feet.
At this lock, excavated in the rock foundations of a spur of the northern hills, the summit level, reaching back through the San Francisco basin, up the San Juan, and across the lake to the first lock on the west side, a distance of 144·8 miles, ends, and the canal, lowered 53 feet by the lock, passes by easy curves down the widening valley of the Deseado to the next lock, less than a mile beyond. Here another drop of 27 feet occurs, and then the canal follows the still widening and gradually descending valley in a north-easterly direction for less than three miles to the third and last lock at the mouth of the valley. This lock lowers the canal to the sea level, and from here it takes a direct course across the flat low basin of the San Juanillo and the Lagoon region, to the harbour of San Juan del Norte, or Greytown, about 11½ miles distant.
The surface drainage to be provided for in this division is not extensive, and it is especially small on the western slope of the “divide,” where three short artificial channels will divert it all into the San Francisco Lake at some distance from the canal. Across the “divide,” and as far as the first intersection of the canal and the Deseado, the natural drainage is away from the canal. From this point to the San Juanillo the canal will be protected on both sides by drains formed partly by the present bed of the Deseado, and partly by artificial channels. The remainder of the canal, through the lowland from the San Juanillo to Greytown, will be protected by embankments formed by the material deposited by the dredgers, an artificial channel being cut on the south to divert the San Juanillo, and another on the north to give Laguna Bernard and its tributaries an independent outlet to the sea. From the last lock to Greytown the canal is enlarged, as at Brito, on the west side, forming an extension of the harbour 11½ miles inland. The material to be excavated in this division is sand, gravel, and alluvial soil (all dredgable material) for a distance of 12 to 15 miles from Greytown, then clay, gravel, and rock in the deeper cuts, and finally, in the “divide,” cut rock, which will be utilised as on the west side, in the construction of the embankment, in the breakwater at Greytown, in pitching the canal slopes, and in concrete for the dam and locks.
About 27 miles of the actual canal will be ordinary excavation, and it is proposed that the remaining 13 miles will be largely, if not entirely, excavated by dredgers. In the western division, the excavation of the portion of the canal between the last lock and the Pacific by dredgers will solve the problem of the drainage of the work for that division, as on the remaining excavation, being above sea-level, the question of drainage will be perfectly simple.
In the eastern division, as in the western, the portion of the canal between Greytown harbour and the first lock, a distance of 11½ miles, will be dredged.