An excellent plan, when it can be adopted, is to have more than one line of operations, so that the heading up of the water is divided between them.
In India closures of streams having depths of 6 or 8 feet are effected by means of rough trestles made from trunks of small trees and placed at intervals in the stream like bridge piers, one leg of the trestle inclined upstream and one downstream. Each pair of adjacent trestles is connected by a number of rough, horizontal poles. Against these are placed bundles of brushwood. Earth is at the same time collected and is rapidly added at the last. The chief danger is the undermining of the bed by scour. This is prevented by driving in stakes and placing brushwood against them. Closures of small channels or of breaches in the banks of canals are effected by means of staking and brushwood. Where dangerous breaches are liable to occur, it is a good plan to have a barge, fitted up with a small pile-driver and carrying a supply of sheet piles, ready at a convenient spot.
Hurdle dykes, first used on the Mississippi, were employed on the Indus in 1902 to close partially the main channel of the river. There were to be three dykes, each dyke consisting of three lines of very long piles—some were 60 feet long,—driven into the bed of the stream, which was to be protected with mattresses made of fascines and extending right across it, with their heads above flood-level. The idea was not to wholly stop the flow of the water, but to obstruct it so much that silt would deposit, the channel become choked up, and the water find a course down another channel. The work was begun in March 1902, and was in progress in May of the same year when an unusually early flood put a stop to it. The dykes had at this time advanced considerable distances from the right bank of the stream, but none had been completed. Two dykes out of the three were for the most part carried away. The river, however, took a new course, starting from a point far upstream, the western channel became a creek, and the remains of the dykes were soon embedded in silt.
In any case in which the provision of a proper mattress has been omitted, or when the mattress has been destroyed, or when a breach has occurred in an embankment, whenever, in short, it is evident that the gap cannot be closed until some other escape for the water is provided, it may be possible to provide such an escape by cutting partly through the dam or embankment on the downstream side at another place, and thoroughly protecting the place and extending the protection downstream and away from the dam or embankment. The water can then be let in, and the closure of the old gap attempted. If a closure is effected, the protected gap can then be closed. Sometimes it may be desirable to make such a protected gap beforehand and with deliberation.
Dams for closing streams which are dry can be made similarly to flood embankments ([Chap. XII., Art. 7]). Sand does very well, provided it is protected by a covering of clay or by fascining.
3. Instances of Closures of Streams.—In 1904 the Colorado River broke into the Salton Sink—a valley covering 4000 square miles. Unsuccessful attempts were made to close the stream by two rows of piles with willows and sandbags between them, by a gate 200 feet long, supported on 500 piles, and by twelve gates each 12 feet wide. A “rock-fill” dam was then constructed on a mattress 100 feet wide and 1·5 feet thick. The river, which was 600 feet wide, broke through, but was stopped by the construction of three parallel rock-fill dams in the gap (Min. Proc. Inst. C.E., vol. clxxi.).
Fig. 25.
At the site of the railway bridge over the river Tista in Bengal, it was necessary to close the main stream ([fig. 25]), which flowed at the left side of the channel, while the bridge had been built at the right. The bed was of sand, width 500 feet, depth 6 feet, and discharge 3700 cubic feet per second. The first attempt to close the stream was made at M N, a floor of stone 200 feet long, 20 feet wide, and 2 feet thick, being laid in the middle of the stream, and dams of earth, sandbags, and stones being run out from each bank. As the gap decreased in width the bed was torn up and the work failed. The heading up was 3 feet 9 inches. It was recognised later that the site should have been at the bifurcation higher up, and that the stone floor should have been laid on a mattress.
In the next working season the dams C D and E F G were made. The dam C D was of earth. Two walls, each consisting of a double line of bamboos with the spaces between the lines filled with bundles of grass weighted with earth, were run out 50 feet in advance of the earthwork near the lines of the toes of the slopes. Along the line of the upper wall a mattress of broken bricks 10 feet in width, and 1 foot thick, was laid, and was kept 50 feet in advance of the wall. A total length of 1000 feet of embankment was made in five months and pitched on its upstream side. The end was strongly protected by a mass of stone. The embankment F G was of earth. The dam E F consisted of three lines of piles driven 10 feet into the bed. A mattress weighted with stones extended for 20 feet upstream of the dam and 40 feet downstream. A gap of 150 feet was left at D E, and was not protected by a floor of any kind. A channel, parallel to F G and extending to K, had been dug to a width of 200 feet. During the floods the heading up at D E was about 2·5 feet, and the water was 30 feet deep. The line E F was greatly damaged and was repaired. The cut F G K gradually enlarged, and by the end of the floods more water was going down it than down the main stream. The gap D E was finally closed by means of a line of bamboos and grass, the bed being protected by a carpet, 100 × 50 feet, made of common cloth weighted with sandbags. The success of the operations turned on the scouring out of the cut F G K. It is remarkable that the gap D E did not become wholly unmanageable in the floods (Min. Proc. Inst. C.E., vol. cl.).