Fig. 55.
The Bengal Dooars Railway runs near the foot of the Bhutan Himalayas, and crosses some broad river channels which, after the excessively heavy rains which occur, are filled by streams of very high velocities. One such channel or set of channels ([fig. 55]), more than half a mile wide, is provided with a bridge whose waterway consists of ten spans of 60 feet each. The railway embankment across the remainder of the channel having been breached in many places in 1903, protection was afforded by T-headed spurs and other groynes, the first arrangement, which withstood the floods of 1904, being as shown in the figure. The triangular apex of the A-shaped groyne, south-east of the bridge, was added in 1905 because, in its absence, the water struck the bridge obliquely. After the addition there was a great deposit of silt in the neighbourhood of the four T-headed spurs. Next year the river, in a great flood, rose over the top of the railway embankment near these spurs, and finally caused a breach 600 feet wide. The embankment was afterwards raised. The velocity through the bridge seems to have approached 18 feet per second. The bridge had at first no floor. A floor was added, but was much damaged by the floods (Min. Proc. Inst. C.E., vol. clxxiii.). The level of the floor is not given, but it would seem to have been desirable to make it at a very low level. The rising of the stream over the railway embankment was attributed to the silting up near the T-headed spurs. The addition of the triangular portion above referred to would seem to have somewhat assisted this process. If all the trouble could have been foreseen, it might have been best to build an additional bridge 2000 feet south-east of the existing bridge. The groynes were composed of the wire-network rolls, described in [Chap. VI., Art. 3], piled pyramid fashion.
CHAPTER XII
DRAINAGE AND FLOODS
1. Preliminary Remarks.—Arts. 2 and 3 of this Chapter deal with the calculation of flood discharges, Art. 2 dealing with small streams, in which the water has to be got rid of, and Art. 3 with large streams. The remaining articles discuss the methods of predicting floods and of preventing them from doing damage. When the discharge figures have been arrived at in any case, the necessary masonry works can be designed in accordance with the principles described in [Chaps. X.] and [XI]. For remarks regarding the design of channels and banks, see [Chap. IX., Art. 4], and also [Art. 6] of the present Chapter.
In England, land near a stream or flooded area is said to be “awash” when the flood water rises to within 3 feet of the surface of the ground. The drainage of such land is apt to be unsatisfactory. If land is flooded or awash, it may be desirable to shift the outfall of a branch drain to a point lower down in the main outfall.
2. Small Streams.—In dealing with small streams, such as branch drains or natural streams not far from their sources, the engineer is concerned only with their maximum discharges. He has to design culverts, bridges or syphons to pass the streams under roads or other works, or to design channels or waste weirs for them. In a settled country there may be already some works in existence on the same stream, and these may form a guide, or it may be possible to obtain local information as to the height or volume of floods. Even in such a case rainfall figures will be most useful. In districts where there is no settled population, and in any case where the stream is ill-defined, and the flow fitful, the rainfall figures may afford the only, or at least far the best, means of estimating the discharge.
The rainfall to be considered in all these cases is the maximum likely to fall in a short period of time. The catchment areas dealt with are small, say 5 square miles or less. It must be assumed that the fall of rain extends to all parts of the catchment area, and that its duration is sufficient for the water from all parts of it to reach the site of the work. The different valleys or divisions of the catchment area should be considered separately, and regard must be had, not only to the area of each division, but to its length and declivity measured along the course of the stream which drains it. On these two factors depend the time taken by the rain water to reach the site of the work. The rate at which the rain water flows over the ground into rills or small subsidiary streams may be taken to be ¼ mile per hour in flat land, and 1 mile per hour on steep hill sides. The velocity of the current in the rills and larger streams is generally 2 to 4 miles per hour. It can, when necessary, be calculated roughly from the size and slope of the stream. To be on the safe side, the highest probable figure can be taken.
The time taken by the water to flow from the furthest points of the catchment area to the site of the work having been arrived at as above, the next thing is to estimate the probable maximum intensity of the rainfall during that time over the whole catchment area. The only figures immediately available will be the mean annual rainfall, or perhaps the maximum fall in twenty-four hours, but it has been shown ([Chap. II., Art. 5]) how the probable maximum fall over a shorter period may be estimated.
The next thing to be calculated is the “run-off,” i.e. the probable proportion of the rainfall which will at once run off. This may be less than the proportion which will eventually become “available,” because some of it may go to feed the underground supply from which springs are fed. The proportion running off a small area in a short time would, under most circumstances, be rather difficult to estimate, but in the case under consideration, only the probable maximum figure is required. This occurs when the ground is saturated. Under these circumstances the ratio of the run-off to the total fall may be somewhat as follows:—