River and Canal Engineering, the characteristics of open flowing streams, and the principles and methods to be followed in dealing with them.

by E. S. Bellasis

For a description of flood embankments along the great shifting rivers of Northern India, see Punjab Rivers and Works.

Note to Art. 5.—Floods can sometimes be mitigated by sinking pits in the flooded area so that the flood water comes in contact with permeable strata and is absorbed by them.

CHAPTER XIII
RESERVOIRS AND DAMS

1. Reservoirs.—The object of a reservoir is to store water for town supply or for irrigation or other purposes. Reservoirs for the water supply of towns are divided into “impounding reservoirs” and “service reservoirs,” the latter being of comparatively small size, and their object being to store, near to the town, a supply sufficient for a short period. Instead of one impounding reservoir there may be several, formed by various dams and one discharging into another. When a reservoir is mentioned without qualification, an impounding reservoir is meant. A reservoir is generally made by blocking up a valley by means of a dam of earth or masonry. The site of the dam should be selected at a place where the valley is narrow. The lowest portion or “bottom water” of a reservoir is usually not drawn upon, because it is less pure than the rest, and it has to be left, in dry weather, for the fish. It is not included in calculating the capacity of the reservoir.

In Great Britain, when the water of a stream is impounded, “compensation water” has to be given back to the stream lower down. This compensation water is generally given in the form of a constant supply, and amounts to perhaps a quarter of the available supply. It has to be included in calculating the daily supply taken out of the reservoir. The advantage to the stream in having this addition to it during dry weather is very great.

It has already been seen ([Chap. IX., Art. 1]) that in an earthen bank which has to retain water the leakage generally decreases rapidly and the bank becomes almost impermeable. The same is true of the surface of a valley, in the case of most ordinary soils, provided that it is kept submerged. Any portions which become exposed to the sun and weather are likely to crack and give rise to percolation. Thus a reservoir formed by the construction of a dam resting on the surface of the ground may be more or less water-tight according to circumstances. There are many which are sufficiently water-tight. But in most cases the dam—or an impervious core-wall—is carried down to an impervious stratum. A masonry dam is carried down to rock.

In the case of dams of considerable height the soil should be examined by borings. If there is an inclined stratum not well connected with that below it, unequal settlement of the dam may occur; and this may also happen if there is a thick stratum of clay, owing to its compressibility.

Except for very high dams—those, for instance, more than 110 or 120 feet in height measured from the ground to the water-level—an earthen dam is cheaper than a masonry dam. It is also more easily raised and strengthened—though this operation has also been effected on masonry dams—in case, for instance, of the silting up of the reservoir, a process which is slow in England, but not so slow when water containing much silt is dealt with, as in the case of irrigation reservoirs in India. Sometimes a dam consists of a wall of masonry or concrete with earth behind it as a support. Whatever kind of dam is used, its construction always demands very great care. Serious disasters, with much loss of life, have occurred owing to failures of dams.

A reservoir with an earthen dam is provided with a waste weir for the purpose of passing off flood water, which might otherwise overtop the dam and destroy it. Generally the waste weir is a continuation of the line of the dam. Its crest has to be below the high-water level of the reservoir, but not lower than can be helped, and its length has therefore to be considerable. Sometimes it is provided with grooved piers between which planks are placed in the season when floods are not likely to occur.

In connection with irrigation reservoirs in Western India, it has been pointed out by Strange (Min. Proc. Inst. C.E., vol. cxxxii.) that a long high-level waste weir is best suited to cases in which the replenishment of the reservoir is uncertain, and that in cases where it is nearly certain, the high-level weir prevents the water-level in the reservoir being quickly lowered in the case of an accident or for the purpose of effecting repairs, impounds the earliest floods, which are most charged with silt, and causes the water area to be a maximum, and therefore gives all floods the maximum time in which to deposit silt. He accordingly suggests that the crests of waste weirs in these reservoirs should be shortened and lowered and provided with falling shutters (this had been done in one reservoir and has since been done in another), and that sluices be added with sills at a still lower level than the lowered crest. These proposals seem to be entirely reasonable, though of course it would be necessary to have skilled supervision over the working of the sluices. Sometimes the waste weir is made in a separate place, being separated from the dam by a saddle.