Rural Hygiene

by Henry N. Ogden

The writer has recently heard of a dam on a small stream being made by the continual dumping of field stone from the farm into the brook at a certain definite place. This stone, of course, assumed a slope at each side and settled in place from year to year as the dam grew. The mud and silt of the stream filled up the holes between the stones, so that the dam was finally practically water-tight. This made a cheap construction and had the additional value of serving to use up stones from the fields. It was necessary, since the spring floods poured over the top of this dam, to protect the top stones, and a plank crest was put on, merely to keep the dam from being washed away.

The third type of dam is entirely of concrete or stone masonry, concrete to-day being preferable because more likely to be water-tight. The problem with a concrete dam is to get a foundation such that the impounded water will not leak out under the dam, imperiling the very existence of it. The ideal foundation, of course, is rock, and in a great many locations can be found in the small gulleys where the limestone and shale peculiar to this region will answer as well as more solid rock for dams not more than ten feet high; but with gravel banks on the sides or with soft sandy bottom, or where the clay soil becomes saturated with water at times, the gulley offers great difficulties for the construction of a dam. It will be wise, under such conditions, to carry a cut-off wall, not necessarily more than twelve inches thick, well into the bank, that is, about ten feet on each side, and under the dam this cut-off wall ought to go down until it reaches another stratum of sand or clay or rock. This cut-off wall, then, surrounding the main dam, shuts off the leakage, and the dam itself can be built without danger of undermining. In many large dams this cut-off wall is carried down more than a hundred feet, especially where the depth of water behind the dam is great. For small dams, a row of plank driven down behind a timber sill across and in the bed of the stream will often be sufficient.

Fig. 40.—Section of a flood dam.

The cross section of the main dam, in cases where flood water in the spring runs over the dam, should be such that the bottom thickness is about one half the height, and Fig. 40 (after Wegman) shows a suitable cross-section of a dam ten feet high. Figure 41 (after Wegman) shows a cross-section intended to carry the water over the dam, especially in times of flood, without danger of erosion.

Sometimes, in a narrow gorge with rock sides, it is possible to save masonry by building the dam in the form of an arch upstream, the resistance to the force of the water being then furnished by the abutment action of the rock sides, instead of by the weight of the dam, as in ordinary construction. For a dam ten feet high, the necessary thickness of the curved dam would probably not be more than twelve inches, while the ordinary gravity dam would be three or four feet thick. The workmanship on the former, however, must be of a very superior order.

Fig. 41.—Section of a flood dam.

It is never desirable to allow the water flowing over the dam to fall directly on the ground in front, since the falling water will rapidly carry away this soil and undermine the front of the dam. For this reason, the lower section of the dam is made curved, as shown in Fig. 41, giving the water a horizontal direction as it leaves the dam instead of a vertical. A plank floor is often added to carry even further from the dam any possible erosion (Fig. 40). Where it can be done, it is a good plan to provide a small body of still water below the dam, so that the force of the falling water may be distributed through the water on to the soil below.

There are other forms of dams often used. For example, brush dams, formerly common, are made by cutting off the tops of trees and dropping them in place and loading them with stones so as to make a mass of interwoven branches. These branches hold together particles of earth which are dumped in and form a dam.