Rural Hygiene

by Henry N. Ogden

Another dam that has been much used in rural communities is the old-fashioned crib dam, where logs are piled up crib fashion, held together at the corners by iron pins, a bottom spiked on, and the crib then filled with stone, a succession of these cribs across the stream forming the dam. Dirt is filled in on each side of this crib work, and, in some cases, cross timbers are set in, and both sides of the dam covered with tongue-and-grooved planking. But such dams are not permanent, and their construction involves an expense nearly equal to that of a permanent structure, and consequently they are not to be recommended.

Waste weirs.

When the dam is made of earth with or without a core wall and when no opportunity exists for carrying the waste water around the dam, a waste weir of masonry through the dam must be provided, so that freshets may be carried off without destroying or washing out the earth work.

The size of this weir is a matter of considerable concern, since its ability to carry off the high water is fundamental. The capacity of such waste weirs depends on the volume of flood-water, and this, in turn, depends on the area of the watershed. This volume cannot be predicted with any absolute certainty, but, in general, it may be said that the maximum run-off in the eastern part of the United States, from small areas not exceeding twenty-five square miles, will be about one hundred cubic feet per second per square mile, so that the freshet flow for a watershed of twelve square miles would be twelve hundred cubic feet per second. Ordinarily, the height of the weir is taken to be from two to four feet and the length made sufficient to care for the volume of discharge.

If the depth of water flowing over the weir is taken at one foot, the length of weir in feet necessary to carry the flood flow may be computed by multiplying the number of square miles of watershed by thirty. Then an area of twelve square miles would need a length of waste channel of three hundred sixty feet; in most cases, for small dams, longer than the dam itself.

If the depth be taken at two feet, then the number of square miles of watershed must be multiplied by ten to get the length of weir, so that a shed of twelve square miles would mean a weir one hundred twenty feet long.

The factor for a depth of three feet on the weir is six, making for the same area the length of weir seventy-two feet, and for four feet depth the factor is four. There is no more important part of the construction of a dam than that involved by a proper design of a waste weir, since a failure either to provide proper area or to so build as to withstand the erosive action of the running water will inevitably wash away the dam.

When the valley is narrow and the watershed large, the waste weir will occupy the entire width of the dam, and then it becomes necessary to construct the dam in masonry. On the other hand, when the watershed is small and the width of the valley great, then it is proper to make the waste weir only a certain portion of the entire width of the dam, making the rest of the dam either masonry or earth, as may be convenient.

Gate house.

In connection with a reservoir and at the back of the dam at the bottom of the bank, it is convenient to have what is called, in larger installations, a "gate house"; that is, a masonry or wooden manhole through which the water-pipe leading out from the reservoir passes and in which a gate is placed to shut off the water. In larger installations, it is usually possible to admit water at this point from different levels of the reservoir into the water-pipe, so as always to get the best quality of water, but for a small plant that is not necessary. A gate or valve, however, should always be provided, and while this may be on the bank of the pond with the intake pipe extending twenty or thirty feet into the pond, the valve should not be omitted. The end of the pipe extending into the pond should be placed about two feet above the bottom of the pond, instead of resting in the mud, in order to get a better quality of water.