n 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.

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