all 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.

[Illustration: 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.

[Illustration: 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 ofte