siderable period. These peculiarities must be known before an
adequate drainage system can be planned.

It is almost universally true in the United States that precipitation
at a very high rate will be for a relatively short duration, and
during these short periods, which usually do not exceed thirty
minutes, a portion of the water that falls on the areas adjacent to
the road and that drains to the road ditches will soak into the soil
and therefore not reach the ditches along the road. The extent to
which the water is taken up by the soil will vary with the porosity
and slope of the land and the character of the growth thereon.
Cultivated land will absorb nearly all of the water from showers up to
fifteen or twenty minutes duration; grass land a somewhat smaller
percentage; and hard baked or other impervious soil will absorb a
comparatively small amount. Rocky ground and steep slopes will absorb
very little storm water.

The surface of the road is designed to turn water rapidly to the
ditches, but when the material is the natural soil, there is always
considerable absorption of storm water. Surfaces such as sandclay,
gravel and macadam do not absorb to exceed 10 per cent of the
precipitation during short showers. Bituminous surfaces, brick and
concrete pavements, do not absorb an appreciable amount of storm
water.

Generally it is best to assume that if a rain lasts for forty-five
minutes or more, all of the water will run off, as the soil will reach
a state of saturation in that time. This is not true of deep sand, but
is for nearly all other soils.

The ditch capacity needed will therefore depend upon the area drained,
the character of the soil, the slopes and the rainfall characteristics
of the region, and upon the nature of the road surface.

For a required capacity, the cross section area of the ditch will vary
inversely as the grade, because the velocity of flow increases with an
increase in the grade of the ditch. If the surface water must be
carried along the road for distances exceeding five or six hundred
feet, the ditch must be constructed of increasing capacity toward the
outlet in order to accommodate the accumulated volume of water.

The velocity of flow varies not only with the grade, but with the
shape of the cross section, cleanness of the channel, the depth of the
water in the channel, alignment of the channel and the kind of
material in which the channel is formed. It is not necessary to go to