tem for a proposed road
improvement ought to be designed with as much care as any other
element, and, to do so, a study must be made of all factors that have
any bearing on the drainage requirements and the probable
effectiveness of the proposed drainage system. The well established
principles of land drainage should be followed so far as applicable.

The basic principle of road drainage is to minimize the effect of
water to such an extent that there will always be a layer of
comparatively dry soil of appreciable thickness under the traveled
part of the road. This layer should probably never be less than two
feet thick and for soils of a structure favorable to capillary action
it should be at least three feet thick. The means employed to
accomplish the requisite drainage will be as various as the conditions
encountered.

=Surface Drainage.=--The drainage method which is by far the most
nearly general in application is that which utilizes open ditches, and
the system which employs these ditches is usually referred to as
surface drainage. The full possibilities of this method of minimizing
the effects of storm water are rarely fully utilized in road
construction. Very frequently, deterioration of a road surface is
directly attributable to failure to provide adequately for the removal
of the storm water or water from the melting of snow that has fallen
on the road, or water that flows to the road from land adjacent
thereto. Surface water can usually most cheaply and expeditiously be
carried away in open ditches, although special conditions are
occasionally encountered which require supplementary tile drains. The
cross section commonly adopted for roads lends itself naturally to the
construction of drainage ditches at the sides of the traveled way, and
these are usually the principal dependence for the disposal of storm
water.

=Run-off.=--The capacity required of side ditches to insure
satisfactory surface drainage will be affected by the amount and
nature of the precipitation in the region where the road is built. The
annual rainfall in a region may amount to several feet, but may be
well distributed throughout the year with an absence of excessive
rainfall for short periods, that is, flood conditions may rarely
occur. In other areas, the annual rainfall may be comparatively small
but the precipitation occurs at a very high rate, that is, flood
conditions may be common, or it may be at a low rate extending over a
con