Figure 3-10.-Check dams.
falls upon the surface, or traveled way, is drained by
crowning the surface; that is, constructing the traveled
way so that the middle is higher than the edges.
The traveled way in curves is drained by
superelevating the surface; that is, constructing the
traveled way so that the inside edge of the curve is lower
than the outside edge.
The water that drains from the surface continues
over the shoulders. The shoulders always have a slope
greater than, or at least equal to, the surface slope. This
slightly increases the speed of the draining water and
therefore increases the rate of drainage. The water then
flows from the shoulder down the side of the fall, if in a
fill section of a roadway. If the section is in a cut, the
water flows into a roadway ditch. Roadway ditches are
not normally in a fill section.
Roadway Ditches
The functioning of a roadway ditch is the most
important factor in roadway drainage. If this ditch,
which runs alongside the roadway, becomes obstructed
or is inadequate for the volume of water, then the road-
bed becomes flooded. Not only can this block traffic,
but it can also wash away surface and shoulder material.
There are several factors to consider in determining
the size and type of roadway ditches, such as volume of
water to be carried, the slope of the backslope, soil types,
the “lay of the land,” and the maximum and minimum
ditch grades.
The slopes of the surface, shoulders, and backslopes
affect the volume. A steep slope increases the rate of
runoff, thereby causing a greater instantaneous volume
of water in the ditch. On the other hand, a lesser slope
decreases the rate of runoff but exposes more surface
area on the backslope, which increases the amount of
runoff.
The choice of slopes to be used is governed by other
factors, however. The foremost factors are whether the
additional excavation is needed in the roadway
construction and the type of soil. A lesser slope would
be required if the cut is in sand instead of clay or rock.
The usual cut slope, or backslope, is 1 1/2:1 (1 1/2 foot
horizontal, 1 foot vertical). This slope maybe decreased
for sandy soil or greatly increased for rock cuts. The
usual ditch slope, from the shoulder to the bottom of the
ditch, is 3:1. All these soil types have different amounts
of runoff. The runoff from a sandy soil is small, but from
a clay soil or solid rock it is large.
An important design factor is the ditch grade itself.
The minimum grade is 0.5 percent, and the desirable
maximum grade is 4 percent. A grade greater than 4
percent would cause excessive erosion due to the greater
velocity of the water. In this case, low dams of wood or
stones, called check dams (fig. 3-10), are built across
3-11
