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

the bottom of the ditch to slow down the water. In general, a moderate velocity is desirable because it prevents excessive erosion and can offset the pending effect of slower moving water.

One factor involving the volume of water that cannot be controlled is the rainfall itself. The more intense the rainfall and the longer the duration, the greater the volume of water the ditch has to carry. Talking to local residents and observing high-water marks along streams are helpful to the engineer in determining the heaviest rainfall to expect in a particular area.

The engineer must consider not only the factors involving the volume of water but also the design of the ditch itself. Two common types of ditches are the V-bottom and the flat bottom, or trapezoidal, ditch. Examples of these ditches are shown in figure 3-11. Under similar conditions, water flows faster in a V-bottom ditch than in a trapezoidal ditch. The side slope for a shallow V-bottom ditch is 4:1 or greater. For

Figure 3-13.-Types of culverts.

The flat bottom is generally 2 feet wide but can range from 1 foot to 6 feet or more.