Roadway Ditches

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