Figure 3-10.-Check dams.falls upon the surface, or traveled way, is drained bycrowning the surface; that is, constructing the traveledway so that the middle is higher than the edges.The traveled way in curves is drained bysuperelevating the surface; that is, constructing thetraveled way so that the inside edge of the curve is lowerthan the outside edge.The water that drains from the surface continuesover the shoulders. The shoulders always have a slopegreater than, or at least equal to, the surface slope. Thisslightly increases the speed of the draining water andtherefore increases the rate of drainage. The water thenflows from the shoulder down the side of the fall, if in afill section of a roadway. If the section is in a cut, thewater flows into a roadway ditch. Roadway ditches arenot normally in a fill section.Roadway DitchesThe functioning of a roadway ditch is the mostimportant factor in roadway drainage. If this ditch,which runs alongside the roadway, becomes obstructedor 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 determiningthe size and type of roadway ditches, such as volume ofwater to be carried, the slope of the backslope, soil types,the “lay of the land,” and the maximum and minimumditch grades.The slopes of the surface, shoulders, and backslopesaffect the volume. A steep slope increases the rate ofrunoff, thereby causing a greater instantaneous volumeof water in the ditch. On the other hand, a lesser slopedecreases the rate of runoff but exposes more surfacearea on the backslope, which increases the amount ofrunoff.The choice of slopes to be used is governed by otherfactors, however. The foremost factors are whether theadditional excavation is needed in the roadwayconstruction and the type of soil. A lesser slope wouldbe 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 foothorizontal, 1 foot vertical). This slope maybe decreasedfor sandy soil or greatly increased for rock cuts. Theusual ditch slope, from the shoulder to the bottom of theditch, is 3:1. All these soil types have different amountsof runoff. The runoff from a sandy soil is small, but froma 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 desirablemaximum grade is 4 percent. A grade greater than 4percent would cause excessive erosion due to the greatervelocity of the water. In this case, low dams of wood orstones, called check dams (fig. 3-10), are built across3-11
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