American Rural Highways

by T. R. Agg

The surface of the road is designed to turn water rapidly to the ditches, but when the material is the natural soil, there is always considerable absorption of storm water. Surfaces such as sandclay, gravel and macadam do not absorb to exceed 10 per cent of the precipitation during short showers. Bituminous surfaces, brick and concrete pavements, do not absorb an appreciable amount of storm water.

Generally it is best to assume that if a rain lasts for forty-five minutes or more, all of the water will run off, as the soil will reach a state of saturation in that time. This is not true of deep sand, but is for nearly all other soils.

The ditch capacity needed will therefore depend upon the area drained, the character of the soil, the slopes and the rainfall characteristics of the region, and upon the nature of the road surface.

For a required capacity, the cross section area of the ditch will vary inversely as the grade, because the velocity of flow increases with an increase in the grade of the ditch. If the surface water must be carried along the road for distances exceeding five or six hundred feet, the ditch must be constructed of increasing capacity toward the outlet in order to accommodate the accumulated volume of water.

The velocity of flow varies not only with the grade, but with the shape of the cross section, cleanness of the channel, the depth of the water in the channel, alignment of the channel and the kind of material in which the channel is formed. It is not necessary to go to great refinement in the design of the side ditches for the ordinary case where the water is carried along the road for only a few hundred feet. The ditches are made of ample capacity by using the commonly accepted cross section for a road, which will be discussed in a later paragraph. But where large areas must be drained by the road ditches, it is desirable carefully to design the side ditches. The basis for that design is too lengthy to be included herein, and reference should be made to a standard treatise on the subject.

Ordinary Design of Ditches.—For grades of one per cent or less on roads in the humid area, the bottom of the ditch should be at least three and one-half feet lower than the traveled surface of the road, except for very sandy soil. For grades greater than one per cent, this depth may be decreased one foot, and for grades of four per cent and upward, the depth may be still less. These general rules for depth are susceptible of variation but are believed to be the minimum except in arid or semi-arid climates. It is far better to be too liberal in ditch allowance than to be too conservative. In arid or semi-arid regions, the ditch design will be based on the necessity of providing for flood flow and preventing damage through erosion. Ordinary drainage requirements will be satisfactory with the ditch about one foot deep.

If the topography is such that it is evident considerable storm water will flow from the adjacent land to the road ditches, the design must be modified to take this into account. Sometimes such water can be diverted by ditches well back from the road, and thus prevented from flowing into the side ditches along the roadway. It is especially desirable to divert water, which would otherwise flow down the slope of a cut, by means of a ditch on the hill-side above the upper edge of the slope of the cut.

Ditches are not effective unless they afford a free flow throughout their length and have an outlet to a drainage channel of ample capacity. Therefore, ditch grades should be established by survey, especially if the gradient is less than one per cent, and the construction work should be checked to insure that the ditch is actually constructed as planned. A few high places in the ditch will greatly reduce the effectiveness, although these may appear at the time of construction to be slight. Constricted places, such as might be due to a small amount of loose earth left in the ditch, are always to be avoided.

Where the side ditch passes from a cut to the berm alongside a fill, the ditch should be excavated throughout in the undisturbed natural soil, five feet or more from the toe of the slope of the fill, and along the filled portion of the road there should be a berm of three or four feet between the toe of the slope of the fill and the near edge of the ditch.

Underground Water.—In a preceding paragraph, mention was made of the fact that only a part of the storm water runs off over the surface of the ground, the larger part being absorbed by the soil. The water thus absorbed flows downward through the pores in the soil until it is deflected laterally by some physical characteristic of the soil structure. The movement of underground water is affected by many circumstances, but only two conditions need be discussed herein.