DRAINAGE METHODS FOR COUNTY EOADS. 15 



As an example of how the tables may be used, consider a side ditch 

 of the cross section shown in figure 3, in moderately stable loam iyoil 

 and let it be assumed that the ditch will drain the surface on each 

 side ft>r an average distance of 12 feet from the center, or that the 

 total width drained is 24 feet. Also assume the maximum rainfall 

 at 1 inch per hour, the maximum run-off at 100 per cent, the allowable 

 velocity in the ditch at 1 foot per second, and the longitudinal slope 

 of the ditch at 0.05 or 5 per cent. From table IB we see that in order 

 for the velocity not to exceed 1 foot per second, the depth of water 

 in the ditch must not exceed about If inches, and that this depth fixes 

 the capacity of the ditch at about 500 cubic feet per hour. Under the 

 given assumptions, the allowable quantity of water would flow into 

 the ditch in a distance of 250 feet, and, if a greater length of ditch 

 must be drained without the 'water being turned off, some provision 

 for protecting the ditch from erosion should be made below the 250- 

 foot point. 



In practice it frequently will be necessary to modify the computa- 

 tions by taking into consideration such contingencies as changes in 

 the gTade or curves in the alignment of the ditch, or the matter of 

 water being turned into the ditches from intersecting roads or adja- 

 cent fields. Where the grade of a ditch flattens out in descending a 

 hill, for example, the velocity of the water will be checked where the 

 change occurs, and a part of whatever soil it has in suspension will 

 be deposited. Curves in alignment produce a tendency for the water 

 to cut sideways, away from the center of the curve, and for this 

 reason water following along the inside ditch of a curved roadway is 

 very liable to erode the shoulder, and eventually may undermine the 

 road crust, unless provision be made to prevent this. In general, 

 wherever there are marked changes in grade, it is good design to 

 proportion the ditches accordingly, especially where a descending 

 grade flattens out or a curve occurs in the alignment. This is a 

 detail quite commonly neglected, but by observing it the velocity of 

 the flow can be maintained sufficiently uniform to prevent both the 

 deposit of sediment and the erosion of the ditch or grade. The prac- 

 tice of turning into side ditches water from adjacent fields and inter- 

 secting roads is very objectionable, especially on steep grades, and 

 usually can be avoided. Where it can not be avoided, ample allow- 

 ance for the additional water should be made in the computations. 



In order to supplement the data contained in the tables a limited 

 number of observations have been made on roads in several localities, 

 with a view to ascertaining just how the erosion of side ditches actu- 

 ally is affected by different conditions of soil and grade. The sec- 

 tions of road selected for study were all so situated that the area 

 drained by the side ditches consisted of only the roadway and the 

 excavation slopes formed in grading the road. Studies were made in 



