636 SOILS: PROPERTIES AND MANAGEMENT 



semicircle. In this form the ditch is one-half as deep as 

 it is wide at the surface. This brings the minimum sur- 

 face in contact with the moving water. The tendency 

 of the banks to cave near the top, as well as the diffi- 

 culty of constructing such a form, has led to the modifi- 

 cation of the walls to an inclined slope that is normally 

 one to one, or an angle of forty-five degrees. This angle 

 is further modified by the nature of the soil through which 

 the ditch passes, and is steeper in clay soil and less 

 steep in loose sandy soil. Where the land is very flat 

 and near the level of the water in the outlet channel, it 

 may be desirable to deepen the ditch considerably below 

 the minimum level of water in order to increase the flow 

 during freshets. 



The shape may be further modified where the volume 

 of water to be carried varies excessively. A wide channel 

 may be provided to accommodate the flood water, and 

 in the bottom of this channel a smaller channel may be 

 provided for the normal flow, of such a size that it is more 

 likely to be kept clean and free than would a ditch of 

 larger cross section in which the water would be shallow. 



An open ditch should be kept as straight as possible so 

 as to avoid erosion of the banks where turns occur. Change 

 of direction should begin gradually and should have the 

 maximum curvature at the middle of the turn. It should 

 then pass gradually on into the straight line of the new 

 direction. 



The grade will naturally conform in a large measure to 

 the surface of the ground, but it may need to be modified 

 from the natural grade where the slope is so steep as to 

 cause serious erosion. This difficulty receives special 

 attention in constructing canals to carry irrigation water. 

 Sandy soils having low cohesion are most subject to 



