12 



BULLETIN 124, V. S. DEPARTMENT OF AGRICULTURE. 



D is usually made from 1 foot to 1 foot 6 inches, with W sufficient to 

 give the required capacity. Where the ditch is not located sufficiently 

 far from the road to avoid danger to traffic, and the grade is steep, 

 it may be desirable to increase W and diminish D, in order to secure 

 the required cross-sectional area and at the same time prevent erosion. 



Slope li: I 



SECT/ON OF EARTH DITCH 



Fig. 1. 



The section shown in figure 3 gives a ditch of comparatively small 

 capacity, but the same amount of water flowing in a ditch of this 

 shape will cause less erosion on steep grades than in a very narrow 

 ditch of the cross section shown in figure 1. Another advantage of 

 this wide, flat ditch is that it may be constructed easily and main- 



5 lope liu 



SECT/ON OF EARTH DITCH 



Fig. 2. 



tained by means of a road grader, while the other types of ditches 

 require more or less pick-and-shovel work. One of its principal dis- 

 advantages is that it must be extremely wide on light grades in order 

 to carry any consideraDle quantity of water, and in deep cuts it may 

 be very expensive to obtain the necessary width. 



v 1 ^^ '- ■■ 1 1 1 im > Wf7 



S/ope li:l 



'7^/m> 



SECTION OF EARTH DITCH 



Fig. 3. 



Figure 2 shows a modification of the section shown in figure 3, 

 which is intended to meet conditions intermediate between those de- 

 scribed as being suited respectively to the other two sections. 



From the velocity-quantity tables, pages 7 to 9, it is apparent 

 that for a ditch of given cross section the velocity of flow is de- 

 pendent upon (1) the quantity of water the ditch carries, and (2) the 

 longitudinal slope of the ditch. The velocity which causes erosion 



