12 BULLETIN "24, U. 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. 
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^ v . //AV/7WAV rf ^ 
V-W-A 
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- 
S/ope fiu 
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 carn^ any consideraDle quanthw of water, and in deep cuts it may 
be very expensive to obtain the necessary width. 
^ Slope 
Slope li- 1 
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 
