16 BULLETIN 721, U. S. DEPARTMENT OF AGRICULTURE. 
Maryland, Ohio, North Carolina, and South Carolina, but the data 
so far secured do not warrant any attempt to take the locality into 
account in presenting the results. The conclusions which seem war- 
ranted at present may be stated briefly as follows : 
1. Except where the soil is sandy, the run-off, so far as the roadway and 
slopes are concerned, should be assumed as 100 per cent of the rainfall. 
2. For sandy soils the run-off varies considerably. Where the soil is as much 
as 3 or 4 feet in depth and the grade does not exceed 5 or 6 per cent, it appears 
that the surface run-off may usually be entirely neglected. For less depths of 
sand j 7 soil the run-off may for present purposes be assumed as inversely propor- 
tional to the depth. This does not apply to soils consisting of very fine sand 
with a large percentage of silt, as such soils may be almost impervious to water 
and always erode very easily. 
3. The power of soils to resist erosion is much less when the ditch is first 
constructed than later. This is true especially where vegetation is allowed to 
grow in the ditches. 
4. Although the velocity of flow is independent of the length of ditch, the 
length affects the quantity of water passing any given point, and consequently 
affects the amount of erosion. Further, the tendency to erode on the steep sec- 
tion of a ditch, however short, will be practically the same as if the steep grade 
prevailed throughout the length drained. 
5. The relative carrying capacity of differently shaped ditches appears to be 
indicated with a fair degree of accuracy in the velocity-quantity tables given 
on pages 7 and 8, though the observed data bearing on this point are not 
sufficiently complete to warrant an exact comparison. 
6. It is theoretically possible to construct an earth side ditch of such cross 
section as to carry any given' amount of water on any given grade without ero- 
sion. But the impracticability of widening the ditch indefinitely and the diffi- 
culty of maintaining a wide flat-bottomed ditch necessary to produce this result 
in practice usually serve to place a rather low limit on the capacity. 
7. Where ditches of ordinary cross section are employed, the most satis- 
factory way to prevent erosion is to provide outlets for the water at proper 
intervals, to be determined in the manner already discussed. 
8. Where for any reason outlets for the ditches can not be provided at proper 
intervals it usually is much more satisfactory to provide a paved gutter than to 
attempt to widen the ditch beyond the limits already described as conforming 
to good practice. The main reason for this is that a wide ditch, though prop- 
erly designed and constructed, easily may become partially obstructed, so that 
the channel is effectively narrowed and erosion results. Once started this may 
increase very rapidly. 
9. Side ditches may be protected to some extent from erosion by means of 
"bars," "breakers," or "drops" constructed across the ditch at such intervals 
that the slope from the bottom of one breaker to the top of the next one below 
does not exceed what is allowable for the soil and maximum flow of water in- 
volved. Such breakers frequently have been constructed of wood and sometimes 
of stone or concrete. Plate I is a photograph illustrating the method em- 
ployed. It should be noted that where drops are used much difficulty is ex- 
perienced sometimes in preventing the water from finding its way around their 
ends and cutting into the shoulders or banks. 
10. Where doubt exists as to the necessity for employing a paved gutter it 
frequently is permissible to withhold decision until time can demonstrate 
whether or not the side ditch is adequate. This is true especially where the 
