16 BULLETIN 724, U. S. DEPARTMENT OF AGRICULTURE. 



Maryland, Ohio, Xorth 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 gi-ade does not exceed 5 or 6 per cent, it appears 

 that the surface run-oft" may usually be entirely neglected. For less depths of 

 sandy 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 constriact 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 pi-ovide 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 ditcli at sucli intervals 

 tluit 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. Phite I is a photograph illustrating the method era- 

 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 slioulders or banks. 



10. Where doubt exists as to the necessity for employing a paved gutter if 

 frequently is permissiltle to withhold decision until time can demonstrate 

 whether or not the side ditch is adequate. This is true especially where the 



