DRAINAGE METHODS FOR COUNTY ROADS. 



13 



varies considerably with the character of the soil traversed by the 

 ditch, and, since it is impracticable to describe soils in exactly com- 

 parable terms, no exact rules can be established for determining the 

 safe velocity for water flowing in earth ditches. The heavy zigzag 

 lines drawn through the tables are just below the maximum velocities 

 which would ordinarily be considered safe for the most favorable 

 soil conditions, and table 5 shows other safe velocities which may 

 be assumed for given soil conditions. 



Table 5.- 



-Safe maximum stream, velocities for different soils and other 

 materials. 



Kind, of material. 



Mean velocity 



.in feet per 



second beyond 



which erosion 



is Likely to 



occur. 



Remarks. 



Fine sand and silt 



Sandy loam, clay loam, and light clay 



Stiff clay and other firm soils (not subject to 



freezing). 

 Coarse gravel and crushed stone 



Cobble gutters sand filled 



Cobble gutters grout filled 



Solid rock and concrete - . 



0.5 to 1.0 



1.0 to 1.5 

 1.5 to 3.0 



3.0 to 5.0 

 5.0 to 7.0 



10. to 15. 

 15. to 25. 



These values would apply to some alluvial 



soils. 

 Average soil conditions. 

 These values should be used for only very 



resistant soils in warm climates. 

 These values apply to such materials as are 



ordinarily used for road crusts. 

 The sand fUler would be retained only by 



virtue of the rough surface. 



