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 hkely to 
occur. 
Remarks. 
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 filler would be retained only by 
virtue of the rough surface. 
Sandy loam, clay loam, and light clay 
Stiff clay and other firm soils (not subject to 
freezing). 
Cobble gutters grout filled 
