DEAINAGE METHODS FOE COUNTY EOADS. 
25 
this purpose, and the culverts and inlets should be placed at such 
intervals as to prevent the ditch from eroding or overflowing. The 
distance apart depends on the capacity of the gutter as designed. If 
the gutter be large enough it may be possible to use few cross drains 
or culverts and space them at intervals of several hundred feet. 
Under other conditions they may be required at intervals of as little 
as 50 to 100 feet. 
The design should provide an inlet that will carry all that the cross 
drain will, and usually there is little difficulty in securing this condi- 
tion. It is not essential that 
(the inlet and cross drain be 
designed to clear the gutter if 
this be continuous, and the 
inlet entrance consequently 
may be a flat grating or open- 
ing designed to admit only a 
part of the flow. Usually, 
however, if the expense of in- 
lets is incurred it is best to 
clear the ditches at each point, 
and the capacity of the cul- 
vert and inlet should be cal- 
culated accordingly. It is 
also good practice to make the 
gutter continuous, so that if 
the inlet becomes partly or 
temporarily clogged the water 
may continue along the gut- 
ter. On very steep grades it is 
well to design the. inlet open- 
ing with a baffle on the lower 
side, so that all water flowing 
in the gutter to that point 
will be taken. In the baffle 
there should be a depression 
along the upper edge to pro- 
CONCRETE DROP INLET 
■0" 
Fig. 8. 
Scale |"=l' 
vide relief in case the inlet becomes interrupted. 
Owing to the difficulty sometimes had in clearing such culverts and 
inlets of obstructions it is well to give careful consideration to the 
probable conditions under which they will operate. If they are 
frequently under a head the inlet will run full but little difficulty 
will be experienced in keeping the culvert clear even at a very low 
slope: but if the prevailing storms are light and omV a small flow 
occurs ordinarily in the culvert the pipe should be given sufficient 
