DRAINAGE METHODS FOR COUNTY ROADS, 11 
The foregoing tables on page 7 are not applicable to short cross 
drains and culverts, because the condition of approach channels and 
outlets affect the capacity of these to a very great extent. Data for 
use in designing structures of this kind are furnished in other 
bulletins of the Bureau of Public Koads. 
DESIGN OF SURFACE DRAINAGE. 
Surface-drainage systems for roads consist of side ditches along 
road, paved gutters which are a development of side ditches and 
replace them, open intercepting ditches constructed to prevent water 
from reaching the road, and lateral or relief ditches to carry off the 
water which collects in the side ditches or in the intercepting 
ditches. Culverts and inclosed drains, which are constructed for the 
purpose of removing storm water from the side ditches or gutters, are 
essentially a part of the surface drainage system and are not to b'e 
confused with subdrains, which have an entirely different purpose. 
CROWN. 
The amount of crown which the surface of a road should be given 
in order to drain itself properly is very important in connection 
with surface drainage and one of the early elements of considera- 
tion in drainage design. But, since this feature is necessarily in- 
fluenced by other considerations than efficient drainage, it will not 
be discussed at length in this paper. The Bureau of Public Roads has 
already published bulletins discussing the detailed design and con- 
struction of many types of improved road and reference may be had 
to these 1 in determining the amount of crown necessary for the 
various types. 
SIDE DITCHES. 
Side ditches usually are necessary where roads are located in cuts 
or where they follow the natural surface of the ground. It is also 
frequently desirable to construct ditches approximately paralleling 
the road, even when in fill, in order to conduct water from a neigh- 
boring cut or berm ditch by a ready channel to some accessible 
natural drainage line. Tables 1A and IB show the carrying capacity 
of ditches having various forms and slopes, and figures 1, 2, and 3 
show the three cross-sectional shapes most commonly employed. 
A ditch having the trapezoidal cross section shown in figure 1 has 
a greater carrying capacity for the same cross-sectional area, but 
unless the bottom be made relatively wide it erodes more easily than 
the ditches shown in figures 2 and 3. This section is adapted in 
general to locations where the slope is comparatively light and the 
amount of water to be drained is relatively large. For side ditches 
1 Office of Public Roads Bulletins 43 and 45 ; Yearbook separate 737. 
