DEAiNAGE METHODS EOR COUNTY KOADS, 11 



The foregoing tables on page 7 are not applicable to short cross 

 drains and culverts, because the condition of approach channels and 

 outlets aifect 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 Roads. 



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 be 

 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 dirainage 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 ^ in determining the amount of crown necessary for the 

 various types. 



SroE 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 lA 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 figxire 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. 



