DRAINAGE METHODS. FOR COUNTY EOADS. 19 
carried. The influence of these factors has been discussed under the 
heading " Side ditches." 
The proper type of gutter to employ depends on a number of con- 
siderations. Due attention should be paid in its design to having 
it of adequate size and properly correlated with the other parts of 
the surface drainage. If a ditch on a steep grade be replaced at 
some point by a paved gutter, the latter should of course be designed 
in harmony with the ditch it replaces. If a paved gutter receives a 
heavy discharge from side roads or elsewhere, the necessary changes 
should be made to prevent flooding. Often the distance of the gutter 
from the surface affects its design and type. A gutter paved simply 
to prevent erosion, and situated perhaps 10 feet from the edge of 
the surface, does not require the bearing strength of a gutter imme- 
diately adjacent to the surface and used often as an effective part 
of the traveled way. The surfacing material itself sometimes affects 
the gutter design, inasmuch as principles of harmony and uniformity 
of surface must be considered; and occasionally proposed methods 
of construction of the wearing surface affect the design of the gutter. 
All of these details must be considered, as well as the general ones 
of available materials and economic cost. 
Gutters may be constructed of Portland cement concrete, brick, 
cobble stones, angular fragments of stone, or other similar material. 
In some cases the} 7 ' are formed simply by constructing a curb along 
the edge of the road surface, as is done sometimes in city pavement 
construction. This method can oe employed only where the road 
surface is made of material capable of withstanding the eroding 
action of water and where it is not important that the earth 
shoulders be kept open to vehicle traffic. 
Gutters are made from 2 feet to 6 feet wide and from 3 inches to 
10 inches deep, depending to a great degree on the type of design, 
and the size is governed by the same considerations as govern the 
size of earth-side ditches. That is, they should have a cross section 
sufficient to drain the road without danger of overflow, and, in 
order to determine this cross section, it is necessary to consider the 
area to be drained, the maximum rate of fainfall and run-off, and 
the character and slope of the gutter. Tables 2 and 3 give the 
capacity of gutters having various shapes, slopes, and cross-sectional 
dimensions. The values given in these tables were computed for 
cobble and concrete, respectively, but may be used in designing other 
t} 7 pe of gutters, provided the relative surface roughness of each 
type is properly considered. In Tables 6 and 7 will be found velocity- 
quantity figures for an alternative design for concrete gutters. In 
making the computations the cobble gutters were assumed to have 
their characteristic rough finish, and the concrete gutters a smooth, 
