DEAINAGE . METHODS. FOR COUNTY ROADS. 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 harmonj^ 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 glitter 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 ciases they 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 be 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 com]3uted for 

 cobble and concrete, respectively, but may be used in designing other 

 type 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, 



