DRAINAGE METHODS FOR COUNTY ROADS. 5 



older States where the system of State roads has changes in the 

 foundation design at frequent intervals; and is, of course, doubly 

 apparent from the location and relation of the constantly changing 

 units of the surface drainage system. 



In order, therefore, to plan intelligently a system of drainage for 

 any particular road, it is necessary to take into consideration not 

 only the local character of the soil composing the roadbed, but also 

 the topography of the adjacent land, the amount and rate of rainfall, 

 and the availability of different materials suitable for use in con- 

 structing drains. 



Topography has a very important influence in determining the 

 relation of surface and subdrainage, and when one or the other 

 drainage is more practicable'. When the topography is such that 

 the side ditches can be given a grade of as much as 1 or 2 per cent, 

 subsurface drains seldom are necessary, except in unusually heavy 

 clay soils and where springs occur; while, on the other hand, where 

 the grade is level or nearly so, subsurface drainage may be required 

 when the soil composing the roadbed otherwise would be compara- 

 tively easv to drain. 



The topography of the land adjacent to a road determines to a 

 considerable extent also the necessary capacity of the surface drains 

 and the location of the appropriate structures. Watersheds having a 

 fan shape, for example, empty the water which falls upon them into 

 the drainage structures much more rapidly than long, narrow drain- 

 age areas, because of the difference in time required for water from 

 the more remote parts of the area to reach the outlet. Steep slopes 

 deliver water at the outlets much more rapidly than flat areas ; and 

 impervious soils, by refusing to absorb precipiitation, deliver greater 

 percentage of rainfall at the outlet. 



The amount and rate of rainfall are the primary factors in de- 

 termining the quantity of water drained off from the land, or 

 " run-off," as it is called. Since the capacity necessary for a given 

 drain is fixed by the maximum run-off which it will be called upon 

 to accommodate, a knowledge of the rainfall is essential in planning 

 a system of surface drainage. In the design of surface drains and 

 culverts it is usually necessary to consider only the maximum rate 

 of rainfall and the maximum duration of time through which indi- 

 vidual rains continue, but in the case of subsurface drains the 

 duration of a wet season and its effect on the accumulation of ground 

 water also may be of importance. During the most extreme local 

 storms occurring in some sections of the United States the precipi- 

 tation sometimes amounts to Y or 8 inches in 24 hours, an.d in a few 

 cases has been known to exceed an inch in one hour. Where the 

 topographic conditions are favorable to a rapid run-off, such as is 

 generally the case with an improved road, from 80 to practically 



