DRAINAGE METHODS FOR COUNTY EOADS. 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 subclrainage, 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 easy to drain. 
The topograph} 7 of the land adjacent to a road determines to a 
considerable extent also the necessary capachV^ 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 precipitation, 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 7 or 8 inches in 24 hours, and 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 
