HIGHWAY CONSTRUCTION 39 



to be passed. It' the culvert, is too small, it is liable to raiise a washout, 

 entailing interruption of traffic and cost of repairs, and possihlv inav 

 cause accidents that will require payment of large sums for damages. 

 On the other hand, if the culvert is made unnecessarily large, flu- 

 cost of construction is needlessly increased. 



The area of waterway required depends (1) upon the rate of 

 rainfall; (2) the kind and condition of the soil; (3) the character 

 and inclination of the surface; (4) the condition and inclination of 

 the bed of the stream; (5) the shape of the area to be drained, and 

 the position of the branches of the stream; (6) the form of the mouth 

 and the inclination of the bed of the culvert; and (7) whether it is 

 permissible to back the water up above the culvert, thereby causing 

 it to discharge under a head. 



(1) It is the maximum rate of rainfall during the severest storms 

 which is required in this connection. This varies greatly in different 

 sections of the country. 



The maximum rainfall as shown by statistics is about one inch 

 per hour (except during heavy storms), equal to 3,630 cubic feet per 

 acre. Owing to various causes, not more than 50 to 75 per cent of 

 this amount will reach the culvert within the same hour. 



Inches of rainfall X 3,630 = cubic feet per acre. 



Inches of rainfall X 2,323,200 = cubic feet per square mile. 



(2) The amount of water to be drained off will depend upon the 

 permeability of the surface -of the ground, which will vary greatly 

 with the kind of soil, the degree of saturation, the condition of the 

 cultivation, the amount of vegetation, etc. 



(3) The rapidity with which the water will reach the water 

 course depends upon whether the surface is rough or smooth, steep 

 or flat, barren or covered with vegetation, etc. 



(4) The rapidity with which the water will reach the culvert 

 depends upon whether there is a well-defined and unobstructed 

 channel, or whether the water finds its way in a broad thin sheet. 

 It the water course is unobstructed and has a considerable inclination, 

 the water may arrive at the culvert nearly as rapidly as it falls ; but 

 if the channel is obstructed, the water may be much longer in passing 

 the culvert than in falling. 



(5) The area of waterway depends upon the amount of the area 



