40 HIGHWAY CONSTRUCTION 



to be drained ; but in many cases the shape of this area and the posi- 

 tion of the branches of the stream are of more 'importance than the 

 amount of the territory. For example, if the area is long and narrow, 

 the water from the lower portion may pass through the culvert before 

 that from the upper end arrives; or, on the other hand, if the upper 

 end of the area is steeper than" the lower, the water from the former 

 may arrive simultaneously with that from the latter. Again, if the 

 lower part of the area is better supplied with branches than the upper 

 portion, the water from the former will be carried past the culvert 

 before the arrival of that from the latter; or, on the other hand, if 

 the upper part is better supplied with branch water courses than 

 the lower, the water from the whole area may arrive at the culvert 

 at nearly the same time. In large areas the shape of the area and 

 the position of the water courses are very important considerations. 



(6) The efficiency of a culvert may be very materially increased 

 by so arranging the upper end that the water may enter into it without 

 being retarded. The discharging capacity of a culvert can be greatly 

 increased by increasing the inclination of its bed, provided the channel 

 below will allow the water to flow away freely after having passed 

 the culvert. 



(7) The discharging capacity of a culvert can be greatly increased 

 by allowing the water to dam up above it. A culvert will discharge 

 twice as much under a head of four feet as under a head of one foot. 

 This can be done safely only with a well constructed culvert. 



The determination of the values of the different factors entering 

 into the problem is almost wholly a matter of judgment. An estimate 

 for any one of the above factors is liable to be in error from 100 to 

 200 per cent, or even more, and of course any result deduced from 

 such data must be very uncertain. Fortunately, mathematical exact- 

 ness is not required by the problem nor warranted by the data. The 

 question is not one of 10 or 20 per cent of increase; for if a 2-foot pipe 

 is sufficient, a 3-foot pipe will probably be the next size, an increase 

 of 225 per cent; and if a 6-foot arch culvert is too small, an 8-foot will 

 be used, an increase of 180 per cent. The real question is whether 

 a 2-foot pipe or an 8-foot arch culvert is needed. 



Valuable data on the proper size of any particular culvert may 

 be obtained (1) by observing the existing openings on the same 



