116 RAILROAD LOCATION AND CONSTRUCTION — MACKENZIE. 



drift-wood, for the water will rise, flow over, and fall into the 

 •opening between the obstruction and the head or end wall of the 

 culvert. 



Fixing- tjie sizes of culvert-openings is a matter requiring great 

 Care. If possible, the engineer in charge should personally ex- 

 amine the ground and the character of the country drained by 

 the stream, whether rough, rolling or flat. If county or geolo- 

 gical maps exist, the drainage area can be gotten approximately. 

 Then as a rough approximation we use Major E. T. D. Myers' 

 formula : 



A = C p^^T 



A = Area of opening in square feet. 



M = Drainage area in acres. 



C = Variable coefficient = 1 for flat ground, and Ij^o for hilly 

 compact ground. 



Highway-bridges and openings over the same stream are 

 examined, and the highest freshet-level obtained from " the 

 oldest inhabitant." With this information, Myers' formula, and 

 some brains, the openings will not be far wrong. 



Culverts are seldom made less than 2|x2^ ft., so that a man 

 can ffet through to clean them out, and their fall not less than 

 Sins, per 100 ft. The head walls of box culverts are carried down 

 4 ft. below paving, and the side walls 3 ft., to prevent frost from 

 acting on them. Where they are over 3 ft. span, the two upper 

 courses are corbeled out to reduce the span of the covers. Pro- 

 portions and quantities are about as follows : 



