6 BULLETIN 724, TJ. S. DEPARTMENT OF AGRICULTURE. 



100 j)er cent of the total precipitation may be drained from the 

 watershed during a storm. The extreme rate occurs principally in 

 paved cities in the closely built up business sections, but 80 per cent 

 run-off may occur in very steep, impervious or compact!}' shaped 

 watersheds. Under the worst conditions, therefore, it may be neces- 

 sary to design surface drains and culverts with capacity sufficient 

 to remove about 6 inches of water from a watershed in 24 hours 

 and to take care of an extreme precipitation of about 1 inch in 1 

 hour. Under other circumstances, the maximum rate of rainfall 

 ma}' be considerably less than that mentioned above, and the maxi- 

 mum run-oil during storms may be only about 15 or 20 per cent of 

 the total precipitation on the watershed, though this low run-off 

 does not apply often to road drainage. 



CAPACITY OF DRAINS. 



The amount of water required to cover an acre of land to a depth 

 of 1 inch is. 3,630 cubic feet. If the maximum rainfall and run-off 

 and the area to be drained can be determined, therefore, the total 

 volume of water which a given drain should be designed to accommo- 

 date can be computed readily. From what has been said, it is ap- 

 parent that any estimate of the run-off necessarily will be a. rather 

 rough approximation, and due allowance on the safe side always 

 should be made as a safeguard against possible inaccuracies in such 

 estimates. '•' 



The size of drain necessary to carry a given volume of water in a 

 given time may be estunated from the formula v=c\/rs, in which v= 

 mean velocity of water; r=hydraulio mean radius of the drain; s= 

 the slope of the drain ; and c=a coefficient, the value of which depends 

 on the character of the surface of the drain, the slope, and the hy- 

 draulic mean radius. Kutter's formula is the one generally consid- 

 ered most reliable for determining the value of the coefficient, c. Ac- 

 cording to this formula when English units are used. 



,i,e+:^}mi+hm 



c- 



1 + 



(41.6 + -^) 



■^Jr 



in which 5=slope ratio of drain; ?^hydraulic mean radius in feet, 

 and n=a constant which depends on the character of the surface of 

 the drain. Under average conditions the value of n may be assumed 

 as follows : for open ditches, 0.025 to 0.040 ; for cobble gutters, 0.020 

 to 0.035 ; for concrete gutters, 0.015 to 0.020; for tile pipe, 0.015. The 

 following tables have been computed by means of the above formulae 

 especially for use in planning- drainage systems. 



