54* MACHINERY 



2. Over a weir or plank, free overfall. 

 Q = HX L X V 



where H = height of still water above crest in feet. 

 L = length of crest in feet. 

 V— mean velocily of water approaching the crest in 



feet per second = |X5 ^ H. 

 Q = quantity discharged over the crest in c. ft. per 

 second. 



In gauging, the water must all be made to pass over a 

 rectangular aperture in a thin board. The height must be 

 measured from the top of the crest to the level of the surface 

 where it is not affected by the curve of the overfall. 



w Storage. 



It is usual to assume that an acre of crop will require an 

 acre-fathom (6 ft.) of storage in a tank if the latter is the 

 source of supply. Rain is required to supplement this supply 

 as also to make up for the loss due to evaporation winch 

 may be taken as *4 inch as a maximum figure on a very hot 

 day. It is less proportionately in deep tanks than in shallow 

 ones. 



Capacity of a tank may be roughly taken to be = area of 

 waterspread at F.T.L. (full tank level) X £ of the depth of 

 the lowest sluice. 



Velocity and discharge of minor irrigation channels. 



The average velocity of water flowing in a channel may be 

 taken as 4/5 of the surface velocity which may be easily ascer- 

 tained by means of a float floated down a measured distance. 

 The average velocity of an earthen channel should ordinarily 

 be more than one foot per second and less than three feet 

 per second. If less than one foot per second, there will be 

 a free deposit of silt and the channel will be choked with 

 aquatic plants. If more than three feet per second, the 

 water will cut its own banks and bed and take a new course. 



The velocity of a channel depends upon the depth of 

 water flowing as well as the fall of bed. If the depth is 

 great as in large rivers, the bed-fall will have to be small so 

 that the velocity may not exceed about three feet per second. 

 In small channels where depth is limited the bed-fall will 

 have to be great to secure the proper velocity. The bed-fall 

 is usually half to two feet per mile in large rivers, five to t«n 

 feet per mile in canals, and 20 to 25 feet per mile in very 

 small channels, 25 feet per mile = 1 in 211 or say 1 in 200. 



