110 WATER CONDUCTORS AND ACCESSORIES 



Evaporation is small as compared with seepage, which increases 

 with the depth of the water and with the wetted perimeter, but 

 decreases with an increase in velocity. While evaporation, there- 

 fore, can be neglected, the effect of seepage must usually be con- 

 sidered in determining the capacity of a canal. 



The velocity decreases with an increase in the wetted perim- 

 eter, and when the fall is great it may be advisable to use a 

 shallower section to reduce the velocity, or vice versa. If the 

 actual slope of the country is so great that the corresponding 

 velocities would cause erosion, it is necessary to limit the grade to 

 a value which would not give an excessive velocity, and to con- 

 centrate the excess fall at suitable drops along the canal. 



Flumes. Where the contour of the country is very irregular 

 or the soil very hard and difficult to excavate, flumes are some- 

 times used for diverting the water. While the first cost of such 

 structures may be very low where timber is cheap, their upkeep is, 

 however, usually much higher than for a canal, and every pre- 

 caution must, therefore, be taken in their design and construction. 



The velocity of the water, which can be found from the for- 

 mulae given in the previous section, may be much higher than for 

 unlined canals, and the higher the velocity the smaller cross-section 

 is required. When the water, therefore, enters a flume from a 

 canal, it becomes necessary to provide a sufficient drop in the upper 

 end of the flume for the increased velocity head. This may be 

 found from the formula: 



where 



Ji = drop necessary to increase the velocity in feet; 

 ^1= velocity of flow in flume in feet per second; 

 v 2 = velocity of flow in canal hi feet per second; 



g = acceleration of gravity = 32. 16. 



Similarly there should be a gain in head when the water again 

 enters a canal from a flume, although this is not realized to a very 

 great extent and can be neglected. 



Flumes may be classified according to the material of which 

 they are built, into: 



Rectangular wooden flumes. 

 Semi-circular wood-stave flumes. 



