OPEN DRAINS. 



IS3 



since it is a virtual addition of head, accelerates the 

 velocity. The following table shows the effect that 

 increase of depth has upon the mean velocity in a rect- 

 angular channel lo feet wide with a grade of 3 feet per 

 mile. 



MEAN VELOCITY OP WATER AT DIFFERENT DEPTHS IN RECT 

 ANGULAR DITCH 10 FT. WIDE, GRADE 3 PT. PER MILE. 



Depth in. Feet. 



Mean Velocity in 

 Feet per Second. 



0.5 



1-5 

 2.0 



2.5 

 3-0 

 4.0 



5-0 

 6.0 



8.0 



1.4 



^ 3 

 2.6 



2.8 



2.9 



3 2 



3-4 



3-^ 



3.B 



Relation of Breadth and Depth of Channel to Surface 



and Mean Velocity, 

 As the result of many experiments by careful 

 hydrauHcians, the following table from '*Fanning*s 

 Hydraulic Engineering" may be given to show the 

 relation of breadth, depth, surface velocity and mean 

 velocity to each other for rectangular smooth channels 

 when the water is from 5 to lo feet deep. Let b-=. 

 breadth, d = depth, V = surface velocity at thread of 

 current, and v = mean velocity. 



When 6 = 2d then v = .920 V. 



" b - 



3 ^ ' 



' V — .910 V. 



" b - 



4 d ' 



' V — .896 V. 



« ^ _ 



^d ' 



' V — .882 V. 



" b - 



6d ' 



' V = .864 V. 



" b = 



7 d ' 



' V — .847 V. 



" b - 



8 d ' 



' V - .826 V. 



" b - 



9 d ' 



' V = .80s f^- 



" b - 



10 d ' 



' V — .780 V. 



