152 ENGINEERING FOR LAND DRAINAGE. 



has come to be recognized a necessary adjunct of suc- 

 cessful drainage. 



Mean Velocity of Flow. 



The velocity of water flowing in an open channel is 

 retarded by the contact of the particles with the bottom 

 and sides of the channel, these resistances being greater 

 or less according to the nature of the material through 

 which the channel is dug and the irregularities pre- 

 sented by the bottom and sides and having contact 

 with the water. It has been demonstrated that the 

 films of water from the bottom of the channel upward 

 toward the surface and from the sides toward the cen- 

 ter of the channel form vertical and horizontal curves 

 respectively with the advanced portion of the curves 

 in the center line of the stream. If these curves were 

 platted the resistance of the sides and of the bottom of 

 the ditch would have the appearance of holding back 

 the water so that no two films of water would have the 

 same velocity. The greatest velocity of a stream is 

 found in the thread of the current just underneath the 

 surface, all other parts having a less velocity in pro- 

 portion as they approach the bottom and sides of the 

 channel. In considering the discharge of a channel we 

 must use the mean velocity of flow, which in a trape- 

 zoidal earth channel is about eight tenths of the surface 

 velocity, and is the velocity found at a point in the cen- 

 ter line of the stream and a little more than half way 

 from the surface to the bottom. The bottom velocity 

 is about seven tenths of the surface velocity, depending 

 much upon the kind of material which forms the bot- 

 tom. An increase in the depth of the water in a ditch, 



