1 54 ENGINEERING FOR LAND DRAINAGE. 



The mean velocity will be a little less for a trape- 

 zoidal channel and will decrease as the side slopes 

 are flattened. When the breadth is twice the depth 

 we have a naean velocity of flow equal to 92 per cent 

 of the surface velocity measured in the middle of the 

 channel. 



Curvature of Ditches. 



The proper curve to give ditches when they are de- 

 flected from a straight line is a matter which merits 

 careful attention. It is desirable that the adjustment 

 of curve to velocity of flow be such that the banks will 

 not require artificial protection. The relation of bank 

 erosion to curvature of the ditch and the velocity of 

 flow is intricate owing to the great difference in the 

 stability of earth when subjected to the action of waten 



Circular curves are described by the number of de- 

 grees of arc which a chord of 100 feet subtends. The 

 degree of a curve is determined by the central angle 

 which is subtended by a chord of 100 feet. For curves 

 of from I degree to 10 degrees the radius maybe found 

 by dividing 5730 feet (the radius of a i -degree curve) 

 by the degree of the curve. The following is a list of 

 curves and their corresponding radii, which may be 

 used as a basis in constructing ditches with limitation 

 as hereafter described : 



Degree of Radius Degree of Radius 



Curve. m Feet Curve. m Feet. 



7*" 819 14*" 410 



8° 716 IS" 383 



9^ 637 16° 359 



10,.- 574 17" 338 



II 522 iS"" .320 



12'' 478 19"*. 310 



13° 442 20° 2ZZ 



