I $6 



ENGINEERING FOR LAND DRAINAGE. 



force is thrown against the other forces or filaments 

 toward the interior of the stream and assist in breaking 

 the force and deflecting the current. The section of 

 curve first struck will receive the greatest force, and be 

 subject to greater erosion if the curve were a segment 



Ditch 



Fig. 30. — Action of Current on Ditch Banks at Curves. 



of a circle. For this reason the up-stream part of the 

 curve should be deflected from the tangent by using a 

 curve of greater radius than the remainder of the curve, 

 in order that all parts may be subject to uniform erosion. 



When the points of tangency have been fixed upon, 

 the curve may be ' ' run in by the eye ' ' better than by 

 the instrument, and the center line located by measure- 

 ments from the tangents in the manner shown in Fig. 29. 



How short a curve may be used in large ditches such 

 as are constructed for drainage districts without en- 

 dangering the stability of the banks at the curve is a 

 question that cannot be answered with mathematical 

 certainty for the reasons previously stated. Deduc- 

 tions from close observations of both natural and arti- 

 ficial streams which flow through alluvial soils are the 



