STEP METHODS FOR BACKWATER CURVES 



95 



Fortunately, the majority of problems encountered in practice do 

 not involve complicated water-surface profiles. Often the curve is 

 nothing more than flow at the normal depth, slightly modified by the 

 irregularities of the channel. If the velocity head is comparatively 

 small, the profile can be computed for a short distance in the wrong 

 direction without introducing serious errors. Strict adherence to the 

 rule is desirable, however, for step computations carried in the wrong 

 direction tend inevitably to diverge from the correct elevations. 



(a I Uniform Channel 



(6) Slightly Irregular Channel 



^Point of Control 



^^WAW/AW^AW,?^ 



<e> Markedly Non- uniform Channel 

 Fig. 901. Effect of Channel Irregularities. 



The converse is also true. Computations started at an elevation 

 that is incorrect for the given discharge, and carried in the right direction, 

 will become more nearly correct after every step. This fact is con- 

 venient when no elevation is known within or near the reach to be 

 investigated. An elevation may be assumed at a section far enough 

 away from the reach that by the time the step computations have been 

 carried to the initial section of the reach, the elevations will be correct. 

 A check may be had by starting with a different elevation at the distant 

 point. If the distance is sufficient, the computed elevation at the 

 initial section of the reach will be practically the same as before. 



The computation of water-surface curves by step methods offers an 

 opportunity for a great variety of procedures. There are but three 



