GRIMM'S METHOD 103 



All in which both velocity head changes and friction are taken into 

 account will give about the same results. If the velocity of flow is low, 

 so that the velocity head is small compared with the average depth, the 

 computations may be simplified by ignoring the effect of velocity head 

 changes. It is impossible to fix a definite limit for this; proportionately 

 greater velocity head changes may be ignored when the velocity is 

 decreasing in the downstream direction than when it is increasing in the 

 downstream direction. If there is doubt, a check computation should 

 be made in which the velocity head changes are taken into account. 



Grimm's method. Grimm's step method^ may be used when profiles 

 of the stream in its natural state, without the backwater effect, are 

 available. The profiles should be for several different discharges, with 

 water-surface elevations completely covering the range in which the 

 backwater curve is expected to lie. It is thus necessary to have profile 

 data for a larger discharge than that for which the backwater curve is 

 desired. Such data may be synthesized, if not otherwise available, by 

 using one of the methods for extending rating curves described by Hoyt 

 and Grover.^ 



The data for Grimm's method are often cheaper to obtain than the 

 data required for the standard step method. It is most useful when the 

 backwater effect is intermittent, as that caused by navigation dams, or 

 by tributary flow. Where occasional backwater effects are important, 

 continuous records of the profile are kept. The data are always up-to- 

 date, and frequent roughness determinations and cross-section measure- 

 ments of shifting channels are not necessary, as they might be if the 

 standard step method were to be used. 



With respect to the changes of velocity head due to irregularities in 

 the channel, so troublesome in the standard step method, Grimm's 

 method is probably quite accurate. However, it does not take into 

 account the effect of the change of velocity head due to the backwater 

 effect, and for this reason should only be used for those backwater 

 curves in which the velocity is well below critical, and is decreasing in 

 the downstream direction. 



In the application of Grimm's method, the stretch is broken up into 

 reaches, which, as for all step methods, should be short. They may be 

 interpolated between the stations where profile observations were taken. 

 Curves showing the relation between discharge and elevation are drawn 

 for each section. Using the subscript n to refer to normal flow con- 



^ " Backwater Slopes Above Dams," by C. I. Grimm, Engineering News-Record, 

 V. 100, p. 902, June 7, 1928. 



* River Discharge, by J. C. Hoyt and N. C. Grover, John Wiley & Sons, third 

 edition, 1914. 



