ILLUSTRATIVE EXAMPLE 



105 



example on page 77 will be used to illustrate the method. The channel has a 

 bottom width of 100 feet and side slopes of 1 to 1. Manning's n is 0.022. The 

 uniform grade is 0.0004, and the discharge of 6,220 c.f.s. normally flows at a 

 depth of 10 feet. Determine the backwater curve above a dam at which the 

 depth is 25 feet. 



There is no need to draw the flood profiles, for they will be straight lines 

 parallel to the bottom. The discharge-elevation curves will all be similar, 



60,000 



50.000 40,000 30,000 ^^ 20,000 



Distance Upstream from Dam, in Feet 



Fig. 904. An Ml Curve by Grimm's Method. 



(See computations in Table 901.) 



differing only in elevation. Let the zero datum for elevations be at the base 

 of the dam. 



To illustrate the method for natural watercourses, reaches of different lengths 

 will be used. The first reach is taken as 5,000 feet. From Fig. 904 it is seen 

 that Qn for a normal surface elevation of 25.00 at section 1 is 29,300. The 

 normal fall in reach is 2 feet. The fall corresponding to the slope at section 1 is 



^=f.X(^1^2x('^?^Y = 0.090 



.. X (^\ = 2 X (^ 



\QJ \29,300/ 



The approximate elevation of the backwater curve at section 2 will be 



25.00 + 0.09 = 25.09 



