Manning's Equation
And Manipulation of Empirical Equation



At a change to a gentler slope in a stream, flow will remain reasonably constant causing wetted perimeter (WP) to increase faster than the cross sectional area (CSA).  Therefore causing flooding which in turn changes the "n" value (roughness value from Manning's equation), especially in wooded areas.  This will cause more water to rise over the banks of the stream.  A profile of mild water rises including arrival times of peaks could be useful in predicting future flooding and time of peak arrivals.  Since this analysis is projected from mathematically manipulated empirical equations, further study is warranted.  Also knowing the flow will help to predict concentrations of infiltration of continuous contaminants into the stream.


*Finding flow and velocity for height and a cross section of the water way.
 
 

 

*Finding flow and velocity for height and a cross section of the water way.


 


*height is related to CSA and WP


Steps to measure Q (flow):
 

1.  Get crossection of channel and banks

2.  Draw this on graph paper

3.  Set up height measurement above turbulent flow.  Assume turbulent flow to be 2" above height
     of rocks in stream

4.  Height should determine V (velocity) and Q (flow).  They should be related to Manning's
     Equation at this point.

Assume n1 = n2    if the water is within the banks     (valid assumption)

Assume S1 = S2   slope must stay relatively the same


This equation drops to be the following with these assumptions:

with "h", we can determine both CSA and WP

 

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