retained in SAM.hyd because of its simplicity. It is adequate for simple cross-
section shapes, and it is programmable for complex cross-section shapes.
Total Force Method
This method was proposed by Pavlovskii, by Muhlhofer, and by Einstein and
Banks (Chow, 1959). It is based on the hypothesis that the total force resisting
the flow is equal to the sum of the forces resisting the flow in each panel. The
resulting composite n-value is
(∑ )
1/2
k
2
Pi ni
(∑ )
i= 1
n=
Equation 2-27
1/2
k
Pi
i= 1
where
Pi = Wetted perimeter in wet panel i
ni = n-value in wet panel i
P = Total wetted perimeter in cross section
Conveyance Method
With the conveyance method, a composite roughness coefficient is calculated
based on weighted conveyances in three subsections. The conveyance method
separates the overbanks from the channel so the calculations can be confined to
strips, or subsections within the cross section, having similar hydraulic
properties. The conveyance for each subsection can be calculated and the values
summed to provide the conveyance for the entire cross section. The Manning
equation is used to calculate conveyance instead of the Chezy equation used in
the alpha method. Composite hydraulic radius is calculated as total area divided
by total wetted perimeter, rather than as a conveyance weighted parameter -- thus
the compositing characteristics are concentrated in the Manning's roughness
coefficient. Conveyance for the overbanks and channel are calculated in English
units using the following equations:
23
Chapter 2
Theoretical Basis for SAM.hyd Calculations
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