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# Mannings Equatiin

## Mannings Equatiin

(OP)
Given the flow in cfs and the geometry of the channel, n,  and slope, how does HydroCAD calculate velocity?

### RE: Mannings Equatiin

You will find complete equations in the HydroCAD Owner's Manual, as well as the help system.

### RE: Mannings Equatiin

Sorry, I should have added some detail.  The actual calculation depends on the context:

1) The velocity reported in the reach summary is simply the flow (CFS) divided by the cross-sectional area at the flow depth.  All these parameters are known at each step in the routing, so the calculation is straight-forward.  For complete details please click on the specific item in the reach summary.

2) Manning's equation is used to determine the initial stage-discharge relationship:  For any depth, Manning's equation is used to calculate the flow velocity.  Multiply by the cross-section to get the discharge at each depth, as shown on the stage-discharge plot for any reach.  You can also display the velocity by selecting it from the right-click menu.

### RE: Mannings Equatiin

You should also know that the Manning Eq may be used for pressure flow provided you use the slope of the hydraulic grade line, rather that the slope of the free water surface.

good luck

### RE: Mannings Equatiin

(OP)
THE QUESTION RELATES TO THE CROSS -SECTION AREA.
In appendix A, it defines the methodology for three common geometries.
How does it calculate area for a custom cross-section?

### RE: Mannings Equatiin

queque,

For a custom reach cross-section, the geometry is defined by a number of straight line segments.  The area is calculated by stacking trapezoidal shapes inside this outline, up to the desired depth.  The sum of all the trapezoid areas is the total cross section.  Basically, its the same procedure you would use to calculate the cross-section from a hand-drawn sketch.

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