Using the nomograph in figure 4-5, we must first locate a turning point on the turning line. This line is the center

line on the nomograph. The turning point is found by locating the slope(s) in ft/ft in the column at the far left of

the chart and the roughness coefficient ("n") on the far right column and connecting them with a straight line.

Where the line crosses the turning line is the turning point. This turning point will remain the same as long as S

and "n" do not change.

The hydraulic radius is found by connecting the velocity in the second column from the right, and the turning

point by a straight line through to the R scale and reading the hydraulic radius (R) off the second column from the

left. This gives the required R for any given V in Manning's Equation. In this example, R equals 0.87 which

shows the calculator method for solving the radius.

and yx, you can determine R more quickly and accurately than with the

If your calculator has the function

nomograph provided in the workbook Transposing Manning's Equation,

Since n and S are constant for various trial iterations, you need only enter different values for V to arrive at an

acceptable value for R. Therefore, if S and n are constant, the above equation for R becomes:

To calculate R, use the above equation for numerous iterations until you have "bracketed" flowrate Q within

acceptable limits. Below are keystrokes which will work on most calculators.

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