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Table Of Contents
Chapter 5 Working with Hydrology Commands
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Calculating Riser Values
Risers can be used as outlets (or principle spillways) for detention ponds or
other water storage facilities. The goal when designing a riser is to size the riser
and pipe diameters so that the peak discharge is less than the allowable rate for
the chosen design storms. When you enter the required information into the
Riser Calculator, the calculator determines the optimum riser and pipe
diameters for you.
The Riser Calculator determines whether the top of the riser stand pipe is acting
as a weir or a riser based on the flowrate. Use the Settings option in the Riser
Calculator to define the weir and orifice coefficients to use in the calculations.
You can view the results of the riser calculation in a few different ways. The
calculator dialog box itself reports diameter, flow, and head values for the riser
and the pipe. The Output button creates a file that lists all of the information in
the Riser Calculator dialog box, along with weir and orifice flowrates. In
addition, you can view the results of the calculations as a rating curve.
Changing the Riser Settings
In the riser settings, you specify which orifice and weir coefficients are used in
the riser calculations. You can also choose to enable a feature that checks to see
that the riser diameter is at least 1.25 times greater than the pipe diameter.
To change the riser settings
1 From the Hydrology menu, choose Riser Calculator to display the Riser
Calculator dialog box.
2 In the Riser Calculator dialog box, click Settings to display the Riser Settings
dialog box.
3 In the Orifice Coefficient box, type an appropriate orifice coefficient.
4 In the Weir Coefficient box, type an appropriate weir coefficient.
NOTE
You must determine the weir coefficient based on the geometry of the weir
you are designing.
5 To automatically check that the riser diameter is at least 1.25 times greater than
the pipe diameter, select the Riser Diameter check box.
6 Click OK to return to the Riser Calculator dialog box.
Calculating Riser Values
Using the Riser Calculator, you can design a riser based on base flow, riser
elevation, and several other values. After you input the required values, the
Riser Calculator calculates diameter, flow, and head values for the riser and the
pipe. For information on the Riser Head formula, see Riser Head Flow Formula
and Riser Equations and Steps Used to Calculate Riser Values in this chapter.