HydroCAD^® Stormwater Modeling - Since 1986

Modeling Pumps

Although HydroCAD does not have a specific "pump" capability, a pond with a pumped outlet can be easily modeled by using a special (user-defined) outlet device.

A special outlet can be used to model specialized situations where one of the standard outlet devices (weir, orifice, culvert, etc.) may not be appropriate.  Rather than using the built-in discharge equations, a special outlet allows the user to define an arbitrary rating curve of discharge vs. head.

In the case of a pump, we need to define a stage discharge-relationship that produces a sudden increase in flow when the pump turns on.  To configure a special outlet device for this purpose:

1) Edit the desired pond, select the Outlets tab, and create a "Special" outlet.  (This may be the only outlet, or may be combined with an overflow weir, culvert, or any other arrangement of outlets.)

2) On the Special outlet screen, set the "Invert Elevation" to the elevation where you want the pump to turn on.

3) Enter the following values in the head/discharge table:

  Line  Head    Q
   1    0.00  0.000
   2    0.01  x.xxx  <- Enter pump rate here
   3    1.00  x.xxx  <- and here as well

Line #1, in conjunction with the invert elevation, establishes the level at which the pump starts to turn on.  When the water reaches line #2 (1/100 of a foot higher) the pump will be fully "on".  Line #3 is used to prevent extrapolation of the discharge at higher heads, or you can adjust this line if you want to simulate higher pump rates as the pond level increases.

Note: If you're using HydroCAD 7.1 or later, you can omit line #3 and deselect the "Extrapolate" option instead.

After creating the special outlet, OK the pond description, open a report window, and examine the stage-discharge plot.  If you've set up the device correctly, the plot will show no discharge up to the invert, and then a sudden jump to the full pump rate.  The rate should then remain constant as the level increases.

Note: This procedure simulates a fixed on/off point without hysteresis.  Modeling of pumps with separate turn on/off points is being considered for a future HydroCAD release.

4) If applicable, you can also model an increasing pump rate as the pond fills above the turn-on point.  This is done by adding additional lines (4, 5, 6, etc) to the table shown above.  When entering this data, remember that the pond head is a measure of the pond water level above the invert (turn-on point), while the pumped head is measured from the water surface to the pump discharge:

    Pond Head + Pumped Head = Discharge head (above invert)

When determining discharge from the pump rating table, always use the pumped head given by:

    Pumped Head = Discharge head - Pond Head

[Home]   [Support]

Copyright © 2008 HydroCAD Software Solutions LLC (privacy)   (legal notices)