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HydroCAD Stormwater Modeling - Since 1986


Modeling Pumps with HydroCAD

HydroCAD can perform complete pump modeling and hydrograph routing, including the use of separate on/off elevations, pipe losses, tailwater effects, and even multiple pumps.

Just select "pump" from the list of outlet devices and enter the appropriate parameters.  For further details, including a discussion of tailwater interactions, click the Help button on the pump screen.  Also see the "Pump Examples" project that is preinstalled with HydroCAD, and the pump modeling tips near the bottom of this page.

Note: Pump modeling was first available in HydroCAD 9.0, with further enhancements in HydroCAD 10.0 build 24.  Earlier versions of HydroCAD cannot model the pump switching hysteresis and should not be used to model pumps.


The following reports were generated from the "Pump Examples" project that is preinstalled with HydroCAD.

Pond 1P - Preset WSE with no inflow

This hydrograph demonstrates a simple draw-down scenario in which the pond has no inflow hydrograph, but the starting water surface elevation has been preset to 105', as shown on the (black) elevation line.

Since this satisfies the pumps "on" elevation, the pump starts running immediately at time zero.  As the pump operates, the WSE in the pond decreases, increasing the pump head, which causes the flow rate to steadily decrease.  Finally, the WSE gets to the "off" elevation and the pump turns off, causing the outflow to stop.

Pond 2P - Constant inflow

In this example the pond starts empty, but there is a constant inflow of 0.4 CFS.  The WSE increases steadily until the pump turns on at 105'.  With the pump running the WSE starts to drop until it reaches the "off" elevation at 102.5'.  The pond then begins to fill again and the cycle repeats as long as the inflow in sustained.

Pond 3P - Typical inflow hydrograph

In this example we have a typical runoff hydrograph flowing into the pond.  The inflow causes the WSE to increase until it reaches the pump's "on" elevation of 105' at 15 hours.

Since the inflow initially exceeds the pumps capacity, the WSE continues to rise until an equilibrium condition is reached at 18.88 hours.  At this time the inflow has fallen below the pump capacity so the WSE starts to decrease, until it reaches the "off" elevation at 102.7' and the WSE stabilizes.

Pond 4P - Pump cutoff at overflow weir

This example demonstrates the use of the high-level cutoff feature added in HydroCAD 10.0 build 24.  Once again, the pond is being supplied with a typical runoff hydrograph.  The WSE increases until it reaches the "on" elevation of 105', but the WSE continues to rise until it reaches the maximum elevation of 107.1', at which time the pump shuts off.  This initial pump cycle is shown by the blue (primary) outflow between approximately 14 and 15 hours.

As the pump is reaching it's maximum cutoff of 107.1' an overflow weir had begun to handle the higher outflows, as depicted by the secondary (red) outflow curve.  Water continues to flow over the weir until the WSE drops below the pump maximum of 107.1', and the pump resumes operation for a final cycle between 32 and 44 hours.

Pond 5P - Two pumps in a "Priority Group"

This pond is configured with two pumps in a priority group.  The lower pump starts to operate at 15 hours when the WSE reaches it's on elevation of 106', as shown by the primary (blue) outflow.

The WSE continues to rise until the second pump turns on at 109' (secondary red outflow) and the lower pump is automatically turned off.

The higher pump continues to run until the WSE drops below it's Off elevation of 105'.  At this point neither pump is running and the WSE begins to climb again, until it reaches the on elevation of 106' for the lower pump which runs again from 22 to 37 hours.


Pump Modeling Tips

  1. In order to model pump switching and respond to tailwater effects, be sure to use the Dynamic Storage-Indication method (DSI) for pond routing.
  2. Make sure the Off Elevation is above the bottom of the pond storage definition, otherwise the pump may not turn off completely.
  3. To avoid short-cycling, make sure you provide an adequate distance (and storage) between the on and off elevations.  If the values are too close (or the same) the pump will cycle on and off rapidly.
  4. For a better view of the pump operation and switching, right-click the pond hydrograph and select the elevation curve.
  5. You can have multiple pumps in a single pond.  If the pumps use the same device routing (i.e. primary) the flows will be combined and reported together.  In some cases you may prefer to use separate routing (primary, secondary, etc.) so that you can view the operation of each pump separately.
  6. To model multiple pumps discharging into a shared manifold, model the manifold as a separate zero-storage pond with a tube or culvert outlet.
  7. If you only want to have a single pump operating at any time, select Priority Group for all the applicable pumps.  This will give priority to the active pump with the highest On Elevation and all lower pumps will be turned off.
  8. A pump can also be configured to turn off at a specified Maximum Elevation.  This feature can be used if you prefer to have high flows handled by another outlet device, such as an overflow weir.  Otherwise this field must be left blank.
  9. Make sure you don't exceed the flow or head limits of each particular pump.  HydroCAD will issue a warning message if the pump is operating beyond the range of the specified rating curve.


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