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Modeling Thirsty Duck flow controls in HydroCAD

Thirsty Duck is a line of buoyant flow control devices, designed to produce a constant flow rate regardless of water surface elevation.  When used as an outlet control device on a detention pond, this increases the storage utilization, allowing the use of a smaller storage volume while still attaining the same discharge rate.

Designed and manufactured by Thirsty Duck LP of Trinity, Florida, the devices are available in several designs to suit a wide range of flow requirements.  The photo at left shows an ER 200 in a typical high flow/low head application.  This design employs a telescoping internal weir which is maintained at a constant head by the action of external floats.  For low-flow applications, the TD 100 series (shown at right) uses a bellows-mounted orifice which is maintained at a constant head by the integral float assembly.

Modeling Thirsty Duck devices in HydroCAD

The objective of a Thirsty Duck flow control device is to produce a relatively constant flow rate regardless of water surface elevation.  This behavior is shown in a typical stage-discharge  curve for the ER series, shown at right.

This behavior can be modeled in HydroCAD by entering the specific rating curve into a special outlet device.  However, many situations are easier to model using the constant flow outlet device which was added in HydroCAD-10.  With a constant flow outlet, only three parameters are required:

1) The invert elevation at which flow will begin to occur
2) The phase-in depth required to attain full flow
3) The full flow discharge rate

Once the invert elevation is attained, the discharge will increase linearly over the phase-in depth until the full flow is achieved.  This behavior approximates the transition that occurs as the Thirsty Duck becomes fully buoyant and achieves its target operating head.  If greater accuracy is required during the transition phase, a special outlet device can be used to define the exact rating curve.  But for most applications a constant flow outlet will provide comparable results with less time and effort.

For the TDP series, the restraining action of the bellows causes a slight increase in head and flow at higher stages.  For maximum accuracy, these devices are best modeled using one of the preinstalled rating curves in HydroCAD 10.0 build 13:

1) Create a special outlet
2) Click "Load From File" and select the Thirsty Duck TDP 184 or 248
3) Set the Discharge Multiplier to the nominal flow rate (measured at 5 foot head)
    0.005-0.43 CFS for the TDP 184 and 0.41-1.83 CFS for the TDP 248
4) Set the Invert Elevation to the WSE where flow commences

Detailed Thirsty Duck information

For further details on Thirsty Duck products please visit www.thirsty-duck.com
A complete video demonstration of these modeling techniques is also available on YouTube.

For more information about HydroCAD

Please see the following pages for further HydroCAD information:

    Pond outlet devices
    Pond storage calculations
    General support page
    HydroCAD features and capabilities
    Free evaluation program
    Pricing and ordering

Images courtesy of Thirsty Duck LP.
Thirsty Duck is a trademark of Thirsty Duck LP.
HydroCAD is a registered trademark of HydroCAD Software Solutions LLC.

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