Modeling FAQ

How do I model a Thirsty Duck Buoyant Flow Control Device?

You model the Thirsty Duck BFD by inputting a custom developed stage vs. discharge rating curve into your hydrologic/hydraulic model. Instructions for specific software packages are shown below.

How do I have a custom rating curve developed?

You can download a Thirsty Duck Rating Curve Generation Tool from the Thirsty Duck website here or you can call or e-mail Thirsty Duck for assistance. If you will provide some basic information about your system, a Thirsty Duck engineer will produce a rating curve for you. There is no fee, charge or other obligation for this service and the calculations are not a secret.

How do I input the rating curve into my model?

If you are using one of the Thirsty Duck Rating Curve Generation Tools, simply cut and paste the data from the "Export" tab into your model. If a Thirsty Duck engineer provides the rating curve for you, it will be provided in either PDF or Microsoft Excel format and so you can simply cut and paste the data into your model. (Note: The Western Washington Hydrology Model does not allow you to cut and paste rating curve data, you need to enter the rating curve in Comma Separated Value (CSV) file.)

What information do I need to provide to have a rating curve developed for my project?

For dry detention systems:

• Pond Bottom Elevation
• Minimum discharge elevation/Water Quality Elevation
• Peak Allowable Discharge Rate(s) and Return Frequency.
• Proposed Design High Water Elevation
• Proposed Top of Bank Elevation or Maximum Water Elevation
• Outfall pipe diameter, length, Manning's n value, approximate invert elevations
• Tailwater Elevation or Range of Tailwater Elevations

For wet detention systems:

• Control Elevation/Low Water Elevation
• Water Quality Elevation
• Peak Allowable Discharge Rate(s) and Return Frequency
• Proposed Design High Water Elevation
• Proposed Top of Bank Elevation or Maximum Water Elevation
• Outfall pipe diameter, length, Manning's n value, approximate invert elevations
• Tailwater Elevation or Range of Tailwater Elevations
• Bleed Down Orifice Diameter and Invert Elevation

How do I model Thirsty Duck over a range of tailwater elevations?

Thirsty Duck is attempting to work with the major modeling software developers to allow a moving orifice; however, we have developed the following work around procedure for users of AdICPR (Advanced Interconnected Pond Routing) by Streamline Technologies, Inc.:

1. Provide your Thirsty Duck engineer with the range of tailwater elevations over which your project is to be analyzed.

2. Your Thirsty Duck engineer will produce a family of (multiple) rating curves over the range of tailwater elevations to be analyzed.

3. In AdICPR, create an Operating Table by selecting Operating Table from the Routing drop down menu. Enter a unique name for this Operating Table.

4. Change the Type drop down to Rating Curve and the Function drop down to Tailwater(X)-Headwater(Y)-Discharge(Z) Relationships.

5. Cut and paste the rating curves provided into the Operating Table.

6. Create a Rating Curve by selecting Rating Curve from the Routing drop down menu. Enter a unique name for this Rating Curve. Choose the From Node and the To Node for the Thirsty Duck device.

7. Enter the name of the Operating Table created in step 3 in the Operating Table field. Use an elevation 0.005 above the minimum discharge elevation elevation for the Elev On, and the minimum discharge elevation for the Elev Off.

AdICPR will use linear interpolation between your family of curves to determine the discharge rate for tailwater elevations not explicitly entered into the operating table.

NOTE: A bug in AdICPR occurs with linear interpolation of families of rating curves. If an entered rating curve contains a section that is horizontal, such as the constant discharge of a Thirsty Duck device, AdICPR produces erratic and unpredictable flows. To avoid this bug, it is recommended that each rating curve be trimmed to remove extraneous values in the constant discharge section, and then the final value be increased by a very small value. See this example. All values from 100.13 to 104.99 were removed, then the flow at 105.00 was increased by 0.000001.

How do I model Thirsty Duck with HydroCAD?

1. Open your project file in HydroCAD.

2. Add a “Special & User-Defined” outlet device to the pond to be controlled by the Thirsty Duck device. Set a unique description for the outlet.

3. You will now need to enter a rating curve into HydroCAD. There are two ways to do this. You can enter the rating curve information by hand. This is obviously time consuming, especially for a device that has a large non-linear area. See the above method for trimming a rating curve. Alternatively, you can import a comma separated value (.csv) table directly into HydroCAD.

4. To create a .csv table, create a new, blank spreadsheet. Copy the data from the Thirsty Duck rating curve into the new spreadsheet. Ensure that the data starts at the top left corner (Cell A1,) and that there are no titles above that value. Also, make sure that the flow for the first stage is 0.00.

5. Use “Save as” to save the file as a .csv. Save the file in a known location with a unique name.

6. In the HydroCAD Special & User-Defined Outlet screen, click the Load from File... button, then find and select the file created in step 5.

7. Click “OK” to complete the process.

How do I model the Thirsty Duck with Advance Interconnected Pond Routing (AdICPR)?

1. Open your project file in AdICPR.

2. In AdICPR, create an Operating Table by selecting Operating Table from the Routing drop down menu. Enter a unique name for this Operating Table.

3. Change the Type drop down to Rating Curve and the Function drop down to Tailwater(X)-Headwater(Y)-Discharge(Z) relationships.

4. Cut and paste the rating curve provided into the Operating Table.

5. Create a Rating Curve by selecting Rating Curve from the Routing drop down menu. Enter a unique name for this Rating Curve. Choose the From Node and the To Node for the Thirsty Duck device.

6. Enter the name of the Operating Table created in step 3 in the Operating Table field. Use an elevation 0.005 above the minimum discharge elevation for the Elev On, and the minimum discharge elevation for the Elev Off.

How do I model Thirsty Duck with the Western Washington Hydraulic Model (WWHM3)?

1. Find the interval used for the stage-storage relationship of the pond the Thirsty Duck device is to be used in. Set the calculation interval in the Thirsty Duck spreadsheet to this value.

2. Generate the rating curve for your device.

3. If you have any other control devices such as weirs or slots, add the flows between the two here. Please be sure that the flows are being added at the same stages.

4. Because you must enter a custom discharge curve to model the Thirsty Duck device, you must enter your pond as a SSD (Stage-Storage-Discharge) table.

5. If you have entered your pond as a non-SSD table, such as a trapezoidal pond, use the Show Pond Table option in the pond data to generate a stage-storage table. This will also give you the interval required in step 1.

6. Use a spreadsheet editor to add the rating curve's discharge data into the fourth (discharge) column of your pond's stage-storage table to generate a complete SSD table. Please be sure that the stages of the discharge match the stages of the pond. Save this as a comma separated value table with a unique name in a known place.

7. Enter this as a SSD table in WWHM3, by choosing to load a file in the SSD table window. Find the file created in step 6 and choose it.

Thirsty Duck is proud to be a HydroCAD Partner.

What if I use a modeling package not listed here?

Contact a Thirsty Duck engineer and we will provide assistance with any questions you may have.