User notification "Downstream TW out of range. Looking for TW value that is less than lowest table value."

  Applies To 
  Product(s): PondPack
  Area:  Calculations
  Original Author: Mark Pachlhofer, Bentley Technical Support Group


When I compute my model I get a user notification that states, "Downstream TW out of range.  Looking for TW value that is less than lowest table value.  Looking for TW:  X.XX ft³.  Lowest table value: XX ft.  Used TW Elevation:  XX ft.  Results may be invalid.  You need to add more TW rating points, starting at the lowest possible water surface elevation connected on downstream end."


This is an indication that there is something wrong with one or more of your outlet structures and if accompanied by another user notication that says, "The pond has a diversion with both interconnected and level pool outlet structures. It is recommended that you use either all interconnected or all level pool outlet structures with a diversion from a pond. " then you should follow the steps below. If it's not accompanied by this or a similar message please try to resolve all the user notifications and examine your composite outlet structures closely for mistakes.


Change all outlet structures to use Interconnected ponds as the tailwater type

  1. Open the composite outlet structure manager (Components > Composite Outlet Structures).

  2. Change the Tailwater type for every single outlet structure in your model to "Interconnected Ponds" for all the ponds that are interconnected. If the outlet structure was originally set to a type of "Constant Tailwater" and is connected to an outfall downstream then you will need to change the outfalls boundary condition type to "time-elevation" then create a time elevation curve as follows to represent this constant elevation: For a time a 0.00 hours set the elevation to whatever the constant tailwater elevation is suppose to be. Next create another row that has the end time of the simulation as indicated in your calculation options 'Total Simulation Time" property and set the elevation equal to the same elevation as the first row. Lastly, create a final row set to 1 hour after the simulation finishes and set the elevation equal to .01 ft.(m) less than the elevation for the row above it. The picture below represents a time-elevation curve for a constant tailwater of 1460 ft. on a model that has a total run duration of 24 hours.


     3.     In the composite outlet structure manager under the Tailwater section set the "Interconnected Pond Type" Property to "Use Pond/Outfall for Tailwater Range"

     4.     In the Pond/Outfall ID property click the drop down arrow in the input field and choose "Select from Drawing"   then go out and carefully select the correct associated outfall so the Minimum(Tailwater) and Maximum(Tailwater) fields are populated.

    5.     Make sure to do this for each outlet structure and then re-compute your model

See Also

PondPack user notification: Mass balance for routing volumes vary by more than 0.5%

Calculation failure or bad results with a mix of ICPM and Level Pool ponds