User notification "Downstream TW out of range..."

  Applies To 
  Product(s): PondPack
  Version(s): V8i, CONNECT Edition
  Area:  Calculations
  Original Author: Mark Pachlhofer, Bentley Technical Support Group

Problem

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."

Or:

"Downstream tailwater out of range.  Looking for tailwater that is greater than highest table value.  Looking for tailwater XX ft.  Highest table value XX ft."

Background

This may be an indication of a problem with the configuration of one or more outlets (not necessarily the ones mentioned in the notification), or a problem with a mix of ICPM and level pool routes.

that there is something wrong with one or more of your outlet structures and the configuration may need to be reviewed, or possibly due to a problem .

Solution

Review Outlet Structure headwater and tailwater configuration

The particular outlet referenced by the user notification in question may not necessarily be the one that needs to be fixed. It has been known to identify a seemingly random outlet that may be some distance upstream or downstream of the outlet in question.

First, review all outlet link elements and check for the following:

  1. Ensure that the correct composite outlet structure is selected in the pond outlet link element (the line connecting the square pond outlet node to the next-downstream element)
  2. If an outlet link is set to use a stored EQT table, ensure that it is correct, or change it to use a composite outlet structure
  3. In the composite outlet structure properties, check the headwater and tailwater range and make sure they match up to the upstream and downstream elements.

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

If the issue is 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 may have an invalid ICPM/level pool configuration.

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

  2. Change the Tailwater type for every outlet structure to "Interconnected Ponds".

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 outfall 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 to something 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.

 

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

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