TUFLOW BC Advice

The following links provide useful boundary condition guidance to TUFLOW users.

• TS1 File Format
• TUFLOW NetCDF Rainfall Format
• TUFLOW Rainfall Control File Examples
• Multiple Domain 2D/2D Boundary Configuration Guidance
• 1D/2D Boundary Configuration Guidance (SX and HX)

Frequently Asked Questions (FAQ)

I wish to use a PO line to monitor the flow exiting a model. Should I snap the PO line to the downstream boundary for this purpose?

Plot Output (PO) Q lines report model flow results for water crossing the digitised line. Computationally, TUFLOW transposes the digitised PO line to the closest adjacent cell face for the output calculation. In addition to this, TUFLOW requires an active 2D cell on either side (upstream and downstream) of the cell face for the calculation. Collectively, these requirements are necessary for accurate flow calculations irrespective of the orientation of the PO line relative to the cell alignment.
It is best practice to snap 2D boundary condition lines, such as HT, QT and HQ, to the extent of the 2D code active area polygon at an orientation roughly perpendicular to the expected flow direction. This model design requirement means the 2D boundary condition lines are located along the outermost 2D cells in a model. Snapping a 2D PO line to the 2D boundary condition may align segments of the Plot Output (PO) Q result inspection line along the outermost cell faces of the model. This does not comply with TUFLOW’s requirement for an active 2D cell on either side of the Plot Output (PO) Q result inspection line. As a result, snapping a 2D PO line to the 2D boundary condition line may underestimate the flow exiting a model. For this reason, Plot Output (PO) Q lines should NOT be snapped to 2D boundary condition lines. The PO line should be defined immediately upstream of the downstream boundary. Use the po_check and grd_check files to review the PO line location to ensure there are active cells each side of the PO line.

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Can TUFLOW model a waterfall?

Yes, TUFLOW can model waterfalls, but it requires careful handling of supercritical flow conditions.

As water approaches a waterfall, it speeds up and transitions into free fall. To ensure realistic behaviour, the model must allow for this acceleration rather than imposing an artificial depth at the brink.

Here are three suggested approaches in TUFLOW:

1. HQ (Water Level versus Flow) Boundary: Defining a custom HQ curve ensures the model correctly represents the flow acceleration before the waterfall.
2. Steep Slope HQ Boundary: Setting a higher slope value in the automatic HQ boundary can also generate supercritical conditions near the outlet.
3. 2D HPC Weir: Modelling a small drop in 2D cell elevations with a weir structure can better reflect natural waterfall hydraulics.

It is also recommended to run sensitivity tests with different boundary setups to check how the downstream conditions influence the model results.

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