Zpt Description

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This page gives a brief description of the Zpts. A more detailed description is contained in section 4.4 of the 2010 TUFLOW manual.

The model topography is defined by elevations at the cell centres, mid-sides and cell corners. The TUFLOW Zpts are shown below as red points, the TUFLOW grid is shown as black squares. At each of these elevation points (Zpts) the elevation needs to be inputted into the TUFLOW model.
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The elevation points at locations around the TUFLOW cell are used for different purposes computationally. The following locations or types are of Zpts are referred to:

  • ZC ("C" type), this is the cell centre or the middle of the cell;
  • ZU ("U" type), this is the cell side in the x direction;
  • ZV ("V" type), this is the cell side in the y direction; and
  • ZH ("H" type), this is the cell corner.


One of the most important aspects of TUFLOW modelling is to understand the roles of the elevation points. A description of each of the zpt locations is given below, these locations a shown graphically in the figure at the end of the page.

ZC Point

The ZC point:

  • defines the volume of active water (storage). The cell volume is based on a flat square cell that wets and dries at the ZC height (plus the cell wet / dry depth);
  • controls when a cell becomes wet and dry (not cell sides can also wet and dry); and
  • is used to determine the bed slope when testing for upstream controlled flow regime (refer to TUFLOW manual).

ZU and ZV Points

The ZU and ZV points:

  • control how much water is conveyed from one cell to another;
  • are where the momentum equation terms are centred and where upstream controlled flow regimes are applied;
  • are deactivated if the cell has dried (based on the ZC point) and cannot flow; and
  • can wet and dry independently of the cell wetting and drying, which allows for modelling of "thin" obstructions such as fences. Such "thin" (relative to cell size) obstructions can be modelled by setting the cell side elevations, this does not change the storage in the model (calculated at ZC point).

ZH Point

The ZH points:

  • play no role hydraulically;
  • are, by default, the only elevations written to the SMS .2dm (2D Mesh file). By default, all output is interpolated / extrapolated to the cell corners. This default can be overwritten, by using the .tcf command:

Map Output Format == SMS High Res

The location of the four Zpt types are shown in the figure below. For more information, please refer to Section 4.4 of the 2010 TUFLOW manual.

Zpt Locations.png