TUFLOW Topography Guidance

Common Questions Answers (FAQ)

Should I use Z Lines or Z Shapes for topographic modifications?

The Z Line feature is a predecessor of Z Shape. As it has only one attribute, elevation, the GULLY or MIN / RIDGE or MAX / THICK / ADD options have to be specified in the control file at the end of the “Read GIS Z Line” command. This limits the Z Line layer to be used for multiple applications and separate command, including separate GIS layer, has to be used for every option combination. The development of Z Lines has ceased and some alternative options might not be available with new features, such as Sub-Grid Sampling.
The new Z Shape feature has four attributes to capture all possible option combination within one GIS layer and a single command “Read GIS Z Shape” in the control file. As such, Z Shapes are recommended to use over Z Lines.
Z Lines can be modified into Z Shapes by copying the GIS lines and points into 2d_zsh layer and specifying the options in the Shape_Option attribute.

What procedure does TUFLOW follow for interpolating DEM elevations?

• The DEM elevation is assumed to be in the centre of the DEM grid cell as shown by the four red circles in the picture below.
• The four DEM elevations surrounding the yellow Zpt in the attached are triangulated by forming four triangles all having a common vertex (green circle) located at the middle of the four DEM elevations.
• The elevation of the common vertex (green circle) is equal to the average of the four DEM elevations (red circles).
• As the Zpt falls within the top triangle the elevation assigned to the Zpt is the planar (linear) interpolation of the three elevations of the top triangle.

Can I use SRTM data for modelling rivers if no other sources are available?

SRTM is a very coarse grid. May be suitable of catchment discretisation of large catchments (100km2+) but generally should never be used for 1D or 2D flood modelling. For river bathymetry, SRTM or LiDAR won't penetrate accurately below the water surface so ground survey or bathymetric survey will be required. For the floodplain SRTM data may able to be used, provided its vertical accuracy is checked against ground survey data (SRTM is usually very inaccurate in the vertical). For this reason, approaching local agencies if they have bathymetry data (Local Council, County or State Department) might be the next step. In some countries these agencies host the data online for download.

How should I model buildings?

There are numerous different industry standard approaches to represent buildings in a 2D rainfall model. Australian Rainfall and Runoff Guideline, Project 15 (Representation of Buildings in 2D Numerical Flood Models discusses some of the available options.

When including buildings in rain on grid model, there are additional factors to consider. For more details refer to: What is the best approach for modelling buildings in rain on grid model?

Common TUFLOW modelling approaches are summarised below:

• Higher Roughness Value
  

Modelling buildings with higher roughness is a widely used approach for larger models. To gain an understanding of the most appropriate roughness value for a specific catchment, we recommend calibrating the model to field data. This is discussed in Section 2.1 in the Australian Rainfall and Runoff Guideline

The guidelines have noted a Manning's n value of 0.3 is a reasonable value to adopt for representing buildings. However, the reference used in the guidelines is most likely using the Smagorinsky or a Constant turbulence model, in which the Manning's n does not affect the eddy viscosity. Since the 2023 TUFLOW release, the Wu turbulence model has been set as default. In this model, the higher Manning's n would increase the bed friction as well as the eddy viscosity.

We recommend conducting a sensitivity analysis to understand how the model responds to different roughness values between -/+ 20%.

This approach is appropriate for models with large cells that are not able to capture the footprints of buildings in detail. However, when used in high-resolution models, this approach is not capable of capturing complex flow patterns around buildings in comparison to other methods.

• Raising the topography of the building footprint
  

Raising the topography to represent buildings would also be an appropriate approach if the model has a fine cell resolution that is sufficient to capture the footprint of the buildings. However, this approach would not take into account any storage effects of water within the building. The impact of losing this additional storage within the building could vary.

Appropriate to use in cases where the model resolution is too coarse to capture the flow paths between the buildings.

• Inactive cells for buildings
  

Buildings can also be represented by inactive cells; however, this approach would also not take into account storage effects similar to raising the topography of the building footprint.

This approach would also require a fine cell resolution and would not capture friction along the buildings when using the Wu turbulence model.

• Represent the buildings on three sides
  

A thin breakline (2d_zsh) can be digitized around three sides of the building. This approach will account for storage within the building as well as capture the flow paths between buildings, given the model uses a fine cell resolution.

How should I model fences?

A layered flow constriction (2d_lfcsh) can be used to represent fences and guardrails by simulating a portion of water to be blocked by still allowing for some water to pass through.

This is some guidance on how to configure 2d_lfcsh points and line layers to represent fences:

Extract the ground elevation at points along the fence line and add the height of the fence at each of these points. The values calculated from elevation + fence height will be used as the L1_Obvert values of the 2d_lfcsh points layer.
Note, that the 2d_lfcsh lines will need to be snapped to these points.

When using points and lines:

• Points layer, set:

Invert = 99999

L1_Obvert = discussed above

L2_Depth = depth of this layer (if required)

L3_Depth = depth of this layer (if required)

• Lines layer, set:

Invert = 99999

Blockage and FLC as required.

If you encounter unexpected results when using a 2d_lfcsh layer, refer to this page: I don't see results that I expect when using 2d_lfcsh layer

Another approach to model fences is to digitise Z shape (2d_zsh) thin lines. This will fully block water from passing through the fence. This approach also provides an option to model a fence collapse with a variable Z shape (2d_vzsh).

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