Difference between revisions of "TUFLOW Remapping"

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<poem lang="fr" style="float:left;">
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The 2020 release of TUFLOW included new Quadtree mesh and Sub-grid Sampling (SGS) functionality. The SGS feature now supports the
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hydraulic analysis of partially wet cells on the flood fringe. Currently, cells that are partially wet are displayed in model output
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as being fully wet. This page introduces how to use the ASC_to_ASC remap function, and discusses limitations of the method.</poem><br><br>
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[[File: Remap_Advice_LinkedIn.jpg ||450px|right]]
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<br><br><br><br><br><br>
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__TOC__
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= Introduction =
 
= Introduction =
With the release of TUFLOW 2020, the combination of Quadtree mesh and Sub-grid Sampling (SGS) method has offered great flexibility to build a model with a range of mesh sizes. SGS samples the digital elevation model (DEM) at a number of points within a cell and computing the water surface elevation as a function of cell stored volume. With SGS, cells and faces can be considered as "partially wet", and this means the impact of the sub-grid scale geometry can be represented by cells with larger sizes. The example below shows a Quadtree model with 10/20m cell size can conduct water as smoothly as a 2.5/5/10/20m model along a narrow stream.<br>
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With the release of TUFLOW 2020, the combination of Quadtree mesh and Sub-grid Sampling (SGS) method has offered great flexibility for rapidly building flood models of varying resolution and increased topographic detail. Now a wide range of mesh sizes can be applied in a model based on the geographic feature and the area of interest in a project. Especially with the SGS method, cells and faces can be treated as 'partially wet', and the impact of the sub-grid scale geometry may be represented by cells with larger sizes.<br>
 +
The example below shows that a Quadtree model with 10/20m cell size (right) can convey water as smoothly as a 2.5/5/10/20m model along the narrow and meandering gully line.<br>
 
[[File:Fig1 H sgs.png|700px]]<br>
 
[[File:Fig1 H sgs.png|700px]]<br>
 
'''Figure 1 Water level simulation results with SGS. Left: 2.5/5/10/20m Quadtree model. Right: 10/20m Quadtree model.'''<br><br>
 
'''Figure 1 Water level simulation results with SGS. Left: 2.5/5/10/20m Quadtree model. Right: 10/20m Quadtree model.'''<br><br>
On the other hand, the same model running without SGS method creates notably different results in the distance of water travelled and the area of inundation on floodplain due to the poor geometry representation.<br>  
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On the other hand, the same model running without SGS applied creates notably different results in the distance of water travelled and the area of inundation on the floodplain due to the poor topographic representation (one elevation per cell centre and cell face).<br>  
 
[[File:Fig2 H nonsgs.png|700px]]<br>
 
[[File:Fig2 H nonsgs.png|700px]]<br>
 
'''Figure 2 Water level simulation results without SGS. Left: 2.5/5/10/20m Quadtree model. Right: 10/20m Quadtree model.'''<br><br>
 
'''Figure 2 Water level simulation results without SGS. Left: 2.5/5/10/20m Quadtree model. Right: 10/20m Quadtree model.'''<br><br>
These examples indicate the mesh size sensitivities of a SGS model are significantly reduced, and thus modellers are increasingly using coarser mesh at areas far away from the location of interest. However, this has also create a challenge for how to output water depth at those coarser cells. The example below shows even the 10/20m mesh and the 2.5/5/10/20m mesh models produce similar water level, but the depth map output is much 'smoother'in the model with finer meshes. <br>
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These examples show how the mesh size sensitivity are significantly reduced if using TUFLOW's implementation of SGS. Therefore, modellers can confidently use a coarser mesh at areas away from the area of interest without adversely affecting the results. However, this has created a challenge on how to map results in areas of coarser mesh. For example, the images below show that whilst the 10/20m mesh and the 2.5/5/10/20m mesh SGS models produce similar water levels, the depth output is much 'smoother' in the 2.5/5/10/20m model due to the finer computational mesh. <br>
 
[[File:Fig3 D sgs zoom.png|700px]]<br>
 
[[File:Fig3 D sgs zoom.png|700px]]<br>
 
'''Figure 3 Water depth simulation results with SGS. Left: 2.5/5/10/20m Quadtree model. Right: 10/20m Quadtree model.'''<br><br>
 
'''Figure 3 Water depth simulation results with SGS. Left: 2.5/5/10/20m Quadtree model. Right: 10/20m Quadtree model.'''<br><br>
  
In TUFLOW, the depth output is interpolated from the depths at nearest cell centre and cell corners surrounding the output grid. The interpolated depth may not perfectly represent the actual depth, since TUFLOW doesn't store the under laying high resolution DEM values at the moment. <br>
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In TUFLOW, the depth output is interpolated from the depths at the nearest cell centre and cell corners surrounding the output grid. Because TUFLOW currently doesn't store the underlying high resolution DEM values other than those needed for the SGS hydraulic computations, the interpolated depth may not perfectly represent the actual depth as illustrated in the figure below. <br>
 
[[File:Fig4 sgs depth interporation.png|500px]]<br>
 
[[File:Fig4 sgs depth interporation.png|500px]]<br>
 
'''Figure 4 TIN interpolation used for water depth map output with SGS.'''<br><br>
 
'''Figure 4 TIN interpolation used for water depth map output with SGS.'''<br><br>
While we are developing high resolution SGS output in the future release to address this issue, we have also added a new functionality in the [[ASC_to_ASC|ASC_to_ASC]] utility to 'remap' a water level grid to a finer DEM grid. This page introduce how to use the [[ASC_to_ASC|ASC_to_ASC]] remap function, and also discuss the limitation of the method.
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This page introduces how to use the [[ASC_to_ASC|ASC_to_ASC]] remap function, and discusses limitations of the method.
  
=Remap Water Level to Finer DEM=
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=Remapping Water Level to a Finer DEM=
The remap function in the [[ASC_to_ASC|ASC_to_ASC]] remaps a water level grid to a high resolution DEM using TIN interpolation. It outputs a high resolution water level grid and a high resolution depth grid.<br>
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The remap function in [[ASC_to_ASC|ASC_to_ASC]] remaps a TUFLOW water level grid to a high resolution DEM using TIN interpolation. The output is a high resolution water level grid and a high resolution depth grid.<br>
 
  <tt>asc_to_asc.exe -remap -wl lowres_h.asc -dem DEM_highres.asc</tt>
 
  <tt>asc_to_asc.exe -remap -wl lowres_h.asc -dem DEM_highres.asc</tt>
In this command:<br>
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where:<br>
 
'''"-wl <wl_file>"'''<br>
 
'''"-wl <wl_file>"'''<br>
Sets the coarser resolution water level grid to remap from.<br>
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sets the coarser resolution water level grid to use for the remap.<br>
 
'''"-dem <dem_file>"'''<br>
 
'''"-dem <dem_file>"'''<br>
Sets the finer resolution DEM.<br>
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sets the finer resolution DEM.<br><br>
The figure below overlays the remapped water depth on top of the 10/20m model water level output, and compared it to the original water depth output. As can be seen, smooth water depth along the gully and flood fringes is produced by the post process utility.<br>
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The first image below shows the remapped water depth on top of the 10/20m model water level output, and compares it with the original water depth output in the second image. As can be seen, much smooth water depths along the gully and the flood fringes are produced.<br>
 
[[File:Fig5 D sgs remap zoom.png|700px]]<br>
 
[[File:Fig5 D sgs remap zoom.png|700px]]<br>
 
'''Figure 5 Remapped vs original water depth for 10/20m mesh SGS model.'''<br><br>
 
'''Figure 5 Remapped vs original water depth for 10/20m mesh SGS model.'''<br><br>
  
The next example presents the water level output and the remapped depth output in a river flood modelling. As can be seen, large portions of small buildings are submerged by water level output due to the relative size of the mesh and the buildings. However, remapped depth shows smooth fringes around the buildings.<br>
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The next example presents the water level output and the remapped depth output from a flood model using SGS with the buildings included in the DEM. As can be seen in the first image, large portions of the smaller buildings appear submerged by the SGS water level output due to the cells containing these buildings being partially wet. However, the remapped depths (second image) clearly shows the buildings and produces a much higher resolution flood map.<br>
 
[[File:Fig6 IR.png|1050px]]<br>
 
[[File:Fig6 IR.png|1050px]]<br>
 
'''Figure 6 Water level output and remapped water depth in a river flood model.'''<br><br>
 
'''Figure 6 Water level output and remapped water depth in a river flood model.'''<br><br>
  
=Remap Other Output Grids=
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=Remapping of Other Map Output Grids=
The utility can also remap extra grid files (e.g. a hazard output) to the resolution of the DEM file.<br>
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The utility can also remap additional map output grids (e.g. velocity, hazard and others) to the resolution of the DEM file.<br>
  <tt>asc_to_asc.exe -remap -wl lowres_h.asc -dem DEM_highres.asc lowers_hazard.asc</tt>
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  <tt>asc_to_asc.exe -remap -wl lowres_h.asc -dem DEM_highres.asc lowres_v.asc lowres_hazard.asc</tt>
This command reads in an extra grid 'lowers_hazard.asc' and remaps it to the finder DEM resolution.The figure below compares the original hazard output from the 10/20m SGS model and the remapped hazard output.<br>
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This command reads in an additional grids 'lowres_v.asc' and 'lowres_hazard.asc' and remaps it to the finer DEM resolution. The figure below compares the original and the remapped hazard outputs from the 10/20m SGS model.<br>
 
[[File:fig7 ZAEM1 sgs.png|700px]]<br>
 
[[File:fig7 ZAEM1 sgs.png|700px]]<br>
'''Figure 7 Remapped vs original hazard output for 10/20m mesh SGS model.'''<br><br>
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'''Figure 7 Original vs remapped hazard output for 10/20m mesh SGS model.'''<br><br>
It is important to note that, for output types other than depth, this utility does NOT interpolate the result from the coarser grid to the finer grid, but only extended/reduced the output extent to the dry/wet extent. Therefore, the resolution of the remapped hazard above remains the same as the original output grid. The interpolation is not conducted for the following reasons:
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It is important to note that, for any output types other than depth, this utility does NOT interpolate the result from the coarser grid to the finer grid, but only extends/reduces the output extent to the dry/wet extent. Therefore, the detail of the remapped hazard above remains the same as the original output grid. The interpolation is not carried out for the following reasons:
* Hazard categories are usually depended on both water depth and velocity, and it is not straight forward to interpolate a cell averaged velocity to sub-cell scale with varying water depth.  
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* Hazard categories are usually dependent on both water depth and velocity, and it is problematic to interpolate/extrapolate the computed cell averaged velocity to sub-cell scale velocities with varying water depth.  
* Even if this can be done based on empirical relationship between depth and velocity, the obtained velocity is much less reliable than a actual output from a model with finer mesh.
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* Should a higher output resolution be required for outputs utilising velocity, a finer mesh in the area should be used.
Therefore, we highly recommend refining the mesh size directly a the location where user wants to obtain finer velocity or hazard output. The figure below shows the hazard output from the 2.5/5/10/20m model, and as can be seen the result is much smoother along the gully. <br>  
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The figure below shows the hazard output from the 2.5/5/10/20m model, and as can be seen the hazard result is much smoother and more accurate along the gully where the finer cell sizes occur. <br>  
 
[[File:SGS 02-20m ZAEM1.png|400px]]<br>
 
[[File:SGS 02-20m ZAEM1.png|400px]]<br>
'''Figure 8 Original hazard output for 2.5/5/10/20m mesh SGS model.'''<br>
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'''Figure 8 Original hazard output for 2.5/5/10/20m mesh SGS model.'''<br><br>
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''Tip: multiple file names or wildcard are allowed for the extra grid files for remapping.''
  
 
=Model Mesh Size vs Remap Result=
 
=Model Mesh Size vs Remap Result=
Beside the quality of hazard output discussed above, the model mesh size can still impact the remapped output in different ways.
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Beside the quality of velocity based outputs discussed above, the model mesh size can also impact the remapped output at locations with steep slope as discussed below.
==Road Crest==
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==Road Crests==
At location that bed elevation changes sharply, the remapped water depth may become negative if the output grid size is too large.<br>
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When water flows over a road crest, the remapped water depth may become negative if the output grid size is coarse. The figure below shows the remapped depth over a road crest from models with mesh sizes ranging from 5m to 1.25m. As can be seen, some areas don't have remapped depths despite the water clearly over-topping the road.<br>
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[[File:Fig9 road mesh size.png|1050px]]<br>
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'''Figure 9 Remapped depths over a road crest from models of different mesh resolutions.'''<br><br>
  
[[File:Fig9 road mesh size.png|1050px]]<br>
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As illustrated in the chart below, the DEM has a much finer resolution and the elevation changes rapidly across the road crest, so when interpolating a coarser water level grid to the finer DEM, the interpolated water level may become lower than the local DEM level. In the 2D solver, this type of location is treated as upstream controlled weir flow with the upstream depth used for the calculation. However, the information that the flow is upstream controlled is not known when remapping, hence the appearance of dry patches on the downstream face of the road crest.<br>
 
[[File:Fig10 road mesh size2.png|500px]]<br>
 
[[File:Fig10 road mesh size2.png|500px]]<br>
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'''Figure 10 Modelled water level line over the road crest.'''<br>
  
==Upstream Catchment==
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==Steep Catchments==
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In direct rainfall models, substantial benefits are being realised from applying SGS. The cell faces now correctly capture the lowest elevations along the gullies to preserve the sub-cell scale flow paths, which can significantly improve the hydrologic response for a whole catchment model (see Duncan Kitts' LinkedIn post: https://www.linkedin.com/pulse/sub-grid-sampling-step-change-way-we-create-apply-hydraulic-kitts/). However, if the cell size becomes too large in areas of significant topographic change, only a small portion of the cell may be wet. This makes the depth plotting extremely challenging and the remapped depth may become negative even though water is flowing through the cell. The blue contour below shows the remapped water depth of a 60m mesh direct rainfall model. As can be seen the flow paths are not continuous along some valleys. The remapped water depth from a 20m mesh model is also plotted as the pink contours underneath the 60m result demonstrating that how the flow paths have become clearer as the mesh size is refined even though the results from these two models are very similar when using SGS. <br>
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[[File:Fig11 Innisfall remapped D.png|600px]]<br>
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'''Figure 11 Remapped depth at upstream catchment from different mesh size models.'''
  
 
=Conclusion=
 
=Conclusion=
When the spatial resolution of a model is increased (i.e. cell size reduced) review of result sensitivity at 1D/2D SX link locations is recommended. This can be done quickly and easily by plotting 1D results and checking for unwanted oscillations. This page demonstrated some useful methods for stabilising 1D/2D boundary (SX) links, in particular where the 1D structure is large in comparison to the 2D cell size. Available options that were introduced included reviewing the 1D timestep, using 1D nodes to define the 1D/2D boundary link, SX boundary lines, SX storage factors and SX boundary polygons.
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The benefits of using the combination of Quadtree mesh and Sub-grid Sampling method are many. In this page we focused on the ability of representing the sub-grid scale geometry by SGS method, which allows the user to apply a coarser mesh to reduce the total simulation time without adversely affecting the results. However, the interpolation of map output from a coarser mesh can be challenging as illustrated in the examples above. While we are developing a high resolution output feature within TUFLOW to tackle this issue, the new remapping functions of the [[ASC_to_ASC|ASC_to_ASC]] utility can be used to post process water level outputs to high resolution depth results. This produces smooth depth output along main flow paths and flood fringes. However, the SGS method and remapping tool is not a panacea, and a computational mesh with sufficient resolution is still needed to produce reasonable and meaningful simulation results, especially where high resolution velocity based map outputs are needed.<br>
 +
 
 +
'''Tip''': Run your model once with small cell size in a test mode (-t) to produce DEM_Z with all topography modifications with the same resolution as the original DEM for use in the TUFLOW Remapping function.<br>
 +
 
 +
Finally, should you have some interesting results using SGS, Quadtree or the remapping feature that you would like to share with the TUFLOW community, please feel free to email [mailto:support@tuflow.com support@tuflow.com].

Latest revision as of 12:22, 8 August 2024

The 2020 release of TUFLOW included new Quadtree mesh and Sub-grid Sampling (SGS) functionality. The SGS feature now supports the
hydraulic analysis of partially wet cells on the flood fringe. Currently, cells that are partially wet are displayed in model output
as being fully wet. This page introduces how to use the ASC_to_ASC remap function, and discusses limitations of the method.



Remap Advice LinkedIn.jpg







Introduction

With the release of TUFLOW 2020, the combination of Quadtree mesh and Sub-grid Sampling (SGS) method has offered great flexibility for rapidly building flood models of varying resolution and increased topographic detail. Now a wide range of mesh sizes can be applied in a model based on the geographic feature and the area of interest in a project. Especially with the SGS method, cells and faces can be treated as 'partially wet', and the impact of the sub-grid scale geometry may be represented by cells with larger sizes.
The example below shows that a Quadtree model with 10/20m cell size (right) can convey water as smoothly as a 2.5/5/10/20m model along the narrow and meandering gully line.
Fig1 H sgs.png
Figure 1 Water level simulation results with SGS. Left: 2.5/5/10/20m Quadtree model. Right: 10/20m Quadtree model.

On the other hand, the same model running without SGS applied creates notably different results in the distance of water travelled and the area of inundation on the floodplain due to the poor topographic representation (one elevation per cell centre and cell face).
Fig2 H nonsgs.png
Figure 2 Water level simulation results without SGS. Left: 2.5/5/10/20m Quadtree model. Right: 10/20m Quadtree model.

These examples show how the mesh size sensitivity are significantly reduced if using TUFLOW's implementation of SGS. Therefore, modellers can confidently use a coarser mesh at areas away from the area of interest without adversely affecting the results. However, this has created a challenge on how to map results in areas of coarser mesh. For example, the images below show that whilst the 10/20m mesh and the 2.5/5/10/20m mesh SGS models produce similar water levels, the depth output is much 'smoother' in the 2.5/5/10/20m model due to the finer computational mesh.
Fig3 D sgs zoom.png
Figure 3 Water depth simulation results with SGS. Left: 2.5/5/10/20m Quadtree model. Right: 10/20m Quadtree model.

In TUFLOW, the depth output is interpolated from the depths at the nearest cell centre and cell corners surrounding the output grid. Because TUFLOW currently doesn't store the underlying high resolution DEM values other than those needed for the SGS hydraulic computations, the interpolated depth may not perfectly represent the actual depth as illustrated in the figure below.
Fig4 sgs depth interporation.png
Figure 4 TIN interpolation used for water depth map output with SGS.

This page introduces how to use the ASC_to_ASC remap function, and discusses limitations of the method.

Remapping Water Level to a Finer DEM

The remap function in ASC_to_ASC remaps a TUFLOW water level grid to a high resolution DEM using TIN interpolation. The output is a high resolution water level grid and a high resolution depth grid.

asc_to_asc.exe -remap -wl lowres_h.asc -dem DEM_highres.asc

where:
"-wl <wl_file>"
sets the coarser resolution water level grid to use for the remap.
"-dem <dem_file>"
sets the finer resolution DEM.

The first image below shows the remapped water depth on top of the 10/20m model water level output, and compares it with the original water depth output in the second image. As can be seen, much smooth water depths along the gully and the flood fringes are produced.
Fig5 D sgs remap zoom.png
Figure 5 Remapped vs original water depth for 10/20m mesh SGS model.

The next example presents the water level output and the remapped depth output from a flood model using SGS with the buildings included in the DEM. As can be seen in the first image, large portions of the smaller buildings appear submerged by the SGS water level output due to the cells containing these buildings being partially wet. However, the remapped depths (second image) clearly shows the buildings and produces a much higher resolution flood map.
Fig6 IR.png
Figure 6 Water level output and remapped water depth in a river flood model.

Remapping of Other Map Output Grids

The utility can also remap additional map output grids (e.g. velocity, hazard and others) to the resolution of the DEM file.

asc_to_asc.exe -remap -wl lowres_h.asc -dem DEM_highres.asc lowres_v.asc lowres_hazard.asc

This command reads in an additional grids 'lowres_v.asc' and 'lowres_hazard.asc' and remaps it to the finer DEM resolution. The figure below compares the original and the remapped hazard outputs from the 10/20m SGS model.
Fig7 ZAEM1 sgs.png
Figure 7 Original vs remapped hazard output for 10/20m mesh SGS model.

It is important to note that, for any output types other than depth, this utility does NOT interpolate the result from the coarser grid to the finer grid, but only extends/reduces the output extent to the dry/wet extent. Therefore, the detail of the remapped hazard above remains the same as the original output grid. The interpolation is not carried out for the following reasons:

  • Hazard categories are usually dependent on both water depth and velocity, and it is problematic to interpolate/extrapolate the computed cell averaged velocity to sub-cell scale velocities with varying water depth.
  • Should a higher output resolution be required for outputs utilising velocity, a finer mesh in the area should be used.

The figure below shows the hazard output from the 2.5/5/10/20m model, and as can be seen the hazard result is much smoother and more accurate along the gully where the finer cell sizes occur.
SGS 02-20m ZAEM1.png
Figure 8 Original hazard output for 2.5/5/10/20m mesh SGS model.

Tip: multiple file names or wildcard are allowed for the extra grid files for remapping.

Model Mesh Size vs Remap Result

Beside the quality of velocity based outputs discussed above, the model mesh size can also impact the remapped output at locations with steep slope as discussed below.

Road Crests

When water flows over a road crest, the remapped water depth may become negative if the output grid size is coarse. The figure below shows the remapped depth over a road crest from models with mesh sizes ranging from 5m to 1.25m. As can be seen, some areas don't have remapped depths despite the water clearly over-topping the road.
Fig9 road mesh size.png
Figure 9 Remapped depths over a road crest from models of different mesh resolutions.

As illustrated in the chart below, the DEM has a much finer resolution and the elevation changes rapidly across the road crest, so when interpolating a coarser water level grid to the finer DEM, the interpolated water level may become lower than the local DEM level. In the 2D solver, this type of location is treated as upstream controlled weir flow with the upstream depth used for the calculation. However, the information that the flow is upstream controlled is not known when remapping, hence the appearance of dry patches on the downstream face of the road crest.
Fig10 road mesh size2.png
Figure 10 Modelled water level line over the road crest.

Steep Catchments

In direct rainfall models, substantial benefits are being realised from applying SGS. The cell faces now correctly capture the lowest elevations along the gullies to preserve the sub-cell scale flow paths, which can significantly improve the hydrologic response for a whole catchment model (see Duncan Kitts' LinkedIn post: https://www.linkedin.com/pulse/sub-grid-sampling-step-change-way-we-create-apply-hydraulic-kitts/). However, if the cell size becomes too large in areas of significant topographic change, only a small portion of the cell may be wet. This makes the depth plotting extremely challenging and the remapped depth may become negative even though water is flowing through the cell. The blue contour below shows the remapped water depth of a 60m mesh direct rainfall model. As can be seen the flow paths are not continuous along some valleys. The remapped water depth from a 20m mesh model is also plotted as the pink contours underneath the 60m result demonstrating that how the flow paths have become clearer as the mesh size is refined even though the results from these two models are very similar when using SGS.
Fig11 Innisfall remapped D.png
Figure 11 Remapped depth at upstream catchment from different mesh size models.

Conclusion

The benefits of using the combination of Quadtree mesh and Sub-grid Sampling method are many. In this page we focused on the ability of representing the sub-grid scale geometry by SGS method, which allows the user to apply a coarser mesh to reduce the total simulation time without adversely affecting the results. However, the interpolation of map output from a coarser mesh can be challenging as illustrated in the examples above. While we are developing a high resolution output feature within TUFLOW to tackle this issue, the new remapping functions of the ASC_to_ASC utility can be used to post process water level outputs to high resolution depth results. This produces smooth depth output along main flow paths and flood fringes. However, the SGS method and remapping tool is not a panacea, and a computational mesh with sufficient resolution is still needed to produce reasonable and meaningful simulation results, especially where high resolution velocity based map outputs are needed.

Tip: Run your model once with small cell size in a test mode (-t) to produce DEM_Z with all topography modifications with the same resolution as the original DEM for use in the TUFLOW Remapping function.

Finally, should you have some interesting results using SGS, Quadtree or the remapping feature that you would like to share with the TUFLOW community, please feel free to email support@tuflow.com.