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<poem lang="fr" style="float:left;">
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.</poem><br><br>
[[File: Remap_Advice_LinkedIn.jpg ||450px|right]]
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__TOC__
 
= Introduction =
With the release of TUFLOW 2020, the combination of Quadtree mesh and Sub-grid Sampling (SGS) method has offered great flexibility tofor buildrapidly abuilding modelflood withmodels aof rangevarying ofresolution meshand sizes.increased SGStopographic samplesdetail. theNow digitala elevationwide modelrange (DEM)of atmesh asizes numbercan ofbe pointsapplied withinin a cellmodel andbased computingon the watergeographic surfacefeature elevationand asthe a functionarea of cellinterest storedin volumea project. WithEspecially with the SGS method, cells and faces can be consideredtreated as "'partially wet"', and this means the impact of the sub-grid scale geometry canmay 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>
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>
'''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 methodapplied creates notably different results in the distance of water travelled and the area of inundation on the floodplain due to the poor geometrytopographic representation (one elevation per cell centre and cell face).<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>
These examples indicateshow how the mesh size sensitivitiessensitivity ofare asignificantly SGSreduced modelif areusing significantlyTUFLOW's reduced,implementation andof thusSGS. Therefore, modellers arecan increasinglyconfidently usinguse a coarser mesh at areas far away from the locationarea of interest without adversely affecting the results. However, this has also createcreated a challenge foron how to outputmap waterresults depthin atareas thoseof coarser cellsmesh. TheFor example, the images below showsshow that evenwhilst the 10/20m mesh and the 2.5/5/10/20m mesh SGS models produce similar water levellevels, but the depth map output is much 'smoother' in the 2.5/5/10/20m model withdue to the finer meshescomputational mesh. <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>
 
In TUFLOW, the depth output is interpolated from the depths at the nearest cell centre and cell corners surrounding the output grid. TheBecause interpolatedTUFLOW depthcurrently maydoesn't notstore perfectlythe representunderlying high resolution DEM values other than those needed for the actualSGS depthhydraulic computations, sincethe TUFLOWinterpolated doesn'tdepth storemay thenot underperfectly layingrepresent highthe resolutionactual DEMdepth valuesas atillustrated in the momentfigure below. <br>
[[File:Fig4 sgs depth interporation.png|500px]]<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 introduceintroduces how to use the [[ASC_to_ASC|ASC_to_ASC]] remap function, and also discuss thediscusses limitationlimitations of the method.
 
=RemapRemapping Water Level to a Finer DEM=
The remap function in the [[ASC_to_ASC|ASC_to_ASC]] remaps a TUFLOW water level grid to a high resolution DEM using TIN interpolation. ItThe output outputsis 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>
In this commandwhere:<br>
'''"-wl <wl_file>"'''<br>
Setssets the coarser resolution water level grid to remapuse fromfor the remap.<br>
'''"-dem <dem_file>"'''<br>
Setssets the finer resolution DEM.<br><br>
The figurefirst image below overlaysshows the remapped water depth on top of the 10/20m model water level output, and comparedcompares it towith the original water depth output in the second image. As can be seen, much smooth water depthdepths along the gully and the flood fringes isare produced by the post process utility.<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>
 
The next example presents the water level output and the remapped depth output infrom a river flood modellingmodel using SGS with the buildings included in the DEM. As can be seen in the first image, large portions of smallthe smaller buildings areappear submerged by the SGS water level output due to the relativecells sizecontaining ofthese thebuildings meshbeing andpartially the buildingswet. However, the remapped depthdepths shows(second smoothimage) fringesclearly aroundshows the buildings and produces a much higher resolution flood map.<br>
[[File:Fig6 IR.png|1050px]]<br>
'''Figure 6 Water level output and remapped water depth in a river flood model.'''<br><br>
 
=RemapRemapping of Other Map Output Grids=
The utility can also remap extraadditional gridmap filesoutput grids (e.g. avelocity, hazard outputand others) to the resolution of the DEM file.<br>
<tt>asc_to_asc.exe -remap -wl lowres_h.asc -dem DEM_highres.asc lowers_hazardlowres_v.asc lowres_hazard.asc</tt>
This command reads in an extraadditional gridgrids 'lowers_hazardlowres_v.asc' and 'lowres_hazard.asc' and remaps it to the finderfiner DEM resolution. The figure below compares the original and the remapped hazard outputoutputs from the 10/20m SGS model and the remapped hazard output.<br>
[[File:fig7 ZAEM1 sgs.png|700px]]<br>
'''Figure 7 RemappedOriginal vs originalremapped hazard output for 10/20m mesh SGS model.'''<br><br>
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 extendedextends/reducedreduces the output extent to the dry/wet extent. Therefore, the resolutiondetail of the remapped hazard above remains the same as the original output grid. The interpolation is not conductedcarried out for the following reasons:
* Hazard categories are usually dependeddependent on both water depth and velocity, and it is not straight forwardproblematic to interpolate/extrapolate athe 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.
* 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.
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 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>
'''Figure 8 Original hazard output for 2.5/5/10/20m mesh SGS model.'''<br><br>
 
''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 hazard output discussed above, the model mesh size can still impact the remapped output in different ways.▼
==Road Crest==▼
At location that bed elevation changes sharply, the remapped water depth may become negative if the output grid size is too large.<br>
 
▲=Model Mesh Size vs Remap Result=
▲Beside the quality of hazardvelocity outputbased outputs discussed above, the model mesh size can stillalso impact the remapped output inat differentlocations with steep slope as discussed waysbelow.
▲==Road CrestCrests==
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>
[[File:Fig9 road mesh size.png|1050px]]<br>
'''Figure 9 Remapped depths over a road crest from models of different mesh resolutions.'''<br><br>
[[File:Fig10 road mesh size2.png|400px]]<br>▼
 
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|400px500px]]<br>
'''Figure 10 Modelled water level line over the road crest.'''<br>
 
==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. <br>
[[File:Fig11 Innisfall remapped D.png|600px]]<br>
'''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|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>
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.
 
'''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].
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