TUFLOW Remapping: Difference between revisions

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>

Whilst we are developing a high resolution SGS map output option for a future release, we have in the meantime added new functionality to the [[ASC_to_ASC|ASC_to_ASC]] utility to 'remap' a water level grid to a finer DEM grid. This page introduces how to use the [[ASC_to_ASC|ASC_to_ASC]] remap function, and discusses limitations of the method.

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 lowres_v.asc lowres_hazard.asc</tt>

This command reads in an additional gridgrids '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>

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

Beside the quality of hazardvelocity outputbased outputs discussed above, the model mesh size can also impact the remapped output at locationlocations with steep slope as discussed below.

==Road CrestCrests==

When water flows over a road crest, the remapped water depth may become negative if the output grid size is too 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 areaareas doesndon't have remapped depthdepths despite the water is clearly over-topping the road.<br>

'''Figure 9 Remapped depthdepths over a road crest from models of different mesh size modelsresolutions.'''<br><br>

As illustrated in the nextchart figurebelow, the DEM has musha smallermuch finer resolution and the elevation changes rapidly atacross the road crest, so when interpolationinterpolating thea coarser water level gridsgrid 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' andwith properthe upstream depth is used for the calculation. However, thisthe isinformation not reflected inthat the depthflow outputis yetupstream andcontrolled willis benot furtherknown addressedwhen inremapping, hence the highappearance resolutionof SGSdry outputpatches featureon scheduledthe indownstream face of the futureroad releasecrest.<br>

==UpstreamSteep CatchmentCatchments==

In direct rainfall models, modellerssubstantial canbenefits alsoare benefitbeing realised from applying relatively large mesh size and SGS method at the upstream region of a catchment. The SGScell faces now correctly capture the lowest elevations along the gullies to preserve the sub-cell scale flow paths, which improvescan significantly improve the hydrologic response for a whole catchment model (see Duncan Kitts et. al., 2020, will add link to Duncan's paperLinkedIn inpost: https://www.tuflowlinkedin.com/Library.aspx when it is uploadedpulse/sub-grid-sampling-step-change-way-we-create-apply-hydraulic-kitts/). However, if the cell size getsbecomes too large atin theareas upstreamof significant topographic regionchange, only a small portion of the cell becomesmay be wet due to the large difference in elevation inside a cell. This makes the depth plotting extremely challenging and the remapped depth may become negative even though water is flowing through athe valleycell. The blue contour below shows the remapped water depth of a 60m mesh whole catchment 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 redpink contourcontours underneath the 60m result. Thedemonstrating that how the flow pathpaths hashave becomesbecome clearer as the mesh size is refined even though the results from these two models are very similar when using SGS. <br>

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 depthmap output offrom ata coarse SGScoarser 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>

Finally, bothshould SGSyou andhave remappingsome areinteresting relativelyresults newusing featuresSGS, inQuadtree TUFLOW and we are also learning fromor the usersremapping whofeature apply them in their projects. Ifthat you find some results interesting and would like to share with the TUFLOW community, please feel free to email [mailto:support@tuflow.com support@tuflow.com].