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>

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 atother than those needed for the momentSGS hydraulic computations, the interpolated depth may not perfectly represent the actual depth as illustrated in the figure below. <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>

In this commandwhere:<br>

Setssets the coarser resolution water level grid to remapuse fromfor the remap.<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 with the original water depth output sidein bythe second sideimage. As can be seen, much smooth water depthdepths along the gully and the flood fringes isare produced by the post process utility.<br>

The next example presents the water level output and the remapped depth output infrom a river flood model 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 and thepartially buildingswet. However, the remapped depthdepths has(second clearimage) dry/wetclearly borders aroundshows the buildings and produces a much higher resolution flood map.<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 outputs from the 10/20m SGS model.<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 the acomputed cell averaged velocity to sub-cell scale velocities with varying water depth.

Therefore, we highly recommend refining the mesh size directly if users want to obtain finer velocity or hazard output at certain locations. 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>

''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].