TUFLOW Remapping: Difference between revisions

With the release of TUFLOW 2020, the combination of Quadtree mesh and Sub-grid Sampling (SGS) method has offered a great flexibility for rapidly building flood modelmodels of varying resolution and buildingincreased topographic detail. Now a wide range of mesh sizesizes 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.<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>

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 area of interest without adversely affecting the results. However, this has also createcreated a challenge foron how to interpolatemap waterresults depthin outputareas at 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, the depth output is much 'smoother' in the 2.5/5/10/20m model withdue to the finer meshescomputational 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 modelmodels, 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 of upstreamsignificant regiontopographic change, 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>

[[File:Fig11 Innisfall remapped D.png|500px600px]]<br>

'''Figure 11 Remapped depth at upstream catchment from different mesh size models..'''