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'''Figure 8 Original hazard output for 2.5/5/10/20m mesh SGS model.'''<br><br>
''TipsTip: multiple file names or wildcard are allowed for the extra grid files for remapping.''
=Model Mesh Size vs Remap Result=
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>
[[File:Fig9 road mesh size.png|1050px]]<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 information that the flow is upstream controlled is not reflectedknown inwhen remapping, hence the depthappearance outputof yetdry andpatches on the downstream face of the road crest. Consideration of this effect will be further addressed induring the development of the high resolution SGS output feature scheduled infor thea future TUFLOW release.<br>
[[File:Fig10 road mesh size2.png|500px]]<br>
'''Figure 10 Modelled water level line over the road crest.'''<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 Kitts et. al., 2020, will add link to Duncan's paper in https://www.tuflow.com/Library.aspx when it is uploaded). 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>
[[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 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].
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