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Quadtree and Sub-Grid Sampling FAQ: Difference between revisions

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The second part of the answer is if you wish to coarsen up parts of your model but retain the same cell sizes in your focus area. To achieve this you can increase your base cell size to your largest cell size you wish to use, then add additional levels of nesting layers for your Quadtree mesh noting that the 2020-01 release of TUFLOW allows for up to 9 levels of nesting, so your smallest cell size can only be one-eighth of your base cell size.
 
=Should the same model using Quadtree with smaller cell count be always be faster than HPC?=
Not necessarily. By default, running a model onwith a mesh (Quadtree) rathermesh identical thanto a grid (HPC) grid is always slower, on average 20%. Quadtree really comesstarts intoto anhave effectmajor benefits once there is at least three levels of cell size and judicious refinement -resulting aroundin 80% of cell count reduction. AsThere is one major benefit of Quadtree developmentregardless isin anthat ongoingin tasknearly thisalways mighthas furthera improvemuch insmaller GPU RAM memory footprint because unused 2D cells within the futurebounding rectangle used by a HPC grid are not stored in memory whereas for a grid they are.
 
=When should I not use Quadtree?=
The main advantages of using Quadtree are shorter run times (with considerable cell count reduction compared with using a HPC grid), lower GPU RAM footprint and smaller size of output files. If longeryou're runnot timesachieving ofthis modelby withoutusing Quadtree are not an issue and output file sizes are fine then there is nolittle strongbenefit argumentin tousing use Quadtreeit. Some models with cell count reduction of only around 30% might bemay even runningrun slower with Quadtree than the original HPC model.▼
 
=Why I should not use a combination of SGS and Quadtree?=
Based on the benchmarking thus far there seems to be little reason not to use SGS with Quadtree. The one exception is if the underlying resolution of the DEM is of similar (or coarser) resolution to the 2D cells the there is little reason to use SGS as there is no detailed sub-cell terrain to sample. SGS was not made the default in the 2020-01 release because for some models, especially those with coarse cell sizes over highly variable terrain, you can see substantially different results due to SGS picking up a lot more detail of the terrain within a 2D cell. Using Quadtree further supports the case of using SGS as it is very likely by using coarser cells within your model than you would have using a HPC grid, you will achieve a much better result and good results convergence (as discussed further above) by using SGS. It's highly likely that SGS will be made the default in a future release once there has been extensive industry benchmarking demonstrating using SGS to be consistently superior to not using.
▲The main advantages of using Quadtree are shorter run times (with considerable cell count reduction) and smaller size of output files. If longer run times of model without Quadtree are not an issue and output file sizes are fine then there is no strong argument to use Quadtree. Some models with cell count reduction only around 30% might be even running slower with Quadtree than the original HPC model.
 
=The resolution of my underlying DEM is 1m and modelmy 2D cell size is 1m. Is there any benefit in using SGS?=
NoNone or very little benefit. The SGSDEM sampleresolution pointsreally withinneeds onceto be finer than the 2D cell wouldsize haveresolution for SGS to be of any benefit. In this case the sameSGS sampling resolution would effectively be one point per cell elevationarea/face, so there wouldn't be any precision benefit and the model will have an unnecessarily longer initialisation time.
 
=Does using SGS increase model runtimes?=
Turning SGS on can inreaseincrease run times byt 20-30%, but in some cases the improvement in model stability can actually cause a reduction in run times. Also, being able to go to a larger grid size and still keep the hydraulic accuracy means that runs can be performed much faster.
 
=I'm using SGS and my water level extent is larger than depth extent. Why?=
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