Difference between revisions of "Quadtree and Sub-Grid Sampling FAQ"
| Line 21: | Line 21: | ||
=Why should I create more Z Shape ridge/max lines with SGS models?= | =Why should I create more Z Shape ridge/max lines with SGS models?= | ||
| − | It is possible that without correct breaklines, SGS model allows a leak over a levee or embankment that model with smaller cell size does not. The more 2d_zsh max/ridges are used the better. A rough estimate of SGS testing has required twice the number of ridge lines compared to a traditional model. This also includes locations where there were no ridges previously. Breakline feature in asc_to_asc utility might be useful | + | It is possible that without correct breaklines, SGS model allows a leak over a levee or embankment that model with smaller cell size does not. The more 2d_zsh max/ridges are used the better. A rough estimate of SGS testing has required twice the number of ridge lines compared to a traditional model. This also includes locations where there were no ridges previously. <u>[[ASC_to_ASC#Extract_Breaklines_from_DEM | Breakline feature]]</u> in asc_to_asc utility might be useful for creating additional breaklines. |
Revision as of 11:46, 2 September 2020
This Page is under construction
How coarse can base cell size be for Quadtree model with Sub-Grid Sampling (SGS)?
Even when using SGS to improve conveyance, only one velocity is used for the whole cell. Every model has a cell size range, from very fine to very coarse, beyond which it just doesn't make any sense to go. Cell size sensitivity testing is recommended to establish this range.
Should the same model using Quadtree with smaller cell count be always faster than HPC?
Not necessarily. By default, running model on a mesh (Quadtree) rather than a grid (HPC) is always slower, on average 20%. Quadtree really comes into an effect once there is at least three levels of cell size and judicious refinement - around 80% of cell count reduction. As Quadtree development is an ongoing task this might further improve in the future.
Why I should not use a combination of SGS and Quadtree?
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.
I'm using SGS and my water level extent is larger than depth extent. Why?
The default for SGS models is to map the water level in its full extent and the rest of the data types as trimmed. This can be changed with the below commands:
SGS Map Extent Trim == All
SGS Map Extent Full == h d v
Remap function that is part of the asc_to_asc utility can be used to post-process the result grids to a finer resolution DEM. See TUFLOW Remapping.
Why shouldn't I use Z Shape gully/min lines with SGS models?
If the DEM is fine enough, SGS can preserve the gully along the cells without using any gully line. In such case using a gully line may overestimate the width of the flow path as it makes the entire cell and face flat.
Why should I create more Z Shape ridge/max lines with SGS models?
It is possible that without correct breaklines, SGS model allows a leak over a levee or embankment that model with smaller cell size does not. The more 2d_zsh max/ridges are used the better. A rough estimate of SGS testing has required twice the number of ridge lines compared to a traditional model. This also includes locations where there were no ridges previously. Breakline feature in asc_to_asc utility might be useful for creating additional breaklines.