Quadtree and Sub-Grid Sampling FAQ - Revision history

Teegan.Burke: /* What is best practice when applying Quadtree cell sizes within a channel? */

2026-06-16T01:14:00Z

What is best practice when applying Quadtree cell sizes within a channel?

← Older revision		Revision as of 11:14, 16 June 2026
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	=How coarse can the base cell size be for a Quadtree model with Sub-Grid Sampling (SGS)?=		=How coarse can the base cell size be for a Quadtree model with Sub-Grid Sampling (SGS)?=
	There are two answers to this question depending on your modelling objectives. For example, by doubling your base cell size, and not changing the Quadtree levels, you will be doubling the cell sizes throughout your model. Provided this doubling of cell sizes does not conflict with other objectives (remember, only one velocity is calculated per cell face, so a consequence of doubling cell size is to reduce the quality of velocity based outputs such as hazard), whether or not it is okay to increase the base cell size simply comes down to whether results convergence can be proven. By results convergence we mean you can increase (or decrease) your cell sizes without unacceptably changing the model results. If you do see a significant change in results that is considered unacceptable, then you need to possibly make your cell sizes smaller (not larger!). A well-designed model ~~mesh~~ is one where you can decrease cells sizes without seeing an unacceptable change in results. SGS will greatly help with achieving results convergence and very much increase your ability to use a coarser base cell size or coarsen up parts of your model.		There are two answers to this question depending on your modelling objectives. For example, by doubling your base cell size, and not changing the Quadtree levels, you will be doubling the cell sizes throughout your model. Provided this doubling of cell sizes does not conflict with other objectives (remember, only one velocity is calculated per cell face, so a consequence of doubling cell size is to reduce the quality of velocity based outputs such as hazard), whether or not it is okay to increase the base cell size simply comes down to whether results convergence can be proven. By results convergence we mean you can increase (or decrease) your cell sizes without unacceptably changing the model results. If you do see a significant change in results that is considered unacceptable, then you need to possibly make your cell sizes smaller (not larger!). A well-designed model grid is one where you can decrease cells sizes without seeing an unacceptable change in results. SGS will greatly help with achieving results convergence and very much increase your ability to use a coarser base cell size or coarsen up parts of your model.
	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 be (1/2)<sup>8</sup>, or 1/256, of your base cell size.		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 grid noting that the 2020-01 release of TUFLOW allows for up to 9 levels of nesting, so your smallest cell size can be (1/2)<sup>8</sup>, or 1/256, of your base cell size.
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	=With SGS, can I have less cells across the main channel than is recommended for HPC without SGS?=		=With SGS, can I have less cells across the main channel than is recommended for HPC without SGS?=
	You can get accurate results with less 2D cells across the width of flow if you use SGS with a high resolution DEM or preferably a TIN that is based on accurate field surveys. The quality of the channel’s bathymetry is critical to the quality of the modelling, ideally the TIN would extend to dry ground if the data are available to avoid bumpy terrain data from the DEM along the edge of the TIN within the channel (this is even more so for concrete trapezoidal channels). Benchmarking to check similar results are achieved using a finer ~~mesh~~ is strongly recommended to demonstrate the coarser ~~mesh~~ is not affecting results.		You can get accurate results with less 2D cells across the width of flow if you use SGS with a high resolution DEM or preferably a TIN that is based on accurate field surveys. The quality of the channel’s bathymetry is critical to the quality of the modelling, ideally the TIN would extend to dry ground if the data are available to avoid bumpy terrain data from the DEM along the edge of the TIN within the channel (this is even more so for concrete trapezoidal channels). Benchmarking to check similar results are achieved using a finer grid resolution is strongly recommended to demonstrate the coarser grid resolution is not affecting results.
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	Where this behaviour is observed, a useful sensitivity test is to refine the channel so that a single Quadtree cell size is used across the channel bed and along the section of interest. The Quadtree transitions can then be moved outside the channel, for example onto the overbank or floodplain areas where the hydraulic gradients are less critical.		Where this behaviour is observed, a useful sensitivity test is to refine the channel so that a single Quadtree cell size is used across the channel bed and along the section of interest. The Quadtree transitions can then be moved outside the channel, for example onto the overbank or floodplain areas where the hydraulic gradients are less critical.

	This does not mean that every channel must use the finest cell size everywhere. However, for narrow channels, low flow conditions, bends, crossings, or locations where water level and velocity results are important, maintaining a consistent cell size within the channel can help avoid artificial changes in conveyance and improve results convergence. As always, the final cell size should be checked by benchmarking against a finer ~~mesh~~ to confirm that the adopted resolution is not adversely affecting the results.		This does not mean that every channel must use the finest cell size everywhere. However, for narrow channels, low flow conditions, bends, crossings, or locations where water level and velocity results are important, maintaining a consistent cell size within the channel can help avoid artificial changes in conveyance and improve results convergence. As always, the final cell size should be checked by benchmarking against a finer grid to confirm that the adopted resolution is not adversely affecting the results.

	= How does memory usage compare between Quadtree and HPC? =		= How does memory usage compare between Quadtree and HPC? =

Teegan.Burke: /* Should the same model using Quadtree with smaller cell count always be faster than HPC? */

2026-06-16T01:10:53Z

Should the same model using Quadtree with smaller cell count always be faster than HPC?

← Older revision		Revision as of 11:10, 16 June 2026
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	=Should the same model using Quadtree with smaller cell count always be faster than HPC?=		=Should the same model using Quadtree with smaller cell count always be faster than HPC?=
	Not necessarily. By default, running a model with a single level Quadtree ~~mesh~~ identical to a HPC grid is always slower, on average 20%. Quadtree really starts to have major benefits once there is at least three levels of cell size and judicious refinement resulting in 80% of cell count reduction. ~~There~~ is ~~one~~ ~~major~~ ~~benefit~~ of ~~Quadtree~~ ~~regardless~~ in that in ~~nearly~~ ~~always~~ ~~has~~ a ~~much~~ ~~smaller~~ ~~GPU~~ ~~RAM~~ ~~memory~~ ~~footprint~~ ~~because~~ ~~unused~~ 2D ~~cells~~ ~~within~~ ~~the~~ ~~bounding~~ ~~rectangle~~ ~~used~~ by a ~~HPC~~ grid ~~are~~ ~~not~~ ~~stored~~ in ~~memory~~ ~~whereas~~ ~~for~~ a ~~grid~~ ~~they~~ ~~are~~.		Not necessarily. By default, running a model with a single level Quadtree grid identical to a HPC grid is always slower, on average 20%. Quadtree really starts to have major benefits once there is at least three levels of cell size and judicious refinement resulting in 80% of cell count reduction. The speed of a simulation is also affected by the adaptive timestepping - introducing cell sizes that are smaller in the refined Quadtree grid than those in the HPC model will require a smaller minimum timestep, which gets applied to the whole model. How a Quadtree grid interacts with hydraulic controls may also cause changes to the model’s adaptive timestepping.
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	This does not mean that every channel must use the finest cell size everywhere. However, for narrow channels, low flow conditions, bends, crossings, or locations where water level and velocity results are important, maintaining a consistent cell size within the channel can help avoid artificial changes in conveyance and improve results convergence. As always, the final cell size should be checked by benchmarking against a finer mesh to confirm that the adopted resolution is not adversely affecting the results.		This does not mean that every channel must use the finest cell size everywhere. However, for narrow channels, low flow conditions, bends, crossings, or locations where water level and velocity results are important, maintaining a consistent cell size within the channel can help avoid artificial changes in conveyance and improve results convergence. As always, the final cell size should be checked by benchmarking against a finer mesh to confirm that the adopted resolution is not adversely affecting the results.

			= How does memory usage compare between Quadtree and HPC? =
	<br>
			A common benefit of Quadtree is that it nearly always has a much smaller GPU RAM memory footprint, when comparing a single level Quadtree grid to an identical HPC grid. This is due to Quadtree only storing a grid of active cells (within the code boundary) compared with HPC and Classic single fixed grid models, that use the domain’s bounding rectangle to allocate memory, which may include large areas of inactive (redundant) cells and consume memory.

			Judicious refinement and using larger cells in non-focus areas in Quadtree will decrease the number of active cells and therefore the RAM requirement. When using Quadtree, the .qcf command <font color="blue"><tt>Quadtree Mesh Processing Method </tt></font> <font color="red"><tt>==</tt></font><tt> Memory Efficient</tt> can reduce the amount of CPU RAM memory, although the model initialisation might take longer.

			= How do results compare between Quadtree and HPC? =
			The Quadtree and HPC solvers use the same fundamental solution scheme, though require slightly different code implementations to accommodate for the fact both solvers are applied to differing grid designs (HPC operating on a fixed grid, whereas Quadtree uses a grid of varying cell sizes). For example, slightly different code is required by the Quadtree solver for line geometry topography update (Read GIS Z Shape) cell selection rules, using a "spider web" approach, rather than the HPC solver's "cross-hair" approach.

			As a result of these subtle differences between the HPC single grid and Quadtree solvers, they produce near though not identical results if both are used for the same single grid (same cell size, orientation and extent).<br>

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Abrar.Alttahir: /* What is best practice when applying Quadtree cell sizes within a channel? */

2026-06-12T01:29:41Z

What is best practice when applying Quadtree cell sizes within a channel?

← Older revision		Revision as of 11:29, 12 June 2026
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	Where this behaviour is observed, a useful sensitivity test is to refine the channel so that a single Quadtree cell size is used across the channel bed and along the section of interest. The Quadtree transitions can then be moved outside the channel, for example onto the overbank or floodplain areas where the hydraulic gradients are less critical.		Where this behaviour is observed, a useful sensitivity test is to refine the channel so that a single Quadtree cell size is used across the channel bed and along the section of interest. The Quadtree transitions can then be moved outside the channel, for example onto the overbank or floodplain areas where the hydraulic gradients are less critical.

	This does not mean that every channel must use the finest cell size everywhere. However, for narrow channels, low flow conditions, bends, crossings, or locations where water level and velocity results are important, maintaining a consistent cell size within the channel can help avoid artificial changes in conveyance and improve results convergence. As always, the final cell size should be checked by benchmarking against a finer mesh to confirm that the adopted resolution is not adversely affecting the results.~~<br>~~		This does not mean that every channel must use the finest cell size everywhere. However, for narrow channels, low flow conditions, bends, crossings, or locations where water level and velocity results are important, maintaining a consistent cell size within the channel can help avoid artificial changes in conveyance and improve results convergence. As always, the final cell size should be checked by benchmarking against a finer mesh to confirm that the adopted resolution is not adversely affecting the results.

			<br>

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Abrar.Alttahir at 01:29, 12 June 2026

2026-06-12T01:29:05Z

← Older revision		Revision as of 11:29, 12 June 2026
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	=How to decide between using SGS, Quadtree and 1D open channel?=		=How to decide between using SGS, Quadtree and 1D open channel?=
	The number of 2D cells across channel is critical. Based on the <u>[https://downloads.tuflow.com/_archive/Australian_Rainfall_Runoff_Project15_TwoDimensional_Modelling_DraftReport.pdf ARR 2D modelling guidelines]</u> the aim is to have 6 cells across the river, creek or drain (if not using SGS). However, use of SGS does allow use of a much fewer cells across a waterway whilst preserving the total conveyance of the waterway. The only downside of SGS is that if the velocity distribution across the waterway, or for example the super-elevation of the water surface around a bend, is important then sufficient number of cells is needed to accurately reproduce the velocity distribution or water level variation. Quadtree grid can be useful for this purpose.		The number of 2D cells across channel is critical. Based on the <u>[https://downloads.tuflow.com/_archive/Australian_Rainfall_Runoff_Project15_TwoDimensional_Modelling_DraftReport.pdf ARR 2D modelling guidelines]</u> the aim is to have 6 cells across the river, creek or drain (if not using SGS). However, use of SGS does allow use of a much fewer cells across a waterway whilst preserving the total conveyance of the waterway. The only downside of SGS is that if the velocity distribution across the waterway, or for example the super-elevation of the water surface around a bend, is important then sufficient number of cells is needed to accurately reproduce the velocity distribution or water level variation. Quadtree grid can be useful for this purpose.
	<br>

	=How can I convert 1D open channel into Quadtree?=		=How can I convert 1D open channel into Quadtree?=
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	# Read the 2d_zsh line and points cross section inputs into the model using the "Read GIS Z Shape" command in the tgc file. <br>		# Read the 2d_zsh line and points cross section inputs into the model using the "Read GIS Z Shape" command in the tgc file. <br>
	# Read the 2d_zsh merge polygon into the model using the "Create TIN Zpts Write TIN" command in the tgc file.		# Read the 2d_zsh merge polygon into the model using the "Create TIN Zpts Write TIN" command in the tgc file.

	<br>
			= What is best practice when applying Quadtree cell sizes within a channel? =
			It is generally recommended to use a consistent Quadtree cell size within a channel, particularly where the channel hydraulics are an important focus of the model results.

			Although SGS can preserve the channel conveyance using sub-grid elevation sampling, TUFLOW still calculates one velocity per cell face. If the Quadtree cell size changes within the channel bed, or across the active flow width of the channel, this can locally change the effective flow width represented by the 2D cells. In some cases this may produce recirculation, bumpy water levels, or inconsistent water level results between scenarios, even where the channel geometry and flow are similar.

			Where this behaviour is observed, a useful sensitivity test is to refine the channel so that a single Quadtree cell size is used across the channel bed and along the section of interest. The Quadtree transitions can then be moved outside the channel, for example onto the overbank or floodplain areas where the hydraulic gradients are less critical.

			This does not mean that every channel must use the finest cell size everywhere. However, for narrow channels, low flow conditions, bends, crossings, or locations where water level and velocity results are important, maintaining a consistent cell size within the channel can help avoid artificial changes in conveyance and improve results convergence. As always, the final cell size should be checked by benchmarking against a finer mesh to confirm that the adopted resolution is not adversely affecting the results.<br>

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Abrar.Alttahir: /* My SGS Sample Target Distance is set to 1 metre, but the model output resolution is larger than 1 metre. Why? */

2025-04-30T07:20:30Z

My SGS Sample Target Distance is set to 1 metre, but the model output resolution is larger than 1 metre. Why?

← Older revision		Revision as of 17:20, 30 April 2025
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	=My SGS Sample Target Distance is set to 1 metre, but the model output resolution is larger than 1 metre. Why?=		=My SGS Sample Target Distance is set to 1 metre, but the model output resolution is larger than 1 metre. Why?=
	The SGS Sample Target Distance sets the spacing at which elevations are sampled within each 2D cell, not the ~~model~~ ~~output~~ resolution. SGS works by sampling the terrain at a ~~smaller~~ scale ~~inside~~ each computational cell to improve how topography is represented~~, especially across cell faces~~. The hydraulic behaviour across the cell is determined using these smaller elevation samples, without changing the size of the 2D cells themselves.~~<br>~~		The SGS Sample Target Distance sets the spacing at which elevations are sampled within each 2D cell. It does not define the 2D cell resolution. SGS works by sampling the terrain at a finer scale within each computational cell to improve how topography is represented. The hydraulic behaviour across the cell is determined using these smaller elevation samples, without changing the actual size of the 2D cells themselves.

	For example, ~~using~~ an 8 m cell size and a 2 m SGS Sample Target Distance, the DEM is sampled at 5 points across each face. This results in 25 elevation points being used to define the ~~volume~~ versus elevation relationship ~~within~~ ~~the~~ 2D ~~cell~~, and ~~the~~ ~~same~~ points ~~are~~ used ~~along each face~~ to define the ~~conveyance~~ versus elevation relationship. ~~This~~ ~~helps~~ ~~better~~ ~~capture~~ more ~~details~~ from input topography while maintaining the ~~chosen input~~ grid size.~~<br>~~		For example, with an 8 m cell size and a 2 m SGS Sample Target Distance, the DEM is sampled at 5 points across each cell face. This results in 5 elevation points being used to define the flow area versus elevation relationship at each 2D face, and 25 elevation points used to define the volume versus elevation relationship within the 2D cell. This allows more detail from the input topography to be captured while maintaining the original grid size.
	If a smaller output resolution (such as for velocity or velocity related outputs like hazards) is required, this is controlled by the 2D cell size and/or the nesting level of the Quadtree polygon, not by the SGS sampling settings. SGS improves the internal hydraulic calculations within each cell but does not alter the 2D grid or the output resolution.

	<br>
			However, TUFLOW still stores only one velocity value per face and one depth value per cell. This means the resolution of mesh based outputs (e.g. XMDF) and standard grid outputs (e.g. ASC, FLT, TIF) is not reduced to match the SGS Sample Target Distance.

			When SGS is enabled, users can output high resolution water level and depth outputs based on the cell centre water level and the sampled sub-grid elevations (see: [[TUFLOW HR Output]]). However, if higher resolution velocity or velocity derived outputs (such as hazard) are required, this must be achieved by reducing the 2D cell size, not by adjusting the SGS sampling settings.

	=Does using SGS increase model runtimes?=		=Does using SGS increase model runtimes?=

Pavlina Monhartova at 03:00, 29 April 2025

2025-04-29T03:00:50Z

Show changes

Pavlina Monhartova: /* My SGS Sample Target Distance is set to 1 metre, but the model output resolution is larger than 1 metre. Why? */

2025-04-29T02:55:41Z

My SGS Sample Target Distance is set to 1 metre, but the model output resolution is larger than 1 metre. Why?

← Older revision		Revision as of 12:55, 29 April 2025
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	# Read the 2d_zsh merge polygon into the model using the "Create TIN Zpts Write TIN" command in the tgc file.		# Read the 2d_zsh merge polygon into the model using the "Create TIN Zpts Write TIN" command in the tgc file.
	=My SGS Sample Target Distance is set to 1 metre, but the model output resolution is larger than 1 metre. Why?=		=My SGS Sample Target Distance is set to 1 metre, but the model output resolution is larger than 1 metre. Why?=
	The SGS Sample Target Distance sets the spacing at which elevations are sampled within each 2D cell, not the model output resolution.		The SGS Sample Target Distance sets the spacing at which elevations are sampled within each 2D cell, not the model output resolution. SGS works by sampling the terrain at a smaller scale inside each computational cell to improve how topography is represented, especially across cell faces. The hydraulic behaviour across the cell is determined using these smaller elevation samples, without changing the size of the 2D cells themselves.<br>
		⚫	For example, using an 8 m cell size and a 2 m SGS Sample Target Distance, the DEM is sampled at 5 points across each face. This results in 25 elevation points being used to define the volume versus elevation relationship within the 2D cell, and the same points are used along each face to define the conveyance versus elevation relationship. This helps better capture more details from input topography while maintaining the chosen input grid size.<br>

		⚫	If a smaller output resolution (such as for velocity or velocity related outputs like hazards) is required, this is controlled by the 2D cell size and/or the nesting level of the Quadtree polygon, not by the SGS sampling settings. SGS improves the internal hydraulic calculations within each cell but does not alter the 2D grid or the output resolution.
	SGS (Sub Grid Sampling) works by sampling the terrain at a smaller scale inside each computational cell to improve how topography is represented, especially across cell faces. The hydraulic behaviour across the cell is determined using these smaller elevation samples, without changing the size of the 2D cells themselves.

⚫	For example, using an 8 m cell size and a 2 m SGS Sample Target Distance, the DEM is sampled at 5 points across each face. This results in 25 elevation points being used to define the volume versus elevation relationship within the 2D cell, and the same points are used along each face to define the conveyance versus elevation relationship. This helps better capture more details from input topography while maintaining the chosen input grid size.

⚫	If a smaller output resolution (such as for velocity or ~~depth~~ outputs) is required, this is controlled by the 2D cell size and the nesting level of the Quadtree polygon, not by the SGS sampling settings. SGS improves the internal hydraulic calculations within each cell but does not alter the 2D grid or the output resolution.

	Note SGS provides the most benefit when the input DEM resolution is significantly smaller than the 2D cell size. If the DEM is similar to or coarser than the 2D cell size, SGS offers little improvement.
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Abrar.Alttahir at 02:38, 29 April 2025

2025-04-29T02:38:00Z

← Older revision		Revision as of 12:38, 29 April 2025
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	# Digitise a 2d_zsh merge polygon around the 2d_zsh lines and points that define the open channel. Tutorial Module 2 provides instructions on how to create this 2d_zsh merge polygon: [[Tutorial_M02#Part_2_-_Other_Topographic_Updates \|Tutorial M02 Part2 - Other Topographic Updates]] <br>		# Digitise a 2d_zsh merge polygon around the 2d_zsh lines and points that define the open channel. Tutorial Module 2 provides instructions on how to create this 2d_zsh merge polygon: [[Tutorial_M02#Part_2_-_Other_Topographic_Updates \|Tutorial M02 Part2 - Other Topographic Updates]] <br>
	# Read the 2d_zsh line and points cross section inputs into the model using the "Read GIS Z Shape" command in the tgc file. <br>		# Read the 2d_zsh line and points cross section inputs into the model using the "Read GIS Z Shape" command in the tgc file. <br>
	# Read the 2d_zsh merge polygon into the model using the "Create TIN Zpts Write TIN" command in the tgc file. ~~<br>~~		# Read the 2d_zsh merge polygon into the model using the "Create TIN Zpts Write TIN" command in the tgc file.
			=My SGS Sample Target Distance is set to 1 metre, but the model output resolution is larger than 1 metre. Why?=
			The SGS Sample Target Distance sets the spacing at which elevations are sampled within each 2D cell, not the model output resolution.

			SGS (Sub Grid Sampling) works by sampling the terrain at a smaller scale inside each computational cell to improve how topography is represented, especially across cell faces. The hydraulic behaviour across the cell is determined using these smaller elevation samples, without changing the size of the 2D cells themselves.

			For example, using an 8 m cell size and a 2 m SGS Sample Target Distance, the DEM is sampled at 5 points across each face. This results in 25 elevation points being used to define the volume versus elevation relationship within the 2D cell, and the same points are used along each face to define the conveyance versus elevation relationship. This helps better capture more details from input topography while maintaining the chosen input grid size.

			If a smaller output resolution (such as for velocity or depth outputs) is required, this is controlled by the 2D cell size and the nesting level of the Quadtree polygon, not by the SGS sampling settings. SGS improves the internal hydraulic calculations within each cell but does not alter the 2D grid or the output resolution.

			Note SGS provides the most benefit when the input DEM resolution is significantly smaller than the 2D cell size. If the DEM is similar to or coarser than the 2D cell size, SGS offers little improvement.
	<br>		<br>
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Pavlina Monhartova: /* When should I use a combination of SGS and Quadtree? */

2024-08-08T02:22:46Z

When should I use a combination of SGS and Quadtree?

← Older revision		Revision as of 12:22, 8 August 2024
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	=When should I use a combination of SGS and Quadtree?=		=When should I use a combination of SGS and Quadtree?=
	Based on the benchmarking so far there seems to be little reason not to use SGS with Quadtree. The combination of SGS and Quadtree is particularly powerful for large models that require higher resolution in the focus area as this allows much larger cells away from the focus area without affecting the results.<br>		Based on the benchmarking so far there seems to be little reason not to use SGS with Quadtree. The combination of SGS and Quadtree is particularly powerful for large models that require higher resolution in the focus area as this allows much larger cells away from the focus area without affecting the results.<br>
	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, substantially different results can be seen due to SGS picking up a lot more detail of the terrain within a 2D cell. Using Quadtree with SGS, and by using coarser cells, much better results and good results convergence can be achieved~~. 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 it~~.<br>		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, substantially different results can be seen due to SGS picking up a lot more detail of the terrain within a 2D cell. Using Quadtree with SGS, and by using coarser cells, much better results and good results convergence can be achieved.<br>
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Abigail.lillo at 00:03, 25 January 2024

2024-01-25T00:03:00Z

← Older revision		Revision as of 10:03, 25 January 2024
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	=How to decide between using SGS, Quadtree and 1D open channel?=		=How to decide between using SGS, Quadtree and 1D open channel?=
	The number of 2D cells across channel is critical. Based on the <u>[https://downloads.tuflow.com/_archive/Australian_Rainfall_Runoff_Project15_TwoDimensional_Modelling_DraftReport.pdf ARR 2D modelling guidelines]</u> the aim is to have 6 cells across the river, creek or drain (if not using SGS). However, use of SGS does allow use of a much fewer cells across a waterway whilst preserving the total conveyance of the waterway. The only downside of SGS is that if the velocity distribution across the waterway, or for example the super-elevation of the water surface around a bend, is important then sufficient number of cells is needed to accurately reproduce the velocity distribution or water level variation. Quadtree grid can be useful for this purpose.		The number of 2D cells across channel is critical. Based on the <u>[https://downloads.tuflow.com/_archive/Australian_Rainfall_Runoff_Project15_TwoDimensional_Modelling_DraftReport.pdf ARR 2D modelling guidelines]</u> the aim is to have 6 cells across the river, creek or drain (if not using SGS). However, use of SGS does allow use of a much fewer cells across a waterway whilst preserving the total conveyance of the waterway. The only downside of SGS is that if the velocity distribution across the waterway, or for example the super-elevation of the water surface around a bend, is important then sufficient number of cells is needed to accurately reproduce the velocity distribution or water level variation. Quadtree grid can be useful for this purpose.

			=How can I convert 1D open channel into Quadtree?=
			Follow these steps to create a TIN from ESTRY cross section inputs that can be read into a Quadtree model: <br>
			# Use the following script that has been developed to convert ESTRY cross sections inputs into 2d_zsh line and points: https://gitlab.com/tuflow-user-group/tuflow/data-pre-processing/estry_to_zsh <br>
			# Digitise a 2d_zsh merge polygon around the 2d_zsh lines and points that define the open channel. Tutorial Module 2 provides instructions on how to create this 2d_zsh merge polygon: [[Tutorial_M02#Part_2_-_Other_Topographic_Updates \|Tutorial M02 Part2 - Other Topographic Updates]] <br>
			# Read the 2d_zsh line and points cross section inputs into the model using the "Read GIS Z Shape" command in the tgc file. <br>
			# Read the 2d_zsh merge polygon into the model using the "Create TIN Zpts Write TIN" command in the tgc file. <br>


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