TUFLOW 2D Cell Size Selection - Revision history

Anne.Kolega: /* Test Case 2 - Results */ i.e.

2026-04-02T02:07:40Z

Test Case 2 - Results: i.e.

← Older revision		Revision as of 12:07, 2 April 2026
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	== Test Case 2 - Results ==		== Test Case 2 - Results ==
	The cell convergence test results are summarised in the table below. They have been presented in a histogram form. The histograms are calculated by subtracting the peak (maximum) flood level result raster grid for the larger cell resolution simulation from the 10ft cell resolution result. The result difference has been sampled at every pixel in the raster datasets where flood information is present in both result files (ie. they overlap).<br>		The cell convergence test results are summarised in the table below. They have been presented in a histogram form. The histograms are calculated by subtracting the peak (maximum) flood level result raster grid for the larger cell resolution simulation from the 10ft cell resolution result. The result difference has been sampled at every pixel in the raster datasets where flood information is present in both result files (i.e. they overlap).<br>

	{\| class="wikitable "		{\| class="wikitable "

Chris Huxley: /* How much will my runtime change when changing cell size? */

2022-05-30T01:46:56Z

How much will my runtime change when changing cell size?

← Older revision		Revision as of 11:46, 30 May 2022
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	However, there is one last and important caveat when refining cell size. The explicit algorithm used by TUFLOW HPC requires that the timestep is also subject to Peclet (diffusion) number control, not just Courant (velocity) number control. As Peclet number is inversely proportional to cell size squared, whereas Courant number is just inversely proportional to cell size, the model will transition from Courant number control to Peclet number control once the cell size becomes small enough. This means that the factor of 8 on time for a halving of cell size will eventually become a factor of 16. This was not a feature of TUFLOW Classic. In general, we do not recommend refining cell size well into the region where timestep control is completely dominated by the Peclet number (the Nd column in the HPC output). The flows in the primary channels are usually well converged by this point, and the 2D map outputs may not reveal a lot of additional detail in the velocity fields.<br>		However, there is one last and important caveat when refining cell size. The explicit algorithm used by TUFLOW HPC requires that the timestep is also subject to Peclet (diffusion) number control, not just Courant (velocity) number control. As Peclet number is inversely proportional to cell size squared, whereas Courant number is just inversely proportional to cell size, the model will transition from Courant number control to Peclet number control once the cell size becomes small enough. This means that the factor of 8 on time for a halving of cell size will eventually become a factor of 16. This was not a feature of TUFLOW Classic. In general, we do not recommend refining cell size well into the region where timestep control is completely dominated by the Peclet number (the Nd column in the HPC output). The flows in the primary channels are usually well converged by this point, and the 2D map outputs may not reveal a lot of additional detail in the velocity fields.<br>
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Chris Huxley: /* Common Questions Answered (FAQ)= */

2022-05-30T01:45:31Z

Common Questions Answered (FAQ)=

← Older revision		Revision as of 11:45, 30 May 2022
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	= Common Questions Answered (FAQ)==		= Common Questions Answered (FAQ)=
	==How much will my runtime change when changing cell size?==		==How much will my runtime change when changing cell size?==
	Solution time for a model is dependent on a number of factors: the solution scheme used, the size and duration of the model, the nature of the hydraulics within the model, the model features selected, and the hardware that it is executed on. However, once a model is developed at a relatively coarse resolution, there are some guidelines that can assist with predicting what the solve time will be as the resolution is refined. These can be useful for estimating whether the refined model will run overnight or need a few days.<br>		Solution time for a model is dependent on a number of factors: the solution scheme used, the size and duration of the model, the nature of the hydraulics within the model, the model features selected, and the hardware that it is executed on. However, once a model is developed at a relatively coarse resolution, there are some guidelines that can assist with predicting what the solve time will be as the resolution is refined. These can be useful for estimating whether the refined model will run overnight or need a few days.<br>

Chris Huxley: /* How much will my runtime change when changing cell size?= */

2022-05-30T01:35:38Z

How much will my runtime change when changing cell size?=

← Older revision		Revision as of 11:35, 30 May 2022
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	= Common Questions Answered (FAQ)==		= Common Questions Answered (FAQ)==
	=How much will my runtime change when changing cell size?==		==How much will my runtime change when changing cell size?==
	Solution time for a model is dependent on a number of factors: the solution scheme used, the size and duration of the model, the nature of the hydraulics within the model, the model features selected, and the hardware that it is executed on. However, once a model is developed at a relatively coarse resolution, there are some guidelines that can assist with predicting what the solve time will be as the resolution is refined. These can be useful for estimating whether the refined model will run overnight or need a few days.<br>		Solution time for a model is dependent on a number of factors: the solution scheme used, the size and duration of the model, the nature of the hydraulics within the model, the model features selected, and the hardware that it is executed on. However, once a model is developed at a relatively coarse resolution, there are some guidelines that can assist with predicting what the solve time will be as the resolution is refined. These can be useful for estimating whether the refined model will run overnight or need a few days.<br>

Chris Huxley: /* How much will my runtime change when changing cell size? */

2022-05-30T01:35:18Z

How much will my runtime change when changing cell size?

← Older revision		Revision as of 11:35, 30 May 2022
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			= Common Questions Answered (FAQ)==
	==How much will my runtime change when changing cell size?==		=How much will my runtime change when changing cell size?==
	Solution time for a model is dependent on a number of factors: the solution scheme used, the size and duration of the model, the nature of the hydraulics within the model, the model features selected, and the hardware that it is executed on. However, once a model is developed at a relatively coarse resolution, there are some guidelines that can assist with predicting what the solve time will be as the resolution is refined. These can be useful for estimating whether the refined model will run overnight or need a few days.<br>		Solution time for a model is dependent on a number of factors: the solution scheme used, the size and duration of the model, the nature of the hydraulics within the model, the model features selected, and the hardware that it is executed on. However, once a model is developed at a relatively coarse resolution, there are some guidelines that can assist with predicting what the solve time will be as the resolution is refined. These can be useful for estimating whether the refined model will run overnight or need a few days.<br>

Chris Huxley at 01:34, 30 May 2022

2022-05-30T01:34:25Z

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 * Detailed integrated 1D/2D urban modelling may adopt the 15ft (4.6m) or slightly finer 12ft (3.7m) cell resolution if the cumulative simulation time for all model events/scenarios does not adversely impact the project timeline. This timeframe consideration is particularly relevant in locations such as Australia where an ensemble methodology is currently replacing single event approaches as a way to address uncertainty associated with hydrologic assumptions.
 * Broad-scale quick run simulation for urban flood forecasting purposes may adopt a coarser 20ft cell resolution or investigate the suitability of a 25ft cell resolution to achieve a quicker simulation turnaround time.<br>
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Chris Huxley at 04:49, 22 June 2021

2021-06-22T04:49:26Z

← Older revision		Revision as of 14:49, 22 June 2021
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	* Detailed integrated 1D/2D urban modelling may adopt the 15ft (4.6m) or slightly finer 12ft (3.7m) cell resolution if the cumulative simulation time for all model events/scenarios does not adversely impact the project timeline. This timeframe consideration is particularly relevant in locations such as Australia where an ensemble methodology is currently replacing single event approaches as a way to address uncertainty associated with hydrologic assumptions.		* Detailed integrated 1D/2D urban modelling may adopt the 15ft (4.6m) or slightly finer 12ft (3.7m) cell resolution if the cumulative simulation time for all model events/scenarios does not adversely impact the project timeline. This timeframe consideration is particularly relevant in locations such as Australia where an ensemble methodology is currently replacing single event approaches as a way to address uncertainty associated with hydrologic assumptions.
	* Broad-scale quick run simulation for urban flood forecasting purposes may adopt a coarser 20ft cell resolution or investigate the suitability of a 25ft cell resolution to achieve a quicker simulation turnaround time.<br>		* Broad-scale quick run simulation for urban flood forecasting purposes may adopt a coarser 20ft cell resolution or investigate the suitability of a 25ft cell resolution to achieve a quicker simulation turnaround time.<br>
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			{{Tips Navigation
			\|uplink=[[ TUFLOW_Modelling_Guidance \| Back to TUFLOW Modelling Guidance]]
			}}

Pavlina Monhartova: /* Test Case 1 - Discussion */

2019-09-30T07:26:52Z

Test Case 1 - Discussion

← Older revision		Revision as of 17:26, 30 September 2019
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	{\| class="wikitable "		{\| class="wikitable "

	\| '''Cell Size''' \|\| '''Model Size <br> (Cell Count)'''\|\| '''Simulation Run Time <br> (seconds)''' \|\| '''Judgement of Convergence <br> (Yes / No)'''		\| '''Cell Size''' \|\| '''Model Size <br> (Cell Count)'''\|\| '''Simulation Run Time <br> (seconds)''' \|\| '''Judgement of Convergence <br> (Yes / No)''' \|\| '''Required GPU Memory <br> (GB)'''
	\|-		\|-
	\| 10m (33ft) \|\| 188,240 \|\| 284 \|\| Yes		\| 10m (33ft) \|\| 188,240 \|\| 284 \|\| Yes \|\| 0.15
	\|-		\|-
	\| 20m (66ft) \|\| 47,080 \|\| 98 \|\| Yes		\| 20m (66ft) \|\| 47,080 \|\| 98 \|\| Yes \|\| 0.04
	\|-		\|-
	\| 50m (164ft) \|\| 7,540 \|\| 32 \|\| Yes		\| 50m (164ft) \|\| 7,540 \|\| 32 \|\| Yes \|\| 0.01
	\|-		\|-
	\| 100m (328ft) \|\| 1,880 \|\| 15 \|\| Yes		\| 100m (328ft) \|\| 1,880 \|\| 15 \|\| Yes \|\| <0.01
	\|-		\|-
	\| 150m (492ft) \|\| 840 \|\| 10 \|\| No		\| 150m (492ft) \|\| 840 \|\| 10 \|\| No \|\| <0.01
	\|-		\|-
	\| 200m (656ft) \|\| 480 \|\| 9 \|\| No		\| 200m (656ft) \|\| 480 \|\| 9 \|\| No \|\| <0.01
	\|-		\|-
	\| 250m (820ft) \|\| 300 \|\| 7 \|\| No		\| 250m (820ft) \|\| 300 \|\| 7 \|\| No \|\| <0.01
	\|}		\|}
	Since simulation time is not a limiting factor in this case, selection of cell size only needs to consider the result accuracy. What is appropriate in terms of accuracy is influenced by the intended use of the model. For example, if lot scale assessment of inundation and structural damage risk is required, selection of the 10m (33ft) cell size may be appropriate to provide high resolution mapping of the velocity gradients. Alternatively, if the model is only intended to inform broad-scale inundation risk, the 50m (164ft) or 100m (328ft) cell size may be sufficient.		Since simulation time is not a limiting factor in this case, selection of cell size only needs to consider the result accuracy. What is appropriate in terms of accuracy is influenced by the intended use of the model. For example, if lot scale assessment of inundation and structural damage risk is required, selection of the 10m (33ft) cell size may be appropriate to provide high resolution mapping of the velocity gradients. Alternatively, if the model is only intended to inform broad-scale inundation risk, the 50m (164ft) or 100m (328ft) cell size may be sufficient.

Pavlina Monhartova: /* Test Case 2 - Discussion */

2019-09-30T06:11:27Z

Test Case 2 - Discussion

← Older revision		Revision as of 16:11, 30 September 2019
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	2m (6.6ft) to 5m (16.4ft). <br>		2m (6.6ft) to 5m (16.4ft). <br>
	{\| class="wikitable "		{\| class="wikitable "
	\| '''Cell Size''' \|\| '''Model Size <br> (Cell Count)'''\|\| '''Simulation Run Time <br> (hours)''' \|\| '''Judgement of Convergence <br> (Yes / No)'''		\| '''Cell Size''' \|\| '''Model Size <br> (Cell Count)'''\|\| '''Simulation Run Time <br> (hours)''' \|\| '''Judgement of Convergence <br> (Yes / No)''' \|\| '''Required GPU Memory <br> (GB)
	\|-		\|-
	\| 10ft (3.0m) \|\| 5,821,533 \|\| 9.2hr \|\| Yes		\| 10ft (3.0m) \|\| 5,821,533 \|\| 9.2hr \|\| Yes \|\| 1.74
	\|-		\|-
	\| 12ft (3.7m) \|\| 4,042,960 \|\| 7.5hr \|\| Yes		\| 12ft (3.7m) \|\| 4,042,960 \|\| 7.5hr \|\| Yes \|\| 1.20
	\|-		\|-
	\| 15ft (4.6m) \|\| 2,587,629 \|\| 4.4hr \|\| Yes		\| 15ft (4.6m) \|\| 2,587,629 \|\| 4.4hr \|\| Yes \|\| 0.77
	\|-		\|-
	\| 20ft (6.1m) \|\| 1,455,869 \|\| 1.8hr \|\| Yes		\| 20ft (6.1m) \|\| 1,455,869 \|\| 1.8hr \|\| Yes \|\| 0.44
	\|-		\|-
	\| 30ft (9.1m) \|\| 647,003 \|\| 0.65hr \|\| Mostly		\| 30ft (9.1m) \|\| 647,003 \|\| 0.65hr \|\| Mostly \|\| 0.20
	\|-		\|-
	\| 50ft (15.2m) \|\| 232,957 \|\| 0.23hr \|\| No		\| 50ft (15.2m) \|\| 232,957 \|\| 0.23hr \|\| No \|\| 0.07
	\|-		\|-
	\| 75ft (22.9m) \|\| 103,525 \|\| 0.08hr \|\| No		\| 75ft (22.9m) \|\| 103,525 \|\| 0.08hr \|\| No \|\| 0.03
	\|}		\|}

Chris Huxley at 02:53, 10 October 2018

2018-10-10T02:53:17Z

← Older revision		Revision as of 12:53, 10 October 2018
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	\| 15ft (4.6m) \|\| 2,587,629 \|\| 4.4hr \|\| Yes		\| 15ft (4.6m) \|\| 2,587,629 \|\| 4.4hr \|\| Yes
	\|-		\|-
	\| 20ft (6.1m) \|\| 1,455,869 \|\| 3.~~9hr~~ \|\| Yes		\| 20ft (6.1m) \|\| 1,455,869 \|\| 1.8hr \|\| Yes
	\|-		\|-
	\| 30ft (9.1m) \|\| 647,003 \|\| 1.~~3hr~~ \|\| Mostly		\| 30ft (9.1m) \|\| 647,003 \|\| 0.65hr \|\| Mostly
	\|-		\|-
	\| 50ft (15.2m) \|\| 232,957 \|\| 0.~~5hr~~ \|\| No		\| 50ft (15.2m) \|\| 232,957 \|\| 0.23hr \|\| No
	\|-		\|-
	\| 75ft (22.9m) \|\| 103,525 \|\| 0.~~3hr~~ \|\| No		\| 75ft (22.9m) \|\| 103,525 \|\| 0.08hr \|\| No
	\|}		\|}