Difference between revisions of "TUFLOW 2D Cell Size Selection"

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== Test Case 1 - Results ==
 
== Test Case 1 - Results ==
Test results have focused on reporting locations 4 and 5. Their location furthest downstream makes them most sensitive. Any divergence in result due to poor representation of the upstream topography will accumulate, amplifying the result difference associated with a change in cell resolution.  
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Test results have focused on reporting locations 4 and 5. Their location furthest downstream makes them most sensitive of all 7 reporting locations. Any divergence in result due to poor representation of the upstream topography will accumulate, amplifying the result difference associated with a change in cell resolution.  
  
 
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Revision as of 13:43, 16 August 2018

This page is currently under construction.

Introduction

This page of the TUFLOW Wiki discusses 2D cell size convergence. Cell size convergence refers to the tendency for model results to trend towards a common answer as cell size decreases. This behaviour occurs due to topographic features that influence the hydraulic flow behaviour better approximating reality as resolution increases. The series of creek cross-section images below demonstrate this. As model resolution increases from 20m to 5m the modelled topography progressively matches the real-world geometry more closely.
Mesh Converge XS 20m.pngMesh Converge XS 10m.pngMesh Converge XS 05m.png

Unfortunately it isn't practical for all models to be designed at an infinitely fine resolution due to the cost it has on simulation speed. Increasing a model resolution will make a simulation run slower. As a rule of thumb, halving the cell size in a model will typically increase the simulation run time by a factor 8. This is due to the number of cells increasing by a factor of (4) four and the necessity for a calculation timestep half that of the larger resolution, translating to (2) twice the number of calculations (4 x 2 = 8). The challenge for modellers is knowing what resolution is necessary to achieve results that are fit for purpose with sufficient accuracy. This Wiki page uses two test cases to discuss this topic.

Test Case 1

This test case has been sourced from the UK Environment Agency 2D Hydraulic Model Benchmark Test dataset. It is referred to as Test 5 in the Environment Agency (EA) dataset. The EA designed to simulate flood wave propagation down a river valley following the failure of a dam. The valley DEM is ~0.8 km by ~17 km and the valley slopes downstream on a slope of ~0.01 in its upper region, easing to ~0.001 at lower elevations. The model uses a single manning n value of 0.04 across the entire domain. The model topography, EA reporting points and the simulation inflow hydrograph is shown below.
Mesh Converge Model Description 001.png Mesh Converge Model Elevation 001.png

The model has a single inflow at the top of the catchment.
Mesh Converge Model Inflow 001.png
The water flows downhill through the respective EA reporting points. Reporting location 4 and 5 are at the bottom of the model, furthest from the inflow locations.
Mesh Converge SMS 0.5hr.png Mesh Converge SMS 1.5hr.png

The EA benchmark testing original assumed a 50m cell resolution. For the purpose of this assessment a range of cell sizes have been used to determine the impact changing grid resolution has on the model result. The following grid resolutions were used:

  • 10m resolution (188240 cell count)
  • 20m resolution (47080 cell count)
  • 50m resolution (7540 cell count)
  • 100m resolution (1880 cell count)
  • 150m resolution (840 cell count)
  • 200m resolution (480 cell count)

Test Case 1 - Results

Test results have focused on reporting locations 4 and 5. Their location furthest downstream makes them most sensitive of all 7 reporting locations. Any divergence in result due to poor representation of the upstream topography will accumulate, amplifying the result difference associated with a change in cell resolution.

Cell Size Location 4 Result Location 5 Result Mesh Resolution Figure
10m Mesh Converge Exg P4 010.png Mesh Converge Exg P5 010.png Mesh Converge Grid 010.png
20m Mesh Converge Exg P4 020.png Mesh Converge Exg P5 020.png Mesh Converge Grid 020.png
50m Mesh Converge Exg P4 050.png Mesh Converge Exg P5 050.png Mesh Converge Grid 050.png
100m Mesh Converge Exg P4 100.png Mesh Converge Exg P5 100.png Mesh Converge Grid 100.png
150m Mesh Converge Exg P4 150.png Mesh Converge Exg P5 150.png Mesh Converge Grid 150.png
200m Mesh Converge Exg P4 200.png Mesh Converge Exg P5 200.png Mesh Converge Grid 200.png
250m Mesh Converge Exg P4 250.png Mesh Converge Exg P5 250.png Mesh Converge Grid 250.png

Rural Assessment Result Discussion

Urbanised Catchment Scenario Assessment

Urban Model Description

Urban Assessment Results

Urban Assessment Result Discussion