Varying the resolution of a 2D mesh

Step-by-step guide

Points to consider regarding resolution of 2D mesh:

• Higher resolution mesh more accurately represents underlying topography, but takes longer to run than lower resolution mesh.
• Always a compromise between detail and speed of simulation and analysis.
• 2D results can become extremely large, with result points in the millions.
• Bigger models require better hardware, so model limit may be available hardware.
• Tools to help manage model size include spatially varying mesh size and employing terrain-sensitive meshing.

To demonstrate the impact of varying mesh detail on a model simulation time, these three models are identical, except for variations in mesh resolution:

Rural model: Uses "rural" sizes of 250m2 to 75m2.

Urban model:

• Uses "urban" sizes of 100m2 and 25m2, so likely contains two to three times as many elements.
• Run time increased from 15 minutes to 40 minutes.

Hybrid model:

• Urban resolution applied only to area of interest, with rural resolution applied outside that area.
• Run time increase to 17 minutes less than half the time of urban resolution model.
• Hybrid model allows for enhanced detail where required, with only modest impact on simulation speed.

For managing the mesh size, Terrain-sensitive meshing can be used to increase the resolution of a 2D mesh in areas that have a large variation in height, without increasing the number of elements in relatively flat areas of the same zone.

1. From the Database, double-click 2D River Model to open the network on the GeoPlan.

Note: A 2D mesh has already been created for this model.

To apply the terrain-sensitive meshing:

2. From the Scenarios toolbar, click Create scenario.

3. In the Create new scenario popup, add a New Scenario Name of “Terrain meshing”.
4. Click OK.
5. Double-click an area of the 2D zone again.
6. From the Multiple Selection popup, select 2D Zone.
7. Click OK.
8. In the Properties window, enable Terrain-sensitive meshing.

An inline validation warning is generated, as mesh parameters have changed since meshing was last performed.

NOTE: Terrain-sensitive meshing parameters are specified for the entire 2D zone and include the minimum element area specified for the 2D zone. These parameters cannot be set for individual mesh zones.

9. From the Model menu, select Meshing > Mesh 2D Zones.
10. Drag the 2D Ground Model from the Database into the Mesh 2D zones dialog box and drop it onto the Ground Model group box.
11. Click OK to mesh the 2D Zone.
12. In the Schedule Job(s) popup, click OK.

13. From the Window menu, select Job control window to display the status of the mesh job and some information on the generation.

IMPORTANT: The mesh generation process samples the ground model in each candidate triangle. If the range of heights within the triangle exceeds the maximum height variation specified for the 2D Zone, then the triangle is split, increasing the resolution of the mesh in areas where terrain height varies rapidly.

Once the mesh has completed, load it into the model:

14. From the Job control window, click the Mesh ready status.
15. In the Manage mesh results window, click Load mesh to generate the terrain-sensitive meshing.

16. Click Close.

NOTE: The mesh is significantly different from the one generated without terrain-sensitive meshing, most noticeably along the steep sides, but also along the bed of the channel.

17. Explore the changes in the mesh, using the ground model for comparison.
18. When finished, click Validate to make sure there are no errors in the Base and Terrain meshing networks.
19. Click Commit changes to database to save changes.
20. Add a comment, such as “Terrain-sensitive meshing”.
21. Click OK.