Creating 2D Meshes

A 2D mesh of elements can be generated in areas for which more detailed analysis is required. The created mesh is used to model 2D flows.

Modelling 2D flows is much more computationally intensive than modelling 1D flows, therefore mesh generation should be limited to areas of particular interest.

During a 2D simulation, for each mesh element, depth, velocity and direction of flow will be calculated. Flooding from the collection system to the mesh is modelled at 2D node locations; surcharging of the 2D flooding manhole results in an inflow to the 2D mesh element in which it resides.

For details on setting up 2D zones and related objects prior to generating a mesh, see the 2D Mesh Generation Methodology topic.

Create a 2D mesh

1. Select the 2D zones to be meshed. All other zones, such as roughness zones or mesh zones, within the selected 2D zones will be included automatically.
2. Select Mesh 2D zones from the ModelMeshing menu. The InfoWorks or SWMM Mesh 2D Zones dialog will be displayed.
3. Optionally select the polygon and polyline objects representing voids, break lines and wall data. The options for each feature are:
   • Files: click to browse for a GIS file.
   • Layers: select a layer from the dropdown list if it is already loaded in the GeoPlan, or click to display the GIS Layer Control dialog.
   • Polygons/Polylines: select a polygon/polyline category from the dropdown list. Only polygons/general lines with the selected category will be used when creating the 2D mesh.
   • None: do not use any data to represent this feature.

   Note:Porous walls, and for InfoWorks networks only Base linear structures (2D), will always be used when creating the 2D mesh, regardless of dialog settings.

Note: If specifying data sources for break lines and walls (polyline types), both data sources must be either from GIS (files or layers) or from network polylines.

1. For InfoWorks networks only:

   Check the Lower 2D mesh element ground levels higher than adjacent bank levels option, to automatically adjust the ground level of mesh elements adjoining a bank section that is lower than the ground level calculated from the ground model. The ground level of the mesh element will be adjusted to match that of the bank.

   If this option is not selected, then the simulation engine automatically raises any portion of any bank that is below the adjoining mesh element ground level to the same level as the mesh element.

2. Select a ground model to be used to calculate the heights of mesh element vertices. Click to browse for a ground model file, or drag and drop a ground model item from the Explorer Window into the selection box.
3. Click OK.

A mesh job will be created for each zone selected for meshing, and the Schedule Job(s) dialog is displayed to allow you to select where and when to run the jobs.

The progress of the mesh jobs is displayed in the Job Control window, showing the network, zone and scenario that are being meshed and the current status of the job. Mesh jobs are queued along with simulation jobs and can be reordered, rescheduled or cancelled in exactly the same way.

Mesh results are not loaded into the zone(s) in the network immediately on completion of the job (a popup message will be displayed briefly when mesh results are ready to be imported). To view the mesh log for a job, click in the Status cell of the mesh job in the Job Control window or select Meshing | Load mesh job results from the Model menu to display the Manage Mesh Results dialog.

Manage mesh results

The Manage Mesh Results dialog is used to mange mesh results. It can be used to view the log for a mesh job, or to load successfully completed mesh jobs to a 2D zone(s).

To display the dialog, click in the Status cell of the mesh job in the Job Control window or select MeshingLoad mesh job results from the Model menu.

To load mesh results to a 2D zone, select the mesh job from the Manage Mesh Results dialog and click Load mesh. The mesh will be loaded to the network. Summary information from the mesh log will be written to the Mesh summary field of the 2D zone.

Mesh 2D zones log

To view the mesh log for a mesh job prior to loading results to the network, select the mesh job from the Manage Mesh Results dialog and click Show log.

For each mesh job, immediately prior to generation of the mesh, the data used for creating the mesh will be validated. If there are any errors in the data, meshing will not take place. The mesh log for the job will contain error messages.

Errors may be generated if:

• Zones overlap. Intersections of the zones are checked, and an error is generated if any of the following zone overlap:
  • Mesh zones
  • Mesh level zones
  • Roughness zones - occurs when using the classic method of mesh generation or when using the clip method and a roughness zone has either not been assigned a Priority value or has been assigned the same priority as another roughness zone in the same 2D zone

  And for InfoWorks networks only:

  • Infiltration Zones (2D)
  • Turbulence Zones (2D)
  • Permeable Zones (2D)
• All the vertices of a mesh element lie within an undefined area of the ground model, which means that the ground level of the element cannot be determined.
• The size of the mesh data exceeds its storage limitation. This may occur if the total size of vertices, edges, triangles or elements exceeds the limit. Whether the limit is exceeded depends on various factors including the amount of aggregation and the complexity of the geometry. For example, if there is no aggregation, the number of elements will equal the number of triangles. But if there is a lot of aggregation, there will be far fewer elements, using up less space, so more triangles could be generated, if required, before the limit is exceeded.

  If the total of the vertices, edges and triangles do not exceed the storage limit, the elements will be generated. As a rough guide, normally around 30 million elements can be created before an error, due to the size of the elements to be stored, is issued.

  Tip:

  You can reduce the reduce the number of elements by increasing the Minimum element size in the properties for the applicable InfoWorks or SWMM 2D Zone.

It is necessary to correct the errors before the mesh can be generated.

If there are warnings, the mesh will be created successfully and the mesh log will contain output from the mesh process listing the 2D Zones meshed. For each 2D Zone, this includes:

• Number of vertices and triangles created
• Number of wall edges created
• Mesh zones used
• Mesh level zones used
• Roughness zones used

And for InfoWorks networks only:

• Mesh element ground levels have been adjusted to match adjacent bank levels
• Infiltration zones (2D) used
• Turbulence zones (2D) used
• Permeable zones (2D) used

Validation warnings will also be listed. Warnings may be generated if polygon boundaries or walls intersect each other:

• Shape recognition error / suspect area or error polygons found in the polygons used to represent voids.
• Unable to calculate an interior point for void formed by polygon. (The void has been ignored during the meshing process. It is likely that the void is a very small area, therefore ignoring it will have very little effect on simulation results.)
• Another object, e.g. Mesh zone, crosses the 2D zone boundary. (When objects cross it is necessary to add a vertex at the crossing point. This may result in a tiny area of overlap when more than two object cross.)

And for InfoWorks networks only:

• Element ground levels have been lowered to match the level at an adjoining river bank

In order to resolve these warnings, edit the geometry of objects using the polygon and polyline editing tools to snap and insert vertices.

For InfoWorks networks, the Diagnostic Polygons Tool can also be used to investigate errors further.