Draw and Assign 2D Mesh Zones Command

GeoHECRAS allows the user to assign or draw a 2D Mesh Zone polygon boundary and define the meshing scheme and corresponding parameters (e.g., element size, etc.). This allows the user to refine the mesh more easily in areas of interest and maintain that refinement as the model continues to be developed. For example, in a critical infrastructure area, a mesh zone could be created and defined, and a much smaller element size can be used to capture the flow direction and velocities that occur during a flood event.

In order to define a 2D Mesh Zone, a polygon is created to define the boundary of the mesh zone and a cell spacing system is provided to be used in the 2D Mesh Zone generation routine. 2D Mesh Zones can be used to densify an area where more detailed results are desired due to rapid changes in terrain or water surface elevation. Additionally, the 2D Mesh Zones allow the user to simplify an area where the water surface elevation will not change significantly, and users want to reduce the number of computation points in the 2D flow area.

Creating 2D Mesh Zones

To create 2D Mesh Zones, the Draw 2D Mesh Zones or Assign 2D Mesh Zones command can be used. Both commands work similarly.

Drawing 2D Mesh Zones

The Draw 2D Mesh Zones command allows the user to manually draw individual polygons on the Map View as 2D Mesh Zones.

Follow the steps below to use the Draw 2D Mesh Zones command:

1. From the Input ribbon menu, click the 2D Flow Areas dropdown menu, and then select the Draw 2D Mesh Zones command.
   
2. The Draw 2D Mesh Zones dialog box will be displayed.
   
3. Click the [Draw] button adjacent to the 2D mesh zone polygon field to draw a polygon inside any 2D flow area. Use the Create curvilinear polygon checkbox option to draw the polygon using curvilinear segments.
4. The Draw 2D Mesh Zones dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user what to do next.
5. Draw the 2D mesh zone polygon on the Map View. While drawing elements, the user can use the [Ctrl] key to switch between the curvilinear and the linear digitizing. Refer to this article in our knowledge base to learn more about drawing elements on the Map View.
6. When finished, press the [Enter] key or right-click and select Done from the displayed context menu. The Draw 2D Mesh Zones dialog box will be redisplayed and the status of the 2D mesh zone polygon read-only field will be changed from Not Drawn to Drawn.
   
7. The software auto-names the newly created 2D Mesh Zone polygon. However, the user can provide a different name for it in the 2D mesh zone name entry field.
8. From the 2D meshing scheme dropdown combo box entry field, select the mesh type. GeoHECRAS can create the following 2D mesh types:
   • Uniform Mesh
   • Hexagonal Mesh
   • Adaptive Mesh
9. The Element spacing entry field defines the cell spacing used in the 2D Mesh Zone generation routine. By default, the software uses a value of 30 ft. The user can enter a different value or click the […] button to measure the cell spacing from the Map View.
10. The Fix adjacent 2D flow area edges checkbox is used to add internal breaklines between adjacent 2D model domains and prompts adjacent 2D elements from the 2D model domain to align with each other.
11. Click on the [Apply] button and the 2D Mesh Zone boundary will be created. Repeat the above steps to draw additional 2D Mesh Zones.

Assigning 2D Mesh Zones

The Assign 2D Mesh Zones command allows the user to manually assign individual polygons as 2D Mesh Zones. To use this command, a polygon that can be selected must already exist on the Map View or a polygon shapefile layer must be loaded to represent mesh zones.

Follow the steps below to use the Assign 2D Mesh Zones command:

1. From the Input ribbon menu, click the 2D Flow Areas dropdown menu, and then select the Assign 2D Mesh Zones command.
   
2. The Assign 2D Mesh Zones dialog box will be displayed.
   
3. Click the [Pick] button adjacent to the 2D flow zone polyline/polygon field.
4. The Assign 2D Mesh Zones dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next. The user can then select polylines or polygons from the Map View.
5. When finished, press the [Enter] key or right-click and select Done from the displayed context menu. The Assign 2D Mesh Zones dialog box will be redisplayed and the status of the 2D flow zone polyline/polygon read-only field will be changed from Not Selected to Selected.
   Note that the 2D Mesh Zone Specifications section of the Assign 2D Mesh Zones command is similar to that of the Draw 2D Mesh Zones command. Hence, refer to the Draw 2D Mesh Zones command section of this article to learn about the options available in this section.
6. Click on the [Apply] button and the software will treat the polyline or polygon as a 2D Mesh Zone boundary.