Mesh Development for HEC-RAS 6.x

Objective - In this tutorial you will learn how to (1) use a 1D HEC-RAS model to inform mesh development, (2) how to export a mesh and import into HEC-RAS version 6, and (3) how to add a bridge in HEC-RAS version 6.

Data Files

The data for this tutorial are provided below.

1DModel.zip

These data are for demonstration purposes only and should not be used for engineering analysis.

These data are for the confluence of the Illinois River and Fox River at Ottawa, IL.

Steps

Step 1: Export HEC-RAS 6.x Features

1. Open HEC-RAS 6.x
2. Open the project "1DModel.prj"
3. Open RAS Mapper
4. Right-click on the Cross Sections Layer and choose the Export Layer | Save Layer to Shapefile menu option
   
   1. Pick a folder and provide a Name ("crosssections")
5. Start Editing the "Base" geometry  
6. Create Bank Lines
   
   1. Expand the Rivers layer
   2. Right-click on the Bank Lines layer and choose the Create Bank Lines from XS Bank Stations menu option
      
   3. Right-click on the Bank Lines layer and choose the Export Layer | Save Layer to Shapefile menu option
   4. Pick a folder and provide a Name ("banklines")
7. Create Edge Lines
   1. Expand the Cross Sections layer
   2. Right-click on the Edge Lines layer and choose the Create Edge Lines at XS Limits menu option
      
   3. Right-click on the Edge Lines layer and choose the Export Layer | Save Layer to Shapefile menu option
   4. Pick a folder and provide a Name ("edgelines")
8. Stop Editing the "Base" geometry  
9. Close RAS Mapper
10. Close HEC-RAS 6.x

Step 2: Create a 2D HEC-RAS Model

1. Start HEC-RAS
   1. Create a new Project
   2. Provide a Name
   3. Set the Projection using the provided .prj file
   4. Press the Create button
2. Create and add the Terrain
   1. Create a new Terrain
   2. Add the tif from from the base HEC-RAS project
   3. Press the Import button

Import Features to Conceptual Mesh

1. Create a new Geometry
2. Import the shapefile features to the Conceptual Mesh
   1. Select the Conceptual Mesh
   2. Right-click on the Conceptual Mesh choose the Import | Polylines/Polygons menu item
   3. Browse to the GISData folder 
      1. Select the crosssections.shp file
      2. Select the banklines.shp file
      3. Select the edgelines.shp file
      4. Press OK when the files are selected
   4. Choose the "Import and Merge"
      This option will attempt to find the intersections of the lines imported and create a node-arc topology
      
   5. Press the Import button
      

Inspection of the Conceptual Mesh will bring several issues to light. 

Fix Conceptual Mesh

Rather than bringing all of the features at once, bring in the shapefiles that might really matter (model perimeter and bank lines).

1. Create a new Geometry
2. Import the Edge Lines shapefile features to the Conceptual Mesh
   
   1. Right-click on the Conceptual Mesh choose the Import | Polylines/Polygons menu item
   2. Browse to the GISData folder 
      
      1. Select the edgelines.shp file
   3. Press the Import button
      
3. Complete the Conceptual Mesh
   1. Create arcs that close the region at the upper end
   2. Connect the arcs at the junction
   3. Delete unnecessary nodes
      
4. Import the Bank Lines shapefile features to the Conceptual Mesh
   
   1. Right-click on the Conceptual Mesh choose the Import | Polylines/Polygons menu item
   2. Browse to the GISData folder 
      
      1. Select the bank lines.shp file
   3. Press the Import button
      
5. Fix the end points of all of the new arcs
   1. Nodes will need to split existing lines and be connected
   2. Hanging arcs will need to be deleted 
6. Improve the Conceptual Mesh to follow the Levee and other high ground
   

Generate the Computational Mesh

1. Start with creating a good mesh for the channel are defined by the bank lines.
   1. Use the Quad mesh type (suggest parameters are shown below)
   2. Main River - Length 400ft,  3 Cells wide
      
   3. Tributary - Length 400ft, 1 Cell wide
      
2. Mesh the overbank floodplain areas
   1. Suggested Arc properties for the Overbank areas are 400ft on all Arcs
   2. Use Quads where appropriate, otherwise use Triangular meshing
3. Press the Regenerate Mesh button to create the computational Mesh
   
4. Continue to edit the mesh as desired

Export the Mesh for use in HEC-RAS 6.x

Once the mesh has been created and refined to satisfaction, export the mesh for use with HEC-RAS 6.x.

1. Right-click on the Mesh and choose the Export | HEC-RAS Mesh (Version 6) menu option
   
   The mesh will be created in the "Exports" folder in the RAS project directory
   The default filename with be the Mesh name with "(Exported Mesh)" appended 
   

Step 3: Import Mesh to HEC-RAS 6.x

1. Open HEC-RAS 6.x
2. Open the project
3. Open RAS Mapper
4. Create a new Geometry by right-clicking the Geometries group and choosing Create New Geometry.
   1. Associate the Geometry with the Terrain and Land Cover
5. Start Editing the Geometry of interest. 
6. Right-click the 2D Flow Areas layer and choose the 2D Flow Area Editor menu item
   
7. Press the Import Mesh button
   
8. Select the exported mesh - There is a backup in the GISData folder (Mesh_Initial.hdf file)
   The mesh will be imported
9. Stop Editing

Note that the imported mesh will be displayed in RAS Mapper.   As shown below, the mesh is "Brown" color and has a Lock icon displayed indicating it cannot be edited (you can, but you should not).

The 2D Flow Area will be named "2D Area 1" by default.  Further, if you look at the 2D Flow Area Editor, you will find the Mesh has been locked from editing.

You may now proceed to create Boundary Conditions and Reference Lines, enter flow data, and run the simulation.

Step 4: Mesh Limitations and Considerations

A mesh that has been imported from HEC-RAS 2025 "cannot" be modified in HEC-RAS version 6.  If you choose to unlock the mesh and try editing it, you will find the RAS 6 meshing will create a different mesh.  Therefore, if you want to make changes to the mesh, you should do so in RAS 25 and re-import.  You may wish to modify your mesh - building a good model takes iteration and refinement.

Evaluate the difference in meshing

1. In RAS Mapper, perform the following
2. Save the Geometry As a new geometry
3. Unlock the Mesh
4. Start Editing the 2D Flow Area
5. Move or Delete a computation point
6. Identify how the computational mesh changes
   
   1. There may be problems with number of cell faces exceeding 8 or other issues which would need to be fixed by hand
7. Stop Editing 

Step 5: Add a Bridge

Most likely, you will want to model a bridge or other hydraulic structure in your 2D model.  You can still do that.  You can add a 2D connections (to model a bridge, culvert, or levee) just do NOT enforce the structure as a breakline.  HEC-RAS 6.x will "attach" the 2D connections to the nearest set of faces.  To more accurately model the bridge, use an arc in the Conceptual Mesh in HEC-RAS 2025 (to enforce cell faces) and re-export the refined mesh.  

Refine the Mesh at Bridges 

Some example bridge locations are shown below. Practice at these bridge locations.

1. Open the HEC-RAS project
2. Select the Conceptual Mesh
3. Edit the Mesh with the Editing tool
   1. Split existing Arcs
   2. Add new Arcs at the bridges
      
   3. Enter parameters for each Arc/Region 
   4. Regenerate the Mesh
      
4. Zoom to the confluence and we will focus on the bridge on the main river.
5. Add an Arc to represent the bridge
   
6. Provide Arc (Count) and Region (Quad) properties to the new arcs/regions
7. Regenerate the Mesh
   

Export the New Mesh

1. Export the Mesh by right-clicking on the Mesh and choosing the Export | HEC-RAS Mesh (Version 6) menu option

Import Mesh to HEC-RAS 6.x and Add Bridge

1. Open the HEC-RAS version 6.x project
2. Open RAS Mapper
   1. Create a New Geometry (name it MeshwithBridge)
   2. Start Editing the Geometry
   3. Import the mesh into the HEC-RAS version 6.x geometry
      1. You can just import over the existing Perimeter. (There is a backup in the GISData folder (Mesh_FinalwBridge.hdf file)
   4. Add the bridge location as a 2D Connections layer
      1. Draw the centerline of the 2D connection directly over the cell faces, making sure to connect it with high ground
      2. Complete the Name, Width and Structure Type (Bridge 1D - Family of RCs) data
         
   5. Stop Editing in RAS Mapper 
3. At this point, you need to complete the bridge parameters using the Bridge Editor in the Geometric Schematic 
   1. Open the Geometric Schematic
   2. Open the newly created geometry (MeshwithBridge)
      
   3. Open the SA/2D Connection Editor
      1. Set n values for Bridge Cross Sections by selecting the Options | External and Internal Bridge Cross Sections
         
      2. Use an n value of 0.035
         
      3. Provide Deck and Pier information
         1. Top of Road Elevation - 512ft
         2. Deck Width - 12ft
         3. Piers - 2 piers, 6ft wide, spaced 600ft apart
            
      4. Set the HTab Parameters
   4. Close the SA/2D Connection Editor
   5. Close the Geometric Schematic

Step 6: Create Boundary Conditions

1. Open RAS Mapper
   1. Start Editing the Geometry
   2. Create Boundary Conditions locations for each River
      
   3.  Provide names for each
   4. Stop Editing
2. Open the Unsteady Flow Editor and enter BC data (data are provided in the FlowData.xls file)
   Note that the data are 6hr data starting on 01DEC2020 24:00
   
   1. Enter a Flow Hydrograph for the Fox River 
   2. Enter a Flow Hydrograph for the Illinois River
   3. Provide a Normal Depth boundary on the Illinois River (slope = 0.001)
      
3. Save the Unsteady Flow file
4. Close the Unsteady Flow Editor

Step 7: Create a Plan and Simulate