Mesh Applications

Note: This workshop was updated for the Feb 2026 build with mesh updates

Objective - In this tutorial you will learn to apply the meshing approaches from the previous workshop to common river conditions like islands, expanding floodplains, and bridges. 

Data Files

The data for this tutorial are provided below.
Pier Expansion Workshop.zip

And in the model library under class/workshops/2_5_Mesh_Applications

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

Steps

Meshing Through Islands, Flow Splits, and Expansion/Contractions

Island Modeling

1. Open Pier Expansion Workshop Files
   IF you are not currently in the class (or need
   You can download the zip file or download through the Development Window with the following Model ID
   class/workshops/2_6_Mesh_Application
   Select Development Tools
   
   Go to Model Library navitage to Class→ Workshops and right click on 2_6_Mesh_Applications and select Open
   
   
   
2. Look at the results from the 20,000 and 60,000 cfs simulations.
   
   
   What features do you need to accommodate with your Mesh?

   Your mesh needs to lay out cells around the flow split/island, accommodate the bridge piers, and account for the flow expansion.

3. Right click on the Geometry, Duplicate and Rename to something like "Quad Channel"
   
4. Click on your Conceptual Mesh and select the Edit tool to work on your conceptual mesh.
5. Digitize the outside of the channel, do not worry about the island for now.
6. Define Quad cells in the channel and choose a resolution.
7. Regenerate the mesh.  
8. 
9. Digitize the Island and select triangular cells with a 200 ft cell size for it.
   
   The island mesh is ok, but we lost our quads in the channel by splitting it.
   How can you keep quads in the channel?
10. Separate the side channels with Arcs at the upstream and downstream separation points.
    You may need to reset each of the four regions that now make up the channel back to quads.
    
11. Let's improve the channel mesh by fixing the number of cells. 
    1. Try 3 cells to the left of the island, 2 to the right, and 5 upstream and downstream.  
    2. Click on the "width" (transverse) arcs and set their count.
       
12. The triangles across the island are fine, but let's try to add quads across the island.
    How would you add quads to the island? 
13. Make triangle sections at the upstream and downstream end of the island so the most of the island is a 4-sided region.
    
14. Set the cell count on both of these new arcs at the upstream and downstream end of the island and switch the region to quads.
    

Modeling a Floodplain Expansion/Contraction

1. Next let's use Quads to model expansion and contraction in the left bank.
   This will work better if you add an Arc where the left overbank region necks down.
   
2. Make the thin northern left overbank region a single cell quad region.
3. What happens?
   
   What did that do to the rest of your left overbank area?

    This forced the whole left overbank 1-cell wide (or, depending on your other assumptions, more cells, but likely too few).

   

   How do you fix this?

   Add an intermediate arc with more cells specified.

4. Add one or more Arcs to capture the flow expansion in this zone. 
5. Give each arc the cell spacing you want.
   Remember to try to make these cells perpendicular with flow.
   
   
   

Modeling Bridges and Piers

Modeling Piers on a Coarse Grid

1. Next we are going to work on the bridge crossing at the north end of your model.

   Why aren't these piers well represented in the model?

   

2. First, Draw Arcs that follow the centerline of the road and bridge.  

   You can experiment with the satellite imagery and the opacity to see the road and terrain at the same time.

   

   

3. Regenerate the mesh.  

4. The simplest way to align your mesh with your piers is to drop nodes in the middle of the piers.

   Press "S" and double click (you can use Ctrl+Z to stop drawing and only leave the node.

   

   This Conceptual Model Node forces a Computational Node in the footprint of the pier which tends to align the cell faces.

5. Next we are going to construct two more detailed bridge and pier models. 

   The first won't be super detailed, but more than the previous model.

   For that, draw a 4 Arc region around the bridge.

6. Press the + on the Geometry to create a new geometry file and give it a name like "Detailed Bridge- Quad."

   

7. Draw a Bridge centerline across the region, following the road on either side.

8. This may change the aspect ratio of each of the smaller regions splitting creates, which means that the "Length" is no longer in the direction of the water.

   You may need to switch the Length and Width sizes.

9. Place nodes in the center of each pier like we did above.

10. Regenerate the mesh.  

Moderate Detail Pier Modeling

1. The simplest pier modeling approach above only works because the piers are not skewed.  A more detailed model extends arcs along the piers.

2. Draw Two Arcs on each pier starting at the centerline (the center of the pier) and extending to the end of the full-width portion of the pier (not the tip)

   

3. Regenerate the mesh.  

   How does it look?
   How could you make it better?

4. Connect the ends of the piers with each other and with the bounding nodes to form 6 regions through the bridge.

   

5. Then you will want to give each of these interior bridge regions the same lateral cell spacing as the main channel but make them 1 cell thick in the longitudinal direction.

   But because they are longer in the lateral dimension than the stream-wise dimension, you will need to make the "Length"=20ft and the "Width" = a count of 1.  Set the arcs to these values.

   

6. With only 6 of these internal bridge regions, it is not a lot of work Now lets tag these Arcs so we can change them together.  

7. You can edit multiple arcs at the same time and find them again later by tagging them.

   This video illustrates the steps

   Tagging Arcs in RAS2025.mp4

   1. Try to tag the stream-wise and transverse arcs separately.

   2. Choose the "Fruit Ninja" tool and draw an S to select all nine transverse arcs that make up the bridge.

      

   3. Make sure you have Arcs selected and Press the Add Tag button to tag these.

      

   4. You can just give them all a consistent string or make the tag a T/F Boolean 

   5. Search by Tag, use Shift to select the search results, and change the cell spacing for all of the bridge face arcs together (Size=20)

   6. Do the same for the streamwise arcs (Count = 1)

8. Regenerate the mesh.  

Detailed Pier Modeling

Finally, let's build an extremely detailed model through the bridge (more detailed than you will probably ever need) to give you experience using blank cell types to simulate structures.

1. Draw another small region around just the bridge.
   1.  Press the + on the Geometry to create a new geometry file and give it a name like "Super Detailed Bridge- Triangle."
   2. Give it a cell size of 10.
      
2. Zoom into the pier.  Digitize a region that is just inside the edge of the pier.
   
    
   
   Why do we draw the blank region inside the edge of the pier?

   The blank cell does not have any wetted perimeter.  Therefore, the wetted perimeter must come from the intersection of the terrain and 

3. Choose the blank cell type and regenerate the mesh.  
   
   
   Did you get an error? Why?

   Even though your region is blank, the Arc has a cell spacing...which is the default 100.  
   This was incompatible with the cell spacing of 10 and/or the length of the pier.

   

4. Change the Arc cell size to 10 and regenerate the mesh.  
   
   
5. If you have time you can try another, finer, cell resolution
   

Solution Files: 

Pier Expansion Workshop Solution.zip

Adding a Tributary (If you have extra time)

If you have extra time, try adding a tributary to these starting files.

Data Files

The data for this tutorial are provided below.
Tributary Workshop.zip

And in the model library under class/workshops/2_5b_Tributary

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

Steps

1. Open the Tributary Mesh Workshop file.
   This model has a very coarse mesh.  The main channel is aligned with flow but the tributary uses a coarse triangular mesh.
   Your job here is to use quads to align this expanding tributary/floodplain with the flow direction.  
   
2. Right click on the Geometry, Duplicate and Rename to something like "Quad Trib"
   
3. Click on your Conceptual Mesh and select the Edit tool to work on your conceptual mesh.
4. Press "S" to get the "Split" tool to start a new arc on the main channel, right-bank, arc.
   If Start your Arc (e.g. place a Conceptual Mesh Node) at a "Mesh Node" (as pictured below) it will not change the location of the cells.
   
5. Draw your arc along the southern (rights) bank of the tributary
6. Draw another arc along the northern (left) bank of the tributary
7. Regenerate the mesh.  
   Notice how the mesh cells realign so a mesh node falls on the computational mesh node. 
   
   
   
   How many things are called "nodes"?

   The RAS2025 meshes have at least three components we call nodes.  

   

   1. On the conceptual mesh, we call the place where Arc join "Conceptual Mesh Nodes".
       Users control the location of Conceptual Mesh Nodes.
   2. Each location where cell faces intersect is a "Computational Mesh Node" or just "Mesh Node.
       Users do not control these.  They generate automatically in response to the conceptual mesh.
       Nodes on the Conceptual mesh always enforce nodes on the computational mesh.
   3. Arcs are also made of nodes.
       These "Arc Nodes" are vertices when you are editing.

   While confusing, these names are consistent with the mesh and topology tradition and literature so we use them to reflect that literature and align with the theoretical tradition.

8. Convert this tributary into a Quad region.
   
   
   The cell faces are not very smooth. What can you change to smooth them?

   The cell faces are not very smooth.  This is an artifact of using a coarse resolution with the Laplacian smoothing on large, curved cells.
   
   If you increase the Laplacian Smoothing factor it will improve these faces.

9. This works if you are ok with the length of your main channel cells setting the width of your tributary cells.  But lets say you want a detailed model of the tributary, so you want your cells to be 20ft wide and 40ft long.
   Try setting up a mesh with detailed cells in the trib.
10. Duplicate and Rename your geometry.
11. Select the tributary region and set the Width = 20 and the Length = 40.
    
12. Regenerate the mesh.  
    
    
    Where would you like to improve this mesh?

    

13. Draw Arcs connecting the Conceptual Mesh Nodes associated with the Tributary on the main channel left bank to the main channel right bank- perpendicular with flow.
     
14. Select the Upstream Arc of the Tributary, and set the spacing to Count=5 and turn Stricter Cell Spacing on.
    Also make sure that the downstream arc has stricter cell spacing on
    
15. Regenerate the mesh.  

Trib.mp4

Solution Files: