Create a Mesh

The mesh is what makes it a 2D model. Read the RAS documentation to learn more about the properties of the 2D mesh including computation points and cell faces. In a 1D model, rivers are modelled using cross-sections across the floodplain, distributed along the river. In a 2D mesh, I think to think of the cell faces as 1D cross-sections, and the cell center as a storage area that receives precipitation and allows for infiltration.

I have chosen to do a 2D model here because the lower portion of the Caroni watershed is a very wide and flat floodplain, and I expect there to be multiple flow paths. Also because I prefer doing 2D models. Now if we were modelling just the Maracas Valley or the Aripo Valley, a 1D model can give just as good results as a 2D model since flooding is contained to the channel. I intend to test this at some later date, but for now we carry on with the 2D model of the Caroni basin. Check out Chapter 6 in this document on the advantages of 2D modelling over 1D.

Import the Basin Boundary

which we delineated as one of the first steps of this project. In RAS Mapper, start editing our geometry, right click on Perimeters > Import Features From Shapefile. A wizard will pop up and you can select the boundary shapefile to be imported. If no features show up you may have incompatible features, usually meaning multipart features.

Generate Computation Points

Once we have successfully imported the perimeter, we can generate the Computation Points. We need at least one perimeter to generate the points. A model can have more than one perimeter for more complex models, but here we will just stick with one.

Right click again on Perimeters > Edit 2D Area Properties. Here we need to set the general spacing of the points, which is the distance between them. The rule of thumb is to select a size that can accurately capture the topography of the terrain without sacrificing model runtime. A model with a million points will run much slower than one with a thousand points. Here I chose 100 meters since its one of the more popular Olympic track and field events. Later we will add more detail to the mesh.

Face Perimeter Connection Errors

After generating the points, you may notice that the mesh has Face Perimeter Connection Errors. These occur at sharp bends in the perimeter where the general point spacing cannot account for the angle in the perimeter. These are identified by large red dots on the problem points. These can be solved by adding Computation points in the area, or adjusting the perimeter to be smoother. In the screenshot below I added on point and the problem went away.

Save the Geometry once all the errors have been resolved, indicated by no error messages at the bottom left of the window The 2D Area properties will also not mention any errors.

Our 2D mesh is ready to go! It has 87006 cells in it. Of course, there are several things that can be done to add more detail to the model such as adding breaklines and refinement regions, but for now we just want an error-free mesh to run the model. The next step is to apply some rainfall to it.