Notice that the majority of the mesh was created with 500-ft by 500-ft square cells, but around the perimeter of the 2D area, RAS automatically creates irregularly shaped cells to fit the edges. Here we can see the mesh that was created with 500 ft X 500 ft spacing. Then click “Generate Points in 2D Flow Area, and HEC-RAS will build a structured mesh of constant cell size (this can be adjusted later). Then go ahead and define the grid cell size (both width, DX, and height, DY). Typically you’ll enter the Manning’s n value and then click the “Generate Computation points on regular interval. The defaults usually work very well for the tolerance inputs. The tolerance input boxes allows the user to have some control on how much data goes into each of the cells on the soon-to-be-created 2D grid. Only 1 n value can be used for now in the 5.0 beta version of HEC-RAS for a 2D Area, but future versions will allow multiple n value definition for a single 2D Flow area.
#HEC RAS 2D WINDOWS#
The first windows provides the user options for Manning’s n value for the 2D Area. Once a 2D area has been defined, click on the 2D Flow Area editor button to set up the mesh. Single click the points to define a polygon shape, staying within the bounds of the terrain (very important), and to complete the area, double click. This is drawn exactly the same way storage areas are drawn. Use the 2D Flow Area button at the top of the geometry schematic to draw a new 2D area. More information about world files can be found here: The background image has to be created external to HEC-RAS and must be accompanied by a world file. The background image will also give you a clue about areas in your 2D grid that might need more or less detail.
Boundary cells will also exhibit unstructured characteristics where RAS works to fit the mesh to the mesh perimeter.īefore building your mesh, it can be useful to include a background image of your terrain so that you can make sure you draw your grid within the bounds of your terrain. Typically you’ll begin with a Cartesian or rectilinear structured mesh and where you add more detail and resolution, you’ll have unstructured (irregular) zones within your mesh. RAS is able to work with structured or unstructured meshes with cells that have up to eight sides. Notice that the mesh remains within the perimeter of the terrain. Here in the geometry editor, we can see the computational mesh “draped” over the terrain. The continuous properties of the “infinite” terrain are then discretized onto the mesh so that computations can be made over a finite domain.
Think of a mesh as a net that you drape over your terrain. Your computational mesh is what will allow you to do 2D modeling in HEC-RAS. Written by Chris Goodell | September 16, 2014