Finally, it is often advantageous to define several bank material layers. Some banks have distinct stratigraphy, stacking soil layers. Sometimes vegetation is modeled by introducing a surface layer with the same friction angle as the parent material but with a higher cohesion. To specify layers, select Define Layers, from the Left/Right Bank Material lists available in the HEC-RAS Sediment Data Editor (below). If the Define Layers option is selected, a new table will appear on the right side of the editor (below).
The bank material layer table requires two parameters: a bottom elevation and a material. Each layer exists between its own bottom elevation and the bottom elevation of the higher layer. The top layer will extend from the highest point on the half-cross section to the highest specified Bottom Elevation. (Note - in the current version bottom elevations MUST be entered from low to high).
Then, just like the Bank Material Type option in the main BSTEM editor, a list of bank materials can be accessed by clicking on the Material column (above). Each layer has to have its own material specified, but the materials do not have to be unique and can be any combination of default or user specified material types. Add layers until the last Bottom Elevation extends below the conceivable bottom of the model (i.e., the elevation the model is likely to scour to). The bottom of the deepest layer has to at least extend to the thalweg elevation for the model to run.
Guidelines for Selecting Layer Elevations: Setting layer elevations according to a couple conventions can make an HEC-RAS/BSTEM model more stable. First, set the bottom of the top layer below the lowest point of the overbank, so the layer only intersects the cross section at one point, between the toe and the top of bank (below). Second, set the bottom of the bottom layer below the deepest possible thalweg elevation (e.g., thalweg-max erodible depth) so the model never scours below the defined layers (below).
