The portion of the cross section outside the Edge of Bank Station must conform to three conventions to provide optimal results:

  1. The floodplain must be wide enough to include the maximum failure plane angle. For long term simulations this includes the maximum failure plane from the maximum scour location. As a rule of thumb, include a floodplain wide enough to encompass at least a ten degree angle from the toe station.

  2. The floodplain needs intermediate station-elevation points between the edge of the bank and the end of the cross section (below) for BSTEM to compute failure planes effectively.
    BSTEM requires cross section station-elevation points between the Bank Edge Station and the end of the cross section.
  3. If the floodplain is irregular (like in the figure in Too Much Scour) it should not intersect with a layer. If the cross section includes multiple soil layers, the top layer should include all of the cross section nodes outside of the bank edge station.

  4. HEC-RAS should be able to handle wet depressions and ineffective flow areas outside the bank edge, but the more flodplain complexity in a cross section the more likely that the bracket and Brent method (Teukolsky et al., 2007) will converge on a false maximum or that the cross section update will encouter a problem. So avoid incidental cross section complexity, particularly in the overbank.