As the name suggests, there are two major, interacting components to BSTEM (Bank Stability and Toe Erosion Model):
- Bank Failure: A geotechnical bank failure model that computes failure planes through the bank to determine if the gravitational driving forces exceed the frictional resisting forces (and the interactions of pore water pressure).
Toe Scour: An erosion model that computes the progression of bank undercutting by hydraulic forces. As the toe scours, the bank becomes less stable, so toe scour can initiate bank failure.
These two processes also interact with a third process native to the classic sediment methodology in HEC-RAS computations:
- Vertical Erosion or Deposition: The vertical adjustment of the cross section can also decrease the stability of the bank and interact with toe scour computations. Conversely, a large bank failure could add enough sediment mass to the system to deposit downstream and increase the stability of downstream banks.
Modeling the interactions and feedbacks between these three processes were the main motivation for including the USDA-ARS BSTEM algorithms into HEC-RAS. The science, methods and math of vertical erosion and deposition are covered in the HEC-RAS User's Manual (HEC, 2010) and the HEC-RAS Technical Reference Manual (HEC, 2010a).