BSTEM has two approaches to toe scour. The cohesive algorithms use the excess shear equation to compute scour rates based on measured erodibility data. The cohesionless algorithms apply transport functions to compute scour. The cohesionless methods are generally less accurate, but it is difficult to estimate cohesionless erodibility either in the lab or in the field.
HEC-RAS/BSTEM decides between these methods for each soil layer by computing the percentage of the bank soil that is cohesive (i.e., in the first five HEC-RAS grain classes, less than .063 millimeters is the default grain class definition). HEC-RAS computes this percentage from the gradation defined in the BSTEM Material Parameter Editor (see second figure in the link) from the soil type list, or the narrow gradations of the default soil types. By default, if 20 percent or more of the soil is cohesive, (clay or silt), BSTEM uses the cohesive methods. However, if less than 20 percent of the soil is finer than the cohesive threshold, BSTEM will apply the cohesionless transport equations.
The twenty percent threshold is not entirely arbitrary. Around twenty percent cohesive content, the fine particles fill the soil voids enough for their cohesive properties to dominate the erodibility of the larger particles. However, in reality, the transition from cohesive matrix support to cohsionless framework support is a gradient not a step function. Therefore, the parameter is exposed as a user option in the BSTEM Options in the Sediment Data Editor.
If the transport functions return unreasonable scour rates, as often happens, users can model cohesionless materials with the cohesive erodibility approach. To force the cohesive, excess shear, erodibility method, set the Min % Cohesive to 0 in the BSTEM Options in the Sediment Data Editor.