1. Selecting Pre-Defined Default Parameters

The standalone version of BSTEM includes sixteen default material types that are also included in HEC-RAS. These default material types are each populated with characteristic soil properties distilled from a database of field data collected by the USDA-ARS. The unit weight, friction angle (Φ^'), cohesion, Φ^b, critical shear stress (τ_c), and erodibility are listed in Table 2 below (SI parameters are included at the end of the document). See the description of these parameters in the next section (Soil Strength Parameters).

It is worth noting, however, that these parameters are extremely site specific, and the default parameters are central tendencies of very noisy data sets, particularly for cohesive material types. Therefore, default parameters will often generate substantial errors.

Coupling these bank failure algorithms with the mass balance computations in the mobile bed capabilities in HEC-RAS introduced one additional parameter requirement. Any mass that "fails" into the channel requires a gradation so HEC-RAS can partition it into grain classes for transport. Therefore, idealized gradations were selected for each material type based on their description. These gradations are depicted in the figure below.

Modeling Note/Warning

Models constructed without direct erodibility measurements or calibration data are speculative. Additionally, using default sand materials or coarser will invoke the cohesionless scour methods, which are based on transport functions and tend to over-predict scour. Interpret default material results carefully.

In order to select one of the default material types, from the table on the Sediment Data Editor, BSTEM Tab (see bottom figure here), from the columns labeled Left Bank Material or Right Bank Material click at the cross section of interest. A list of default material types that are available will appear as in the figure below. Ignore the option "DEFINE LAYERS", from the list and select the desired material type and it will associate it with that bank (see figure below).