Because of the inherent variability of these parameters, site specific measurements are recommended. If data is available, customized material types can be associated with a bank. This is analogous to the process for defining sediment gradations in the Initial Conditions and Transport Parameters Tab of the Sediment Data Editor, where gradation records are defined and then associated with the appropriate cross section.

Before selecting customized materials a user must define the materials by clicking Define/Edit BSTEM Sample Parameters. The BSTEM Material Parameters Editor will open (above). To create a new BSTEM material, click New Record and specify the name. HEC-RAS will reject any names that are identical to existing or default material names. Five mandatory intrinsic soil strength parameters (used to compute the failure plane and factor of safety), two mandatory erodibility parameters (used to compute toe scour) and one optional parameter can then be entered.

Soil Strength Parameters

The first five parameters are intrinsic soil strength parameters and are associated with the computation of a critical failure plane and the FS associated with that failure plane. These five parameters emerge from classical geotechnical measurements that most soils labs would be able to handle. HEC-RAS uses four user defined parameters with hydrodynamic and geometric data to compute a factor of safety for a range of possible failure planes by computing the ratio of the resisting forces to the driving forces: cohesion (c'), saturated unit weight (W), the angle of internal friction (Φ'), and the angle representing the relationship between shear matrix suction and apparent cohesion (Φ^b). These four user defined parameters are entered in the BSTEM Material Parameters Editor (above) and are described below.

Unit Weight:

This is the saturated unit weight⁷ (combined weight of the solids and water of the soil when saturated). Note that this is different than the unit weight used elsewhere in HEC-RAS sediment transport computations. The unit weight used elsewhere in HEC-RAS sediment transport computations is the mass of the solids per unit volume.

Friction Angle (Φ'):

The friction angle is a classic geotechnical parameter that is a measurement of the soil strength that quantifies the friction shear resistance of soil. The "angle" of the "friction angle" is derived from the Mohr-Coulomb failure criterion and is the angle of inclination in the classical Mohr diagram. The angle of inclination is a theoretical angle⁸ used to compute soil strength and should not be confused with physically intuitive angles like the angle of repose. This also is not the minimum angle of the failure plane. In cases where groundwater elevation is higher than the water surface elevation the bank can lose frictional strength and be left only with cohesion, allowing for a shallower failure plane angle. The fiction angle can be determined by collecting "undisturbed" cores for tri-axial testing in a soils laboratory or it can be measured in situ with a borehole shear test. The Iowa Borehole Shear device (Thorne, 1981) is a hand held instrument that is commonly used to collect this parameter from hand augured eight centimeter boreholes for BSTEM studies.

Cohesion:

Cohesion is the attractive force of particles in a soil mixture, usually as a result of electrochemical or biological bonding forces. These forces increase the strength of a soil matrix. Cohesion is generally a minor consideration in granular soils but can account for a substantial amount of soil strength in cohesive materials. Cohesion is computed from the same data as the friction angle and, therefore, must be measured either by tri-axial laboratory tests or in situ borehole shear measurements.

Phi b (Φ^b): As soil drains, capillary tension induce negative pore water pressure or matrix suction. Suction resists bank failure and increases the shear strength of the soil matrix. In the bank failure algorithms, suction is quantified as an "apparent" cohesion" or the equivalent increase in cohesion required to generate the same increase in shear strength (below).

Φ^b is a function of soil moisture and maximizes at the friction angel (Φ') at saturation. For most materials Φ^b is generally between ten to thirty degrees depending on soil type. Φ^b is very difficult to go out and fundamentally measure Φ^b. Φ^b has been measured a handful of times in research settings. Most applications start between ten and fifteen degrees but Φ^b goes to a maximum of the friction angle when the material is saturated (Fredlund, 1986). Because of the estimated nature of this parameter it can be used as a calibration factor.

Gradation Sample:

HEC-RAS requires a fifth bank material parameter that is required but not used until after the failure calculation. In order to partition any failed material into grain classes for transport by the sediment transport model, the bank material has to have a bed gradation associated with it. Bed gradations are defined by clicking Define/Edit Bed Gradation from the Initial Conditions and Transport Parameters Tab of the HEC-RAS Sediment Data Editor (below).

Any gradations defined here become automatically available in the Gradation Sample list on the BSTEM Material Parameter Editor (BSTEM Material Parameters Editor.).

Erodibility Parameters

The second set of parameters on the BSTEM Material Parameters Editor (top) are the erodibility parameters. These parameters are specialized for bank failure analysis. Erodibility parameters are measurements of the erodibility of the soils in response to hydrodynamic forcing. Standard soil testing laboratories are not likely to have the capabilities to collect these parameters. However, the USACE Coastal and Hydraulic Lab (ERDC-CHL), other federal agencies, and several universities can quantify these parameters. Bank jet tests (Hanson, 1990) and SEDFLUME laboratory tests (Briaud et al., 2001, Smith et al., 2010) are the best ways to estimate these parameters.

Critical Shear Stress: Critical shear stress is when the bank begins to scour.
Erodibility: The rate of sediment removal in response to a unit shear stress.

In the absence of these parameters Simon et al., 2000 summarized their database of cohesionless measurements to find a relationship between critical shear stress and erodibility:

This relationship is based on the regression depicted in the figure below which includes a great deal of scatter in log space. This underscores the variable and site specific nature of these parameters, therefore local measurement of these parameters is highly recommended. Also note that this relationship does not account for cohesion and, therefore, should not be used for cohesive soils.

Groundwater Parameters (Optional)

Groundwater parameters are optional and are only used if the dynamic groundwater option is selected in the BSTEM section of HEC-RAS. There are two parameters that determine the rate that water can drain from or infiltrate into a bank. In turn, this determines the lag between the rising or falling of groundwater elevation with respect to the water surface elevation.

Hydraulic Conductivity:

Is the standard Darcy "K" parameter used in groundwater modeling. The hydraulic conductivity is a linear parameter that determines velocity of groundwater proportional to a gradient. Hydraulic conductivity can be measured with field or laboratory tests but is also often documented in regional literature or estimated by expert intuition (analogous 'o Manning's n).

Reservoir Length:

Groundwater dynamics follows a simple reservoir routing model that assumes the banks store water. Water can be added to or drained from the rate of the saturated hydraulic conductivity. The "length" of this reservoir is the bank thickness (perpendicular to the river) that contributes water to the river on pertinent time scales.

If the soils are very permeable or the reservoir is small (high K or low L) the groundwater elevation will track the channel water surface elevation (figure a below). If the soils are impermeable or the reservoir is very large, the groundwater will not respond much to flow depth in the channel (figure b below). However, intermediate hydraulic conductivities will introduce a lag between channel water surfaces and groundwater elevation (figure c below). Because groundwater elevations higher than the confining water surface elevation are the critical condition, groundwater lag can induce failures on the falling limb of the hydrograph, both in the field and in the model.