Transport Parameters

The last three variables, the adaptation , critical shear stress, and erodibility are transport parameters. Both the overtopping and piping use these transport parameters. Piping uses cohesionless transport (and ) or excess shear for cohesives (which uses critical shear stress and erodibility). Overtopping (including the post-collapse phase of piping) also uses these parameters. Surface erosion uses for cohesionless and critical shear/erodibility for cohesive, and the headcut algorithms use critical shear and erodibility to determine the vertical erosion rates.
is part of the non-equilibrium transport used for the cohesionless erosion model. Therefore DL Breach only requires if it applies the transport model to a cohesionless embankment.
Critical shear stress and erodibility – on the other hand – are cohesive parameters of the excess shear equation. They are only required for cohesive embankments (or cohesive core materials, which are defined separately, under the Core column.

Adaptation: (unitless)
Non-equilibrium sediment transport models use an adaptation length to compute the rate of erosion. A non-equilibrium model does not just erode the computed capacity but takes time to entrain the full capacity. It uses an adaptation length (Ls) to control this temporal lag, such that:

Where A is the cross section flow area in the breach channel, Ct is the transported sediment concentration, C_t* is the transport capacity (or, strictly, the equilibrium concentration under the flow conditions for the sediment parameters) and Q is the flow through the breach. 

Instead of requiring users to define an adaptation length (which can be an abstract parameter that can be difficult to estimate) DLBreach makes Ls a function of the breach channel width. So users define a linear multiplier () such that:
 

Wu (2016) recommends:

HEC-RAS defaults to 6.0 based on this recommendation.


Critical Shear Stress (Stress Units – lbf/ft2 or Pa):
If the Soil Type is Cohesive, DLBreach uses the excess shear stress to erode the pipe or embankment. The excess shear stress requires two user parameters, Critical Shear Stress and erodibility, to compute erosion from the pipe or embankment:

These parameters are difficult to measure and highly variable. However, Wu et al. (2013) constrained the critical shear stress for breach calculations. He calibrated 30 different breaches by setting critical shear stress to 0.15 Pa (US Customary) and adjusting Erodibility (see next), and recommends that approach to these parameters.


Erodibility (kd) 
If the Soil Type is Cohesive, then the user must use this field to enter an erodibility coefficient for core material. Wu et al (2013) calibrated 30 breaches by setting critical shear stress and adjusting the erodibility. The calibrations (some of which were also sensitive to initial pipe diameter and other, interrelated model variables) required erodibilities that varied between 2.5 and 30 cm³/N-s (US Customary).

Note:

Erodibility Unit Diversity in HEC-RAS Erodibility has a variety of units, and it has different units in different places in HEC-RAS. The cohesive editor of the sediment transport module has units of mass/area/time (e.g. kg/m³/hr or lb/ft³/hr). BSTEM has units of Volume/Force-time (m³/N-s or ft³/lbf-s).