Sediment Transport Potential

Sediment transport potential is the transportable mass of a particular grain class in response to cross channel hydraulic parameters. HEC-RAS computes transport potential for each grain class with one of the sediment transport equations available in the program.

The sediment transport equations are empirical equations or algorithms that translate hydrodynamics into transport. However, most of these equations were developed for a single representative grain size.

To apply these equations to sediment mixtures, with multiple discrete grain classes, HEC-RAS computes transport potential, allying the transport function independently to each grain class present in the system, as if it were the only grain class in the system. Later transport potential is prorated by the prevalence of the grain class, to compute the transport capacity (see discussion below), which is the transport used in the Exner equation. But first HEC-RAS applies the transport function to each available grain class independently, computing a transport potential for each.

HEC-RAS includes eight 1D sediment transport potential functions. The three 2D (Wu, van Rijn, and Soulsby-van Rijn) functions are not available in 1D yet. Since sediment transport is sensitive to so many variables, transport potentials computed by the different equations can vary by orders of magnitude, depending on how the material and hydrodynamics compare to the parameters over which the transport function was developed. As much as possible, select a transport function developed for similar gradations and hydraulic parameters as the project reach. Appendix E in this document include the actual equations and algorithms. This section includes brief, qualitative notes on the use, applicability, and sensitivity of each equation.

Most sediment transport functions are based either on shear stress or stream power. They usually use an excess-shear or excess-power form, which compare the actual shear or power to a threshold. HEC-RAS does not compute any transport for that grain class if it is below the threshold (i.e. the grain class is not "competent"). The stream power equations use two different versions of stream power, the product of velocity and slope (VS) and the product of velocity and shear stress (tV). The six shear stress or stream power equations are:

Table: Transport functions based on excess shear stress and stream power.

*Engulend-Hansen is not an excess form of the stream power equation, but just a function of stream power.