Natural sediments are nonuniformly sized. For modeling purposes, the continuous distribution of sediments are grouped into sediment size classes. Each size class is treated as a dissolved conservative constituent. The sediment size classes may or may not have different diameters, densities, roundness, etc.
The sum of the bed- and suspended-load is the bed-material load. If all of the suspended load is assumed to interact with the bed, then the bed-material load is equal to the total-load. In HEC-RAS, the bed- and suspended-loads are combined into the total-load and solved together. This approach avoids having to solve separate transport equations for the bed- and suspended-loads. Sine there there can be many grain classes, this represents significant computational saving. When the transport bed- or suspended-load dominant transport, the total-load equation reduces to the bed- or suspended-load transport equations respectively.