Sediment transport models require hydraulic parameters. Therefore, HEC-RAS computes hydraulics each time step before it routs sediment or updates cross sections. HEC-RAS 5.0 couples sediment transport computations with either quasi-unsteady hydraulics or unsteady hydraulics. The User's Manual offers pros and cons of both approaches. Unsteady flow is not unique to sediment studies. HEC-RAS can simulate unsteady flow without sediment data and Chapter 2 documents the unsteady equations in detail.
Quasi-unsteady hydraulics, on the other hand, are only used for sediment studies. Therefore, the quasi-unsteady approach is described below.