Model accuracy can be defined as the degree of closeness of the numerical solution to the true solution. Accuracy depends upon the following:
Assumptions and limitations of the model (i.e. one dimensional model, single water surface across each cross section, etc…).
Accuracy of the geometric Data (cross sections, Manning's n values, bridges, culverts, etc…).
Accuracy of the flow data and boundary conditions (inflow hydrographs, rating curves, etc…).
Numerical Accuracy of the solution scheme (solution of the unsteady flow equations).
Numerical Accuracy. If we assume that the 1-dimensional unsteady flow equations are a true representation of flow moving through a river system, then only an analytical solution of these equations will yield an exact solution. Finite difference solutions are approximate. An exact solution of the equations is not feasible for complex river systems, so HEC-RAS uses an implicit finite difference scheme.