In addition to boundary conditions, the user is required to establish the initial conditions (flow and stage) at all nodes in the system at the beginning of the simulation. Initial conditions can be established in two different ways. The most common way is for the user to enter flow data for each reach, and then have the program compute water surface elevations by performing a steady flow backwater analysis. A second method can only be done if a previous run was made. This method allows the user to write a file of flow and stage from a previous run, which can then be used as the initial conditions for a subsequent run.
In addition to establishing the initial conditions within the river system, the user must define the starting water surface elevation in any storage areas and 2D flow area that are defined. This is accomplished from the initial conditions editor. The user can enter a stage for each storage area within the system. 2D Flow areas have several ways of establishing initial conditions within the 2D flow area. For more information on unsteady flow data, please review "Modeling Multiple Bridge and/or Culvert Openings" of the HEC-RAS User's manual.