The unsteady flow computational program in HEC-RAS uses the same hydraulic calculations (cross section properties, bridge and culvert hydraulics, weirs, gated structures, etc…) that HEC developed for steady flow, however, the solution of the unsteady flow equations (Continuity and momentum equation) are solved using a unique skyline matrix solver developed by Dr. Robert Barkau for his UNET (Unsteady NETwork model) program. The unsteady flow simulation is actually a three-step process. First data is read from HEC-DSS, if necessary, and then converted into the user specified computation interval. Next, the RasUnsteady.exe program runs. This software reads the hydraulic properties tables computed by the pre-processor, as well as the boundary conditions and flow data from the interface. The program then performs the unsteady flow calculations. The final step is a program called RasDSSWriter.exe. This software takes the results from the RasUnsteady.exe run and writes the stage and flow hydrographs to an HEC-DSS file.