The Post-Processor is used to compute detailed hydraulic information for a set of user specified time lines during the unsteady flow simulation period. In general, the unsteady flow computations only compute stage and flow at all of the computation nodes, as well as stage and flow hydrographs at user specified locations. If the Post Processor is not run, then the user will only be able to view the stage and flow hydrographs, and inundation mapping (HEC-RAS Mapper), no other output from HEC-RAS. By running the Post Processor, the user will have all of the available plots and tables for unsteady flow that HEC-RAS normally produces for steady flow.

By default, the Post-Processor will compute detailed output for a maximum stage water surface profile. This profile does not represent any specific instance in time, but rather represents a profile of the maximum stage that occurred at each cross section during the entire simulation. This profile is often useful for getting a quick view of the maximum extent of flooding during a specific event. If you only want to get this maximum envelope profile, then simple select "Max. Prof." from the Detailed Output Interval drop down list.

In addition to the maximum water surface profile, the user can request the software to write out a series of instantaneous profiles at a specific time interval. This is accomplished from the Computation Settings section of the Unsteady Flow Analysis window. The user turns on this option by selecting an interval from the box labeled Detailed Output Interval. The Post-Processor will then compute detailed output for each of the instantaneous profiles requested, as well as the maximum stage envelope profile. When the unsteady flow program runs, flow and stage water surface profiles are written to DSS for the entire system, starting with the beginning of the simulation and then at the user specified time interval for the entire simulation.

When the Post-Processor runs, the program reads from HEC-DSS the maximum water surface profile and the instantaneous profiles (stages and flows). These computed stages and flow are sent to the HEC-RAS steady flow computation program RasSteady.exe. Because the stages and flows are already computed, the RasSteady.exe program does not need to calculate a stage, but it does calculate all of the hydraulic variables that are normally computed for steady flow. This consists of over three hundred hydraulic variables that are computed at each cross section for each flow and stage.

WARNING: The purpose of the post processor is to compute hydraulic output variables that are not computed by the Unsteady Flow computational engine. However, the Post Processor is actually just the Steady Flow computational engine. There are a few differences between assumptions made in the steady flow and the unsteady flow computations engines that will affect some of the post processor output. For example, the unsteady flow engine combines the left and right overbanks areas into a single "Floodplain" flow area, while the steady flow engine computes hydraulics for the left and right overbank separately. Therefore, some of the post processor output is computed like the steady flow program performs computations, even though the unsteady flow program makes some different assumptions. Specifically, the computation of left and right overbank cumulative volumes is based on separate reach lengths in the steady flow program, and the post processor. However, the unsteady flow program combines the left and right overbank hydraulic properties in the equations, and uses a simple average of the two overbank lengths to compute volumes between the cross sections in the floodplain area. Also, the bridge computations are performed with a set of pre computed curves in the unsteady flow engine, while the steady flow engine computes the hydraulics through the bridge each time. These differences can lead to some small differences in some of the post processor output, as compared to how the Unsteady flow engine actually performs hydraulic calculations.

At hydraulic structures such as bridges and culverts, the unsteady flow program only reports the stage just upstream and downstream of the structure. During the Post-Processing of the results, the Steady.exe program calculates the hydraulics of the structures by using the computed tailwater and flow, and then performing detailed hydraulic structure calculations. This is done so that the user can see detailed hydraulic information inside of the hydraulic structures for each of the profiles that are being post processed. However, this process can produce slightly different results for the upstream headwater elevation. Occasionally, you may notice a water surface elevation computed from the Post-Processor that is higher at the structure than the next upstream sections water surface. This difference is due to the fact that the unsteady flow simulation uses a pre-computed family of rating curves for the structure during the unsteady flow calculations. The program uses linear interpolation between the points of the rating curves to get the upstream headwater for a given flow and tailwater. The Post-Process performs the calculations through the structure and does not use rating curves (it solves the actual structure equations).

Once the Post-Processor is finished running, the user can view output from all of the HEC-RAS plots and tables. The maximum water surface profile and user specified instantaneous profiles can be viewed by selecting Profiles from the Options menu on each of the output windows (tables or plots). The overall maximum water surface profile will be labeled "Max W.S.", while the instantaneous profiles are labeled by the date and time. For example, a profile from January 5, 1999 at 1:00 p.m. would be labeled "05Jan1999 1300".

WARNING: Specifying a detailed output interval for post processing that is small can lead to long computational times and huge output files. Select this interval wisely, in that you only get detailed output when you really need it.