The Computation Settings area of the Unsteady Flow Analysis window contains: the computational interval; hydrograph output interval; detailed output interval; Mapping Output Interval; a computation level output flag; the name and path of the output DSS file, and whether or not the program is run in a mixed flow regime mode.

The computation interval is used in the unsteady flow calculations. This is probably one of the most important parameters entered into the model. Choosing this value should be done with care and consideration as to how it will affect the simulation. The computation interval should be based on several factors. First, the interval should be small enough to accurately describe the rise and fall of the hydrographs being routed. A general rule of thumb is to use a computation interval that is equal to or less than the time of rise of the hydrograph divided by 20. In other words, if the flood wave goes from its base flow to its peak flow in 10 hours, then the computation interval should be equal to or less than 0.5 hours (30 Minutes). This way of estimating the time step tends to give an upper boundary as to what the value should be.

A second way of computing the appropriate time step is by applying a numerical accuracy criteria called the Courant condition. The Courant condition criteria looks at cross section spacing and flood wave velocity. The basic premise is that the computational interval should be equal to or less than the time it takes water to travel from one cross section to the next. A detailed description of the Courant condition can be found under the Model Accuracy, Stability, and Sensitivity section of this chapter. Use of a time step based on the Courant condition will give the best numerical solution, but it may cause the model to take a lot longer to run.

Additional considerations must be made for hydraulic structures, such as bridges, culverts, weirs, and gated spillways. Within bridges and culverts, when the flow transitions from unsubmerged to submerged flow, the water surface upstream of the structure can rise abruptly. This quick change in water surface elevation can cause the solution of the unsteady flow equations to go unstable. One solution to this problem is to use a very small time step, on the order of 1 to 5 minutes. This allows the module to handle the changes in stage in a more gradual manner. Additionally, when gates are opened or when flow just begins to go over a lateral weir, the change in stage and flow can be dramatic. Again, these types of quick changes in stage and flow can cause the solution of the unsteady flow equations to go unstable. The only solution to this problem is to shorten the computational time step to a very short interval. This may require the user to set the value as low as 1 to 5 minutes. The time step should be adjusted to find the largest value that will still solve the equations accurately. Additional variables that affect stability are the number of iterations and the Theta weighting factor. These two variables are discussed under the calculation tolerances section below.

The Hydrograph Output Interval is used to define at what interval the computed stage and flow hydrographs will be written to HEC-DSS. This interval should be selected to give an adequate number of points to define the shape of the computed hydrographs without losing information about the peak or volume of the hydrographs. This interval must be equal to or larger than the selected computation interval.

The Detailed Output Interval field allows the user to write out profiles of water surface elevation and flow at a user specified interval during the simulation. Profiles are not written for every computational time step because it would require too much space to store all of the information for most jobs. Also, when the Post-Processor is run, the program will compute detailed hydraulic information for each one of the instantaneous profiles that are written. This option is turned on by selecting an interval from the drop-down box next to the detailed hydrograph output label. The selected interval must be equal to or greater than the computation interval. However, it is suggested that you make this interval fairly large, in order to reduce the amount of post-processing and storage required for a detailed hydraulic output. One example for selecting this variable would be, if the time window of the simulation was set at 72 hours, then one might want to set the instantaneous profiles to an interval of every 6 hours. This would equate to 13 profiles being written out and having detailed hydraulic information computed for them.

Mapping Output Interval. This field is used to enter the interval at which the user will be able to visualize mapping output within HEC-RAS Mapper. A limited set of output is written to a separate HDF5 output file that corresponds to the Plan file. For example, if you are running Plan 1, the Plan filename will be something like "filename.p01". An output file will be written out by the unsteady flow computational program, with the name "filename.p01.hdf". This file will contain results from the unsteady flow simulation, written out at the interval the user defines for the Mapping Output Interval. The user will then be able to visualize spatial mapping output in HEC-RAS Mapper based on this time interval.

Computation Level Output. This option will instruct the program to write out a limited set of variables for each cross section at each computational time step to a separate output file. The variables that are written to this file are: water surface elevation; flow; maximum depth of water in the main channel; numerical error in the calculated water surface elevation; numerical error in the calculated flow; average velocity in the channel; and average velocity of the entire section. After a simulation is complete the user can plot and tabulate each of these variables spatially, or as a time series at an individual cross section. Because this output is at the computational time step level, it can be very useful in debugging model stability problems. The user can get to plots and tables of this information by selecting the Unsteady Flow Spatial Plot or the Unsteady Flow Time Series Plot options from the View menu of the main HEC-RAS window. Detailed descriptions of plotting and tabulating this output can be found under the Viewing Results Chapter of this manual.

Warning: Turning the Computation Level Output option on can create very large output files and it will also potentially slow down the computations. This option writes several output variables to a file for all locations in the model and for every computational time step. If you have a large data set and/or are running it for a long simulation time period, this file will be very large.

The field labeled DSS Output Filename is required before an execution can be made. The program will always write some results to a HEC-DSS file, so the user is required to select a path and filename to be used for this information. Hydrographs written to the DSS file are based on the user selected hydrograph output locations, as well as some default locations that HEC-RAS will always output a hydrograph.