Controlling the Quasi-Unsteady Sediment Output Increment

Outputting data periodically can also reduce file size, without eliminating output variables. Control output frequency with the Number of Increments Between Outputs parameter. First select the basic output 'unit' with the Output Increment, then specify the number of increments between outputs:

Output Increment:

HEC-RAS has four options increments to choose from. Two increments are the two time steps used in the quasi-unsteady flow editor (the Flow Duration and Computational Increment) and the other two are regular time increments (Hours and Days). The Computation Increment defined in the Quasi-Unsteady Flow Editor is the default output increment. This option also generates output at the end of each flow duration, whether it is a multiple of the Computation Increment or not. Users can also request output every Flow Duration or some multiple of them.

Both the Computation Increment and the Flow Duration Output Increment can distort animated profile results in time (if they are not constant). Larger or rapidly changing flows are often modeled with smaller Computation Increment, which will generate more output profiles during these morphologically active times. Temporal distortion can be valuable or problematic when visualizing results. Consider pros and cons.

The other two options, hours and days, set the output increment to a constant absolute time. This avoids the temporal distortion in the time series plots and animations but can result in skipping over interesting changes and overemphasize boring, low change, periods.

After selecting the Output Increment, define the number of Output Increments between results using the Number of Increments Between Profile/Time Series Outputs. HEC-RAS will write output for every selected variable at this multiple of the base increment. For example, in first figure in Output Level (the default values) HEC-RAS writes results every ten computation increments.

Modeling Note: Discrete Output for the Increment NOT the Elapsed Time

HEC-RAS reports incremental output like Mass Out, Mass In, or Mass Bed Change (the mass entering, leaving, and depositing/eroding in the control volume in a time step) for the last computational increment. Therefore, if the output increment is longer than the computational increment, HEC-RAS will not sum data between the output increment. Instead a series of 'snapshots' in time are reported. The cumulative variables are usually more helpful for tracking mass through a simulation. To compute increments, select the cumulative outputs and subtract or output every computational increment.

Modeling Note: Setting Output Increment in Unsteady Sediment Transport 

The legacy ("Old") sediment output does not work well with unsteady sediment transport. View unsteady sediment output with the new HDF5 viewer (View->Sediment Output) and control the output increment with the Mapping Output Interval. Sometimes HEC-RAS will provide unsteady sediment results with the legacy viewer if the output increment is set to 1.