The new (HEC-RAS 6.0 and later) Sediment Hostart feature will initialize the model with the cross sections and bed gradations from any output time period in a sediment transport output file (gradation hotstart requires Volume results at Output Level 6).

To read initial bed gradations or cross sections from a 1D sediment output file check the Initialize Data From Sediment Output File.  

Select which data HEC-RAS should import from the result file.
Then, from the Hotstart Type dropdown menu, select which data to initialize.  The options include:

  • Cross Sections Only: This option will read the updated Cross Sections (XS) from a sediment simulation output file.  This will initialize the new model with the updated station-elevation points of the specified time step of the selected simulation.  The hotstart will not read bed gradations from the file, but will use those specified in the Sediment data editor.
  • Gradation Only: This option will read bed gradations from the specified output file.  It will initialize the new model with the mass of each grain class (not just the percentage) in each bed mixing layer at the output time step.  Make sure the model and the output file used the same bed mixing algorithm.
  • Gradation and XS: This option reads in the station elevation data from the  

Press the Browse button to find the sediment output file to use as a hotstart.  Sediment output files have the suffix pxx.hdf (where xx is a number between 01 and 99).  The number of the pxx.hdf file corresponds to the plan number in of the simulation (e.g. the figure above is the result from plan 1 (.p01)).

Specify a date and time.  HEC-RAS will select the output (gradation and/or cross sections) closest to that time from the output file.  It will then initialize the new model with those data.


Output File Must Have Supporting Bed Mass/Volume Variables

If you want to use model results for a gradational hotsart, you must run it at Output 6 and Set the Mass/Volume option to Volume.

HEC-RAS will generate an error if the user requests a gradational hotstart from an output file that does not have all of the required bed layer results by grain class:

The gradational hotstart cannot use results files created with output level 1-3 (unless the user adds the layer masses by grain class).  Run the model at output level 4 to generate a results file complete enough for a gradational hotsart.

The New Model Must Have the Same Cross Sections as the Output File

All hotstart features require that the model used to develop the hotstart file has the same cross sections and other nodes (bridges, inline/lateral structures, etc…) as the simulation file uses. HEC-RAS has some logic to try to reconcile hotstart files with extra or missing cross sections (relative to the starting geometry). But users should just be careful that the geometry used to generate the hotstart file has the same cross sections (and other features) as the geometry file in the plan that uses it. 


Gradational Hotstarts Require the Same Bed Mixing Algorithm

Bed gradation output are algorithm specific.  The Active Layer method cannot hotstart from Copeland or Thomas.  Make sure the model and the output file you hotstart with have the same bed mixing algorithm.