Note

This feature is superseded by the Sediment Hotstart from and Output File. It has been retained for backward compatibility, but new models should use the new feature, which is much more powerful, flexible, and intuitive.

Sediment bed gradations are often vertically discontinuous (e.g. a thin, coarse armor layer often covers finer, well graded/poorly sorted sub surface material). HEC-RAS accounts for cover layers computationally with the bed sorting and armoring algorithm, but it can be useful to start simulations with different gradations in the cover/active layer and the subsurface/inactive layer.
Users can manually define and select separate cover layers using the *Options→Mixing Options* menu or users can import layer-specific bed gradations from a previous HEC-RAS sediment run with the Gradation Hotstart tool.
The Gradation Hotstart has two steps. First, an HEC-RAS sediment model must write out sediment gradation data. Select the Write Bed Gradations to an Output File check box under Gradational Hotstart (below).
Select which data HEC-RAS should import from the result file.
This will write an HDF5 file (*.tmp.rst.HDF extension) with the bed gradation for each mixing layer of each cross section (below). HEC-RAS writes the faction associated with each grain class in rows 0-19 and the total layer mass in row 20, and will read these in automatically as initial conditions if prompted.
HDF5 hotstart file. Each cross section has mass data by grain class for the cover layer and the sub-surface layer.
To use a Gradational Hotstart file as initial bed gradations, check the Read Gradational Data from Hotstart File check box in Gradational Hostart box, then Browse to the HDF5 hotstart file (below).
Selecting a hotstart gradation file as starting cross section gradations.
HEC-RAS always requires user specified gradations in the sediment editor, but HEC-RAS will use hotstart gradations instead of gradations specified in the sediment editor when users select the hotstart feature. However if the geometry includes cross sections that do not exist in the hotstart file, the program will default to the gradations specified in the sediment editor.

Modeling Note - Graduation Warm Up Simulation

The feature can be used to perform a 'warm up' run to develop starting bed gradations for the actual simulation. TD13 (HEC, 1992) and Thomas and Chang (2008) recommend running a 'robustness test' to evaluate the numerical stability of 1D sediment models. Robustness runs involve running a constant representative flow (often the 'channel forming discharge') through model until the solution converges on a final stable geometry. By requesting a gradational hotstart file a robustness test and selecting them as the Hostart gradations of the actual simulation, the robustness test can double as a 'bed gradation warm up' and will reduce the numerical fluctuations early in the simulation as the mixing algorithms converge on stable gradations.