At high concentrations the solid component has a significant effect on the volume of the mixture. This can confound flow conventions if users and modeling teams are not careful. There are two main ways of incorporating solid volume into mud and debris models, and users should select the appropriate approach under Select Bulking Method to reflect that decision.

Incorporate Volume of Solids in Flow Data (Do Not Bulk)

One way to account for the volume of the solids is to include it in the flow. With this approach, the flow (Volume/Time) is the flux of the mixture. This will be a common approach for measured flows, because field measurements (or estimates) will not separate the fluid and solid components. If the flows include the total volume of the mixture, the mud and debris calculations should not bulk them. Increasing the volume based on the concentration would double count the influence of the solids.
The model still requires a concentration for the non-Newtonian equations. But if the flow includes the volume of the solids, select Do Not Bulk under Select Bulking Methods.

Add Solid Volume to Water Flow Data (Bulk Fluid Volume)

In the second approach users define only the water flow in the unsteady flow file, and then HEC-RAS adds the volume of the solids during the non-Newtonian simulation. This approach is common if the flows come from a hydrologic model (or a runoff model like HEC-HMS that computes separate hydrographs and sedigraphs), if Cv is a calibration parameter, or if the modeling team wants to quantify the effect of the mud or debris (by comparing the result to a clear water flow with the same flow file).
If the flow data only account for the water, select Bulk Fluid Volume under Select Bulking Method. HEC-RAS will increase the volume of the boundary flows to account for the solid components based on the user-specified volumetric concentration using the relationship describe in the Bulking section of the Technical Reference Manual.