Converting between mass and volume in HEC-RAS is a relatively simple matter of multiplying or dividing by the unit weight of the material, though the multiplier is different for clay, silt, and cohesionless grain classes. Some users prefer to view the magnitude of erosion, deposition, transport or other variables either in mass or volume units. Volume is the default reporting unit but it can be switched to mass using this option. In recent versions, users can get both by selecting whichever choice is not selected in this drop-down in the Customized Variables tool.

Modeling Note: Calibrating to Volume

 Repeated cross sections are the most common and –often - the most reliable calibration data. Repeated cross sections can be converted to volume change by multiplying the area change by the control volume reach length, or can be summed from upstream to downstream to compare Longitudinal Cumulative Volume Change Curves (See later) HEC-RAS produces. Therefore, while mass change can be more intuitive, volume change is often easier to compare to common calibration data.


Modeling Note: What if I want Mass AND Volume?

The default sediment results will only report the mass/volume results (e.g. Mass/Volume Bed Change) in the form selected in this option. However, users can add the other one with the Customized Output option. So if you select Volume but also want Mass bed change, simply select it and HEC-RAS will write both.