HEC-RAS computes volume from cross section area change or vice versa in two places. It uses these methods to compute the cross-section change, and also bed material accounting. Earlier versions of HEC-RAS followed HEC 6, using Simpson's rule for the cross section change and a rectangular, single cross section, control volume (Single CV) for bed mixing. This sometimes generated volume errors and numerical artifacts, which could include cross section deposition (↑A) while the Exner routines computed negative mass.
Recent versions of HEC-RAS are backward compatible, maintaining the legacy computational approach. But new models use Simpsons' rule by default. Additionally, users can select to apply any of the three approaches to both bed change and mixing, or select the legacy method ("Backward Compatible" – not recommended)