The simplest way to convert volume change to cross section change is to consider each cross section and its control volume independently (this is called the Single CV method in HEC-RAS). However, when distributing the volume longitudinally, it is advantageous to spread the volume out over the control volume in a way that transitions between the cross sections.

The other bed changes methods in HEC-RAS follow HEC6, assuming that the volume change is not equally distributed over the control volume. In order to compute a smooth transition between cross sections, HEC-RAS assumes the mass is distributed in a "wedge", tapering out between the maximum volume change at the central cross section, to zero at the upstream and downstream cross sections (below). This assumption allows HEC-RAS to use a couple common numerical approximations to compute volume from irregular areas and, in this case, the inverse.
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"Wedge" used to distribute erosion or deposition volume longitudinally over the control volume.

Note - Legacy Method Can Cause Strange Behavior

We added new volume to area change methods in HEC-RAS version 6.0 because the default method, inherited from HEC 6, used different methods to compute volume/area change for the active layer and the cross section nodes.  This meant that the area and volume change did not always "match" and in extreme cases, HEC-RAS could compute a positive volume change (deposition) in the control volume but lower the cross section nodes (erosion) - or the opposite.  We have left this method in the model for backward compatibility and it should remain default if the model was created in a previous version.  But the other methods use the same algorithm for the active layer and the cross section change, and are recommended.