Most dredge methods do not remove 100% of sediment but re-entrain sediment that was not actively transporting in the navigation channel. Users can model the fate and transport of these materials with the Reintroduction Dredge feature. Click on the Re-Introduce Part of Dredge Load feature in the Dredging Editor, which will reveal the features in the figure below.
% or Material to Re-Introduce:
HEC-RAS will monitor dredged mass and will re-introduce this, user specified percentage, back into the model as Mass In (mass flowing into the control volume, as if it were coming in from a lateral boundary or upstream cross section). It will re-introduce mass proportional to the gradation of the mass removed (usually the gradation of the active layer).
Grain Class Threshold:
Of course, not all grain classes are likely to be lost during the dredge process. Fines are far more likely to re-entrain and transport downstream while gravel losses are usually too small to care about and immediately re-deposit. The Grain Class Threshold is a filter. Only grain classes finer than this threshed is re-entrained.
Set RS Where Dredge Material is Released:
Dredged materials are not always re-entrained where they are extracted. Push the Set RS button to inject dredge losses somewhere else in the model. All the dredged sediment can be transferred to another cross section if dredged materials are piped or barged downstream (e.g. % Re-Introduced = 100) using this feature.
