SEDIMENT FEED AND RECIRCULATING FLUMES - FUNDAMENTAL DIFFERENCE

Two standard modes of laboratory sediment transport flume operation ar e sediment-feed and recirculating. In the case of uniform sediment, th e final equilibriums achieved in the two modes are equivalent and inde pendent of initial conditions, even though the specified now and sedim ent constraints differ. When the sediment consists of a mixture of siz es, however, this symmetry breaks down. In a sediment-feed flume, both the feed rate and size distribution are specified by the user, and mu st be identical to those of the bed load at final equilibrium. The fin al state thus becomes independent of initial conditions. In a recircul ating flume, however, the equilibrium size distribution of neither the bed load nor the bed surface material can be specified in advance. As a result, the final state depends on the initial conditions and can i nclude the possibility of partial transport. The equilibriums themselv es are equivalent in that they obey the same laws of flow and sediment transport. Their a-priori predictability, however, differs due to the difference in governing constraints. When fractional transport is sca led by the bulk size distribution, the two flume modes may produce ver y different transport relations, even for the same sediment. A consist ent approach requires that the transport be scaled by the size distrib ution of the bed surface.