Experiments on the transport of graded sediment

Four large-scale experiments are reported on the transport of graded sedime nts in a straight channel in the domain of partial sediment mobility (in wh ich not all size fractions are fully mobile). Particular attention is paid to the influence of sediment introduced near the head of the experimental c hannel in inbank and overbank flows, respectively. In addition to sediment transport rate, the composition and texture of the bed and the composition of the bed load were measured at intervals. The differences in sediment beh aviour between the inbank and overbank flows tested were unexpectedly small ; however, the input of sediment near the head of the channel changed the s ediment mobility profoundly. Transport rate was suppressed ahead of a wave of material that propagated through the channel. This suggests that the art ificial introduction of sediment changed the near-bed flow structure for mo re than 15m downstream, impairing the transporting capacity of the stream. The observed changes of surface composition and texture imply that a comple x grain structure is laid down during the recession of sediment-transportin g flows in the field, and this structure is important for prediction of sub sequent sediment behaviour. Better understanding of the near-bed processes and, particularly, the role of the bed condition will have to be assembled before significantly more reliable prediction methods can be made available . However, awareness of the factors that limit the accuracy of present meth ods is valuable in deploying those factors, and this paper contributes to s uch awareness.