SEDIMENT TRANSPORT PROCESSES IN POOL–RIFFLE SEQUENCES

Abstract

This paper reviews the published data for sediment transport in pool–riffle sequences which suggests that a velocity or shear stress reversal hypothesis does not explain all of the published evidence of sediment transport. This conclusion is explored in more detail using observations of sediment transport from the regulated River North Tyne. Sediment transport rates at discharges of <30 per cent bankfull are initially highest on riffles. As discharge increases, sediment transport rates in pools rise sharply, and values locally exceed those on riffles. Tracing experiments using a combination of magnetic and painted tracers are used to show the downstream dispersal of fine and coarse sediments through the pool–riffle sequence. Coarser particles experience longer transport paths and faster virtual rates of travel in pools during bankfull floods. Analysis of entrainment data reveals evidence for additional restraint operating on riffles and causing higher dimensionless entrainment thresholds. An appraisal of the possible mechanisms controlling sediment entrainment, transport and deposition in pool–riffle sequences is made which reveals evidence for the operation of a combination of hydraulic, sedimentological and interactive processes. A descriptive model of sediment transport processes in pool–riffle sequences is presented.