SEDIMENT TRANSPORT PROCESSES IN POOL-RIFFLE SEQUENCES

This paper reviews the published data for sediment transport in pool-r iffle sequences which suggests that a velocity or shear stress reversa l hypothesis does not explain all of the published evidence of sedimen t transport. This conclusion is explored in more detail using observat ions of sediment transport from the regulated River North Tyne. Sedime nt transport rates at discharges of <30 per cent bankfull are initiall y highest on riffles. As discharge increases, sediment transport rates in pools rise sharply, and values locally exceed those on riffles. Tr acing 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 ba nkfull floods. Analysis of entrainment data reveals evidence for addit ional restraint operating on riffles and causing higher dimensionless entrainment thresholds. An appraisal of the possible mechanisms contro lling sediment entrainment, transport and deposition in pool-riffle se quences is made which reveals evidence for the operation of a combinat ion of hydraulic, sedimentological and interactive processes. A descri ptive model of sediment transport processes in pool-riffle sequences i s presented.