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Hull Environment Research Institute (HERI) and Department of Geography, University of Hull, Hull HU6 7RX, UK (e-mail: l.e.frostick{at}hull.ac.uk)
Rates of landscape evolution and landform development depend on the capacity of the main transporting medium, predominantly water and the river system, to move material away from the site of production in the upland slopes. In upland areas the main river bed material is coarse gravels, with various admixtures of finer sand and silt. This paper reports a series of flume experiments to investigate the impact of admixtures of finer material on the entrainment of coarse particles. In all experiments the main framework of the bed was made up of quartz-density gravels with a mean particle size of 8 mm. In some of the experiments unimodal, 0.09 mm mean particle size, quartz sands were introduced upstream of the experimental section and transported into place in order to simulate a common condition in natural river beds, of sand migrating over a static gravel bed. New image analysis techniques were developed to extract data from video recordings of the experimental runs. These revealed important differences in entrainment processes among the experiments with a distinctive contrast between the clean gravel and sand–gravel runs. Observations suggest that the presence of sand increases the rates of gravel entrainment and leads to a distinctive patchiness in bed break-up which will encourage bed form development. In the mixtures, sand removal prior to gravel entrainment destabilizes the bed and allows large areas to become entrained. This contrasts with clean gravels where grains tend to entrain individually. These observations show the importance of bed material character in controlling river form and process and point to its role in controlling sediment flux through the landscape.
