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Abstract

The development of three separate devices at Birkbeck College, London, and their deployment in a gravel-bed stream allows us to pinpoint entrainment thresholds, to measure bedload continuously and to quantify the ingress of matrix fines. This provides a comprehensive picture of processes and gives clear indication of the reasons why gravel beds appear to react unpredictably to flood flows and why a universal bedload equation has yet to be developed. We show that ubiquitous bed microforms (e.g., pebble clusters) delay initial motion so that stream power is 5 times greater than it is when bedload transport ceases. We also show that low-flow ingress of matrix fines increases the difference to 16 times by increasing shear strength of the stream bed. Our continuous monitoring devices indicate a regular pulsation of bedload transport (mean wave period 1.7 hrs) that cannot be attributed to deficiencies in the samplers because it is corroborated by distinctly different and independent instruments; the pulses are not explained by changes in hydraulic parameters. We suggest that they are kinematic waves of mobile bed particles, the first dynamic manifestation of this phenomenon in a natural stream. Our data give a clear indication of those facets of water-sediment interaction that must be investigated in order to arrive at a general bedload transport equation for gravel-bed streams, and the data highlight the naive assumptions currently made in paleohydraulics when using clast size as an indicator of paleo-flow conditions.

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