Sediment Dynamics in a Bedrock Channel

Abstract

Headwater systems often comprise alluvial rivers interrupted by reaches with partly or entirely rock beds, indicate a local change in bedload conveyance relative to sediment supply. Detailed knowledge of sediment dynamics in bedrock-alluvial systems is crucial for a better understanding of bedrock incision and sediment delivery downstream, but field evidence is lacking. This thesis reports a field investigation in this topic. Coarse bedload transport was studied in alluvial, semi-alluvial and bare rock segments of Trout Beck in North England by monitoring two sets of 270 magnet-tagged pebbles over a period of nineteen months. At-a-station hydraulic geometry, flow resistance and shear stress were estimated using stage recorders, water surface profiles and salt-wave measurements in five short sub-reaches. Thresholds of motion were investigated using tracer-pebble data and bedload impact counts. Tracers seeded in the upstream alluvial channel moved more slowly. Tracers seeded in a bare rock gorge dispersed quickly at first but accumulated in a coarse-sediment zone at the start of the next partial cover. Bedload transport was size selective over alluvial or semi-alluvial segments but not over bare rock. Flow resistance, as quantified by Manning’s n or Darcy-Weisbach f, varies considerably with both discharge and bed character. The estimated bedload conveyance is highest in reaches with entirely or mainly rock bed, and lowest in alluvial and boulder-rich segments. The boulder-rich segment has high shear stress, but presumably its threshold stress is also high due to form drag from boulders, as evidenced by very high values of n and f. No standard resistance law describes flow in Trout Beck accurately from measured bed D84. This study demonstrates how bedrock reaches control sediment processes and how they are different from alluvial reaches.