Flow Resistance and Shear Stress in Bedrock Canyons
Abstract
Understanding sediment transport and erosion in bedrock rivers is critical to gaining meaningful insights into landscape evolution. Due to the difficulty of incising into bedrock when compared with alluvial material, bedrock bound channels are often slower to respond to changes in base level, tectonic uplift, and climate when compared with other channel types. For this reason bedrock channels control the rate of landscape responses to these signals, and set the lower boundary condition for all hillslopes in their watershed when they are present. Predicting sediment transport or erosion rates at the local, reach scale in bedrock rivers remains difficult. Observations have revealed unusual flow properties including plunging flows structures, strong secondary flow cells and upwelling coherent flow structures at high Reynolds numbers. Our understanding of flow dynamics in bedrock rivers is hampered by a poor understanding of flow resistance, which has not been widely studied in rock bound rivers. We examined flow dynamics through bedrock canyon morphologies, and explore the relation between surface roughness and flow resistance. Multibeam echosounder (MBES) and Acoustic Doppler Current Profiler (ADCP) data was used to examine the 3D flow field and its relation to bathymetry in Black Canyon on the Fraser River in British Columbia, Canada. Roughness was quantified as the deviations of the boundary from an idealized surface and resistance is quantified from shear stresses. A recurring Constriction-Pool-Widening (CPW) morphology was observed. Visualisations of 3D flow data through CPW morphological units document the fundamental controls on plunging flow structures. A consistent, linear relation between our roughness and resistance metrics, calculated through the CPW morphology was also developed. This work adds to our growing understanding of bedrock canyons and their formation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP53I2249H
- Keywords:
-
- 1625 Geomorphology and weathering;
- GLOBAL CHANGE;
- 1815 Erosion;
- HYDROLOGY;
- 1824 Geomorphology: general;
- HYDROLOGY;
- 1856 River channels;
- HYDROLOGY