An exploration of variability in bankfull river width
Abstract
During a flood the geometry of a river channel constrains the flows of water and sediment, however over many floods channel geometry evolves to reflect the long-term fluxes of water and sediment. This circular relationship is underpinned by erosion thresholds. The size of the channel dictates the relationship between depth and discharge, which generates the shear stress necessary to mobilize sediment, whereas the erosion threshold of the most resistant material constrains the shape of the channel. Stability within a river channel appears to be achieved when the channel geometry evolves such that the probability of a flood eroding the banks is rare. These links between flow, sediment and morphology result in remarkably consistent bankfull width scaling with discharge. However, for any given bankfull discharge across the global compilation we observe an order of magnitude in scatter about the governing trend. Over geologic timescales, this is a limited concern, however from an engineering perspective, being unable to discern why some rivers are 10 m wide and others are 100 m wide for the same discharge is less comforting. Here, we leverage a large dataset of alluvial river hydraulic geometry and an ever-growing catalogue of lidar topography to explore this variability in detail. Initial exploration of reach-scale lithologic and climatic variables provided minimal explanation of the observed scatter. Focusing on field sites with lidar, we extracted high-resolution bankfull river width at the reach scale for a diverse set of rivers spanning the United States. We find that variability in river morphology drives a considerable portion of the observed scatter. At each site, width is generally normal to lognormally distributed with values covering half an order of magnitude of the observed scatter. We then compare sites with the same discharge across different physiographic regions to attempt to parse how and why they differ. Using a series of case studies, we explore how significant disturbances (floods, wildfire, landslides) have impacted channel width distributions both spatially and through time. Taken together, these results highlight the importance of strengthening the links between flow, transport, and morphology for moving towards a probabilistic and dynamic description of river channels.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFMEP52A..02P