Impacts of Channel Morphology and Seasonal Flow History on Sediment Mobilization and Transport
Abstract
Using the onset of bedload transport as a predictive metric for defining channel equilibrium thresholds is complicated by observations that, in addition to discharge, initial sediment mobility and subsequent sediment flux depend upon a variety of conditions including the grain size distribution on the bed and channel reach morphology. While bedload flux is notoriously difficult to measure, if variations in active bedload depth and channel width can be neglected, then a proxy metric for sediment flux is virtual velocity (i.e., the average downstream velocity of bedload sediment). The purpose of this research is to use a natural experiment of a gravel-bedded channel disturbed by dam removal to quantify the conditions required for sediment mobility (initial motion and subsequent flux) under varying bed grain size distributions, reach morphologies, and seasonal flow histories. Sediment mobility is investigated over high discharge events ranging from spring snowmelt to summer thunderstorms, thus ensuring a range of flow conditions, including partial and equal mobility conditions. Spatially explicit bed shear stress was determined using 2D flow models of the 1 km study reach and the locations of ~300 radio frequency identification (RFID) tagged tracer rocks and ~45 accelerometer-embedded tracers were used to quantify sediment mobility across multiple mobilizing flows. Movements of ~45 accelerometer-embedded tracers were used to constrain the critical stage for the onset of tracer mobility. The critical Shields parameter shows a partial dependence on morphology only in step-pools during the fall season. Over seven seasonal time periods of passive tracer transport, virtual velocity ranged from 0.09 m/h to 0.81 m/h. The fraction of mobilized passive tracers (Fm) provides a proxy for determining partial vs. equal mobility conditions and mapping both critical Shields and virtual velocity values over time provides a quantitative metric for channel equilibrium.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMEP41B..07R