Field validation of the virtual velocity approach for estimating bedload flux in gravel-bedded rivers

Keywords: sediment flux, bedload, fluvial geomorphology, rivers,
Session Type: Virtual Paper Abstract
Day: Friday
Session Start / End Time: 2/25/2022 09:40 AM (Eastern Time (US & Canada)) - 2/25/2022 11:00 AM (Eastern Time (US & Canada))
Room: Virtual 20

Abstract

Bedload flux is a substantial component of sediment budgets in mountainous regions but measuring bedload in natural settings is notoriously difficult and, as a result, long-term bedload datasets are rare. Theoretical approaches suggest that bedload sediment flux may be measured indirectly via a function of the “virtual velocity” of grains moving on the bed, active channel width, sediment bulk density, and thickness of the mobile layer of the gravel-bedded channel. Yet, validating the virtual velocity approach is difficult given the paucity of directly measured bedload datasets. In this study, we directly compare virtual velocity, measured by tracer rocks equipped with radio frequency identification (RFID) passive integrated transponders (PIT), to annual bedload flux in a small upland stream in New England where bedload has been directly measured annually since 1985. Heavy precipitation throughout WY 2021 brought flows of 98%, 81%, 120%, and 70% of the magnitude of 1-in-2 year flow (Q2), that mobilized 72%, 32%, 83%, and 5% of tracers, respectively, allowing accurate characterization of bedload mobilization thresholds for a distribution of grain sizes bracketing d50. We show that bedload flux calculated via virtual velocity matches directly-measured bedload flux within ~20% and discuss techniques that may improve bedload flux measurement via virtual velocity. The data presented here offer a simplified approach to measuring bedload compared to traditional sampling techniques that are cumbersome and inaccurate and provides insights about the effect of sediment supply on bed mobility in mountain streams.