Determining the dynamics of coarse bedload transport using passive indirect monitoring: time-dependent variability at event to inter-annual scales

Peter W. Downs,Philip J. Soar,A. Paquier,N. Rivière

doi:10.1051/e3sconf/20184005014

Peter W. Downs, Philip J. Soar + Show 2 more

Open Access

https://doi.org/10.1051/e3sconf/20184005014

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Abstract

The dynamics of coarse bedload transport in rivers is governed by multiple hierarchical factors including catchment-scale controls on sediment production, annually variable hydroclimatic driving of segment-scale sediment supply, and reach-scale factors related to the interaction of hydraulic forces with channel morphology. Exploring hydroclimatic drivers can beneficially utilise passive sensors to record coarse bedload transport over extended time periods and in previously unattainable resolution. For the River Avon (Devon, UK), five-minute coarse bedload frequency data collected using seismic impact plates inherently records the instantaneous variability of bedload transport intensity, patterns of event-scale hysteresis and selective path transport, and the influence of inter-event supply variations. Converting a four-year record of impacts into loads via a probabilistic, data-driven model illustrates the combined influence of hydroclimate and sedimentology on bedload at the inter-annual scale. Despite highly variable water years, the results indicate that ‘bar-building flows’ consistently achieve the peak efficiency for coarse bedload transport whereas bankfull flows are relatively ineffective. Further, annual sediment rating curves combine both supply and transport limiting phases. Sediment transport forecasting is thus sensitive to both flow year type and antecedent controls on sediment supply, with implications for advancing sustainable solutions in river management.

Highlights

The dynamics of coarse bedload transport in rivers is frequently explained as a transportlimited phenomenon
This is despite long-recognised evidence for bedload pulsing resulting in anti-clockwise hysteresis [1], understanding that bedload transport rates are always below levels predicted by tractive potential, even in supply-unconstrained settings [2], and clear indications of aquatic habitat impoverishment resulting from bedload supply limitations in dammed, urban and other human-affected channel settings (e.g., [3])
Deriving annual rating curves for bedload transport using a locally-weighted scatterplot smoothing function applied to the percentiles in preference to a linear fit [12, 13] provides curves that indicate a combination of both supply and transport limiting phases, the balance of which depends on the water year type

Summary

Introduction

The dynamics of coarse bedload transport in rivers is frequently explained as a transportlimited phenomenon. Sensors for indirect passive recording of coarse bedload transport offer the complementary prospect of exploring time dependencies related to hydroclimatic and channel-scale controls at resolutions previously possible only at select locations with extensive and permanent bedload monitoring infrastructure. A geophone with a binary output signal, offer high temporal resolution measurement of bedload initiation, cessation, intensity over extended time periods. They are relatively inexpensive, portable, non-intrusive and, in our experience, a robust means of monitoring coarse sediment transport.

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