Statistics of bedload transport over steep slopes: Separation of time scales and collective motion

TL;DR

This study investigates bedload transport over steep slopes, revealing two distinct time scales linked to collective particle dynamics, using high-resolution data and probabilistic modeling to understand sediment discharge fluctuations.

Contribution

It introduces a probabilistic framework and experimental evidence for separating time scales in bedload transport, highlighting the role of collective motion and entrainment.

Findings

Two distinct waiting time scales observed in sediment transport.

Separation of time scales linked to collective particle behavior.

Theoretical recovery of time scale separation at low entrainment rates.

Abstract

Steep slope streams show large fluctuations of sediment discharge across several time scales. These fluctuations may be inherent to the internal dynamics of the sediment transport process. A probabilistic framework thus seems appropriate to analyze such a process. In this letter, we present an experimental study of bedload transport over a steep slope flume for small to moderate Shields numbers. The sampling technique allows the acquisition of high-resolution time series of the solid discharge. The resolved time scales range from $10^{-2}$s up to $10^{5}$s. We show that two distinct time scales can be observed in the probability density function for the waiting time between moving particles. We make the point that the separation of time scales is related to collective dynamics. Proper statistics of a Markov process including collective entrainment are derived. The separation of time scales is recovered theoretically for low entrainment rates.