Entrainment and deposition of boulders in a gravel bed river

Rivers transports coarse sediment (gravel, cobbles, or boulder) as bedload. During a flood, when the discharge is high enough, the sediment grains move by rolling and bouncing on the river bed. Measuring bedload transport in the field is notoriously difficult. Here, we propose a new method to characterize bedload transport by floods. Using a drone equipped with a high resolution camera, we recorded yearly images of a bar of the Vieux-Habitants river, a gravel-bed river located on Basse-Terre Island (Guadeloupe, French West Indies). These images, combined with high frequency measurements of the river discharge, allow us to monitor the evolution of the population of boulders on the river bed. Based on this dataset, we estimate the smallest discharge that can move the boulders, and calculate the effective transport time of the river. We find that transport occurs about 10 hours per year. When plotted as a function of this effective transport time, likelihood of a given boulder remaining at the same location decreases exponentially, with an effective residence time of 17 hours. We then propose a rough estimate of the average number of boulders that the river carries every year.

Relevance of acoustic methods to quantify bedload transport and bedform dynamics in a large sandy-gravel-bed river

Despite the inherent difficulties in quantifying its value, bedload transport is essential for understanding fluvial systems. In this study, we assessed different indirect bedload measurement techniques with a reference direct bedload measurement in a reach of a large sandy-gravel-bed river. An acoustic Doppler current profiler (aDcp), the dune tracking method (DTM) and hydrophone measurement techniques were used to determine bedload transport rates by using calibration with the reference method or by using empirical formulas. This study is the first work which attempted to use a hydrophone to quantify bedload rates in a large sandy-gravel-bed river. Results show that the hydrophone is the most efficient and accurate method for determining bedload fluxes in the Loire River. Although further work is needed to identify the parameters controlling self-generated sediment noise, the calibration procedure adopted in this study allows a satisfactory estimation of bedload transport rates. Moreover, aDcp and hydrophone measurement techniques are accurate enough to quantify bedload variations associated with dune migration.