Bedload progression in gravel bed rivers using iron slag as a tracer

In fluvial dynamics studies, different methods are used to evaluate bedload transport
and particle travel lengths. However, results are mostly based on a few transported
elements and on a relatively short time scale. Consequently, it is difficult to
extrapolate these results to whole bedload, because of the burying of particles into
the subsurface layer or the trapping of elements in fluvial forms (point bars, riffles,
…), which can immobilise elements during long periods.
Bedload progression has been evaluated in Ardenne rivers using slag elements
produced by the past factories established along rivers between the 14th and the 19th
centuries. Important quantities of slag were dumped close to rivers or even directly
into channels. For several centuries, slag elements were dispersed in the bedload
and transported by floods of varying importance. Consequently, slag can be
considered as a tracer to analyze bedload progression over several centuries. The
size of slag elements has been studied in many Ardenne rivers. The longitudinal size
trend of the largest slag particles allows us to determine the effective competence of
rivers and to analyze the hydraulic sorting. Moreover, downstream of some
metallurgic sites, we have constrained the presence of slag elements to the most
downstream riffles. Because we know from historical studies the periods of activities
of these sites, we may estimate the speed of bedload progression in several gravel
bed rivers from the Ardenne Massif (2-3 km/century).