Bedload dynamics in gravel bed rivers

The aim of this research was threefold: 1) to improve the values of bedload mobilisation discharge in gravel bed rivers, 2) to determine the bedload velocity over the short and long term and 3) to quantify bedload transport and to examine bedload discharge in relation to the stream power and to the excess stream power.

Since 2006, we used the PIT-tag technique in order to mark pebbles from Ardennian rivers. Ten rivers of different geomorphological properties were fitted out with PIT-tags (in 18 different sites) and 67 surveys were carried out after hydrological events.

These data allow a valuable relationship between the stream power during floods and the size of the biggest elements mobilised to be proposed. This relation is obtained for medium sized rivers (50-200 km²) with bed material composed of gravel (D50 between 2 and 10 cm).

Furthermore, we also obtained good relationships between the average transport distance and the excess of stream power reached during the hydrological events. In order to estimate the bedload virtual velocity over a longer time scale, we applied these relationships to the hydrological events recorded by gauging stations over more than 30 years. We obtained values of virtual velocity which are between 2 km/century for a low energy river with a sinuous bed and well developed riffle-pool sequences (Rulles River – 20 W/m² at Qb) and 11 km/century in a more powerful river with a straight channel (Aisne River – 100 W/m² at Qb) and a sub-flat bed. The propagation velocity of bedload is therefore influenced by the energy available. However, it is also necessary to take into account the river pattern, the characteristics of the armoured layer and the presence of potential trapping sites.