Field experiment on the dynamics of fine and coarse sediments over a gravel bar in an alpine river.

Benoît Camenen, Albert Herrero, Guillaume Dramais, Fabien Thollet, Chloé Le Bescond, Emeline Perret, Céline Berni

Wednesday 1 july 2015

9:00 - 9:15h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 8B. Sediment - River

Gravel-bed rivers are often affected by heavy engineering works such as channelization or dam. As a consequence, systems of alternate bars are usually found in such streams. Since alpine rivers are also characterized by very poorly sorted sediments, significant grain sorting is generally observed on the gravel bars. Surface grain size variability and interaction between the different classes of sediments may have a large impact on the gravel bar dynamics. The purpose of this communication is to present some field measurements achieved on the Arc en Maurienne River, France. Main objectives are to characterize the dynamics of fine and coarse sediments for conditions close to the incipient motion of gravels and for different degrees of clogging of the river bed. The field campaign was carried out on a gravel bar located 10 km downstream of St Jean de Maurienne during the dam- flushing event on June 17th, 2014. Six patches including tagged cobles (using PIT-tag) were built up with three different characteristics: cleaned patches formed with gravels only, patches with a natural mixture, and patches artificially clogged with fine sediments. These patches were located on the side of a secondary channel on the gravel bar. During the flushing event, surface velocities and water levels were measured thanks to video analysis (LSPIV) and pressure gauge, respectively. Intense water sampling was also achieved both upstream and downstream the secondary channel. During the post-event survey the day after, the PIT-tag search showed that the coarse particles did not move. On the other hand, a significant amount of fine sediments were deposited on the patches and infiltrated on the clean patches. An estimation of the fine sediment deposition rate is proposed from these measurements.