Hydrodynamics Of The Orne And Sediment Mobilization In The Backwater Zone Upstream Of The Beth Dam (Lorraine, France)

Orne suffered multiple hydro-morphological changes in the XX th century, in relation to the steel industry located in the downstream valley. In Moyeuvre-Grande, the Beth dam, built in 1958, causes in particular a backwater zone 4.5 km in length, locally presenting alluvial banks as well as solid deposits heavily contaminated by past industrial activity. A study in progress aims to determine the remobilization of these polluted deposits under current hydrological conditions and with a view to removing the dam. This work specifically studies the hydrodynamics of the Orne in the slowdown zone caused by the dam. The biennial flood of the lit June 2016 is used as the benchmark event; its maximum flow at 137 m(3) / s corresponds locally to the flow at full edges. The morphometric study of two sites with a sedimentary bank spaced 450 in apart made it possible to calculate the specific power and the traction stress of the watercourse, coupled with grantdometric measurements. The specific power (about 30 W / m(2)) and the average traction stresses (between 15 and 22 N / m(2)) show the relatively low hydrodynamic energy of the watercourse which can however mobilize sediments of centimeter size by bedload and sand medium to coarse in suspension, during a biennial flood. These first results will be used for the hydraulic modeling which will be done near the dam in order to determine the ablation processes that could occur there following the opening of the valves,Hydromorphological changes were applied to the Orne River during the 20 th century to suit the demands of the steel industries. In the downstream region of the Orne valley, near Moyeuv re -Grande, the Beth Dam constructed in 1958 causes a 4.5 kin long backwater zone rich in highly contaminated waste due to the ancient steel making activity. An ongoing study aims to determine the remobilization of these polluted deposits in the current hydrological conditions. Consequently, improved management plans can be proposed before the dam will be removed. This work specifically studies the hydrodynamic of the Orne River in the upstream backwater zone caused by the Beth Dam. The biennial flood of June st2016 serves as a reference; its maximum flow rate of 137 m(3) s corresponds to the bankfull discharge. Morphometric study performed on river bars of two separate 450 in sites allowed to calculate the unit stream power and the shear stress of the river, coupled with grain size measurements. The calculated unit stream power (about 30 W / m(2)) and mean shear stresses (between 15 and 22 N / m(2)) reveal a rather low hydrodynamic energy of the river that can however mobilize centimetric sediments via bed load transport and medium to coarse sands via suspension during a biennial flood. These results are a starting point for future hydraulic modeling that will be carried out near the Beth Dani. The findings will determine the possible ablation processes caused by the opening of the dam gates as well as dam removal.