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Sediment transport on complex bottoms

Sediment transport in the context of river flows is at the heart of the themes highlighted in the research project of the Pôle Écohydraulique at IMFT. Indeed, the Water Framework Directive 2000/60/EC imposes a better management of sediments by aiming to ensure the continuity of their transport along rivers. However, the prediction of fine sediment fluxes faces two specificities of rivers: historically depleted bottoms in fine sediments (made of coarse gravel, pebbles, or the rocky bed itself) and/or covered by vegetation. In the first case, the fine sediment is transported over a complex, non-erodible natural bottom, with a very complicated flow structure (see section “Turbulent boundary layer over natural bottoms”). In the second case, the sediment can be stored by the vegetation which plays a trapping role thanks to its canopy where the turbulent intensity and velocity drop.

As part of David Raus’ PhD thesis (2015-2018, OFB Funding), synthetic sediment transport experiments over a bed of hemispheres were performed. In particular, a fine-grained study of the hydrodynamics has shown that, during the uncovering of the hemispheres, the transport is initially amplified. Indeed, the turbulence level in the vicinity of the sediment is increased by the wake behind the hemispheres. Uncovering rates of at least 50% are in fact necessary for the local conditions to become compatible with a stop of the transport. This shows that in this context, classical transport laws are not applicable, even if we take into account a partitioning of the total friction on the flow into a part related to the emerging hemispheres, and another part attributed to the sedimentary bottom.

In the context of sediment transport in a plant canopy, however, the PhD thesis of H. Romdhane (2015-03/2019, SECADENORD Funding) showed that this partitioning approach could give reasonable predictive results, provided that the vertical structure of the flow in the canopy and above is well-known. In this PhD thesis, the flow structure in the canopy is given by the semi-analytical model of Cassan et al. (Water, 2017) with calibration on measurements above the canopy.

Transport de sédiments synthétiques au-dessus d’un lit d’hémisphères (Thèse D. Raus, 2015-2018, Financement OFB)
Transport of synthetic sediments over a hemispheric bed (PhD thesis D. Raus, 2015-2018, OFB Funding)