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Biofilm and vegetation

Primary production in rivers plays a crucial role in the quality and ecological functioning of hydroecosystems. It also impacts their flow rate, an important parameter in the issue of flooding. Two main actors of the primary production have been studied, the biofilm on one hand and the vegetation on the other hand.

The epilithic biofilm (i.e. covering the stones on the river bottom) was studied in the laboratory at IMFT in collaboration with Laboratoire écologie fonctionnelle et environnement. Through a joint analysis of direct numerical simulations on a hemisphere bottom performed as part of a PhD thesis and growth experiments at IMFT, it could be shown how the colonization dynamics were controlled by the boundary layer in the vicinity of the hemispheres: the biofilm grows around the flow breakpoints in the roughness sublayer of the hemispheres (Coundoul et al., River Research and Applications, 2014). The structure of this boundary layer in the vicinity of the breakpoints still presents a Reynolds dependency, in contrast to the hemisphere-forced boundary layer, which is in the fully rough turbulent flow regime.

Regarding the impact of vegetation on the flow rate, we first characterised in irrigation channel the evolution of the flow rate as a function of vegetation growth. This growth leads to an inability to supply water to irrigators and biases the flow estimation, which leads to excessive withdrawals from the water resource actually available. From fine velocity measurements, we identified a time-dependent growth model that predicts the change in rating curves. The presence of vegetation was related to the increase in turbulence near the bottom and its influence on the vertical velocity profile. These in situ measurements were the first step towards investigating analytical models for velocity profiles over flexible vegetation (Cassan et al., Environmental Fluid Mechanics, 2015). The following work discriminated the most relevant models to predict flows (friction law) and velocity profiles as a function of vegetation characteristics (height, diameter and density). Knowledge of vertical gradients can thus be achieved and allows for a better understanding of their influence on biological processes such as nutrient or pollutant transport.

Study of the effect of a vegetation patch on a rough bottom