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International audience; Hydropower plants can profoundly modify the natural morphology and hydrology of rivers and can alter the functionality of habitats for fish living and reproduction. In particular, hydropeaking leads to rapid and frequent changes of the hydrodynamic conditions and it is crucial to ensure aquatic habitat quality is maintained as much as possible during these periods. One present method to determine mitigation measure (minimum and maximum flow, rate of change) is to perform hydraulic simulations in 1D or 2D in the range of flow variation and to evaluate habitat quality for fish with the microhabitat method. The hydraulic model calibration has to be conducted carefully since the model has to reproduce precisely the hydraulic conditions from low to high flow rates (up to several times the mean flow of the rivers). Within this range, the friction coefficient can evolve greatly because at low flows the size of roughness elements become comparable to the water depth. This phenomenon is observed by performing the modelling and the calibration at 2 stations on a river in French Pyrenees with different cross section shapes, one with progressive overflowing of some banks with large blocks and the other one without such phenomena. Thanks to field measurements of water levels at low and high discharge, the calibration process has shown that the friction coefficient can be multiplied by 2 as a function of the discharge. The paper proposes a methodology to evaluate the most appropriate tool. As water depth is concerned, the 2D simulations (TELEMAC 2D) provide similar results to those obtained with 1D (HEC-RAS) because flow remains unidirectional. Then a sensitivity analysis is carried out to estimate the uncertainty on the fish habitat outputs for a fish species (brown trout in the present study) resulting from several widely used friction laws. These friction laws can lead to different conclusions about habitat suitability depending on the calibrated coefficient. Finally, to perform relevant habitat modelling, it is necessary to measure water levels at several discharges and to describe accurately the spatial variability of roughness height.

[Departement_IRSTEA]Eaux [ADD1_IRSTEA]Hydrosystèmes et risques naturels [ADD2_IRSTEA]Dynamique et fonctionnement des écosystèmes [ADD2_IRSTEA]Systèmes aquatiques soumis à des pressions multiples; International audience; The analysis of concentration-discharge (C-Q) relationships provides useful information on the processes controlling the mobilization and delivery of chemical elements into streams as well as biogeochemical transformations in river networks. Previous metrics developed to characterize export regimes seldom considered the possibility for the C response to Q dynamics to differ between short-term Q variations during storm events and seasonal Q variations during baseflow periods. Here, we present the “C-Qquick-slow” model, which considers the possibility for C-Q relationships to vary across temporal scales. This model was applied in 219 French catchments with various sizes (11–2500 km2), land use and hydrological contexts. We evidenced contrasting export regimes for nitrate (NO3 −), total phosphorus (TP) and soluble reactive phosphorus (SRP), and surprisingly consistent C-Q patterns at the seasonal scale for each parameter. For instance, NO3 −-Q relationships were positive at the seasonal scale in 75% cases and relationships during storms showed either a dilution pattern (24% cases), a non-significant pattern (50%), or a mobilization pattern (12%). TP and SRP relationships with Q at the seasonal scale were almostsystematically negative (95%), and patterns during storm events were in most cases mobilization for TP (77%) or non-significant for SRP (69%). We linked the different C-Q relationships with catchment descriptors and found that indicators of diffuse source loading determined NO3 − seasonal amplitudes, and hydrological drivers could explain the behavior during storms. By contrast, point sources determined P seasonal amplitudes, and diffuse sources controlled P dynamics during storms. The C-Qquick-slow model has the potential to improve nutrient loadestimations because of the good predictability of appropriate C-Q archetypes and the possibility to interpolate low frequency concentration data to a daily frequency

International audience; In order to respond to the growing concern on invasive alien species and their impact in freshwater environments in France, a working group on biological invasions in freshwater environments was created in 2009. Meeting internationally recognised recommendations, its main objective is to increase management capacity by valuating and promoting expert knowledge, digesting and giving access to scientific information and providing guidance on decision-making. Coordinated by the French Biodiversity Agency and the IUCN French Committee, the group brings together more than 60 members from communities of practitioners, scientists and policy-makers. The group’s activities are determined by the shared needs of the formed network. Examples of projects undertaken to date include: the development of an internet platform to provide access to information; the publication of a best practices guide with fully detailed feedbacks from management efforts; and the setting up of an information database focussed on the operational management of introduced aquatic species in France

International audience; In April 2015, the efficiency of a system of low-sloping rack and bypass has been assessed for Atlantic salmon smolts at the small hydroelectric plant (HEP) of Auterrive on the Gave d’Oloron River (France ; turbine discharge: 7.8 m3.s-1 ; rack inclination: 26° relative to the horizontal ; bar spacing: 20 mm ; bypass discharge: 0.5 m3.s-1). 239 hatchery smolts (mean length 185 mm) were PIT-tagged and released 100 m upstream the HEP, in 5 groups at different times of the day. Their passages downstream the HEP by the bypass and as well by the fishpass for upstream migration were monitored with RFID antennae. On average 80.7% of smolts migrated through the HEP bypass and 3.8% of smolts descended through the fishpass. In total, 84.5% (min-max: 75.5–91.9%) of smolts migrated downstream the HEP via safe routes. 50% of them did it in less than 23 minutes since their release and 75% of them did it in less than 2 hours. Fish migration time was similar for afternoon, evening and night releases (median times between 19 and 21min), but was significantly longer when the fishes were released in the morning (median migration time: 3hours 17min). Our findings give credence to the recommended design criteria for low-sloping racks, which is the main solution implemented in France for small HEPs.