Hydrodynamic and Water Quality 1D Modeling for Screening Impact Assessment on Jaunay Reservoir: Results, Limitations and Perspectives

The Jaunay reservoir is a drinking water reserve of 3.7 Mm3 which extends over 114 ha and stretches over 8 km. It results from the construction of a 200 m long dam located in Landevieille (Western France). A 1D MIKE-HYDRO-River model was developed to understand the hydrodynamics of the reservoir over the years 2016 to 2020 and to simulate the dilution of several water quality parameters. The 1D model considers a succession of 70 cross sections and calculates the heights of water and flows every 100 m. The daily flows measured at the Réveillère hydro station located 2 km upstream the reservoir, are injected directly into the upstream boundary condition of the model. A rain-flow model (GR4J) is used to assess lateral flows. Upstream pollutant inputs were determined based on the measures on this station. The water quality modeling was carried out with two approaches: Macropollutants by coupling the EcoLab module to the 1D hydrodynamic model MIKE HYDRO River and dispersion of chlorides and micropollutants with the Advection–Dispersion module of a conservative tracer. The model considers 7 state variables: Biological Oxygen Demand (BOD), Dissolved Oxygen (DO), Chlorophyll a (CHL), Ammonium (NH4), Nitrite (NO2), Nitrate (NO3) and Phosphate (PO4). The model was calibrated over 2019–2020 using in situ measurements. The model was then compared with available measurements for 2019–2020 period and the results present a good correlation for oxygen and nutrients. For Chlorophyll a, the order of magnitude of the base levels and the peaks were well reproduced, but the results do not show the two seasonal peaks (summer bloom and autumn bloom) observed on the measured data. Indeed, the present model was unable to reproduce the autumn bloom correctly, mainly because a remineralization of nutrients and an interaction with sediments which were not considered. Measured and modelized data were then compared for each one of the 7 state variables using a F-test and with 95% confidence they present no significant statistical differences. Despite its limitation, the 1D water quality model could be a simple and efficient solution to estimate the major impacts due to climate change, to the evolution on the activities upstream the reservoir and other projects that may influence on the water quality dynamics.