Michel Poulin

Pluri-annual sediment budget in a navigated river system: The Seine River (France)

This study aims at quantifying pluri-annual Total Suspended Matter (TSM) budgets, and notably the share of river navigation in total re-suspension at a long-term scale, in the Seine River along a 225 km stretch including the Paris area. Erosion is calculated based on the transport capacity concept with an additional term for the energy dissipated by river navigation. Erosion processes are fitted for the 2007-2011 period based on i) a hydrological typology of sedimentary processes and ii) a simultaneous calibration and retrospective validation procedure. The correlation between observed and simulated TSM concentrations is higher than 0.91 at all monitoring stations. A variographic analysis points out the possible sources of discrepancies between the variabilities of observed and simulated TSM concentrations at three time scales: sub-weekly, monthly and seasonally. Most of the error on the variability of simulated concentrations concerns sub-weekly variations and may be caused by boundary condition estimates rather than modeling of in-river processes. Once fitted, the model permits to quantify that only a small fraction of the TSM flux sediments onto the river bed (<0.3‰). The river navigation contributes significantly to TSM re-suspension in average (about 20%) and during low flow periods (over 50%). Given the significant impact that sedimentary processes can have on the water quality of rivers, these results highlight the importance of taking into account river navigation as a source of re-suspension, especially during low flow periods when biogeochemical processes are the most intense.

Impact of hydro-sedimentary processes on the dynamics of soluble reactive phosphorus in the Seine River

This paper focuses on soluble reactive phosphorus (SRP) dynamics along a 225xa0km stretch of the Seine River, including the Paris urban area, for the 2007–2011 period. The impact of hydro-sedimentary processes on SRP concentrations and fluxes is estimated under various hydrological conditions. Sorption interaction parameters between SRP and suspended matter are experimentally determined on river water samples and are included in a hydro-ecological model. Simulated concentrations are compared to weekly measurements at 11 monitoring stations. The introduction of sorption in the model reduces the root mean square error of simulated SRP concentrations by 20xa0% and allows the simulation of particulate inorganic P (PIP) accumulation in the system. With these ameliorations, the model constitutes a reliable management tool, which is compatible with the requirements of new regulations as the European Water Framework Directive. P mass balances are assessed upstream and downstream the major waste water treatment plant of the Paris urban area. P fluxes in the system are mainly driven by hydrological conditions and sediment-related processes. While SRP is the predominant P form during low flow, PIP accounts for more than 70xa0% of the total P during high flow. Moreover, SRP sorption fluxes are of the same order of magnitude as biotic fluxes affecting SRP concentrations. According to the model, and based on all the available data, 75xa0% of the SRP release by the river bed sediments occurs during high flow periods, and PIP exchanges at the sediment–water interface are more than 4 times higher during high flow periods than during low flow periods.