Hyporheic flux, river/groundwater exchange

Impact of dam removal on hyporheic flux and exchange between river and groundwater.

Coordinator

Alain Crave

Context and issues

Fluxes into the hyporheic zone are controlled by the riverbed physical properties and the hydraulic gradients establishing between the river and the neighbouring alluvial water table. The spatio-temporal variations of those gradients creates complex and ephemeral flow cells that drives the transport of matter in the sedimentary bed, impacting in turn physical, chemical and biological processes in a yet partially unknown fashion. Today, in-situ data characterizing vertical gradients of temperature, solutes and dissolved gas in natural river systems remain sparse and numerical models of hyporheic transfers and biological functioning rely on crude assumptions which are urgent to test and refine. The dams removal on the Selune River may imply significant modification of hyporheic processes at different reaches of the river that are currently clogged. LEARN project will document the recovery process into this critical zone for the river resilience.

Objectives of the project

The first  goal of the PhD is to monitor and describe how geochemical processes into the hyporheic zone respond to hydrological (flood, water table), and sediment (clogging, roughness, compactness) forcing variability. The second  goal is to analyse this field data to develop a dynamic numerical model (inspired from published biochemical models) of hyporheic processes according to hydraulic and sedimentary boundary conditions. This model will be calibrated and tested on field data to predict the variability of physical and chemical conditions and the associated microbial activity in climate change typical scenarios.

Methodology

The in-situ monitoring will be based on a methodology that have been developed at Géosciences  Rennes to follow temperature gradients and dissolved gas concentrations. Additional methodology will be developed during to monitor pressure gradients and passive sensors to measure solute and dissolved gas spatial distributions. One of the challenge will be to deploy in-situ monitoring devices in order to follow the spatial and time variability of physical and chemical gradients and dissolved gas concentrations in relation with surface processes variability Numerical modelling will be based on published hyporheic exchange models.

Laboratories involved

UMR 6118 Géosciences Rennes

UMR 6553 ECOBIO

ARMINES Geosciences center

BRGM Brittany department