Extreme meteo-climatic events, redistribution of sedimentary masses and associated pollutants within a coastal system.
Launched in February 2007, the EXTREMA project is devoted to analysing the impact of extreme meteo-climatic events. The conclusions of the main research programmes devoted to the study of global climate changes (WCRP, GEWEX, PIGB, GICC, IMFREX, etc.) show a clear increase in the seasonality (intra-annual variability) and frequency of extreme meteo-climatic events. It appears that these events fall within the scope of global climate changes. The environment is subject to disturbances over highly variable time and space scales, potentially causing major imbalances or changes: intense precipitation affecting atmosphere-soil or soil-river transfers, episodic deposition of atmospheric particles (Saharan dusts), high waters increasing river flows into the coastal marine environment and contributing to mass transfers from the latter to the deep sea environment, floods affecting terrestrial behaviour, storms causing resuspension of sediments in coastal areas, and cold dense water cascading events causing displacement of significant sedimentary masses from coastal margins to the deep sea environment (with canyons constituting privileged transfer paths).
These extreme phenomena produce exceptional mass transfers and massive redistributions of particle-borne contaminants within the geosphere over very brief time intervals. The material flows involved vastly exceed those resulting from average transfer processes and are often equivalent to the total flow over several months or even years.
Various national and international programmes are being conducted to study these impulsional mass transfers. The programmes in which the IRSN participates focus on continental contributions from rivers, sediment resuspension and coastal erosion. Particular attention is given to exceptional phenomena strongly affecting material balances (ORME, EUROSTRATAFORME, PNEC and CARMA
projects). The vulnerability of environments subject to these massive redistributions of pollutants has not yet been assessed.
Challenges and objectives
The EXTREMA project aims to study the natural processes causing episodic material flows, and their impact on the redistribution of pollutants in the various compartments of the geosphere (atmosphere, soil, rivers, coastal marine environment, deep sea environment).
This work is mainly based on quantifying and modelling the solid and liquid material flows acting as transfer vectors for pollutants. The results will be used to assess the vulnerability of the environment to extreme climate events over the next 20 to 30 years, and thereby answer the following questions:
- What is the impact of massive redistributions of sedimentary masses on the transfer of pollutants within a continent-coastal marine environment-deep sea environment continuum?
- Do these redistributions cause a dispersion/dilution of pollutants in the environment or, on the contrary, a localised accumulation/concentration in certain receiving environments?
- Is the vulnerability of the affected environments modified? If so, over which time scale?
- What are the threshold phenomena potentially inducing sudden changes in the distribution of anthropic pollutants in the environment over the medium term?
Project description, methodology
To achieve these objectives, the EXTREMA project implements an integrated approach encompassing the atmospheric, continental and marine environments in the coastal system of the Gulf of Lions. This study area was chosen for the following reasons:
- geosphere vulnerability (at the scale of the Gulf of Lions) to both extreme meteo-climatic events and pollutions,
- existence of a corpus of knowledge and experience acquired by project partners regarding the various environmental compartments of this coastal system,
- existence of available records and on-site experimental systems (observation networks and instrumentation).
This study area comprises various workshop areas: upstream Rhone and Tet catchment basin, estuaries and deltas of these two rivers, and Gulf of Lions from coastline to deep sea environment (base of continental slope).
Research is focused on two large families of contaminants: artificial radionuclides, which will also be used as tracers (Cs-137 and transuranic elements, covered more specifically in the IRSN's EXTREME project), and trace metals (particularly Pb, Zn, Cd and Cu).
The vulnerability of environments to the impact of extreme climate events over the next decades is assessed by modelling mass transfers in response to various climate change scenarios.
The multiple fields of expertise of partner teams (atmospheric physics, chemistry, geochemistry, morphology, sedimentology, oceanography, modelling) will allow the generation of new data and a combined analysis with historic data. The EXTREMA project should therefore yield the following results:
- determination of changes in frequency of extreme meteo-climatic events in the coastal area of the Gulf of Lions, based on historic records from the last century (precipitation, atmospheric dust, high waters, floods, storms, cascading),
- quantification of episodic flows generated by intense meteo-climatic events, as compared to annual average flows (liquid flow, solid flow, artificial radionuclides and associated trace metals),
- characterisation of the nature and typology of extreme flows, including the identification of preferential transfer vectors (suspended and bedload transport, dispersion and translocation, channels, etc.),
- establishment of empirical force-flow relations within the various workshop areas (large catchment basin, basin of limited size), useful both for extrapolations and predictive approaches,
- determination of the intensity of contaminant redistribution within the various compartments of the geosphere,
- identification of concentration/focalisation points and sink compartments,
- assessment of the duration of imbalances produced by extreme climate events (reversibility or irreversibility, threshold phenomena, sudden changes),
- assessment of the vulnerability of coastal environments to global climate changes over the next 20 to 30 years.