Since ten years, international programmes such as LOICZ (Land Ocean Interactions in the Coastal Zone) have defined priorities for the study of interactions between the continent and marine environment. Now, research focuses more specifically on the margins (the interfaces between the continental shelf and the deep abyssal zone) dominated by fluvial deposits, and, more especially, on the margin near the Rhone River.
Deposits from the Rhone into the Gulf of Lions play a major role in structuring ecosystems in the coastal area, in terms of biology and hydrology as well as in terms of sedimentation. Solid deposits from the Rhone, and especially the sand load, are a major factor used to check coastal sedimentary inventory and, therefore, to monitor changes in the coastline. The finest particles primarily fix agricultural contaminants and industrial and urban waste. Most of this fine material is deposited around the edges of the river mouth at depths of 30 to 80 m in an area known as a "prodelta".
Recent studies have shown that the flow of sand and fine particles and, as a result, the related contaminant flow mainly occur during times of high water. Paradoxically, however, the Rhone has been studied in a highly fragmentary manner and no measurements as it continues into the marine environment have been taken with a view to understanding the impact of natural cycles and events on the river. This lack of monitoring makes it difficult to manage the coast area, in terms of changes to the coastline or in the event of pollution carried by the river. In comparison, the rivers and coastal areas affected by the Seine and the Rhine are continuously monitored to study currents and the hydrology of suspended loads and nutritive salts.
Challenges and objectives
The main aim of the CARMA project is to understand the impact of Rhone River alluvia on the dynamics of hydro-sedimentation in the region.
This entails studying the impact of high waters and storms on deep changes in the morphology of the coast and on what happens to fine particles that trap organic matter and contaminants from human activities. This research will enable us to understand and quantify the time and space dynamics of what happens to Rhone River sand and fine-grained particle alluvia, identifying the different pathways of such matter as it leaves the river and deposit zones, measuring the residence times of deposits, erosive flow and redistribution (exported and redeposited sediment flow). This approach involves a complementary and mutually-enriching combination of field measurements and modelling hydrodynamic parameters (current and swell), together with modelling sedimentation and the transport of suspended matter.
Very little is understood about the impact of storms in terms of the process of resuspension, or about changes over time in the physical properties of sediment (fraction settled / exported, permanent burial or remobilisation). Insofar as the finest sediments are linked to contaminants and organic matter, and given that they are intricately related to the development of benthic populations, accurate modelling of the dynamics in the region around the mouth of the Rhone is now a priority.
Project description, methodology
The methodology used for this programme is based on two components:
current profiling, started in Autumn 2006 and involving all the CARMA project partners
and monitoring morphological changes in priority study areas a few hundred metres in width.
These “units” should reveal the complexity of the area under study. The strong points of this project are the use, for the first time ever, of high-performance instrumentation (Doppler current metres, sonic altimetres) in the mouth of the Rhone and the development of related models.
To achieve the project’s objectives, several methods are used to monitor sediment inventory:
comparative mapping of bathymetric depth,
on-site recording of sedimentation rates (altimetres),
hydraulic parameters (current and swell profiling),
sedimentary flow (trapping and ADCP),
physical parameters (turbidity, temperature and salinity probes).
Naturally-occurring radionuclides (210Pb, 7Be, etc.) and artificial radionuclides (137Cs, 239+240Pu, etc.) are used as markers to track continental deposits in this area. For the purposes of the CARMA project, they will be used conventionally to quantify sedimentation processes, but also to study the phenomena of translocation of sediments deposited by the Rhone in the Gulf of Lions.
All the data gathered as part of the project will be used to create three kinds of model:
models of dune erosion at La Gracieuse spit, based on the SBEACH 2DV model (vertical two-dimensional model), which simulates recession of the shoreline and dune erosion during storms. This model has already been tested, calibrated and validated for the beaches in the Camargue as part of a project backed by the national Liteau programme.
models of estimated sand deposits from the Rhone to surrounding beaches, performed using a 2VD model which calculates sediment transit along the coastline, factoring in the effect of river flow rates on swell and wave-breaking propagation (NMLONG model, licensed to the CEREGE). The results obtained will only be orders of magnitude, but they will, above all, make it possible to identify differences in the supply of sand to the beaches via transit along the coast during high waters and/or when conditions are calm.
models of the different stages in the transport of particle matter deposited by the Rhone using the SYMPHONIE 3D model combined with a model of sediment transport and a swell generation and propagation model. This will be used in simulations at each stage of data acquisition in the field in order to calibrate and validate it.
The final report of the CARMA project was published in June 2009.
This project was a fundamental step in the instrumentation strategy of the Rhone mouth and in the observation and recording of sediment transport to the Rhone mouth, particularly during extreme events. CARMA also enabled the development and validation of the hydro sedimentary models SYMPHONIE (LA Toulouse) and MARS3D (IFREMER, thesis by François Dufois).
The French research agency ANR's EXTREMA project is already able to use the feedback from the CARMA project and the scientific and technical dynamic that has been created. An agreement was signed in early 2009 (Contract N° 08/3211103) between Phares and Balises (West Mediterranean subdivision, Direction Départementale du Rhône) and scientific partners IRSN, IFREMER, CEREGE, CETMEF, COM and LSGE to install and equip a fixed buoy with a programmable sediment trap (BFI) in the mouth of the Rhone. This will help data acquisition and provide physical protection for all scientific equipment, from trawling operations in particular. The ANR EXTREMA project is able to use this equipment from autumn 2009. It should also be noted that the results of the CARMA project feed directly into the IRSN EXTREME project.
The benefits for society
In delta areas, the benefits for the local economy and industry and for the natural heritage are many and varied.
First, the environmental benefits: in the Rhone delta, the sediment balance of the beaches is a major factor in protecting the wetlands behind barrier beaches. A reduction in sediment deposits from the Rhone, and poor redistribution to the surrounding beaches, has, for the last thirty years, caused major problems due to erosion of barrier beaches and recession of the shoreline.
Second, the industrial benefits. In fact, these interface areas form the primary protective barrier against sea swell and thus protect industrial and port facilities; more particularly, in the Rhone delta, those found in the Gulf of Fos and in Marseille, as well as shellfish farming in Carteau Cove.
Thirdly, the benefits for the tourist industry. The beaches on either side of the river mouth are a major attraction insofar as concerns tourism and leisure activities centred around the towns of Arles and Port Saint Louis du Rhône. In addition to problems concerning changes to the coastline, the future of sediment deposited by the Rhone in the Gulf of Lions is of great importance to environment management agencies.
This last point is also crucial to the fishing industry, which is highly developed in the Gulf of Lions, an area greatly affected by deposits from the Rhone.