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  • VIEWPOINT OF PARTNERS

    -Olivier Thibault \ Artois-Picardie water agency
    "An analysis of hydromorphological alterations was carried out in each river basin for the 2004 status report, but using very different methods..."


    -Laurent Valette \ Cemagref
    "The Syrah method used to assess the risks of hydromorphological alterations in rivers is based on analysis spanning two scales, the catchment (large scale) and the reach, in order to achieve an overall view..."

Evaluating risks of hydromorphological alterations

In a partnership with the Water agencies and with scientific and technical support from its joint hydroecology centre with Cemagref in Lyon, Onema is currently mapping zones at risk of hydromorphological alteration that could result in failure to achieve good ecological status. The goal of the maps, drafted using the Syrah method, is to identify zones requiring particular attention. In 2013, they will help in revising the status report that must be filed for the WFD.

Various pressures (intensive farming, extraction of gravel, dams and reservoirs, urbanisation) spread over the hydrographical network result in alterations in the physical operation of rivers. The researchers at the Onema/Cemagref centre in Lyon have developed a method, called Syrah*, to assess the risks of hydromorphological alterations in rivers based on their knowledge of the pressures.

Evaluating the risk of alterations on the reach scale in rivers

After publishing a national atlas on engineering work and water uses that could result in alterations (see the Highlights for 2009), the researchers developed a method to assess the risks of hydromorphological alterations on a smaller scale, i.e. that of each geomorphological reach. Almost 230 000 kilometres of rivers in continental France were divided into 69 500 reaches, each representing a section of a river with specific geomorphological characteristics.

An operating mode was assigned to each reach in rivers. This is because pressures do not produce the same impacts in reaches with different geomorphological operating modes and consequently the risks of alteration also differ. For example, engineering works produce less impact on dynamic, mountain rivers with significant fall then on slower rivers located in plains. The researchers thus propose to combine, for each reach, information on pressures (travel routes, levees, water bodies, gravel-extraction zones, bridges, etc.) with data on the typology of the river where the pressures are exerted.

An operational tool for revising the status report

Thanks to the Syrah method, the basin-level technical offices (Water agencies, Onema, DREAL) have an operational tool to map all risks of hydromorphological alteration in their basin and to set up optimised solutions. With technical support from Onema, Syrah will help in revising the status report in 2013.

Informational meetings in the river basins

Onema and the hydroecology centre organised day-long meetings in each of the six basins in continental France to inform the local actors (Water agencies, DREAL, departmental councils, local water commissions, public river-basin territorial agencies, fishing federations, etc.) on the considerable progress in hydromorphological knowledge made possible by Syrah and to familiarise them with the results. These meetings were an occasion the present the methods and its basic principles, as well as examples of its use on the local level.

* Relational system for river hydromorphological audits.

Initial results from field data

Analysis of the risks of alteration took place in step with implementation by Onema personnel of Carhyce, the standardised national field protocol that is used to measure precisely, on the station level, the physical characteristics of rivers. It was deployed in 2009 in 230 stations of the surveillance-monitoring network (RCS) and the initial results obtained in 2010 made clear the value of the data produced. The results revealed a close link between the hydromorphological characteristics of rivers and biological populations, and in particular the role of sediment dynamics and riparian corridors in biological responses. This initial, exploratory work will continue in 2011 with the development of hydromorphological indicators for the surveillance programme. This work, carried out in conjunction with the University of Paris I and Cemagref, will be finished in 2013 in order to have in 2015 a set of calculated indicators for each of the 1 500 stations in the WFD surveillance network.

A hydromorphology manual for water stakeholders

The goal of the book Elements of fluvial hyrdomorphology, published by Onema, is to provide the knowledge required to run river-management and restoration projects. Jean-René Malavoi* and Jean-Paul Bravard**, the authors, reveal the extraordinary complexity of the phenomena involved in "making" a river. They explain the geomorphological characteristics of rivers that create the major types of habitat on which aquatic and terrestrial species depend. The book comprises over 400 illustrations, including photos and diagrams.

* works at the Onema/Cemagref centre in Lyon.
** former professor of geography at the University Lumière-Lyon 2.