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    Pierre Steinbach, engineer at the Centre - Poitou Charentes regional office. Pierre Steinbach manages the inventory and analysis of obstacles to ecological continuity for the entire Loire Bretagne basin


Pierre Steinbach, engineer at the Centre - Poitou Charentes regional office

Pierre Steinbach manages the inventory and analysis of obstacles to ecological continuity for the entire Loire Bretagne basin

Onema is a participant in the inventory of river obstacles and analysis of ecological continuity in all French river basins. Why is such an inventory useful?

The inventory of obstacles will enable us to identify the main breaks in ecological continuity that block the movement of fish and sediment. An inventory is also the means to assess the overall pressure exerted on French rivers. This is because the obstacles to flow deform and break up rivers. They also exacerbate the processes of eutrophication*, evaporation and temperature rise. In operational terms, knowledge on obstacles is a major tool in restoring the ecological quality of rivers because it is required to set priorities for treatment. Finally, the inventory is required by a number of European directives and regulations.

Loire-Bretagne is the first and largest river basin in France to benefit from the inventory. Were there any specific characteristics or difficulties?

The Loire-Bretagne basin covers 28% of continental France and represents 135 000 kilometres of rivers. The difficulty for the inventory was the size of the basin, in which the evaluation must be consistent and homogenous. The 180 technicians must work in the field using the same evaluation method, applied to environments as different as the coastal wetlands in the Vendée and small mountain streams in Lozère. Concerning the impact of obstacles and given the requirements of the European regulation, we looked in particular at their effects on the migration of eels because restoring the population of these migratory fish is a major ecological issue in the basin.

Practically speaking, how did you go about the inventory?

The method was developed by the Orléans regional office to evaluate obstacles to upstream migration. The contact persons from the regional offices in the Loire-Bretagne basin were trained during a seminar organised in Poitiers, in collaboration with the Ecohydraulic centre in Toulouse. Subsequently, further training and management of the field work were provided by each regional office. An evaluation document for each obstacle indicates the location and status of the obstacle, its current economic use and a number of physical characteristics including height of fall at low flow rates, slope and changes in slope, and any equipment installed at the site of the obstacle. What is the flow rate through the turbines during operation and what are the characteristics of the turbines? Is there means to overcome or avoid the obstacle for fish? If necessary, we go back for more information. The central question is to know whether the eels can colonise the basin and then migrate downstream to the ocean and reproduce.

What are the initial results of the inventory?

The inventory for the Loire-Bretagne basin revealed the high number of obstacles on our rivers. On average, there is an obstacle every 3.4 km of river throughout the main hydrographic network. And the inventory is not yet finished. Over 10 000 installations have been recorded and there are probably some 20 000. Many obstacles are located on rivers located on private property, which complicates the inventory work. Our work showed that a vast majority of the installations no longer have any clear economic use and their impact on the natural slope and operation of rivers is considerable.