Widening and sediment reloading of the Drac riverbed in Saint-Bonneten-Champsaur-France
The physical restoration of the riverbed upstream of Saint-Bonnet was a priority in the river contract for the upper Drac signed in 2011. The plan was for a major operation to physically restore the riverbed by widening the bed (lowering the banks) and reloading the river using the coarse sediment taken from the banks.
During the first phase, 3.6 kilometres of alluvial terraces along the Drac were cleared of trees and trunks, representing a total surface area of 27 hectares. The active channel of the Drac was thus considerably widened and the alluvial bars were once again exposed to the river currents. Some small islands with vegetation were maintained to enable the formation of side channels, secondary channels and nesting sites.
The second phase consisted of a vast operation to reload the bed with 450,000 m³ of alluvium removed from the former terraces of the Drac and from zones of sediment accumulation along other rivers in the river basin.
During the third phase, following the return of the water to the bed, the new banks were stabilised using geogrids and bundled wood (fascines), then seeded and planted with over 6,400 willow cuttings and 500 helophytes, notably along the track for maintenance access and walkers on the right bank. In addition, 13 hectares of wetlands and parallel streams were created, primarily in the lower section of the reach.
In order to make the transverse obstacles compatible with the river continuity requirements stipulated in article L. 214-17 of the Environmental code, works was done on the weirs at the Baraques bridge and the recycling center site, located downstream of the reloading zone, during the year prior to the reloading. It consisted amongst others of building a studtype fish pass designed specifically for trout and a passageway for canoes.
Monitoring surveys and results
In the framework of the network of demonstration sites, the Water agency established a monitoring programme before and for five years (n+5) after the works, addressing biological aspects (fish, macroinvertebrates, diatoms) and the hydromorphology. This programme was filled out with a complete survey of active and potential redds for trout, in the main channel and in the reconnected secondary channels. This survey was carried out by the Onema local office*. A multi-partner monitoring plan (CLEDA, Water agency, Onema, Hautes-Alpes department, IRSTEA, Écrins national park, consulting firms) has been set up to complement the EU ALPeril programme that made possible Lidar topographical surveys in 2009 and 2015. In addition, a thesis organised by Onema/CNRS/ University of Paris 7 is now under way to assess the effectiveness of restoration works on rivers characterised by high levels of sediment transport and to study the dynamics of sediment transport.
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initiated byOnema, ETRM, Burgeap, Geolithe
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Type of initiativeOperational level
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StatusCompleted
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CountryFrance
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RegionProvence-Alpes-Côte d'Azur
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LanguageEnglish
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KeywordsConservation / Restauration
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Optional KeywordsFlood risk management, Habitat and biodiversity, Hydromorphology, Monitoring, Spatial planning, Water quality
Environmental flows and water resources, Habitat and biodiversity, Hydromorphology -
Scale of implementationLocal or River section
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Stakeholder involvementPolicy makers - public administrators
Interest groups
Experts -
Level of public participationInform
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FundingNational
Regional -
Start dateFriday, 01 November 2013
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End dateSunday, 01 June 2014