Design of a debris retention basin enabling sediment continuity for small events: the Combe de Lancey case study (France)

Abstract

The Combe de Lancey stream is a relatively calm tributary of the Isère River flowing through the city of Villard-Bonnot near Grenoble (France). In 2005, a long-lasting extreme rainfall event triggered dramatic erosion processes in this 18 km² granitic catchment. A volume of 20,000 m3 of sediment and logs deposited in the paper factory located near the fan apex. This paper focuses on the definition of a new protection system, namely a debris retention structure made of an excavated basin and an open check dam. The design is based on expert knowledge and tested byphysical small scale modelling. The particularity of this case study relies on two points: (i) its design scenarios and (ii) the structure capacity to transfer small events. Attention was paid to define several 100-year return period events used as “design events” for which the structure must have its best effectiveness. Extreme events with higher return periods called “safety check events”, for which the structure must still withstand the event without failure but with acceptable marginal damages, were also modeled. The definition of the scenarios is described in the paper. Secondly, the debris retention basin should have the capacity to transfer small debris floods without trapping sediment in order to prevent downstream incision and heavy maintenance costs. It must however trap nearly totally the sediment and large woods that are erratically supplied by the catchment during extreme events. Classical debris retention basins are usually not able to achieve such a dual objective. Here, two concepts developed in past works, namely a guiding channel and a hybrid open check dam with mechanical – hydraulic control were successfully tested. The paper presents the design and testing procedure of this case study exemplifying the next generation of debris retention structures.