Gestione sostenibile dei sedimenti in fiumi ghiaiosi incisi in Francia

In Europe, most of large rivers have been modified by human activities for centuries. The earlier activities concentrated on works for flood protection and navigation. The channel features underwent se- rious disruptions in the 19th century, when systematic hydraulic engineering measu- res were conducted along reaches of 1 to more than 100 km length. These major and widespread regulation efforts totally modified the morphodynamics and sedi- ment transport behaviour and also initiated channel changes like river bed degradation. Such morphological changes led to abiotic and biotic consequences followed by ecological and economical impacts. Today’s critical situation in terms of management (channel instability problems, limitations of flood regulation works, biodiversity decrease) has made river restoration and sustainable management of the bedload an important issue in the French Alps and their surrounding areas. This policy must take into account the European Water Framework Directive (WFD), which aims to ensure that rivers reach good ecological status by 2015 (EUROPEAN PARLIAMENT AND COUNCIL OF EUROPE, 2000). In the Alpine and piedmont environments, the energy available due to steep slopes is important, and channel features are sensitive to changes in control parameters (peak flow regime and bedload input). The main challenge is therefore related to identifying the processes and key parameters to improve the geomorphic conditions under often restricted boundary conditions. Whereas early attempts at river restoration mainly designed small-scale measures, successful restoration and sustai- nable sediment management projects in high-energy and bedload transport-dominated conditions have to include the full spectrum of scales and should concentrate on initiating self-forming morphodynamics. In this context, we present the problem of the river incision and the bedload deficit, and discuss how this evo- lution has led to the necessity of sustainable management of sediment, and notably the promotion of artificial bedload supply, the bank erosion preservation and restoration, but also measures for mitigating ecological impacts of channel incision. This paper is a synthesis of information and examples mainly reported in BRAVARD et al. (1999), AMOROS et al. (2005), PIEGAY et al. (2005), HABERSACK & PIEGAY (in press).