Source-to-sink sediment transfers, environmental engineering and hazard mitigation in the steep Var River catchment, French Riviera, southeastern France

Steep coastal margins are potentially subject to mass wasting processes inv olving notable landslide activity and sediment evacuation downstream by ste ep-gradient streams. Sediment transfer from short source-to-sink segments, coupled with mountain hydrological regimes, regulate patterns of river chan nel aggradation and coastal sediment supply in such geomorphic settings. On the steep French Riviera margin, sediment transfers from existing landslid es or from various minor mass wasting processes to stream channels may resu lt following bursts of heavy, concentrated rainfall. High-magnitude floodin g and massive sediment transport downstream are generally related to unpred ictable extreme rainfalls. Both mass movements and channel sediment storage pose serious hazards to downvalley settlements and infrastructure. A consi deration of channel sediment storage patterns in the Var River catchment, t he most important catchment in this area, highlights two important shortcom ings relative to environmental engineering and hazard mitigation practices. In the first place, the appreciation of geomorphic processes is rather poo r. This is illustrated by the undersized nature of engineering works constr ucted to mitigate hazards in the upstream bedload-dominated channels, and b y the unforeseen effects that ten rock dams, constructed in the early 1970s , have had on downstream and coastal sediment storage and on sediment dispe rsal patterns and, consequently, valley flooding. Secondly, planners and en vironmental engineers have lacked foresight in valley and coastal managemen t issues on this steep setting, notably as regards the reclaimed areas of t he lower Var channel and delta liable to flooding. Urbanization and transpo rt and environmental engineering works have progressively affected patterns of storage and transport of fine-grained sediments in the lower Var channe l and delta. Meanwhile the problems raised by these changes have not been a dequately addressed in terms of scientific research. A necessary future ste p in bettering the engineering solutions implemented to contain natural haz ards or to harness water and sediment resources is that of fine-scale analy sis of source-to-sink sediment transfer processes, of sediment budgets, of time-scales of storage in stream channels, and, finally, of high-magnitude hydrometeorological forcing events in this area. The way all these aspects have been modulated by engineering practices and socioeconomic development should also be an important part of such an analysis. (C) 1999 Elsevier Sci ence B.V. All rights reserved.