PREVENTING WOOD-RELATED HAZARDS IN MOUNTAIN BASINS: FROM WOOD LOAD ESTIMATION TO DESIGNING RETENTION STRUCTURES

Large wood (LW) represents one of the main problem for risk prediction in Alpine streams mostly because of its potential to clog bridges, culverts and narrow sections during flood events. To prevent wood from reaching critical sections, since long time wood-trapping structures, along with the periodic removal of mature riparian vegetation and in-channel wood, have been built in the European Alps and in Japan (Lange and Bezzola, 2006). However, the overall efficiency and thus the “success” of such structural measures depend on many factors which include LW volumes and rates, timing of LW transport during the flood, individual LW size, interaction between bed, LW and sediment in the proximity of the structure, structure location and orientation with respect to the flow, structure geometry, and local flow characteristics (Rimbock, 2004). It is thus evident how designing an efficient system for wood retention is not an easy task. The main goal of the paper is to describe a rational procedure to be applied in mountain basins for coping with wood hazard, a problem which until now has often been tackled without a systematic framework to guide the final design of retention structures.