Large wood in rivers and its influence on flood hazard

In terms of flood hazard, the presence of large wood (logs, trees, branches and roots) in rivers may aggravate the consequences of flood events. This material may affect infrastructures such as bridges, weirs, etc., especially those intersecting forested mountain rivers. Until recently, a widely accepted practice was to systematically remove wood debris from river channels as a preventive measure. However, studies have shown that this practice may be useless as the material is transported and deposited after each flood and may even not benefit the long term natural balance of the river ecosystem. Therefore, the presence of this woody material in rivers must be managed and included in flood hazard and risk analysis. In this paper we present a comprehensive methodological approach to study the role of large wood in rivers, with a focus on flood hazard. First, to understand the dynamics of wood recruitment, the contributing areas delivering wood to the streams have to be delineated and the recruitment mechanisms studied. Thus, an estimate can be obtained of the potential volume of deliverable wood. To analyse wood transport we present a numerical model, which allows simulates the behaviour of individual pieces of wood together with hydrodynamics. Finally, we analyse the impact of wood on the magnitude of flood events (in terms of water level, flow velocity or flooded areas), using as an example a flood which occurred in December 1997 in the Sierra de Gredos. The results allowed us to reproduce the wood deposit patterns during the event and to reconstruct the bridge blockage. This caused the upstream water level to rise by up to 2 meters and reduced the flow velocity, which favoured debris and sediment deposits. Consequently, the effects of flooding were equivalent to those of a greater magnitude event. This increase in the flood hazard has been numerically quantified.