Advances in the analysis of in-stream large wood: recruitment, transport and influence on flood risk

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 (in Spain) was to systematically remove wood from river channels as a preventive measure. However, it is well known now that this practice may be useless as the material is transported and deposited after each flood. In addition, this practice might be not beneficial for the natural balance of the river ecosystem in a long term perspective. Therefore, the presence of this woody material in rivers must be managed and included in flood hazard and risk analysis. However, there is lack of studies in this respect in Spain, and therefore there is a lack of related bibliography in Spanish. In this paper we present a detailed review of recent methods proposed to analyse large wood in rivers, and in addition we propose 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 processes studied. Thus, an estimate of the potential volume of deliverable wood can be obtained. To analyse wood transport we present a numerical model, which allows simulating the behaviour of individual pieces of wood together with hydrodynamics. These methods have been tested in several rivers in Gredos Mountain Range (Avila, Spain), and we present here a synthesis of the main obtained results. As a final conclusion of these results we can affirm that a good river management requires a proper management of the riparian vegetation and in-stream large wood, and this management should find the equilibrium between the good ecological conditions and the potential risks.