Floods in forested mountain rivers, is it all clear water? The role of organic load in fluvial ecosystems

Floods are without doubt the most significant natural hazard, causing important damages and economic losses. In mountains basins, it is well known that rivers are not carrying just clear water during floods, but significant volume of sediment (both suspended and bedload), and in forested areas, large amounts of instream wood can be also transported. The transport and deposition of this organic load may result in an increased flood hazard (e.g., due to a reduction of the cross-sectional area, backwater effect and overflow, aggradation, avulsion or scouring) as witnessed during the large floods in 2005 in Central Switzerland. However, instream wood is a key element of the fluvial ecosystem, an important physical component that impacts fluvial morphodynamics and enhances the ecological diversity of rivers. But, unless inorganic sediment, organic load has not been present in our understanding of rivers, although, instream wood is an important driver of physical and biological complexity, and the interactions and feedbacks between wood, water and sediment occur at all possible scales, from one single log, to the sub-reach, segment up to the watershed scale. Thus the challenge is to find sustainable conditions that can maintain wood and the good ecological status of rivers while minimizing the potential hazards. Therefore the quantification of the instream wood dynamics (i.e., wood recruitment from hillslopes and fluvial corridor, transport, deposition and remobilization) is crucial for understanding and managing rivers. Unfortunately, wood in rivers has not been frequently monitored or measured, and very few direct observations and data series exist worldwide. This presentation will shortly describe the processes involved in the instream wood dynamics, drawing analogies with inorganic sediment. Moreover, the talk will show the most recent techniques to quantify wood in rivers, such as the use of unmanned aerial vehicles, monitoring using video cameras or numerical modelling. These techniques are now being used within the context of the WoodFlow (2015-2019, founded by the Federal Office for the Environment) project in Switzerland. The purpose of this project is to develop the knowledge and methods to analyse instream wood dynamics in Swiss rivers. This contribution will present an overview of the project and some relevant preliminary results.

Dr. Virginia Ruiz-Villanueva (born in Madrid, Spain in 1981) is postdoctoral research associate at the University of Geneva (Institute for Environmental Sciences) since 2016. She previously worked also as postdoctoral researcher at the University of Bern (2013-2016) and obtained her PhD at the University Complutense of Madrid (Spain) in 2013. She is fluvial geomorphologist with special interests related to hydro-geomorphic processes in general. A particular focus of her current research is dedicated to the monitoring and modelling of instream wood and the analysis of its influence on floods and fluvial dynamics, including river morphology and sediment transport. Moreover she is interested in the response of rivers to flood events, including the analysis and modelling of morphodynamics and geomorphic changes.