The Width of Valleys – WIVA

Rivers flow in valleys of varying width, which they have not only deepened but also widened over time. The widening of river valleys (without a glacial legacy) occurs when the river comes into contact with the edges of the valley. This occurs during extreme floods but also through slow lateral migration of the channels. The evolution of the width of the valleys plays a major role in the transfer rates of sediments to the basins, in the capture of CO2 over geological time or in the evolution of ecosystems. However, due to the different time scales involved (day->ka) and the lack of methods to quantify average lateral erosion over millennia, the processes of valley widening and the corresponding lateral erosion law are very poorly understood. The objective of the WIVA project is to establish an average valley widening rate law and to implement this law in landform dynamics models. This law will be constrained by quantifying for the first time the long-term lateral erosion of valley sides (>ka) by their 10Be content at slowly eroding sites (Andes-France), by experimenting with relief dynamics in a scale model, by monitoring current lateral erosion by innovative methods (Lidar and erosion sensors) at rapidly eroding sites (Siwaliks-N. Zeeland), by 1:1 scale experiments of lateral erosion by coarse sediment impact and by modelling the fluvial dynamics to link these scales of observation. In our consortium we have brought together developers of these approaches and specialists of the studied field sites. By implementing these laws in dynamic landform models we will unlock outstanding and old questions (formation of river terraces, buffering of sediment fluxes towards the basins, role of extreme events in the erosion hazard and on the long-term levelling of the topography) of interest to a broad community on the topographic and sedimentological record of pre-anthropic palaeoenvironmental variation and erosion hazard.