Impacts of a large flood along a mountain river basin: the importance of channel widening and estimating the large wood budget in the upper Emme River (Switzerland)

On 24 July 2014, an exceptionally large flood (recurrence interval ca. 150 years) caused large-scale inundations, severe overbank sedimentation, and damage to infrastructure and buildings along the Emme River (central Switzerland). Widespread lateral bank erosion occurred along the river, thereby entraining sediment and large wood (LW) from alluvial forest stands. This work analyzes the catchment response to the flood in terms of channel widening and LW recruitment and deposition, but also identifies the factors controlling these processes. We found that hydraulic forces (e.g., stream power index) or geomorphic variables (e.g., channel width, gradient, valley confinement), if considered alone, are not sufficient to explain the flood response. Instead, the spatial variability of channel widening was first driven by precipitation and secondly by geomorphic variables (e.g., channel width, gradient, confinement, and forest length). LW recruitment was mainly caused by channel widening (lateral bank erosion) and thus indirectly driven by precipitation. In contrast, LW deposition was controlled by channel morphology (mainly channel gradient and width). However, we also observed that extending the analysis to the whole upper catchment of the Emme River by including all the tributaries and not only to the most affected zones resulted in a different set of significant explanatory or correlated variables. Our findings highlight the need to continue documenting and analyzing channel widening after floods at different locations and scales for a better process understanding. The identification of controlling factors can also contribute to the identification of critical reaches, which in turn is crucial for the forecasting and design of sound river basin management strategies