ABSTRACT
River widening, defined as a lateral expansion of the channel, is a critical process that maintains fluvial ecosystems and is part of the regular functioning of rivers. However, in areas with high population density, channel widening can cause damage during floods. Therefore, for effective flood risk management it is essential to identify river reaches where abrupt channel widening may occur. Despite numerous efforts to predict channel widening, most studies have been limited to single rivers and single flood events, which may not be representative of other conditions. Moreover, a multi-catchment scale approach that covers various settings and flood magnitudes has been lacking. In this study, we fill this gap by compiling a large database comprising 1564 river reaches in several mountain regions in Europe affected by floods of varying magnitudes in the last six decades. By applying a meta-analysis, we aimed to identify the types of floods responsible for more extensive widening, the river reach types where intense widening is more likely to occur, and the hydraulic and morphological variables that explain widening and can aid in predicting widening. Our analysis revealed seven groups of reaches with significantly different responses to floods regarding width ratios (i.e., the ratio between channel width after and before a flood). Among these groups, the river reaches located in the Mediterranean region and affected by extreme floods triggered by short and intense precipitation events showed significantly larger widening than other river reaches in other regions. Additionally, the meta-analysis confirmed valley confinement as a critical morphological variable that controls channel widening but showed that it is not the only controlling factor. We proposed new statistical models to identify river reaches prone to widening, estimate potential channel width after a flood, and compute upper bound width ratios. These findings can inform flood hazard evaluations and the design of mitigation measures.
ABSTRACT
The compound hazard effects of multiple process cascades severely affect Chilean river systems and result in a large variety of disturbances on their ecosystems and alterations of their hydromorphologic regimes leading to extreme impacts on society, environment and infrastructure. The acute, neo-tectonically pre-determined susceptibility to seismic hazards, the widespread volcanic activity, the increasing glacier retreat and the continuous exposure to forest fires clearly disturb entire riverine systems and concur to trigger severe floods hazards. With the objective to refine the understanding of such cascading processes and to prospect feasible flood risk management strategies in such a rapidly changing environment we first classify the large river basins according to a set of disturbances (i.e. volcanic eruptions, earthquakes, glacier lake outburst floods, wild fires and mass movements). Then, we describe emblematic cases of process cascades which affected specific Chilean drainage basins and resulted in high losses as tangible examples of how the cascading processes may unfold in other river basins with similar characteristics. As an attempt to enrich the debate among management authorities and academia in Chile, and elsewhere, on how to sustainably manage river systems, we: a) highlight the pivotal need to determine the possible process cascades that may profoundly alter the system and b) we suggest to refine hazard and risk assessments accordingly, accounting for the current and future exposure. We advocate, finally, for the adoption of holistic approaches promoting anticipatory adaptation which may result in resilient system responses.