Predicting the geomorphological responses of gravel‐bed rivers to flow and sediment source perturbations at the watershed scale: an application in an Alpine watershed

Abstract

AbstractPredicting morphological channel changes using physically‐based models requires extended data for the description of the river channel and for hydrological and sedimentological inputs. At the watershed scale, these data are usually scarce, and such a refined modeling is typically difficult to build. A simpler modeling of the morphological impacts due to the changes in the principal drivers that control channel shape and dynamics is more adaptable. In this study we focused on the morphological responses of gravel‐bed rivers to flow and sediment source perturbation at watershed scale. The aim is to develop and test a tool capable of semi‐quantitatively predicting the morphological river response at the watershed scale due to a set of spatially distributed perturbations. The model considers flow regime (Q) and sediment supply (S) as the two main factors controlling the fluvial morphology in alluvial rivers. Two indicators have been proposed to evaluate the alteration on Q and S, and they are illustrated as vectors on each reach of the river network. The magnitude of the vectors corresponds to the intensity of the perturbation and its direction represents the changing trend that nine selected morphological variables (bed elevation, slope, width, depth, wetted area, width to depth ratio, d50, terrace formation, and colonization of vegetation) are likely to follow from an initial state. The trends or trajectories of changes were assessed based on empirical relations, case studies, and conceptual models. This method was applied to the Isère watershed (5700 km2) at Grenoble (France), a river that hosts large and complex hydropower plant systems constructed during 50s ‐70s. The predictions over 23 river reaches and eight variables were evaluated in the range where the model was capable of predicting the morphological evolution of the river system. Its performance was verified and in the majority of the cases the results were coherent with field surveys and previous observations. The results indicate that this is a complex problem which needs more careful consideration of constraints that are difficult to assess, such as simultaneous and different sources of perturbations, hypotheses of initial dynamic equilibrium, and sediment supply quantification. Copyright © 2017 John Wiley & Sons, Ltd.