Combining multi-physical measurements to quantify bedload transport and morphodynamics interactions in an Alpine braiding river reach

Understanding the interactions between bedload transport and morpholdynamics in braided streams has important applications in river management and restoration. Direct field measurements addressing this question are however scarce as they are often challenging to perform. Here, we report an extensive two-month field campaign in an Alpine braided reach (La Severaisse river, French Alps) that experienced predictable daily peak discharge (48 events observed) generating significant bedload transport and morphological changes during the melting season. We monitored these processes using a wide range of direct and indirect techniques (bedload sampling, continuous seismic measurements, pebbles tracking, topographic surveys, remote sensing using ground control cameras and drone flights). Doing so, surrogate measurements allowed to extend temporally discrete manual bedload sampling, and to extend spatially local riverbed cross section measurements. These measurements provide unique complementary constraints on the targeted physics, at various spatial and temporal scales which enabled us to draw robust conclusions. Data showed a progressive decrease in bedload transport for a given flow rate along the two months period. Simultaneously, river morphology in the braided sections changed from an incised to a more distributed configuration which led to a decrease of local maxima in dimensionless shear stresses in the braided reach for similar flow conditions. This control of bedload transport by maximum local shear stresses was in line with tracked pebble surveys indicating that coarse bedload particles were mostly transported in the main active channel. At the reach scale, this transport was found to be more efficient in laterally confined sections than in braided ones which has important implications in terms of bedload estimation in alternative confined and braided (unconfined) rivers. Finally, this study highlight the interest to combine a large variety of traditional and innovative measurements techniques to better understand complex sediment transport processes in the field. (C) 2019 Elsevier B.V. All rights reserved.