| Résumé: |
Strong stream power, a lack of significant vegetation and the associated highly mobile beds make braided rivers highly dynamic, and may lead to practical problems such as flooding, bridge scour, channel shifts or agricultural land loss. In parallel, it may also produce a high diversity and complexity of habitats, giving braided rivers a high ecological value. Thus, a good understanding of braided river morphodynamics is a crucial challenge for river science. In this paper, we aim to quantify how flow dynamics and channel morphology force erosion and deposition patterns in a braided river reach.\n\nChannel changes in a braided river reach were measured by terrestrial laser scanning during one year from August 2013 to June 2014. Water abstraction for hydroelectric power generation and the subsequent flushing of sediment from the intake created a natural laboratory for quantifying fluvial form-process interactions: periods when the river bed was dry, interspersed with artificial flood events of varying magnitude, facilitated easy acquisition of digital elevation models for quantifying river channel changes using terrestrial laser scanning. Confidence in the change data was obtained through a complete uncertainty analysis based on spatially variable error distribution and the analysis of the coherence of erosion and deposition patterns. In combination with flow simulations in a numerical model, analysis of difference data were used to quantify the mutual forcing between flow discharge, sediment, channel morphology and channel change within a braided river reach. The data reveal: (1) cyclical erosion and deposition in the reach, which can be directly related to sediment availability within the system in relation to the spatial patterns of deposition associated with previous events and underlining the autogenetic character of the braiding process; (2) a more strongly exogenic forcing in the upstream part of the reach, where high stream power forces channel morphology to adapt; (3) morphological forcing associated with cross-section shape and local channel planform, through the concentration and diffusion of flow, but with a spatial lag between this forcing and erosion and/or deposition; (4) hydraulic forcing in narrower sections where flow capacity is paradoxically saturated by high upstream sediment delivery ; (5) internal forcing by sequences of scour-fill, involving changes in cross-section morphology; and (6) a relation between event size and braided river morphodynamics. |
Mémoire [19 Mo]