Changes in braided river morphology resulting from a flood sequence

With increasing hydroclimate variability due to the climate change, fluvial morphology must exhibit adjustments toward changing equilibria. We report a laboratory and numerical study on the mechanism of bed morphology adjustment in response to a sequence of low and high magnitude flood events. Over 60 runs were performed in a mobile bed flume (10 m x 2.5 m), with constant longitudinal slope (0.015) and mean grain size (0.45 mm) in the Total Environment Simulator at the University of Hull, UK. Sediment conditions included floods with equilibrium and deficit loads, and the role of riparian vegetation was also considered. The outcomes of each run were characterized by a detailed digital elevation model, digital imagery and continuous monitoring of the sediment transported through the flume outlet. The Delft3D code in depth-averaged (2-D) mode was used to reproduce different aspects of the braiding process over an up-scaling of the laboratory river. Data analysis allowed us to assess the effect of flood sequence on the braiding intensity and on the width-to-depth ratio of channels, which although variable in time, fluctuated among defined values. Model results showed that the percentage of area of change computed for different river discharges reduced significantly over time, indicating mutual adaptation between hydrodynamics and morphodynamics and the approach towards equilibrium. The rate in which the area of morphological change was established was found to have an exponential form, and diffusive relationships held within the river system were tested for different discharge conditions. The results might provide a useful basis for analysing the similar but more complex long-term dynamics found in natural rivers.