Morphological State Transitions and Sediment Transport Variability in Braided Rivers

Abstract

Abstract Sediment transport in braided rivers shows significant temporal variability, characterized by intense bursts of transport rates even under steady‐state conditions. These fluctuations are often viewed as the consequence of random variations in the local hydraulic conditions and sediment movement. While such explanations emphasize small‐scale processes, they do not account for how larger‐scale channel structure may organize these fluctuations. The role of reach‐scale channel morphology in controlling sediment transport remains unexplored. Here we show that the temporal variability of sediment transport results from switching between a finite set of recurrent morphological configurations. We used the results of a former long‐term (1200‐hr) laboratory experiment conducted under constant water discharge and sediment supply, in which 16 morphological states were identified using image processing. The present paper shows that the temporal evolution of these states can be modeled using a discrete‐time Markov process. We assume that sediment transport depends on the active state of the channel. The state‐dependent transport rates can be simulated using either an empirical probability distribution fitted to the data or a Gamma distribution with parameters dependent on stream power. We introduced a third model that ignores morphological state, and simulates transport rates using the bootstrap technique. We used the three models to generate long time series of transport rates, the ensemble statistics of which were compared with the experimental statistics. The three models perform equally well for predicting the mean transport rate and its variance, but the purely Markovian model is superior for predicting the temporal features of transport rates. These findings indicate that variability in bedload transport rates mainly results from the persistence and succession of a finite set of morphological states.