Effect of Flow Unsteadiness on the Long-Term Evolution of Alternate Bars

Alternate bars are large, migrating bedforms that can spontaneously develop in straightened river reaches due to an instability of the erodible bed. The evolution of alternate bars during a single flood event can be effectively predicted by means of theoretical models. However, how the average properties of bars respond to variations of flow regime in the long term is still unexplored. To address this question, we applied the weakly nonlinear model of Tubino (1991) to three simplified flow series with the same probability density function, but different duration and sequencing of the individual flood events. Results reveal that the average bar amplitude is primarily controlled by the flow probability density function, with the other hydrological parameters playing a secondary role. Conversely, the modification of the bar amplitude in single events and the time needed by rivers to recover from catastrophic events strongly depend on flood duration and sequencing.