Mechanisms of Dune Growth and Decay in Rivers

Abstract

AbstractThe mechanisms responsible for dune growth and decay remain a major unknown in river dynamics. Two theories have been supported by experiments and numerical models. A hydrodynamic explanation suggests that growth and decay occur due to phase lags between bed topography and sediment flux. Alternatively, kinematic processes have been invoked to explain growth by merger and decay by superimposed bedforms cannibalizing larger dunes. Demonstration of either mechanism in natural rivers has been lacking due to a paucity of high‐resolution topography. Here, we provide the first observations of rapid dune growth by kinematic processes during a rising hydrograph limb, followed by gradual adjustments likely through hydrodynamic processes at high flow. As flow wanes, smaller dunes develop and cannibalize larger features, eventually creating a flattened bed near base flow. Kinematic processes occur at a common threshold shear stress. Superimposed dunes formed during falling limbs of hydrographs are signatures of dune decay.