Chute cutoffs in meandering rivers: formative mechanisms and hydrodynamic forcing

Chute cutoffs are autogenic mechanisms typical of many meandering rivers with wide cross sections, large curvature bends, high discharges and high overbank flow gradients. The shortening of the original meander loop through a bypassing channel produces a greater water-surface gradient and, hence, increases the overall transport capacity of the reach, enhancing the downstream sediment delivery. As a consequence, the mean channel width, as well as the planform shape of the meandering bends adjacent to that bypassed by the chute tend to progressively readjust. The occurrence of this type of cutoff is one of the most fascinating and less predictable events in the evolution of rivers, as a multiplicity of control factors are involved in chute cutoff formation. In the last decade, various researchers tried to shed light on the complex mechanisms that lead to chute incision and eventually determine the fate of the bypassed bend and the new chute channel. However, the subject is not yet settled and a systematic physics-based framework is still missing. In this contribution, formative mechanisms are reviewed and two different forcing factors leading to chute cutoffs are highlighted; the inertia and direction of the channelised flow and the topographic heterogeneity of the floodplain. The general features of the involved processes are investigated using a hydrodynamic finite-element model for the two-dimensional flow in the channel and over the floodplain. A linearised two-dimensional hydro-morphodynamic model is used to estimate the channel bed topography in the absence of field data. Two representative case studies are specifically considered, occurred in the Sacramento River (California) and in the Cecina River (Italy). The first concerns a chute cutoff driven by in-channel flow. The second deals with a chute cutoff due to overbank flow and the particular topography of the point bar sediment deposits placed inside the meander loop.