SETUP AND TRIGGERING OF POST-RESTORATION CHUTE CUTOFFS IN A GRAVEL-BED RIVER

Chute cutoffs across meander bends are a key mode of channel adjustment that develop as a result of both a setup, creating suitable channel geometry and floodplain conditions, and a trigger, typically during overbank flow. Here we investigated how flood magnitude and duration impacts cutoff formation and spatiotemporal shear stress patterns in a recently reconstructed gravel-bed river. We simulated various magnitude and duration combinations using a two-dimensional hydraulic model to quantify flood hydraulics associated with two chute cutoffs that occurred in the Clark Fork River, Montana, shortly after the river was routed into a newly constructed, post-dam removal channel. We illustrate the importance of antecedent channel and floodplain conditions (floodplain elevation, vegetation, chute-channel inlet entrance location) and the distribution of shear stresses competent to mobilize sediment in main channel and overbank zones. Floodplain width plays an important role in controlling unit discharge such that overbank areas are more competent in a narrower floodplain conveyance corridor. The effects of flood duration on erosion potential are most prominent in overbank areas, where vegetation conditions can strongly mediate morphodynamics during floods. Advancing understanding of setup and trigger processes associated with chute cutoffs is essential to inform river restoration efforts, which are increasingly incorporating side channels and other lateral connectivity elements.