Hydrodynamics of Meander Chute Cutoffs in Microtidal Mudflats

Abstract

River cutoffs and oxbow lakes represent ubiquitous features in alluvial plains, shaped by unidirectional meandering flow. Extensive studies have been conducted on these features as their presence immediately decreases meander sinuosity and plays a significant role in the long‐term eco‐morphodynamic and sedimentary evolution of landscapes they carved in. In contrast, limited attention has been given to meander cutoffs formed in tidal environments by bidirectional currents, and the corresponding hydrodynamical processes within these features are poorly understood. Here, we present new, first‐handed hydroacoustic data collected through a series of fixed‐point and cross‐sectional flow measurements along a chute cutoff located in the microtidal Diaokou Mudflat, the Yellow River Delta, China. The investigated cutoff bend displays higher hydrologically active than the newly‐formed chute channel at low tide, whereas the chute channel gradually become the main conduit for tidal flow propagation with water stage increasing. Specifically, poorly‐developed flow separation zones are compensated by widespread secondary circulation cells that are more pronounced under the influence of turbulences and complex bedforms rather than channel curvatures. Unlike fluvial cutoffs, which tend to be hydrologically disconnected to parent channels by coarse‐grained bed sediment plugging near the entrances, our study case is characterized by the absence of plug bars and widespread bank slumps instead. Consequently, our results support earlier suggestions that tidal cutoffs seldom disconnect from parent channels due to frequent overbank events and flow confluences, which can keep cutoffs active by flushing away fine‐grained, cohesive sediments, especially in mudflat systems.