A conceptual model of deposition and erosion of mid-channel bar head zone in anabranching river

Head zone of mid-channel bar is a dynamic shoal of water-sediment diversion and alternative erosion-deposition in anabranching river. Erosion-deposition process in bar head relates to water-sediment supply and local topographic condition. Through the geometric generalization of the mid-channel bar head(i.e. longitudinal upslope and lateral downslope), a conceptual model of bedload transport on the bar head is derived using the concept of upslope facilitating deposition and lateral downslope dividing sediment, meanwhile to obtain a critical discharge formula of the erosion-deposition transition. Along with increasing upstream discharge, the deposition rate in bar head increases first up to the maximum value and then gradually decreases, until approaching the critical discharge, and eventually occurring erosion. The deposition rate in the bar head is proportional to the longitudinal channel gradient and bed sediment diameter, inversely proportional to lateral gradient, upslope and downslope. If other conditions are the same, the shape of bar head is more asymmetric with smaller sediment load supply, more beneficial to the erosion. The critical discharge in the Tianxingzhou Island head of Wuhan reach in the middle Yangtze River predicted by this model is roughly in agreement with the hydrological observation. Nevertheless, the Tianxingzhou Island head still occurs to the deposition when the flood discharge surpasses the critical discharge. To this particularity, this study presents a new explanation that the regulation project(i.e. revetment) in the Tianxingzhou Island head plays a pivotal role on resisting the erosion and facilitating the sedimentation in the higher discharge condition.