ABSTRACT
The head zone of mid-channel bar is a dynamic region where water and sediment are divided into two branching channels in anabranching channel. Erosion-deposition process in the bar head relies on upstream water-sediment supply and local topographic condition, i.e. generally experiencing alternate deposition and erosion in non-flood and flood seasons respectively. The geometric shape of mid-channel bar head is simplified by a symmetric triangle with a longitudinal upslope and two lateral downslopes. A conceptual model of bedload transport rate on the bar head is derived based on the concept of upslope facilitating deposition and lateral downslope dividing sediment, meanwhile deriving a critical discharge formula of the erosion-deposition transition point. Along with increasing upstream discharge, the deposition rate in bar head increases up to the maximum and then decreases to reach the critical discharge, and finally occurs to erode. The deposition rate in the bar head is proportionally related to the longitudinal channel gradient and bed sediment diameter, inversely proportional to lateral channel gradient, upslope, and downslope. If other conditions keep constant, the plane shape of bar head is more asymmetric with less sediment load supply, more conducive to the erosion. The critical discharges of erosion-deposition process of bar head zone in the Tianxingzhou of Wuhan reach and the Tiebanzhou of Datong reach in the middle and lowerYangtze River are predicted by using this model since the impoundment of the Three Gorge Reservoir. The predicted critical discharges are roughly in agreement with the observation of the hydrological stations. Since the impoundment of the Three Gorge Reservoir was operated in 2003, the bar head zone of Tianxingzhou and Tiebanzhou has experienced an ongoing erosion process. The erosion volume of the bar head zone (1–7 m contour) of the Tiebanzhou from 2008 to 2011 is 62000m3, erosion depth of 1.2 m, erosion length of 910 m. However, the Tianxingzhou bar head appears to somehow deposit when the upstream discharge surpasses the critical discharge according to the hydrologic station. To this particularity, this study provides a new explanation that when surpassing the critical discharge, the regulation project (i.e. revetment and protection) in the Tianxingzhou bar head plays a crucial role on resisting the flow erosion and supporting the sedimentation in higher discharge condition, in particular, the bar head submerged in flood season.
