中文摘要：

分汊河流的江心洲洲头是分汊河段分水分沙和泥沙冲淤的动态区域,其冲淤过程受来水来沙和地形条件变化呈现冲淤交替。通过几何概化洲头的浅滩逆坡和侧向顺坡,考虑逆坡促淤和顺坡分沙作用,建立洲头浅滩的推移质冲淤的概化模型,推导洲头由淤积转向冲刷的临界流量表达式。上游来流量增加,洲头淤积速率先增加到最大值后逐渐减少,直至达到临界流量,再转向冲刷。洲头淤积速率与上游水面纵比降和床沙粒径成正比,与浅滩逆坡、侧向水面比降与顺坡成反比。若其他条件相同,洲头浅滩形状越不对称和上游来沙量越小,越有利于冲刷。模型预测武汉河段天兴洲的临界流量范围与水文观测值较为接近,对其出现超过某个大临界流量仍出现淤积的特殊性,给出的新解释是天兴洲洲头的整治工程使得在大流量条件下洲头抵抗冲刷和阻挡作用促进泥沙淤积。

英文摘要：

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.