近50年来长江中下游河道演变分析表明,河道演变具有如下特点:河道总体河势基本稳定,局部河势变化较大;河道总体冲淤相对平衡,部分河段冲淤幅度较大;荆江和洞庭湖关系调整幅度加大;人为因素未改变河道演变基本规律;坐崩是长江中下游岸线崩退和护岸工程崩毁的主要形式;人为因素对长江口河道演变的影响增加。

The analysis on fluvial processes of Middle and Lower reaches of Yangtze River in recent fifty years shows that:(1)the river regime of all reaches as a whole is table,but there are relatively obvious changes in some reaches;(2)the erosion and siltation of all reaches is in a state of relative equilibrium,but variatio in some reaches are rather noticeable;(3)the regulating ranges of relation between Jingjiang River and Dongting Lake increased;(4)the regularity of fluvial processes has not been changed under human activities;(5)rotational slip is a major mode of banks failure and river bank protection works;(6)the impact of human activities on the fluvial processes of the Yangtze River estuary is increasing.

在水利枢纽尤其是水库的拦蓄与调节作用下,下游河道的来水来沙条件显著变化,将导致下游河道不平衡输沙,引起河床冲刷与再造。水利枢纽下游河道的河床冲刷与再造过程,兼具床沙冲刷—粗化—交换—悬移质恢复等多过程耦合的微观水沙运动特性,以及泥沙冲淤—床面形态变化—纵比降改变—河型河势调整等多尺度复杂响应的宏观形态变化。国内外相关研究主要通过实测资料分析、理论研究、实体模型试验和数值模拟等手段,从微观机理和宏观规律2个主要方面开展。归纳和总结了国内外水利枢纽下游河床冲刷与再造过程研究成果,对河床冲刷与再造实体模型试验和数值模拟中的模型沙选择、泥沙恢复饱和系数等关键技术问题的研究进展进行了分析,并指出了需要进一步研究的重点和亟待解决的科技难题。

Under the action of impoundment and regulation of large water conservancy projects, especially reservoirs, the incoming flow and sediment conditions of the downstream channel change remarkably, which will lead to unbalanced sediment transport in the channel, and induce scouring and remaking of the riverbed. The riverbed scour and remaking process in the downstream channel of water conservancy project is characterized by the coupling of multiple processes (inclusive of sediment scour, coarsening, exchange and suspended sediment recovery) and the macroscopic morphological changes of multi-scale complex responses (such as sediment scouring and silting, bed surface morphological changes, longitudinal slope changes, as well as river pattern and river regime adjustment). The related researches in China and abroad are mainly carried out from two aspects, namely, microscopic mechanism and macroscopic law, by means of field measurement data analysis, theoretical research, physical model test and numerical simulation. In this paper the research results of river bed scour and remaking process in the downstream of water conservancy projects in China and abroad are summarized; the research progress of key technologies in physical model test and numerical simulation of river bed scour and remaking are expounded; and the key points for further research and the scientific and technological problems to be solved are presented.

大型水库的兴建深刻改变了下游水沙输移特点,进而导致河床演变规律显著调整,水库下游弯曲河型对水沙过程改变响应敏感,是水库下游河床演变、航道整治、河势控制等方面研究的关键区域。本文基于1996-2016年的实测水文、地形资料,对长江三峡水库下游弯曲河型的演变规律及其驱动机制开展研究,结果表明：① 三峡水库蓄水前,下荆江存在“凸淤凹冲”、“凸冲凹淤”两类弯曲河型,而三峡水库蓄水后均表现为“凸冲凹淤”的一致性规律;② 在水库拦沙作用的影响下,下荆江河段平滩河槽存在累积性冲刷现象,冲刷部位集中于枯水河槽与基本河槽之间的低滩,冲淤部位调整主要由变化的流量过程所驱动,上游河势、河道边界以及支流入汇等因素均有一定驱动作用;③ 在三峡水库蓄水后缺乏大洪水的情况下,凸岸水流挟沙力随流量增加逐渐增强,水流对凸岸冲蚀力度在平滩流量级附近（20000~25000 m3/s）达到最强,平滩流量附近流量级的持续时间超过20天时,弯曲河道发生凸冲凹淤现象。而悬沙中造床粗沙的减少,增强了水流冲刷强度,加剧了凸岸的冲蚀程度。

The impoundment of huge reservoirs deeply changes the water and sediment process in the downstream reaches and then influences the evolution of the downstream rivers. The meandering rivers sensitively responded to the variation of water and sediment process. That is why the meandering rivers are the important survey regions of river evolution, channels regulation and flood control projects. Based on the measured hydrological and morphological data from 1996 to 2016, the evolution law and its driving mechanism of the typical meandering river in the downstream reaches of Three Gorges Reservoir (TGR) are studied. By building the relationship between the cross-sections variation and the influencing factors, the mechanism is well examined. The results show that (1) before the establishment of the Three Gorges Reservoir, the meandering rivers have two types of evolution laws named "Convex banks deposition and concave banks erosion" and "Convex banks erosion and concave banks deposition". After the impoundment of the great project, the meandering rivers are featured only by the latter type. (2) After the retaining of water and sediment of TGR since 2003, the low flow channel and bankfull channel in Lower Jingjiang River have been continually eroded and the erosed areas are found in the low shoal of convex banks. The phenomenon is mainly resulted from the variation of water flow and sediment transportation and also influenced by the upstream river region, riverbed boundaries and branches. (3) The growing process of flow discharge from low water to bankfull discharge can lead to the increase of sediment transport capacity. When no big flood of discharge over 35000 m3/s is observed after the impoundment of TGR, the duration of bankfull discharge (22000-25000 m3/s) decides the erosion or deposition of convex banks after the impoundment. When the number of lasting days of bankfull discharge (22000-25000 m3/s) exceeds 20, the convex banks will be eroded. The concentration of coarse sand decrease contributes to the improvement of strength of water.

熊家洲河段位于长江中游下荆江尾端,其平面形态从顺直过渡到微弯再逐渐演变成S型,是目前荆江最为弯曲的河段。受水沙条件、河岸边界条件、自然裁弯和人工裁弯等因素影响,局部河势调整频繁。熊家洲凸岸斜槽裁弯形成新河槽,导致主流与支汊成为共生的分汊河势,改变了熊家洲出口段水流条件,进而影响下游七弓岭弯道崩岸速率和河床冲淤。为全面分析熊家洲河段平面形态演变过程和河床冲淤变化,选取枯水期遥感影像数据及沿程代表性断面进行分析。结果表明:熊家洲河段整体向下游蠕动且河道展宽,熊家洲弯道新生河槽呈冲刷扩大趋势,7 a平均展宽70 m;出流沿程断面深槽从左岸向右岸方向发展,熊家洲下游的深泓线偏向右岸;三峡水库建成运行后,深泓线开始贴近七弓岭凸岸,而原深槽回淤形成2个沙洲,经过七弓岭弯顶后逐渐向凹岸偏移,出熊家洲弯道后河道沿程呈现冲刷下切趋势。研究成果对目前该段河势控制工程的实施提供了参考意见,为下一步研究三峡水库运行的河势控制提出了思路。

Xiongjiazhou reach is located at the tail of the lower Jingjiang River in the midstream of Yangtze River. The morphology of Xiongjiazhou reach transited from straight to slightly sinuous and then gradually evolved into S-shape. It is currently the most sinuous segment in the Jingjiang reach. Subjected to incoming water and sediment load, and affected by riverbank boundary conditions as well as natural and artificial cutoffs, Xiongjiazhou reach has undergone frequent local adjustment in terms of river regime. Since the chute cutoff in the convex bank of Xiongjiazhou reach, a new anabranching channel was formed, leading to the coexistence of the main channel and the anabranching channel, which changed the flow conditions at the outlet of the Xiongjiazhou reach and further affected the bank collapse rate and riverbed erosion-deposition process in Qigongling bend. According to remote sensing imagery data and representative sections in dry season, the entire Xiongjiazhou reach has been creeping towards the downstream and the channel has been widened. The newly formed anabranching channel in the Xiongjiazhou bend is experiencing expanded scouring, with an average width of 70 m in seven years. The deep trough along the outflow section develops from the left bank to the right bank, and the thalweg of downstream Xiongjiazhou is deflected to the right bank. After the impoundment of the Three Gorges Reservoir, the thalweg has been moving close to the concave bank of Qigongling bend, whereas the original deep trough was deposited into two bars emerging above water surface. After reaching the apex of Qigongling bend, the thalweg gradually shifted to the concave bank. The results of this research provide reference for the implementation of the river regime control project in this segment, and offer ideas for the next step in the study of river regime control.

Geomorphic dynamics of alluvial rivers in response to upstream damming have substantial impacts on navigation, habitat protection, and channel stability. The purpose of this study was to determine how flow and sediment regimes, and meander characteristics affect the morphological adjustment of bends in the Lower Jingjiang Reach (LJR) before and after the Three Gorges Project (TGP). Based on detailed field measurements and hydrological and topographic datasets from 1991 to 2016, banks and point bars morphodynamics of 12 continuous bends in the LJR were comprehensively analyzed. Point bars in the LJR mainly experienced a net deposition before the TGP operation, but substantially deteriorated with a net erosion rate of 4.6 million m³ yr^?1 in the post-TGP periods (2003-2016), and erosion on heads and upstream margins of point bars was a general adjustment pattern in the 12 bends. The most significant morphological changes of point bars and banks occurred in 2006-2011, indicating a delayed response of the channel evolution of the LJR to damming. Detailed observations suggested that the medium discharges (16,000-18,000 m³ s^?1) were the most contributive discharges in shaping the morphology of point bars and banks in the LJR after damming. In addition, we revealed the importance of sediment supply on meander deformation of the LJR, driven by sediment exchange over point bars, and more upstream planform deformation tended to occur in bends with high sinuosity (>2.0) in the LJR after damming. The relationship between meander deformation and sinuosity was manifested through the geometric adjustment range of point bars. The morphological adjustments of point bars in the highly curved or compound bends of the LJR were more conducive to cause flow deflections, leading to form concave-bank bars after the TGP operation.