Mmovement track of a unit water body in nature is always winding. It is a natural character of water movement and an internal condition to form the meanders in rivers.Initial shape of a valley,amplitude and frequency of discharge change,mechanism of sediment transportation in rivers and change of basic level of erosion are all important factors influencing meander development.Analysis of the test data of natural models shows that vertical slope of valley,too large or too small,can not make rivers to form meanders.Small transverse slope widen easily river width,favourable to development of branched channel.By contrast,large transverse slope limits width of meander belt,unfavourable to meander development.So does the amplitide of discharge change,if the amplitide is over large,derivation of thalweg track between flood and dry seasons is too large, if over small,the point bar grows slowly,both of them can not get favourable conditions for meander development.Descent of basic level of erosion increases vertical slope of rivers,to make rivers to branch.In special geographic conditions whether a river can develop into meandering river depends on suitable combination of influencing factors,instead of single factor.

以室内概化模型试验为技术手段, 首次尝试采用不同粒径的新型复合塑料沙(细颗粒层通过加入适当比例的黏合剂)模拟了天然二元结构河岸崩塌过程。试验研究表明二元结构河岸崩塌过程大体可分为5个崩塌阶段坡脚冲刷变陡阶段→滩面裂缝形成发育阶段→坡面渐进侵蚀阶段→河岸崩塌阶段→河岸冲刷趋于稳定阶段;在崩塌发生的过程中首先是下层泥沙受冲变陡, 其次是上层河岸泥沙在多种因素影响下发生着不同的崩塌形式;在同级流量条件下, 水位下降过程中河岸稳定性较水位上涨过程中明显减小。该成果可为研究江河崩岸治理措施和防洪减灾提供理论依据。

The process of natural and binary riverbank collapse is simulated with compound plastic sand of different particle sizes (adding adhesive of appropriate proportion to the upper layer) through indoor generalized model tests. Test results reveal that the collapse process can be divided into five phases in general: the slope toe is scoured and steepened, and then cracks develop on the shoal surface, in subsequence, slope surface is progressively eroded, followed by riverbank collapse, and equilibrium of riverbank erosion and deposition. During the collapse, sands in the lower layers were scoured and the bank was steepened, then the upper sands collapsed in different types under various conditions. In the presence of equal water discharge, the bank stability during water level drawdown obviously decreased than that during water level raise. This research finding provides theoretical support for the harnessing of embankment collapse, flood protection and disaster reduction.

三峡水库运用改变了下泄水沙条件,坝下游弯曲河型均呈现“凸冲凹淤”演变规律,以致碍航问题日益突出。为研究弯曲河型滩槽演变特点及其成因,以下荆江河段荆江门弯道为例,利用三峡蓄水后近15 a实测地形资料,采用逻辑推理及统计分析方法分析其影响因素并预测了航道变化趋势。结果表明:三峡工程运用后,凸岸边滩年际间呈持续冲刷趋势,冲刷位置不断下移导致河段展宽引起航道条件恶化;在平滩流量(20 000~25 000 m³/s)持续时间＞15 d的年份,凸岸边滩冲刷后退,反之淤长;冲淤幅度与年均来沙量,尤其细沙(d≤0.125 mm)含量密切相关,基本呈同步变化趋势;荆江一期工程实施后,在当前水沙条件下凸岸边滩会继续受冲,形成枯水双槽,航道条件将恶化。水沙过程的改变是弯曲河型滩槽演变的主要原因,遇大水且来沙量较少的年份,荆江门河段很可能出浅碍航,这可为下荆江弯曲河段航道治理提供一定的参考价值。

The operation of the Three Gorges Project (TGP) had changed the condition of discharge and sediment. Curved reaches in the Jingjiang River all witnessed erosion in convex bank and deposition in concave bank, detrimental to channel navigation. In order to study the evolution characteristics and influence factors of curved reach, we analyzed the evolution causes and estimated the future changes of Jingjiangmen reach of lower Jingjiang River as an example according to measured topographic data in recent 15 years after the impoundment of TGR via logic reasoning and statistical analysis. Results demonstrated that after the operation of TGP, the convex bank of Jingjiangmen reach showed a trend of continuous scouring year by year, and the position of scouring moved downwards continuously, which deteriorated the channel navigation condition. When the duration of bank-full discharge (20 000~25 000 m³/s) was longer than 15 days in a year, convex bank was scoured and moved backwards, while on the contrary, when the duration of bank-full discharge was shorter than 15 days of the year, the convex bank was silted. The annual variations of convex bank had a good corresponding relationship with sediment concentration, especially fine sand content (<i>d</i>&lt;0.125 mm), synchronously increasing or decreasing. After the first phase implementation project of Jingjiang Reach in 2014, given the discharge and sediment conditions, convex bank wound continue scouring, giving rise to double-groove in dry season and more deteriorated navigation condition. In conclusion, changes in discharge and sediment conditions are the major causes of bend reach evolution. In years of long flood duration with small sediment discharge, Jingjiangmen reach would most likely experience navigation obstruction. These conclusions provide reference for the governance of meandering channels in lower Jingjiangmen reach.

节点广泛存在于分汊河段,对汊道的影响至关重要,研究节点对汊道演变的影响,对指导河道治理具有重要意义。长江下游马当河段为典型的微弯分汊河型,先后有骨牌洲、棉外洲、瓜子号洲等将河段多级分汊,依次有小孤山、彭郎矶、马当嘴、马当矶等山体形成控制节点。建立了马当河段二维水沙数学模型,在验证的基础上,从水流及河床冲淤变化的角度,结合河床演变,探索各个汊道分流比调整后对上下游河段的影响,分析了节点在汊道演变中的控制作用。研究结果表明:经过小孤山-彭郎矶节点的控制,马当河段进口边界条件较为稳定,上游河势发生较大变化时,马当河段河势变化微小;经过马当矶-马当嘴节点的控制,棉外洲左右槽分流比变化为23.3%~58.1%时,对节点下游主流和河床冲淤的影响主要集中在2~2.5 km以内,影响范围有限。通过采用数学模型和河床演变分析相结合的方法,探索汊道之间演变的相互影响,分析节点的控制作用,为河道治理提供指导意见。

Nodes, widely exist in bifurcated channels, have essential impact on river bed evolution. We explored the impact of nodes on bifurcated channel evolution and provided guidance for river regulation by combining mathematical modelling and riverbed evolution analysis. Madang reach, which lies in the downstream of Yangtze River, is a typical slightly-bent and bifurcated channel. Many sandbars or shoals bifurcate this reach, such as Gupaizhou sandbar, Mianwaizhou shoal and Guazihaozhou sandbar. There are some nodes formed by mountains in Madang Reach, like Xiaogushan, Penglangji, Madangji, and Madangzui. A 2-D mathematical model of Madang reach was established and verified. From the perspective of water flow and riverbed evolution, the impact of sub-branches’ diversion ratio variation on their upstream and downstream channels, and the nodes’ influence on riverbed evolution were further analysed. Results reveal that by controlling the node of Xiaogushan-Penglangji, the incoming flow at the inlet of Madang reach is stable, and is slightly affected by its upstream variation. Due to the modification of Madangji-Madangzui node, when the diversion ratio of Mianwaizhou shoal varies from 23.3% to 58.1%, its impact on main flow and river bed erosion &amp; deposition only covers a limited distance of 2-2.5km downstream of Madangji.