汊道演变的水沙过程影响分析——以监利河段为例

长江中下游许多汊道河段主支汊易位频繁,但对产生这一现象的主要驱动机理与影响因素一直争议不休。研究采用距平分析法、Mann-Kendall分析与基于分形理论的极差分析法相结合的方式,详细分析了监利河段1951年-2009年间流量和输沙率时间序列年尺度的变化过程。结果表明,流量及输沙率的Hurst数分别为0.943 0和0.923 9,均十分接近1.0,具有很强的持续性。Mann-Kendall分析发现,输沙率序列无明显突变,流量序列仅在1967年出现突变,主要由上游实施裁弯工程所致。系统分析来水来沙的匹配关系发现,该河段主支汊易位时间与水沙序列匹配状态有明显的相关关系,在"水多沙少"时期,监利右汊为主汊;而在"沙多水少"时期,监利左汊为主汊。

Impacts of variations in the time series of flow discharge and sediment transport rate on main channel switches: Case of Jianli anabranching reach

Many anabranching reaches in the middle and lower Yangtze River have been experiencing frequent switches of main flow from one channel to another, while the mechanism and influencing factors of the phenomenon are still elusive. To examine how the phenomenon is related to the changes in hydrological process, this study applies rescaled range analysis, Mann-Kendall test and anomaly analysis methods and examines in detail the changes in the time series of annual flow discharge and annual sediment transport rate based on observations from 1951 to 2009 at Jianli hydrological station in the Middle Yangtze River. It is shown that the Hurst exponents of the time series for flow discharge and sediment transport rate take respective values of 0.9430 and 0.9239, implying that both time series have long-memory or persistent characteristics. Man-Kendall detection shows that a significant abrupt change in flow discharge time series occurred in 1967, mainly resulting from river cutoffs taking place upstream, while there is not any significant abrupt change in sediment transport rate time series during the same period. Both rescaled range analysis and Mann-Kendall test methods show that there is no match between the time of each channel switch and variations in the time series of either flow discharge or sediment transport rate. Nevertheless, it is found that when the time series of flow discharge in a flood-dominated regime while the time series of sediment transport rate in a low regime, the right-hand side channel functions as the main channel, while flow discharge in a low regime but sediment transport rate in a high regime, the left-hand side channel serves as the main channel. This demonstrates that it is the matching regimes of flow discharge and sediment transport rate that determines if main flow switches from one channel to the other.