三峡水库蓄水后关洲河段河势调整对松滋口分流影响

关洲河段属典型的主流年内交替型分汊河道,地处三峡大坝下游,受三峡水库蓄水运用带来的水沙变异影响,河道冲淤响应较早,但因河床组成差异呈现出不同于沙质河床的河势调整规律。松滋口作为分泄长江水沙进入洞庭湖的重要通道,其分流特性变化与上游关洲河段河势调整密切相关,但三峡水库蓄水运用后两者的影响关系研究相对不足。本文基于近10年来的河道实测资料和水文观测数据,分析了三峡水库蓄水运用以来关洲河段的河道演变特点；从关洲河段河道冲淤、左汊发展和左汊主汊的维持时间等三个方面研究了上游河势调整对松滋口分流的影响。研究成果表明：三峡水库蓄水运用以来(2002.10~2013.11),关洲河段河道枯水河槽冲刷幅度较大,冲刷量占总冲刷量的90%,且冲刷主要集中于左侧低滩及左汊,致使左汊中下段深泓高程低于右汊,枯水期同流量下左汊分流比呈现增加的趋势；当上游来流小于20000m₃/s时,同流量下松滋口分流能力下降,但这一影响随着关洲河段河道冲刷的趋于平衡而逐渐削弱；关洲左汊的发展有利于增加较大流量时松滋口的分流比,且这一流量持续的时间越长,即左汊维持主汊地位的时间越长,松滋口年分流比越大。此外,左汊的过度发展可能会引起关洲河段主流不再年内交替,而是将长期稳定在左汊河道,建议有关部门加强对关洲河段左汊河道的监测。

Influence of Channel Adjustment of Guanzhou Reach on Flow Diversion at Songzikou After Impoundment of Three Gorges Reservoir

Guanzhou reach, located in the downstream of the Three Gorges Dam, is a typical anabranching channel with mainstream alternating between the two branches in a year. After the impoundment of Three Georges Reservoir (TGR), the channel of the Guanzhou reach scoured and silted earlier and showed different characteristics from the sandy-bed channel evolution due to its different riverbed material. Songzikou outlet is an important distributary to divert the water and sediment from the Yangtze River into the Dongting Lake and its characteristics of flow diversion are closely related to the upstream river channel adjustments, however, the study on influence between each other after the TGR is relatively insufficient. Using the repeated topography in 2002-2013 and the water-sediment survey data in 2003-2017, the channel evolution of the Guanzhou reach was analyzed after the TGR and the effects of upstream channel adjustments were studied on the flow diversion at the Songzikou outlet from three aspects, such as the channel erosion and deposition in the Guanzhou reach, the development and the mainstream dominated time of the left branch. The research results have shown that 90% of channel erosion quantity of Guanzhou reach was in the low-water channel in 2002.10-2013.11, and the scour was concentrated on the left low beach and the left branch, so that the riverbed elevation in the middle and lower part of left branch was lower than the right branch and the diversion ratio of flow for the left branch increased in the same discharge during the dry season. When the incoming flow was less than 20000 m3/s, the flow diversion capacity of the Songzikou outlet reduced in the same discharge, which weakened gradually with the channel of the Guanzhou reach scouring into an equilibrium state. The development of the left branch was conducive to the increase of the diversion ratio of flow at the Songzikou outlet in larger discharge. Meanwhile, the longer the mainstream was maintained in the left branch, the greater the annual average diversion ratio of flow was at the Songzikou outlet. In addition, the over-development of the left branch may cause the mainstream of Guanzhou reach no longer alternate between two branches in a year, but will be stable in the left channel. The left branch of the Guanzhou reach should be monitored in the future.