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Response of riverbed evolution of Yichang-Jianli reach of the Yangtze River to water and sediment conditions of Yichang Station
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摘要: 三峡水库蓄水运行以来宜昌站水沙条件发生了明显改变，驱动下游河床发生新一轮调整。采用回归分析、机理分析等方法，探究了三峡工程运行前后长江宜昌—沙市段及沙市—监利段河床冲淤变化对宜昌水沙站条件的响应规律。结果表明：宜昌—沙市段冲淤量与宜昌站水沙指标间的关联度强于沙市—监利段且前者关联度随时间增强；研究河段月尺度河床演变对宜昌站水沙条件不存在明显的滞后响应；基于宜昌站月均流量分别构建的两个河段冲淤量回归模型可较好反映河床冲淤变化；根据所构建模型的模拟，三峡工程运行前宜昌—沙市段呈“小水小冲、大水大淤”的冲淤格局，冲淤转换临界宜昌站流量约为23 500 m3/s，三峡水库蓄水后该河段全面冲刷，呈“小水小冲、大水大冲”，但随着床沙粗化冲刷量有所下降；三峡水库蓄水前沙市—监利段亦呈“小水小冲，大水大淤”的冲淤格局，冲淤转换临界宜昌站流量约为22 500 m3/s，且同流量下的淤积量大于宜昌—沙市段，三峡水库蓄水后宜昌站小水时该河段冲刷量略大于上游河段，但宜昌站大水时该河段冲刷量明显更小。Abstract: Since the first impoundment of the Three Gorges Reservoir (TGR), water and sediment conditions of Yichang station have undergone a significant change, which has triggered a new round of riverbed adjustment of downstream reaches. In this study, regression analysis, mechanism analysis and other methods were used to investigate the response of the riverbed evolution of Yichang-Shashi reach (YSR) and Shashi-Jianli reach (SJR) to Yichang water and sediment conditions before and after the TGR. Results show that the correlation between scouring and silting amount (SSA) of YSR and Yichang water and sediment indexes is stronger than that of the SJR and the correlation of the former reach becomes stronger with time. On a monthly basis, the delayed response of riverbed evolution to Yichang water and sediment conditions cannot be observed for both YSR and SJR. Based on monthly average Yichang discharge, the univariate regression model for SSA can well reflect the riverbed evolution of both river reaches. According to the developed models, YSR riverbed before the TGR shows the pattern of “weak scouring under small discharge and strong silting under large discharge” and the critical Yichang discharge corresponding to the change of state is about 23,500 m3/s. After the TGR, YSR riverbed shows the characteristics of “weak scouring under small discharge and strong scouring under large discharge”. The scouring amount of YSR gradually decreases due to riverbed coarsening. SJR riverbed has the same evolution pattern with YSR riverbed before the TGR. However, its critical Yichang discharge of state change is slightly smaller (about 22,500 m3/s) and its silting amount is larger than that of YSR under the same Yichang discharge. After the TGR, SJR silting amount is slightly larger than that of YSR when Yichang discharge is small, while the former becomes much smaller than the latter when Yichang discharge is large.
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图 2 不同河段冲淤量拟合对应的R2
Figure 2. Curve-fitting R2 of scouring and silting amount of different river reaches
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图 3 宜昌站各流量水平下的含沙量箱型图
Figure 3. Boxplots of suspended sediment concentration under different Yichang discharge levels
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图 4 所有宜昌水沙指标对应R2的平方和随滞后响应时长的变化
Figure 4. Changes with time delay of summary of R2 square of all Yichang water and sediment indexes
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图 5 各宜昌水沙指标在3个时段对应R2的平方和
Figure 5. Summary of R2 square of all Yichang water and sediment indexes in three time periods
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图 6 不同时段宜昌—沙市段月冲淤量与宜昌站月均流量相关关系
Figure 6. Correlation between monthly scouring and silting amount of Yichang-Shashi reach and monthly average Yichang discharge in different time periods
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图 7 不同时段沙市—监利段月冲淤量与宜昌站月均流量相关关系
Figure 7. Correlation between monthly scouring and silting amount of Shashi-Jianli reach and monthly average Yichang discharge in different time periods
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图 8 不同河段月冲淤量拟合曲线
Figure 8. Fitting curves for monthly scouring and silting amount of different river reaches
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图 9 不同宜昌水沙指标的经验分布函数
Figure 9. Empirical distribution functions of different Yichang water and sediment indexes
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图 10 2003—2016年不同河段年冲淤量
Figure 10. Scouring and silting amount of different river reaches from 2003 to 2016
表 1 宜昌站水沙条件表征指标
Table 1 Indexes of water and sediment conditions of Yichang station
指标序号 水沙条件表征指标 指标序号 水沙条件表征指标 指标序号 水沙条件表征指标 指标序号 水沙条件表征指标 1 平均流量（m3/s） 5 最大含沙量(kg/m3) 9 最小输沙率(kg/s) 13 平均流量/平均含沙量(m6/(kg·s)) 2 最大流量（m3/s） 6 最小含沙量(kg/m3) 10 平均流量2/平均含沙量(m9/(kg·s2)) 14 最大流量/最小流量(−) 3 最小流量(m3/s) 7 平均输沙率(kg/s) 11 平均含沙量/平均流量2(kg·s2/m9) 15 最大含沙量/最小含沙量(−) 4 平均含沙量(kg/m3) 8 最大输沙率(kg/s) 12 平均含沙量/平均流量(kg·s/m6) 16 最大输沙率/最小输沙率(−) 
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表 2 不同河段月冲淤量拟合曲线信息
Table 2 Fitting curve information for monthly scouring and silting amount of different river reaches
河段 时段 拟合线y=ax2+bx+c R2 a b c 宜昌—沙市段 三峡蓄水运用前 0.026 −681.175 1 366 936.000 0.472 2003年6月—2010年10月 −0.003 −192.957 536 933.521 0.784 2010年11月—2016年12月 −0.010 110.979 −767 995.481 0.908 沙市—监利段 三峡蓄水运用前 0.036 −961.895 3 381 697.538 0.514 2003年6月—2010年10月 0.011 −424.198 1 184 660.943 0.331 2010年11月—2016年12月 0.010 −405.504 1 492 827.979 0.423 注：y为河段月冲淤量（t）；x为宜昌站月均流量（m3/s）；a，b，c为系数。
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