基于经验模型的长江口南支上段压咸临界流量

Estimation of critical discharge for saltwater intrusion in the upper south branch of the Yangtze River Estuary using empirical models

河口区域盐水入侵,威胁三角洲地区城市淡水资源取用及生态环境安全,合理确定压咸临界径流流量,有助于进一步改进流域水库群的调度模式。以长江口南支为例,利用2009-2014年期间60余个潮周期氯度资料,综合分析多种盐度预测经验模型的有效性,确定了抑制咸潮入侵的临界径流流量。结果表明:①咸潮入侵强度随径流与潮差组合而不同,长江口径流流量小于30000 m^3/s时可发生盐度超标现象,69%的盐度超标天数发生在流量小于15000 m^3/s时期,当径流量小于12000 m^3/s时盐度超标几率达65%;②盐度与潮差之间为指数型曲线关系,潮差总体呈半月周期变化,根据潮差累积频率分布得到平均意义上"连续10 d盐度超标"临界条件对应的青龙港潮差约为2.7 m,由此推算得到临界大通流量略大于11000 m^3/s;③采用多种盐度预测经验模型进行计算,避免连续10 d盐度超标对应的临界径流流量区间为11000～12000 m^3/s,平均阈值取为11500 m^3/s;④在三峡及上游梯级水库联合运行后,2008-2015年长江入海最低径流流量有所增加,但仍低于压咸潮临界阈值,建议优化水库调度模式,控制长江入海径流最低流量在11500 m^3/s以上。

In the Yangtze River Estuary, saltwater intrusion threatens eco-environment and freshwater intake. The estimation of the critical discharge for saltwater intrusion control helps improve the regulatory schemes of reservoirs in the Yangtze River Basin. In this study, the upper south branch of the Yangtze River Estuary was selected as a typical area to determine the critical discharge for saltwater intrusion control using different empirical models. Measured salinity data of more than 60 tidal cycles were used to analyze the characteristics of saltwater intrusion processes, and the validity of four empirical models was tested. Results indicated that(1) the intensity of saltwater intrusion changes with different combinations of discharges and tidal ranges, and saltwater intrusion occurs when the discharge at the Datong station is less than30000 m^3/s. Approximately 69% of saltwater intrusion days occur when the discharge is less than 15000 m^3/s. The probability of saltwater intrusion is 65% when the discharge is less than12000 m^3/s.(2) The tidal range in the Yangtze River Estuary shows a distinct feature of a semimonthly(15 days) periodical change, that is, the condition of "saltwater intrusion that ensues for more than 10 days" occurs with two-thirds frequency when the discharge is maintained at a certain critical value. On the basis of statistical analysis, the tidal range that corresponds to twothirds frequency is determined to be 2.7 m at the Qinglonggang station. With the exponential relationship between salinity and tidal range, the critical discharge at Datong has been estimated to be slightly higher than 11000 m^3/s.(3) The periodical changes in tidal range can be roughly described using sinusoidal functions. By introducing the tidal range functions into empirical models for saltwater intrusion, the change process of salinity under a certain discharge can be calculated. By using three empirical models, the calculation results show that the critical discharge at Datong is between 11000 and 12000 m^3/s. Thus, 11500 m^3/s can be regarded as the critical discharge to control the duration of saltwater intrusion to less than 10 days.(4) After the operations of the Three Gorges Project and its upstream cascade reservoirs,the minimum discharge into the sea in 2008-2015 increased but remained lower than the critical discharge. Therefore, the regulatory schemes of reservoirs in the Upstream Yangtze River must be optimized to release additional water during dry season. In particular, the minimum discharge into the sea should be maintained to be higher than 11500 m^3/s.