Abstract:

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 than 30000 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 than 12000 m ³/s. (2) The tidal range in the Yangtze River Estuary shows a distinct feature of a semi-monthly (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 two-thirds 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.

Key words: critical discharge for saltwater intrusion, discharge and tidal range, empirical model, the Yangtze River Estuary