Saltwater intrusion exits in estuary areas. Significant change in runoff variability (RV) has been detected in many basins, but limited research has investigated this change's effect on downstream saltwater intrusion. Using measured time series, this study has statistically detected a significant trend of increased RV and sea level (SL) rise in Nandu River Estuary, China, and analyzed their effect on the saltwater intrusion combining the Finite Volume Community Ocean Model and Monte Carlo method. The Mann‐Kendall test and Theil‐Sen estimator show that the yearly variance of the log‐transformed runoff and mean SL have significantly increased and are expected to increase by approximately 50% and 0.2 m by the 2060s, respectively, and 101% and 0.4 m by the 2100s. SL rise increases the salinity‐exceeding rate (salinity > 0.45‰, the standard for drinking water), and the increase in middle channels are higher than that of upstream and downstream channels. With an increased RV, the model‐predicted salinity‐exceeding rate decreases at downstream and increases at upstream channels, respectively. In our specific site, the location with a constant salinity‐exceeding rate is predicted to move downstream, and the maximum increase of salinity‐exceeding rate will reach 13.3% and 19.1% in the 2060s and 2100s, respectively, at the middle channel section. The results are validated in Yangtze River Estuary and are generalized to other estuaries to determine the change pattern of salinity‐exceeding rate. Moreover, a lower RV helps to extend the channel length with fresh water available and can be used as a management tool.

In long distance water supply projects, the air vessel is an effective and reliable protection measure to control water hammer. Although it can effectively eliminate water hammer during the process of hydraulic transient, the volume of the traditional air vessel is large due to the long distance pipeline, resulting in high investment. In this paper, based on the analysis of the transient in long distance water supply pipelines, a protection method, combining the air vessel with the downstream valve, is proposed to reduce the volume of the air vessel, keeping the system pressure within the limit. Furthermore, an innovative arrangement of the air vessel is presented to overcome the risk of the combined protection method. Besides the upstream air vessel, a downstream air vessel is additionally installed to mitigate positive water hammer due to rapid closure of the downstream valve. To verify the effect of the new method, the numerical model was established according to the parameters of a practical water supply project, and hydraulic transient due to pump trip was simulated. Compared with the traditional method, the combined protection method provides effective water hammer protection and greatly reduces the volume of the air vessel.

The use of air vessels is an effective measure to control water hammer in a long-distance water supply system. The traditional shape of such vessels is cylindrical. In this paper, an innovative spherical air vessel is proposed to improve the force characteristics of the tank. A mathematical model of the spherical air vessel was established using the method of characteristics. A comparison was performed of water-hammer protection performance between the spherical air vessel and the cylindrical air vessel based on a practical water supply project. Furthermore, a sensitivity analysis on the parameters of the spherical air vessel was performed. The results showed that the spherical air vessels had better protective performance compared with the cylindrical air vessels. Under the same protection requirements, the spherical air vessel can reduce the total volume and surface area by more than 10%. In addition, for a fixed volume of the spherical air vessel, the protective effect improves with the increase of the initial gas volume. Increasing the connecting pipe diameter of the air vessel is beneficial for low-pressure protection, whereas it is adverse to high-pressure protection; in contrast, altering the installation elevation has little effect on water-hammer protection.

This study focuses on the output variation and dynamic behavior of hydroturbines in a hydraulically coupled system under hydraulic disturbance. The theoretical formula predicting the output variation of operating turbines after hydraulic disturbance is presented, and the influential factors are discussed. To investigate the dynamic characteristics of the hydraulic turbine governing system (HTGS) and verify the accuracy of the formula, a mathematical model of the hydropower plant with multiple turbines sharing a common hydraulic system was established, and the transient process caused by partial load rejection was simulated on the basis of the operation of a real hydropower plant. The results show that the output variation of the operating turbine is in agreement with the theoretical formula, and the amplitude of overload is significant under the conventional regulating model, i.e., opening control, in the low-head hydropower plants with long headrace tunnel during hydraulic disturbance. These methods and results will provide a theoretical basis for the safe operation and control of such hydropower plants.

In the long-distance water supply system, the air tank can effectively protect the pump failure water hammer, but the large air tank will increase the investment. This paper analyzes the hydraulic transient process of the water supply system with air tanks, and proposes a joint protection method with an air tank behind the pump and air valves at the end of the pipeline. The function and coupling relationship of the air tank and air valves are clarified, the theoretical analysis system and numerical optimization framework of the joint protection layout scheme are constructed, and the potential hidden trouble conditions are discussed. A setting method of combined air valves is proposed to eliminate the possible water and gas problems in the pipeline. Combined with the actual project, the water hammer protection effects of different protection schemes are compared and analyzed. The results show that this joint protection scheme can effectively protect against water hammer damage, and greatly reduce the volume of the air tank compared to a single air tank protection.