Investigation On Joint Characteristics Of Rainstorm And Tide And Staged Control Of Flooding Risk In A Coastal City

Catastrophic flooding resulting from extreme meteorological events has occurred more frequently due to global warming and rapid urbanization and drawn great attention in recent years in urban areas in China, especially in the coastal areas. Rainstorm and storm tide are both inducing factors of flooding, and their joint probability and impact would be critical to determine the flooding risk, and thus to determine drainage system design and management. In this dissertation, taking urban Fouzhou as an example, the correlation characteristics about rainstorm and tide level are analyzed from multiaspects, and then put forward the staged control strategy on flooding control to adapt to the changing environment. The main research work and results include the following aspects:(1) The changes of rainfall, tide level and the tropical cyclone rainfall in Fuzhou City are analyzed, and also the effect of tropical cyclone on rainfall and its contribution are quantified according to the statistics of the rainfall, tide level, and the typhoon data. The results show that the proportion of tropical cyclones rainfall in annual precipitation has a gradually decreasing trend. The tropical cyclones rainfall on October 7, 8, 9 accounts for more than 40% of the totals, and more than 50% of torrential rainstorm respectively on rainy days. The tropical cyclone of Level 4 has the largest contribution to local rainfall in total tropical cyclone rainfall, with annual average rainfall of 76.67 mm.(2) The EM algorithm is employed to solve the parameters of mixing Von Mises distribution model, which is used to fit the temporal distribution of rainstorm, extreme rainfall and tropical cyclone rainfall, the results show that the fitting effect is so good. It also illustrates that the EM algorithm is an effective method to solve the parameters of mixing Von Mises distribution model. Based on the temporal distribution fitted by mixing Von Mises, it is concluded that the occurrence probability of rainstorm, extreme rainfall and tropical cyclone rainfall in July, August, September, is very large, more than 50%, up to 78%.(3) The hydraulic characteristics and the hydraulic connection of the drainage system are analyzed by using the validated simulation model of flood drainage, and on this basis the main reasons for the flooding the Fuzhou City are revealed. Results show that the synchronous runoff from mountain area and urban area due to rainstorm, the heavy rainfall(including tropical cyclone rainfall), the insufficiency of flow ability upstream, and the high tide level impeding the discharge of tide-locking and drainage pumps are the main causes of flooding disasters in urban Fuzhou.(4) The joint impact of different rainstorm and tide level on flooding risk is quantified, and put forward the threshold condition of flooding damage and the occurrence of flooding disaster. Results show that: 1) working pumps efficiently reduce flood risk within a 20-yr return period(RP) precipitation in Fuzhou. Otherwise, the efficiency is very low. Besides, a 100-yr RP is a critical value with the current pumping capacity. When the precipitation is lower than this value, pumping is useful; otherwise, pumping is almost useless. Flood happens when the flood discharge exceeds the flow capacity of the rivers. 2) Pumps with a certain capacity located downstream of a river network have a limited effect on reducing flood. High-capacity pumps may still not be helpful. In this situation, it should increase the upper storage capacity if we want to reduce the damage degree of the flooding in urban Fuzhou.(5) The changes of joint risk probabilities of different rainfall and tide level are quantified by using the joint risk models established by Copula functions, taking into account the changing environment. The results showed that: 1) Heavy rains and high tidal levels barely happen simultaneously. But since 1985, the chance that extreme precipitation and storm tide happen simultaneously has increased significantly, with the average growth rate of more than 300%. 2) For the coastal cities, the design standards of flood control and drainage engineering would change as the environment changes, so in the future design of flooding preparedness, the effect of changing environment on risk factors should first be assessed.(6) The staged optimization model of flooding risk control for urban Fuzhou is established taking account of the environmental changes. The results showed that with the optimal solution, the total cost has a value of 1.426 billion Yuan. If the staged control strategy on flooding risk is not used in this project, the total cost would be 2.148 billion Yuan. This implies that the staged control strategy on flooding risk not only can make the management for flood control and drainage planning more flexible with the changes in the environment, even to a certain extent, more cost savings.