Assessment Of Defense Capability Of Sea Dikes In Shanghai For Tyhpoon

Due to the global climate change, extremely disastrous storm surges happen more intensively and frequently, which also have the trend to go even north at the northern hemisphere. Meanwhile, the accelerated process of urbanization and population increase in the coastal areas faces the needs of the seaward extension of shoreline for more living space. Shanghai, an international metropolis in mid-latitude the low-lying area with high-density population, is vulnerable to typhoon disasters. To protect against the high-level typhoon and prevent large scale destruction, loss of lives and properties for the typhoon exceeding the design standard, first-line seawalls play an essential role in the safety of Shanghai in the current and future time periods.Through the extensive and in-depth research on first-line seawall and defense standards in Shanghai, a city at the inter-junction of Yangtze River Estuary and Hangzhou Bay, and a collection of multiple landing events at Shanghai or neighboring provinces, this study simulated the storm surge and swells in Shanghai caused by typhoons that exceed the defense standards, and analyzed the factors that probably destroy the seawall. The purpose and destination of this study is to improve the defense capabilities of the seawalls and significantly reduce the potential disaster losses by properly heightening and consolidating the existing seawalls. This specific research contents and results are as follows:(1)Based on the extensively analysis on research progress of current wind-tide-wave numerical model, the typhoon numerical model, storm surge model and typhoon-tide-wave coupling model are selected and established for the Yangtze River estuary, and these models are calculated and calibrated by synchronous wind-tide-wave observed data of Typhoon 9711 and other typhoons.In this study, Fujita.T- Takahashi formula and Jelesnianski wind model are established respectively by computer programming. Jelesnianski wind field model with relatively less decay and consistent wind verification is applied on the numerical model. By adjusting the attenuation coefficient and the radius of maximum wind speed, the calculated wind field is more consistent with the observed wind field.The wind-tide numerical model is established. The tidal wave process before landing process under the impact of Typhoon 9711 is analyzed. In the offshore deep water area, the storm surge is mainly forced by the lower air pressure under the typhoon center. With the typhoon's approaching to the nearshore area, wind induced set-up and set-down occur successively, the impact of low pressure decreases relatively, and the anti-clockwise direction flow along the shoreline under the impact of wind field lead to the formation of a strong water exchange on the south and north sides of typhoon center.The impact of water depth fluctuation during tidal process on wave propagation has been taken into consideration by the tide-wave coupling typhoon model. The energy distribution of wave field is consistent with the typhoon wind field, and along with the movement process of typhoon, the wave intensity in deep water is enhanced with the increasement of wind strength or the radius of wind field. The maximum effective wave height is up to 9.0 m in the deep water under the impact of Typhoon 9711. Due to the impact of wave breaking and bottom friction in the shallow water, the wave intensity is reduced correspondingly.(2)To simulate the potential strong typhoon and super strong typhoons may had the worst impact on the city of Shanghai, the extreme tide level and wave in front of seawall can be simulated in these most unfavorable conditions. By the temporal and spatial transformation of Typhoon 9711, the most dangerous path and landing time can be determined for the typical sea walls at the different positions of Yangtze River and Hangzhou Bay, and take this as the most dangerous hypothetical super typhoon may attack on Shanghai. On the basis of simulation results of the path and landing time of strong typhoon, combined with the low pressure and wind field process of super strong Typhoon 5612, the wind-tide-wave process of super strong typhoon is simulated. The extreme storm surge water level and typhoon wave in front of the typical seawall under the impact of typhoon over normal standard are obtained in this study.(3)In the investigation of typical seawall, the harbor seawall is chosen as the studied seawall. By using the tide level and storm wave at designed and over normal standard criteria, the structure stability and wave overtopping discharge of seawall are tested by calculation. On this basis, the safety of the seawall is evaluated. The evaluation shows that the structure stability and wave overtopping discharge of the existing seawalls can basically satisfy the safety requirement, the wave overtopping discharge of seawall under the action of strong typhoon doesn't meet the requirement; under the impact of super strong typhoon, the stability and wave overtopping discharge of all the studied seawalls can't satisfy the safety requirement. Through the study on safety evaluation of seawall under the different intensity of cyclones, the reinforcement suggestions for the possible damaged seawall structure are proposed to improve the overall defense capability of seawall economically and effectively. The results of this study can be applied to typical typhoon-wave-tide model analysis in Shanghai, and the evaluation system of seawall defense capability can be applied on the seawall in the river and coastal region of Shanghai. This study can provide technical support to the related department to improve the defense capability of seawalls with economic and effective engineering applications.