Shanghai is the most important coastal city in eastern China, the main area that will be influenced by the north pacific tropical cyclones. To protect the city from direct invasion of storm surge, 532 km seawalls have been built along the Changjiang Estuary and Hangzhou Bay area. They are the first promising barrier to Shanghai's safety from tidal flood. Currently, the standard for the dike design in Shanghai is mainly of 200 years return period in tide level combined with force 12 wind, with the world climate change and frequent severe weather, the safety of dike structure under the excessive storm surge, if happens, is the most concern by city managers. A 7.0km long dike at the junction of Changjiang Esutary and Hangzhou Bay is chosen to be studied here. A typhoon route which may have negative effect on the dike is given after some numerical simulation. By using storm surge model and typhoon- generate wave model, tide level before dike, wave force and wind speed are calculated under the conditions of severe typhoon (40 m/s at center) and super typhoon (60 m/s at center) with the selected route. According to the tide level and wave conditions of these excessive storm surge in the seawall of Lingang, the feature of armored structure, seawall stability and anti-scouring ability of back-slope from overtopping are studied.

The result shows, the No. 9711 typhoon route with a 1.5 degree latitude translation to the north will be the most harmful to the research area and two different typhoon intensities are set on this rout, separately the intensity equal to No. 9711 as severe typhoon and No. 5612 as super typhoon. Under the condition of severe typhoon, the highest tidal level can be 6.74 m (Shanghai Wusong Datum) and the average wave height be 2.99 m, with the condition of super typhoon separately be 7.68m and 3.50 m.

Under the severe typhoon, the fence board thickness on current seawall, single wing shape block weight and the overall anti-slipping are all satisfy the stability condition, but the anti overturning stability is dissatisfy, and wave overtopping exceed to the normal extent, as a result the back-slope can easily be scoured.

Under the super typhoon, fence board thickness approaches to critical number, other indicator such as single block weight, anti-slipping, anti-overturning are all dissatisfy the stability condition. Also, the overtopping extremely beyond the normal extent and the back-slope can be easily scoured.

In this paper, we also present some advanced advice to deal with excessive typhoon.

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