Study On Risk Assessment Of Seawall Overflowed By Typhoon Storm Surge At Zhejiang Coast

Storm surge disaster is the most severe one in the oceanic disasters of China, while Zhejiang Province is the most serious region of typhoon storm surge disaster along the coast of China Under the background of global warming, the frequency, intensity and impact range of Zhejiang province storm-surge disaster increases. Based on the observational data analysis and storm surge numerical simulation, the risk assessment of storm surge disasters in Zhejiang coastal area is of great significance to carry out.By systematically combing and following the precedents by previous research, we discussed the connotation of Zhejiang coastal typhoon storm surge disaster risk in this paper, aiming at various temporal and spatial scales and different regional features of typhoon storm surge disasters, appropriate evaluation methods has been use to carry out the Zhejiang coastal storm surge and seawall overflowed risk analysis, storm surge seawall overflowed and flood risk demonstration research of typical coastal area and a visual display by the use of GIS platform.The man research work and concl usi ons are as f ol I ows:(1) According to the existing storm surge disaster risk evaluation index system, it systematic analysis of the risk of storm surge disasters in the coastal region of Zhejiang Province.270typhoon storm surge disaster degree was calculated, so as to determine the risk distribution and disaster level distribution of Zhejiang coastal storm surge. Research shows that Zhejiang coastal region is severe storm surge disaster area, while Wenzhou coastal area is especially the highest in the disaster degree, the Gulf of Hangzhou and the coast of Taizhou take the second place. The disaster degree in space has a distribution characteristic that the south is more than north, while the estuary is more than island.(2) Based on60years of Zhejiang coastal average temperature and tidal observation data, the analysis of the i nf I uence on Zhej i ang sea storm surge due to cl i mate change i s the fi rst ti me.Studies show that the average temperature and the intensity of storm surges have obvious positive correlation. Over the past60years, all of Zhejiang coastal landing typhoon number, mean sea level, larger storm surge frequency, ultra warning storm surge frequency and annual extreme high tide level have a very similar trend as mean temperature, which rises. Snce1990, especially, along with the rise of average temperature, the trend just mentioned is more obvious. Zhejiang landing typhoon multiplied, whiletyphoon landing point has a southward trend, which resulting the increase of storm surge disaster risk in Zhejiang coastal region.(3)Using1951to2012national annual average temperature, SSTA in NINO3and4, the number of typhoon landed in China and Zhejiang, and annual highest tide data of Zhapu station from1954to2012, by using the method of wavelet analysis, this paper analyzed the trend and period of factors above. It shows that:long-term variation periods of the national annual average temperature, the number of typhoon landing in Zhejiang, and the highest tide of Zhapu station are very similar, which is more than60years. There is only a span of35-40years of rising period can be separated from these elements in the time series of60years. And this rising period has been presented to end trend after2007. While a short period of10-11years can be found in national annual average temperature, SSTA in NINO3and4, number of typhoon landing in China and Zhejiang, and extreme high tide in Zhapu station.(4)Taking Hamen station as an example, this paper analyzed the storm surge characteristics of strong tidal estuary areas in Zhejiang, and17storm surge cases in Haimen station which has a surge of more than1meter using the application of EMD (Empirical Mode). The results show that the storm surge in Haimen station has obvious characteristics of tidal period, while the nonlinear effect of the storm surge and astronomical tide is significant. When the surge curve of the station is standard, the surge which is caused by the nonlinear interaction is quite small, which proportion in the total value is only about1/6. When the surge curve of the station is mixed-type, the fluctuation characteristics of the surge becomes significant. The surge caused by the nonlinear interaction effect is larger, and its proportion in the total value is about1/3. The surge caused by the nonlinear interaction of volatile surge is maximum, which proportion in the total value can reach more than1/2.(5) Using ECOMSED to establish the two-tide coupling model, after the simulation verification of several historical storm surge cases, it shows that the method can simulate the tide and storm surge well in Zhejiang coastal area, including estuary. It means that the model parameters is reasonable, which can be used for Zhejiang coastal storm surge simulation and seawall overflowed risk assessment and effective analysis of storm surge characteristics of no observational tidal data region. According to the analysis of surge characteristics in different landing areas of typhoon, firstly, it shows that storm surge of the shore which near and on the right side of the landing point is the largest, most of which has a form of the standard type. Besides, the maximum water value is about3times as which on the left landing point. Hence, it has the largest the risk of storm surge disaster in this region. Secondly, storm surge of the shore which far away or on the left side of the landing point is much smaller, which has a form of wave type. Thirdly, the water values in estuary are larger than that around the island in the same typhoon. Hence, storm surge disaster risk of estuary area is greater than the island.(6) By using the frequency analysis and the numerical simulation method, the system framework of operational Zhejiang seawall overflowed risk analysis of storm surge has been established for the first time. Analyzing the combination of storm surge and astronomical tide in different return period, it shows that Zhejiang coastal storm surge seawall overflowed risk mainly concentrated i n the dam whi ch bel ow the standard of20years, whi I e the seawal I overf I owed ri sk of standard in50years and above is quite small. According to statistics, the highest tide level in history is close to or higher than the20years standard dam height of Zhejiang coastal tidal station, while it is lower than the50years standard.(7) Using the combination method of typhoon probability prediction and storm surge two coupled tidal forecast, it is analyzed, for the first time, the probability of Zhejiang storm surge seawall overflowed under the influence of either of typhoon. This method can smooth the deviations due to the storm surge forecast inaccuracies caused by single typhoon path prediction, which reduces storm surge and tidal level prediction error. The storm surge forecast accuracy can be steadily as long as the typhoon forecast accuracy increased. Hence, this method can be used to forecast the storm surge operationally and do the risk assessment of storm surge seawall overflowed.(8) Using skyline and GIS technology, it integrate station observation, typhoon path prediction, two coupled tidal numerical prediction results, the overtopping probability forecasting results and other information. It construct a storm surge disaster warning and assistant analysis system for Zhejiang coastal for the first time, which can display the distribution of either of typhoon storm surge disaster risk zone and seawall overflowed risk coastline. It has been used in the Typhoon Defending Conference of Zhejiang Flood Control Office in2011.