Disaster Mechanism And Risk Assessment Of Storm Surge In The Yellow And Bohai Sea

The 21 st century is the century of the sea.While vigorously developing Marine economy,it is particularly important to effectively reduce the economic losses caused by various marine disasters and ensure the safety of coastal residents’ lives and properties.With the development of science and technology and the increasing concernto marine disasters,the numerical simulation of purely hydrodynamic process can no longer fully meet the needs of decision makers and users,and the risk assessment based on social attributes is receiving more and more attention.However,because storm surges are influenced by different geographical areas,involve many assessment factors,and lack of measured disaster data,there are still many unclear aspects of their disastercausing mechanisms and risk assessment,so they have been one of the research hotspots for scholars at home and abroad.In view of the above,this thesis investigates the spatial and temporal distribution of storm surge,the disaster-causing mechanism of storm surge,and the risk assessment of storm surge in the Yellow and Bohai Sea(north of 35°N).The study is based on in situ observations,satellite remote sensing,and basic geographic data,and the methods used here include statistics,numerical simulation,and disaster models.The following results were obtained.Firstly,the temporal and spatial characteristics of storm surges were studied.Based on the observation data from tide stations and disaster data in the past 30 years,it is found that the occurrence frequency and extreme value of storm surge and warning water level are relatively consistent in spatial distribution,and both of them mainly occur in Bohai Bay and Laizhou Bay.However,there is a big difference between their temporal distributions.The former mainly occurs in the autumn and winter exchange period,while the latter mainly occurs in the flood season.The casualties and direct economic losses caused by storm surges in the Yellow and Bohai Sea are mainly distributed in Shandong Province,and the proportion of disaster losses in the Yellow and Bohai Sea has been increasing in the past 10 years.At the same time,this thesis shows the possibility distribution of red alert based on the warning water levels,and it is concluded that the building construction of tidal protection is poor and people’s awareness for tidal protection is relatively weak for coastal regions along the Yellow and Bohai Sea.Therefore,the research on the mechanism of storm surge and the construction of risk assessment system are more needed in this region.Secondly,according to the temporal and spatial distribution of storm surge,it is shown that northward typhoons cause the most disaster storm surges in the Yellow and Bohai Sea.Therefore,this thesis focuses on the northward typhoon storm surge to study the disaster-causing mechanism.The studies are conducted from two perspectives: the changes of the weather system itself and the geographical position of the weather system.(1)Based on the analysis of historical typhoon storm surge and numerical comparative simulations,the influence of the change of weather system itself(The main study of the typhoon impact range,which were less considered by previous generations)on storm surge was studied.The results show that the typhoon impact range is one of the decisive factor of storm surge.When the typhoon impact range is small,the change of typhoon intensity and maximum wind speed radius has little effect on storm surge.When the intensity of typhoon is fixed,the maximum value of storm surge increases linearly with the increase of typhoon impact range.(2)A method to decompose the storm surge in the Bohai Sea using the location of the weather system is proposed.By analyzing the historical typhoon storm surge process and designing numerical comparison tests,the storm surge in the Bohai Sea is decomposed into two parts: one part is the direct effect of the local wind field located in the Bohai Sea,and the other part is the indirect effect of the external wind field located in the Yellow and East China Seas.The former is caused by wind acting directly on the Bohai Sea,so the spatial distribution varies greatly;while the latter is mainly triggered by the wind field acting on the Yellow and East China Sea regions,the energy generated and the seawater transfer into the Bohai Sea,so the spatial distribution is relatively consistent.When the external wind field is southeast,the maximum value of storm surge along the coast of Bohai Sea is the largest,and the duration of southeast wind is an important factor that affects the maximum value of storm surge in Bohai Sea.Finally,through the study of the temporal and spatial distribution of storm surges and the disaster-causing mechanism,it is determined that the Laizhou Bay bottom is a disaster prone area.Therefore,this paper constructs a quantitative multi-disaster concurrent and chain risk assessment system for this area,and takes storm surge of TY1909 as an example to carry out risk calculation,and discusses the rationality of the assessment system.(1)The multi-disaster concurrent assessment system is an M + N risk assessment system constructed by the newly proposed algorithm of the complementary superposition algorithm in this paper with the disaster-bearing body as the starting point.The system introduces the role of reflecting the sensitivity of different disaster-bearing bodies to differences in the resilience of different disaster-causing factors and the capabilities of disaster prevention and mitigation,and continuousizes the discrete assessment levels by means of function fitting and normal distribution.The risk assessment of storm surge,waves and flooding concurrency was also carried out for Typhoon TY1909 storm surge based on MIKE21 and flood overflow model,etc.The assessment results are consistent with the actual process of carrier losses,indicating that the assessment system can accurately identify the magnitude and distribution of risks in the study area.(2)The chain risk assessment system is built on the basis of a multi-hazard concurrent risk assessment system.According to the chain relationship of disaster events,the chain risk assessment is decomposed into double concurrent risk assessment,and the occurrence probability is iteratively calculated by the complementary superposition algorithm and conditional probability superposition algorithm to obtain the integrated probability of risk occurrence of the disaster-bearing body.Through reasonable assumptions,a chain disaster event triggered by Typhoon TY1909 storm surge is constructed,while chain risk calculation is carried out and the risk assessment process is given.The calculation results show that the chain assessment system and process can concisely obtain the probability of occurrence of disaster events at each stage and obtain risk values from different perspectives.In summary,this study not only gives the distribution and the mechanism of storm surge in the Yellow and Bohai Sea,but also points out the research direction on storm surge forecast.At the same time,through the establishment of a complete set for single disaster,concurrent disaster and chain disaster risk assessment system,this study provides the technical support for disaster prevention and mitigation and auxiliary decision-making.