Methodology Construction Of Compounded Scenarios Simulation For Storm Flooding

Estuarine delta, an intensively merged area that witnesses the complicated interaction between the physiogeographic units including land, river and ocean, supports a highly developed social economy in most cases. Being considerably sensitive to topography evolution due to human activity and sea level rise caused by climate change, estuary deltas are also prone to impacts of the extreme storm surges. The intensive human activities has caused a rapid change of ecological environment, consequently, under a circumstance of climate change, both the frequency of extreme surge events and the destructive potential keep increasing dramatically, which poses great threat to delta areas, meanwhile generates continuous interest in the disaster research circles. Thus, it is quite necessary to carry out effective research on the extreme disaster risk that estuary deltas are facing with.Considering the tendencies in change of the direct and indirect disaster-causing factors of storm flooding, in this paper, an empirical research was conducted in Shanghai, the front region of the Yangtze estuary delta, and the flood formative mechanisms were discussed based on scenario analysis method. On the benchmark time of 2010, storm flooding compounded scenarios were designed for risk predictions in 2030(short-term) and 2050(long-and mid-term). The main work and conclusions of this research include:(1) Prediction of the direct and indirect disaster-causing factors of storm flooding:First, the direct factor was simplified as the intensity of tropical cyclones. Based on the probabilistic and deterministic method, the intensity was predicted according to the Tropical cyclone best path data sets from CMA_STI for decades, i, e, the minimum central pressure was set as 920hPa when a cyclone landed. Besides,12 designed tracks were acquired from the translations of the typical tracks of TC9711, TC8114, TC0012, TC0205 and TC1109, which have brought great economic loss and causalities in history; Secondly, the indirect factors were specified as sea level rise, land subsidence and coastal erosion-accretion change. The predicted maximum sea level rise value in the RCP8.5 scenario proposed in IPCC AR5 was selected for the research in this paper. The regional land subsidence value was calculated with the mean annual rate during 1980-2005, while the subsidence rate of the seawalls was obtained from the historical monitoring data, and the coastal erosion-accretion change was calculated by the mean annual rate during 2002-2010.(2) Construction of flood simulation model for the study area:Basically, MIKE 21 Model was utilized. By covering the East China Sea, the open boundary of the computational domain reached areas that were deep enough, in order to minimize the calculation errors. First, the tide level at the open boundary was calculated using the Global Tide Prediction Model (DTU10). Then, a multiple embedded triangle mesh covering the whole study area was built with topography data of high-precision. Next, Monitoring data (tide level, wind speed etc.) at several tidal gauge stations during TC9711 and TC0216 were, respectively, set as control parameters for the simulation model. Finally, a continuous calibration of the sensitive parameters, such as the bed resistance, eddy viscosity, time step and open boundary condition etc., showed that, the simulation results were well consistent with the measured value. As for tide level simulation, results acquired in situations including and excluding the wave radiation stresses effect were compared, and at the two tidal gage station located in Yangtze Estuary and Hangzhou Bay, the average ratios were only 1.07 and 1.09 respectively, so the effect of the wave radiation stresses was not taken into account for the storm flooding simulation in the article.(3) Methodology of compounded scenarios designation and hazard simulation: Factors including tropical cyclone tracks, central reference points, central pressure, and sea-level rise, land subsidence, erosion-accretion evolution were reasonably assembled to construct the compounded scenarios. Eventually, a scenario matrix containing 108 typhoon storm surge scenarios was obtained. On the basis of the predicted result of sea-level rise, land subsidence and erosion-accretion evolution in 2030 and 2050, the tropical cyclone parameters and the indirect factors mentioned above were used as inputs for the numerical dynamic simulation of storm flooding process. Results showed that, the inundation caused by the storm flooding under the compounded scenarios were much larger than the simple scenario. Moreover, the inundation caused by the four tropical cyclones landing in Shanghai would be the most serious, and the inundated area, mainly distributed in the northeastern of the central city, including Xinjiangwancheng Street, Changbaixincun Street and Quyanglu Street, of high hazard would reach 70.83km2,126.86km2,145.18km2 and 139.72km2 respectively. Basically, the results indicated that, providing no improvements in adaptation capability and engineering countermeasures, the study area will be seriously affected by the storm surge in the future scenarios with continuous change of regional topography and climate.(4) Acting mechanism of direct and indirect factors:Considering the time-space characteristics of surge flooding, a correlation analysis was conducted between the extreme tide level and the corresponding inundation area by using the quadratic model. Results showed that, the highest correlation coefficient reached 0.825 in 2030, while there was no significant relationship for 2050. Then, the separation evaluation method was adopted to reference and evaluation period to quantitatively analyze the contribution of each factor to the storm flooding. Results showed that the influence of erosion-accretion evolution was obvious during 2010-2030, and the highest contribution value was 40.72%; while during 2010-2050, the influence of sea-level rise and land subsidence was higher than that of erosion-accretion evolution.On basis of the simulation results above and GIS spatial analysis method, the inundated areas of high hazard reached 25.88km2 and 180.96km2 in 2030 and 2050, respectively. In 2050, land for public facilities, residential and agricultural use would suffer most of the inundation, accounting for 52.35% of the total area. Regarding population and GDP, the social-economic risk was evaluated, revealing that the area with highest comprehensive vulnerability would reach 45.42km2 in 2050. According to the assessment result and the analysis of the dominant influencing factors, the highly hazardous inundated areas was partitioned into five major impacted zones (Changning, Yangpu, Hongkou, Zhabei and Huangpu), then emergency evacuation simulation, according to the predicted population, in 2050was conducted in each zone. Finally, countermeasures for withstanding the storm floods and risk reduction were put forward, including protecting the water source conservation areas, digging the impounding reservoirs, improving the design criteria of seawalls, enhancing the drainage ability in the urban built-up areas, optimizing the distribution of the emergency shelters, building the hanging-plate gate in Wusongkou and the flood detention areas.