Mechanism Analysis Of Rainstorm Waterlogging In The City Of Shanghai

Shanghai, by the estuary of Yangtze River and the geographic center of the Asia-Pacific urban agglomeration, located in East China Sea, the total area is6340km2. There is a flat low-lying terrain and a criss-cross river network, with four distinct seasons and abundant precipitation, in the Yangtze River Delta region. As the largest city in the Yangtze River Delta with the total resident population of23.02million, and GDP output value of1.92trillion RMB that accounting for4.07%of the country, Shanghai becomes the most prosperous economic development, highest population density, and fastest urbanization level of China. Urban construction is the foundation of economic development, it plays a pivotal role that achieving Shanghai’s comprehensive, healthy, harmonious and sustainable construction, while avoiding urban hydrology disaster that maybe caused by rapid urbanization, in the development of the Yangtze River Delta region as well as the national economy.Rainstorm waterlogging is one of the urban hydrology disasters to restrict the development of major coastal urban of the world,"to see the sea", has become a unique landscape of it. Recent years, in the context of global warming and regional relative sea level rise, coastal cities waterlogging disasters have been the hotspots in international academia as well as the frontiers of science in research field. On the basis of the achievements of previous development in waterlogging disasters research, this research presents the reasons of coastal urban waterlogging and analyzes the temporal evolution characteristics of main driving factors. Moreover, the theoretical model of coastal urban rainstorm waterlogging will be built to simulate and analyze the scenario of waterlogging process in the City of Shanghai based on the over years waterlogging events. The main research work and conclusions can be summed up as follows:1) Summarizes the main driving factor of the coastal urban rainstorm waterlogging disasters, and their mutual relationships and constraints. Driven by the relative sea-level rise, the high and low tide of tidal rivers were raised correspondingly, and the trend was enhanced on the backwater effect of the rivers into the sea, then that will cause the capacity of drainage system reduced in directly. At the same time, under the influence of global change, the sudden precipitation events of the middle and lower reaches of the Yangtze River region was increasing year by year as well as concentrating frequency. Combined with rapid urbanization development to a certain extent, it is not conducive to infiltrate runoff through the underlying surface when a rainstorm occurs, that resulting in a sharp decline of outside drainage capacity in the low-lying areas. In addition, the existing drainage network system were outdated, resulting in difficulties on urban gravity drainage, poor drainage on low-lying, low efficiency on pumping stations, long time and deepening on waterlogging, and increased frequency and severity on waterlogging disasters.2) Choose the City of Shanghai as a typical study area, in-depth analysis of the spatial evolution of rainfall, land use and river network. The results showed that, precipitation as the primary constraint of waterlogging disasters, it was "three up and downs" in inter-annual variability and the total amount decreases slightly but no obvious mutations. Flood season precipitation as an important part of the year has a significant increased at the late of1980s, and summer precipitation since the end of1990s has also increase obviously which particularly has been remarkable in August, while spring rainfall has a clearly reduced since2006. Major year precipitation days have also decreased but flood season rainfall days were rally slightly. At the same time, it was an ease up on days of daily rainfall which exceeding25mm and50mm. It indicated the concentrating distribution and large intensity on precipitation, higher frequency on storm rainfall, and substantial extension on consecutive days. In addition to the Huangpu River and Suzhou Creek, affected by roads, housing, etc., the urban rivers both between west of Huangpu River and south of Suzhou Creek as the center around to dissipate quickly. Density and length of river network continue to decline, and then the function of natural waterlogging discharge was disappeared basically. With the same trend of Shanghai’s urbanization, it means urbanization plays a direct role in river network structure evolution. LUCC leads to increase the degree of road sclerosis, permeability of rainfall sharply reduced, the surface runoff coefficient was continuously consistent with improved urbanization rate that was exceeding0.7ψ already, and the risk of urban rainstorm waterlogging was tremendously rose. In the process of rapid urbanization, backward sewer network system in the City, such as Hongkou, Zhabei, Huangpu and Xuhui, is able to uncomfortable the increasing demand of drainage. In case of rainfall storm with100a or1000a return period, there is still had a serious waterlogging problem despite the points have a gently diminished than before, and it has also became a key factor in storm waterlogging induced.3) As an example on part of the roads which prone to waterlogging in the centre of City, construction the rainfall storm waterlogging model. Used SWMM ID hydrodynamic model and FloodMap2D numerical model, constructing the rainfall storm waterlogging model in the City of Shanghai through the loose coupling. And, calibrating the coupled model with the depth of waterlogging form the monitoring point nearby the cross of Anfu and Changshu road, analyzing four kinds of Manning’s roughness coefficient and its error rate, get the parameters in the model is0.013which is goodness of fit the actual monitoring. Utilize the25th August,2008waterlogging event as the foundation of model verification. The simulation results showed that the result is basically consistent the data collected from real-time monitoring systems by Shanghai Water Bureau in the overall situation although there is existed some errors and lag in inundation scope and waterlogging time, and the depth of water is above0.1～0.3m according with the hypothesis of without considering greening belt. And, most serious of waterlogging appeared at the junction of Anfu and Changshu road, and the most depth of overflow is among0.5～0.6m that due to terrain and other factors. Based on the extreme precipitation data of recent60a form Xujiahui Meteorology Station and analysis method of P-Ⅲ frequency, calculate the return period of the extreme precipitation. It found that the precipitation at different return period had a significant increased,100a return period shortened into50a and1000a return period is only to385a.4) Based on the after testing coupled model, and with the10a,50a,200a and1000a return period as the background, started the process modeling and scenario simulation of coastal urban rainfall storm waterlogging disaster. The conclusion shows that:there is a closed relationship between precipitation and overflow, and the hysteretic has existed. Based on the after testing coupled model, and with the10a,50a,200a and1000a return period as the background, started the process modeling and scenario simulation of coastal urban rainfall storm waterlogging disaster. The conclusion shows that:there is a closed relationship between precipitation and overflow, and the hysteretic has existed. In a process of last3h rainfall storm, the peak of rainfall always appears in1～1,5h, and the corresponding overflow’s peak is appeared around1.5h. The performance of simulation results with10a,50a, and200a return periods as the following three aspects:i)the largest depth and scope of waterlogging are continuously increased with increasing rainfall storm return period; ii)influenced by house, the speed of overflow is slow in the dense area of house but also a relatively long time; iii)crossing or turning of the roads are more easily to become waterlogging than other place, and also last longer. The characteristics and hazards of simulation under the extreme environment (1000a return period) also as the following three aspects:i)there is a wide range of waterlogging in the modeling area in the whole process with a biggest area of23600m2; ii)the depth of water shows an obviously decreased trend after increased, but there is an unobvious fluctuation by the peak of overflow compared with other scenarios; iii)the space distribution of waterlogging range is uneven.5) At the same time, based on different rainstorm return period, generalized to after the different size diameters drainage network simulation system, in order to get the return period of rainstorm under minimum diameter capacity. Through the calculated, the existing drainage sewer system unchanged and only consider buried deep drainage sewer system itself capacity, under the premise of modeling zone10a return period,50a return period,100a return period,200a return period and1000a return period rainstorm return period the drainage system of ideal diameter are Φ=450mm,Φ=600mm,Φ=700mm,Φ=800mm and Φ=1000mm, and while the modification cost are9.086million yuan,15.174million yuan,17.25million yuan,18.598million yuan and20.183million yuan respectively.