As a low-lying city, Shanghai faces threats from typhoon and spring tide under the condition of climate change and land subsidence. With high water level at the toe, the sea embankment is likely to be overtopped and breached, finally resulting in inundation inland.  The objective of this research is to study climate change and land subsidence effects on Shanghai inland inundation due to dike overtopping and breaching under extreme weather condition.  A hydrodynamic model and a wave model have been established by Delft3D-FM and Delft3D respectively. Through validations on historical events, the hydrodynamic model and wave model are proved to be valid. The water level and wave condition along the coast, which are concerned as the results of these two models, are also essential inputs for overtopping and breach discharge calculation. In overtopping and breach discharge calculation, the threshold of breaching is estimated as an overtopping rate of 0.1 m3/m/s. The resulting overtopping and breach discharge gives the boundary condition of the overland simulation. The inundation map over Shanghai area can then be achieved by the overland simulation. A sensitivity analysis of the breach widths is also done.   Ten hypothetical typhoon events are provided by the Met Office Hadley Center under past and future climate conditions. These cases are applied to the whole process to study the effects of climate change on coastal flooding in Shanghai. The relative sea level rise is also considered for both past and future climate conditions.   The results show that places with high water level and low sea dike elevation are more likely to get high overtopping that can finally result in breaching. For Shanghai city, such vulnerable places can be found along Hangzhou Bay, especially in Jinshan District and the south-east corner of Shanghai. Besides, the entrance of Shanghai Yangtze River Tunnel is also vulnerable due to land subsidence. For some extreme cases, the whole Shanghai coast is in danger.  For the past climate and land elevation around the year 2000 with the wind speed return period of 1.3 yr and the breach width assumed to be 300 m, it is simulated that the maximum inundation area in Shanghai can be 1,805 km2 (33.3% of the simulated area in Shanghai). In the future, given the challenge of climate change and land subsidence, the sea level is relatively rising. The intensity of typhoon will generally strengthen. For the future climate and land elevation around the year 2100 with the wind speed return period of 4.5 yr, it is simulated that the inundation area in Shanghai can be 3,388 km2 (62.4% of the simulated area in Shanghai), which is almost twice of the inundation area around the year 2000.  The breach width also affects the inundation situation. If the breach width becomes larger, the inundation situation will be worse. However, as the breach width grows, the increase of the inundation area decreases. ... Read more

Shanghai is one of the numerous megacities worldwide that experience severe flood events triggered by torrential rainfall. To deal with the undesirable consequences of these events and mitigate the flood hazard, the research of flood reduction measures is necessary. In this effort, the hydrodynamic modelling is a useful tool.
In this master thesis a 2D model was developed for the simulation of urban flood events in Jingan District in the downtown of Shanghai. The main objectives of the thesis were the production of the flood hazard maps for numerous rainfall events and the assessment of the proposed flood mitigation measures. Delft3D Flexible Mesh was used as a tool to produce the inundation maps. Also, several data were considered regarding the grid for the numerical calculation, the surface elevation, the local drainage system and the rainfall events. SOBEK was used for the set up and the preprocessing of the sewer system. For the simulations, data from three historical rainfall events were used: August 2005, August 1997 and September 2013 and five rainfall events with return periods of 1, 3, 5, 10 and 50 years were generated by using the Chicago hydrograph.
For the validation of the model, the rainfall event of August 2005 was used. Although the validation of the model was not proven due to the simplifications that were made in the input data and the lack of data, the model showed that some processes can be simulated, and inundation maps can be produced. By comparing the results that occur with and without the inclusion of the drainage system, it was concluded that the local drainage system should be included in the analysis for the assessment of the flood hazard in an urban area, since its presence plays an important role in the flood reduction. The results showed that the maximum inundation depth can decrease by around 45%. As flood reduction measures, the creation of water storage areas and the increase of the drainage capacity were considered. The water storage areas covered around 10%, or less, of the block areas with available space, leading to a water depth reduction that depends on the location in the map. For the increase of the drainage capacity, the value of 1 m3/s was assigned in a single and in multiple locations in a specific area of investigation. The results showed a percentage of water depth reduction around 15.9% and 45.5%, respectively. For the same location the percentage of water depth reduction due to the water storage areas was 22.5%.
Finally, uncertainties were introduced in the model due to the assumptions and the simplifications that were made in the input data. However, this model can work as a base for future researches to accomplish more realistic results, by improving the current model and adding more updated and precise data.
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Shanghai is a city located in a coastal region and to understand the flood risks it is exposed to, it is of most importance to first understand the processes that control the water levels for the different flood scenarios. Historically, the Huangpu River has reached its highest levels during landfalling typhoon events, which create a combined scenario that involves high sea water levels due to storm surge, and high river water levels consequent not only of the storm surge at the river mouth, but also of the runoff generated by precipitation in the upstream regions. This research project will focus on the later, assessing the impact of torrential rainfall during tropical storms on the water levels along the Huangpu River in Shanghai city. By studying the hydrological regime of the area of interest, three main watershed regions are identified for the Huangpu river basin; the Taihu lake basin situated upstream regulates the yearly discharge on the downstream areas of the river and is controlled by means of a flood gate which remains closed once a certain flood risk is identified, an agricultural area covered in its majority by an interconnected lake system, and the river basin which encompasses the remaining contributing regions to the system. A hydrological model is built for the Huangpu river basin following the rational method, identifying from satellite databases the dominant land cover classes of the region, the hydrological soil group based on the different soil contents, and the average slope around the area based on a digital elevation model. Using the precipitation data from typhoon Fitow, the hydrological model was used to estimate the corresponding discharge time-series from the storm to be used as input on a hydrodynamic model of the Huangpu river. The hydrodynamic model of the river was built using D-Flow Flexible Mesh, it was used to assess different scenarios of the river system configuration, allowing to understand not only the overall contribution of rainfall-runoff to the river discharge but also the effect of each of the catchments on the river system. The performance of this model, as well as of the hydrological model was observed by comparing the predicted values with the site measurements at two hydrological stations, one midstream at Huangpu Park which highlighted the strong influence of the tide on the water levels for the river sections closer to the sea, and one upstream at Mishidu where the influence of the rainfall runoff to the water levels could be observed. The hydrological model was then validated using the precipitation data from typhoon Haikui, taking the corresponding discharge time-series for it to the DFM river model and comparing the estimated water levels to the actual measurements. Finally the river profile with the maximum water levels along it as predicted by the DFM model was compared to the scenario modelled with no rainfall-runoff discharge and the measured embankment height to understand the contribution to flood risk of the torrential rainfall during tropical storms on the water levels along the Huangpu River. ... Read more