Tidal inundation during typhoons is the main flood risk to the Huangpu River in Shanghai, although the city is also at prone to flooding caused by typhoon rainfall. The city is protected from tidal flooding by defences along the river, which are designed to ‘1 in 1000 years’, the same design standard of protection as London. However, sea level rise and ground subsidence has led to the need for improved defences and the city is proposing to construct a flood control barrier downriver of Shanghai near the river mouth.
HR Wallingford was asked to research and model options for the project, leading the technical work, while all of the modelling and analysis was carried out in China, by HR Wallingford and its partners. Our research took the TE2100 approach and applied the method to Shanghai. The project was funded by a Newton Fund research grant to support collaborative research between the UK and other countries, in this case between our UK and Shanghai offices, the Shanghai Climate Change Research Centre and SOAS (University of London).
Over the course of 2018 and 2019, we met the with the stakeholders both in China and the UK to discuss the project and to collect data. These data included information on the sources of flooding (high tides; river flows and heavy rainfall); survey data for the Huangpu River; the flood protection system for Shanghai; information on the flood risk area including the topography and land use; and flood damage data.
The present day and future flood scenarios we developed for Shanghai included: tidal water levels; river flows; rainfall; and drainage inflows to the Huangpu River. The team used sea level rises of up to 3 metres in order to consider the flooding that could occur in the very long term (more than 100 years), or in the case of a rapid rise in sea level that exceeds current projections.
Our team then developed two models using InfoWorks ICM software. The first was a 1-dimensional (1D) model of the Huangpu River and the second took the 1D model and combined it with a 2D model of the entire floodplain area, including the city of Shanghai and the surrounding areas, to create a 1D/2D model. The 1D model was used to predict water levels in the Huangpu River with no overtopping of defences in order to determine the flood defence levels that would be needed to prevent flooding and model flood protection options. The 1D/2D model was used to assess flooding and flood damages.
The team then applied the1D/2D model to a range of scenarios including combinations of high tides, river inflows and rainfall in order to determine present and future flooding and flood damages. Whilst the estimates are approximate, the results clearly show that a very high level of flood damage could occur in Shanghai.
The 1D model was used to determine flood water levels along the river for present and future scenarios of tidal water levels combined with river flows and drainage inflows. It soon became apparent that the present-day flood defence crest levels are below the required levels at some locations.
Options for Shanghai
Shanghai, like London, is highly constrained by development on the floodplains, meaning that major engineering works, such as raising of flood defences and a flood control barrier, are the best options to protect the city from flooding. We established design criteria for the flood protection system including standards of protection, and used modelling to develop adaptation pathways for flood protection options, similar to those developed for London.
The options investigated included raising all the flood defences and developing a strategy for raising the defences in stages. The average increase in defence crest levels needed to provide protection to the year 2080 is about 1.1 m, with further defence raising needed after this date, although this would result in high river walls on both sides of the river.
The city of Shanghai’s preferred option is a new flood control barrier, so we also explored this option. There are a set of key parameters must be established as part of the design process. These included: the defence level provided by the structure; the high tide level at which the barrier must be closed; the timing of a barrier closure; management of upriver fluvial and drainage inflows during a barrier closure; and the number of closures per year.
The number of times a barrier closes in a year is important because it affects the risk of failure. When barriers are closed frequently, the time available for maintenance is reduced and the annual probability of failure during a closure increases. The number of closures per year can be controlled by raising the upriver defences, so we developed an adaptation pathway for the potential barrier that included raising of the upriver defences to reduce both the number of closures and to take account of water level increases caused by potential future drainage inflows.
When the sea level rises, barriers are closed more frequently and the impacts on navigation become more serious. For this reason we also investigated a barrier with locks and a tide excluding barrage. In addition, the team considered options for reducing fluvial inflows on the Huangpu River and managing urban flooding from rainfall.
After exploring a number of options, we concluded that raising of the initial defence followed by the construction of a barrier and the raising of associated defences appears to be the most suitable option for Shanghai. Interestingly, this approach was also identified as the best option for London and the Thames Estuary, where the existing barrier will be replaced or improved in about 2070 based on current sea level rise projections. So, it seems that the two cities will continue to tell similar tales about flooding and flood control well into the future.
Environment Agency (2012), Managing flood risk through London and the Thames estuary: The TE2100 Plan, Environment Agency, November 2012.
HR Wallingford (2009), The Estuary Wide Options, Thames Estuary 2100, Technical Report - Appendix D, HR Wallingford on behalf of the Environment Agency, November 2009.