Extreme weather events are expected to become more intense and frequent in the changing climate, which lead to growing risks of storm-related hazards for delta regions like the Guangdong-Hong Kong-Macao Greater Bay area (GBA). In the densely-populated metropolitan region, storm-related natural hazards can cause great loss of lives and damage to properties. It is necessary to identify possible extreme weather conditions considering the effect of climate change and simulate the hazards likely to happen. It is challenging due to the complex mechanism of different kinds of hazards. For example, flooding can be triggered by rainfall, high sea level and wave overtopping at the coastal region during a tropical cyclone. The flood severity can be exacerbated and the capacity of the urban drainag...[ Read more ]

Extreme weather events are expected to become more intense and frequent in the changing climate, which lead to growing risks of storm-related hazards for delta regions like the Guangdong-Hong Kong-Macao Greater Bay area (GBA). In the densely-populated metropolitan region, storm-related natural hazards can cause great loss of lives and damage to properties. It is necessary to identify possible extreme weather conditions considering the effect of climate change and simulate the hazards likely to happen. It is challenging due to the complex mechanism of different kinds of hazards. For example, flooding can be triggered by rainfall, high sea level and wave overtopping at the coastal region during a tropical cyclone. The flood severity can be exacerbated and the capacity of the urban drainage system which plays an important role in the water cycle of an urban region, can be also be affected.

In this research, the extreme weather events in GBA are first analysed. The characteristics of rainfall in GBA in terms of annual total amount, mean rainfall intensity and moderate extreme rainfall, as well as the trends are studied to learn about the past. Significant rising trends are observed in mean daily rainfall intensity and heavy rain days in GBA. Moreover, the 24-hour probable maximum precipitation in a megacity in GBA (Shenzhen) is estimated using the storm transposition method to learn about the upper bound of local extreme rainfall amount. To produce long-term rainfall series for hazard analysis, a rain field generator is developed based on point rainfall observation. The generator is able to reproduce the statistical characteristics of spatial-temporal variation of regional rainfall. The major advantage of the new rain field generator lies in the feature that the model can be easily adapted for climate change. The generator is applied to GBA and the statistical characteristics of the rainfall in GBA can be well reproduced.

The flood hazard definitely has a profound impact for the cities in GBA. The delta region is prone to fluvial, pluvial and coastal flood hazards. In this thesis, multiple flooding triggers, i.e. rainfall, storm surge (high sea level) and wave, are integrated in a model for flood analysis. An equivalent drainage method is proposed to improve the efficiency of large-scale urban drainage modelling and to extend the application for cases without plenty of data on the distribution and geometry of drainage pipelines. The equivalent drainage method simulates the capacity of drainage networks by enhanced infiltration, considering the variation of drainage rates with the weather conditions. Two case studies are conducted in the Qianhai area of Shenzhen and the Tseung Kwan O area of Hong Kong to investigate combined rainfall-storm surge and rainfall-storm surge-wave overtopping events. It is found that the drainage rate can be affected by the rainfall intensity and sea level boundaries. In the combination of low-intensity rainfall and high sea levels, large volumes of backflow into the outfalls are observed and the stormwater is difficult to drain into the sea. The influence of sea level rise and ground subsidence in reclaimed lands, unfavourable combination of storm surges and high astronomical tide, and the effectiveness of flooding mitigation structures are also discussed.

Apart from flooding, multiple hazards that can happen during a severe rainstorm are also simulated in the central metropolitan region of Shenzhen with an integrated numerical model, EDDA, which is able to simulate floods, landslides and debris flows at the same time. Several rainstorm scenarios are designed from historical events and the estimated probable maximum precipitation for hazard analysis. Hazard maps are produced and locations prone to these hazards are identified. It is found that upon the extreme designed rainstorms, the inland inundation can particularly severe and the deposit depths from landslide and debris flow can be large in some urban areas, which can affect residential estates and schools near the hilly region.