The relationship between climate and cities has always been a focus of urban-related research, profoundly affecting all aspects of urban planning and construction. Especially in recent years, the increase in rainfall events caused by global climate change has led to many cities being affected by stormwater flooding. How to guide and dissipate rainwater has become one of the key points of urban planning. For some old coastal urban areas with a high degree of urbanization, due to problems such as high ground hardening rate, small design rainfall of municipal drainage pipe network, and seawater backflow, the permeability and discharge of rainwater are insufficient, which makes it more likely to cause large-scale stormwater flooding in low-lying areas of the city, posing great hidden dangers to the buildings and residents of the old urban areas. Currently, experts and scholars at home and abroad are also committed to studying how cities can respond to the risks of stormwater flooding and proposing many natural-based solutions. China has also proposed the theory of sponge cities based on this background. This article takes the Macau Peninsula, which has the characteristics of an old coastal area with high density, as an example. Four types of storm events with different recurrence intervals are designed, and a city flood risk assessment framework composed of "hazard-exposure-vulnerability" is applied to comprehensively evaluate the flooding situation on the Macau Peninsula. Based on the evaluation results and combined with urban renewal methods, the sponge city planning strategy for the old urban areas is studied. Finally, the sponge city planning map is drawn and a feasibility analysis is carried out to provide strategies and suggestions for reducing the risk of stormwater flooding in old coastal areas with high urban density through the construction of sponge cities. The main research results are as follows.
(1) This article adopts the Chicago rainfall intensity formula to design four types of storm events with different recurrence intervals on the Macau Peninsula. The SCS-CN runoff model is used in conjunction with the drainage capacity of rainwater pumping stations on the Macau Peninsula to calculate the runoff and actual water accumulation. Finally, the water accumulation scenario is simulated using ArcGIS. The result shows that the water accumulation area and depth on the Macau Peninsula increase with the increase of stormwater intensity. In low-lying areas, the depth of stormwater flooding can reach more than half a meter, posing significant safety hazards.
(2) This article adopts the urban waterlogging risk assessment framework of "Hazard-Exposure-Vulnerability," introduces index formula and "depth-damage" curve for quantification, uses the age of residential buildings in the Macau Peninsula as the weight indicator, and combines the analytic hierarchy process to construct a judgment matrix for weighted evaluation and analysis of waterlogging risk in the Macau Peninsula. The results show that as the intensity of heavy rain increases, the waterlogging risk in the areas of Nossa Senhora de Fátima, São Lourenço, Sé, and Santo António in the Macau Peninsula gradually rises from low risk to high risk. Among them, under the intensity of heavy rain occurring once in 500 years, the waterlogging risk in Nossa Senhora de Fátima reaches the super high-risk level, with a risk index of 10.783. When implementing sponge city planning to reduce waterlogging risk, it is necessary to pay special attention to the layout of low-impact facilities and the control of annual runoff in Nossa Senhora de Fátima. By constructing the "depth-damage" curve, the annual average loss of waterlogging in the Macau Peninsula is estimated to be 4.2017 million Macau Patacas, and the cost-effectiveness of sponge city construction is highest when the annual investment is below this standard.
(3) This article conducted a comprehensive assessment of the urban flooding risk in the Macau Peninsula, and combined with information such as its overview, surface runoff direction, building age distribution, and land use planning diagram, set the rainwater annual runoff control rate at 65%, and divided it into 11 sub-catchment areas for sponge city planning. The result of the feasibility analysis of the planning scheme showed that the comprehensive annual runoff control rate was 70.94%, exceeding the target of 65% and meeting the requirements of the planning design. The total cost of investment in sponge city construction was 149.48 million Macau Patacas. If sponge city construction is carried out in the Macau Peninsula without exceeding the annual average loss of 4.2017 million Macau Patacas, it would take approximately 36 years.