Objective In the context of the normalization of global extreme weather, resilient design has become the focus of urban construction and urban design disciplines. Through literature review, it can be found that storm-flood resilience and sponge city are the core issues in the construction of coastal resilient urban areas. However, at present, there are still few studies that can map the "4R" (robustness, redundancy, resourcefulness, and rapidity) resilience characteristics with spatial elements and specific measures in urban design, or evaluate the performance of these specific resilience strategies under different scenarios. The conduction relationship between urban resilience theory and urban design practice needs to be further rationalized and refined. Based on the perspective of urban design, this study aims to explore the corresponding relationship between urban spatial form and stormwater resilience index, and put forward a resilience evaluation idea suitable for coastal urban areas.

Methods The research uses literature research, empirical research, quantitative analysis and other methods. Based on literature induction, extraction and integration of 4R resilience characteristics, a comprehensive evaluation model with 24 indexes including 4 categories of robustness, redundancy, resourcefulness and rapidity was constructed. According to the four principles of coastal city typicality, international influence, resilience construction experience and multi-scenario difference, 9 typical coastal urban cases at home and abroad are selected in a targeted and differentiated way to carry out empirical research globally. They are the lower Manhattan of New York City, the central area of Singapore, the core area of Hong Kong Island, the central area of Boston, the central area of Sydney, the Faishan Bay business District of Qingdao, the Silicon Valley area of Santa Clara, the central area of Copenhagen, and the waterfront area of Zhuhai Lovers Middle Road. Then, this paper researches how each case improves its 4R resilience level through urban spatial form and functional layout is analyzed.

Results According the results, Boston was the only sample city to score above average in all four categories, showing a high level of overall resilience. For other cities, New York and Sydney excel in redundancy and resourcefulness resilience. The resourcefulness and rapidity of Qingdao and Singapore contributed the most to the overall resilience level. Hong Kong attaches the highest importance to accessibility; Copenhagen, Santa Clara and Zhuhai need to focus on their resilience strategies in terms of robustness and speed. The reasons leading to the results are mainly summarized in the following three aspects. 1) Due to dense population and land shortage, high-intensity urban areas pay more attention to spatial redundancy and resource acquisition during disasters. Its urban core functional areas have achieved full coverage of public services and emergency service facilities to a large extent through encrypted road networks, mixed land use, vertical functional layout and other ways. At the same time, reasonable division of land parcels is also conducive to the construction of green belts and green corridors in urban areas. For example, cities such as Singapore and Boston are significantly higher than other cities of the same type in terms of speediness indicators. 2) The coastal space of low-intensity urban areas is sufficient, which can withstand and dissolve the impact of stormwater through better shoreline protection and blue-green network. Low intensity urban areas have the conditions to establish a generous coastal buffer zone, which can avoid coastal storm surge in the first time to attack the urban core area. The green belt separation between the clusters in the urban area is clear, and the retention and penetration ability of rain and flood are greatly enhanced. On the contrary, due to the large land scale and low degree of development and construction, the spatial redundancy of low-intensity urban areas is small, and the distribution of emergency rescue facilities is relatively sparse. 3) There is no significant correlation between the overall performance of 4R toughness and development intensity in the coastal urban area, and both high and low strength samples can enhance the overall toughness level by strengthening their own advantages or appropriately compensating for weaknesses. In high-intensity urban areas, priority should be given to improving the layout of redundant units and emergency resources, and then the robustness and rapidity level should be improved by broadening the coastline and connecting the greenways. In low-intensity urban areas, the road network should be properly encrypted and public service facilities should be evenly distributed to ensure better coverage of resource services in the context of ecological urban construction.

Conclusion Through the empirical evaluation of typical coastal urban samples, the key points of guidance and control and improvement strategies of coastal resilient urban areas in the 4R evaluation dimension are summarized. Coastal urban areas should consolidate coastal infrastructure combining resistance and buffering, strengthen three-dimensional and complex redundant functional units, build an integrated emergency rescue system from coastal to hinterland, and improve the blue-green ecosystem combining point, line and surface. In the future, China's coastal urban areas should break through the short board of planning homogeneity, fully learn from excellent coastal urban construction cases at home and abroad, classify and implement policies through 4R resilience evaluation results, formulate a resilience strategy system for coastal urban areas in line with local environmental characteristics, and finally implement it into specific urban spatial form, functional layout and emergency management mode.