Urban development and climate change continue to aggravate urban pluvial floods, which are becoming more serious (Bulti et al., 2020), causing socioeconomic problems such as economic losses, traffic congestion, environmental pollution, and resident` travel and health risks (Rahmati et al., 2020). Urban land cover change dominated by the increase in impervious surfaces has reduced surface infiltration and increased surface runoff (Arnold and Gibbons, 1996). In the process of urbanization, urban infrastructure construction is not in harmony with urban development in most majority countries. For example, drainage networks in artificial standards cannot deal with heavy rainfall, which has led to the frequent occurrence of urban floods. In addition, climate change, characterized by extreme rainfall and sea level rise, is exacerbating this trend. Furthermore, population and assets gradually accumulate in cities during urbanization (Slater and Villarini, 2016), resulting in a further increase in the scope and intensity of urban flood disasters. Therefore, urban flooding has become an urgent problem of urban flood management and has attracted increasing attention from government departments and stakeholders.
Urban flood risk management, including urban flood risk assessment and risk mitigation, rather than traditional gray engineering measures, is considered the main solution for resolving urban flood problems (Johnson et al., 2007). To date, urban flood risk assessment is an important and effective means to strengthen technological support and improve emergency response capabilities. The purpose of urban flood risk assessment is to analyze current or future flood risk information, identify high-risk areas (Muis et al., 2015), and provide decision support for flood mitigation measures, as well as assess the effectiveness of their mitigation measures (Zhou et al., 2012). Numerous related scholars (Haynes et al., 2008; Koks et al., 2015; Foudi et al., 2015; Fakhruddin et al., 2020) and research institutes have used the frame of “hazard-exposure-vulnerability” for flood risk assessment.
Flood risk = Hazard (H) × Exposure (E) × Vulnerability (V)
Hazard can be described as the flood location, depth, and flow rate. Exposure is mainly used to analyze the affected land-use types, people, buildings, and infrastructure in certain flood events. Vulnerability can be defined as the susceptibility of the elements (people, buildings, infrastructure and so on) at risk of suffering from flood damage. This framework not only focuses on the hazard and exposure of a disaster but also considers the vulnerability of various disaster-bearing bodies.
Previous studies on urban flood risk have mainly focused on fluvial or coastal floods at relatively large scales, such as regional scales or basin scales (Vojinovic et al., 2015; Ganguli and Merz, 2019). With urban development, although the damage or cost of urban pluvial floods is greater than that of coastal or river floods, the frequency of pluvial floods increases (Muthusamy et al., 2019) with urban growth, thus having a greater adverse impact on people, transportation and economic activities. Hence, urban pluvial floods deserve more attention.
Urban pluvial floods usually occur in local areas of the city due to short-term heavy rainfall, and their impact is very extensive. Therefore, urban pluvial flood risk urgently needs a comprehensive assessment (Kulkarni et al., 2014; Vercruysse et al., 2019) on a fine scale (Casiano Flores and Crompvoets, 2020). However, most of the research content has focused on the hazard or exposure to risk, and less attention has been given to vulnerability. There are few comprehensive flood risk assessment studies. Hazards mainly rely on hydrological, hydraulic models or simplified models (Bulti et al., 2020; Feng et al., 2020) to obtain inundation information. Exposure based on the hazard requires spatial analysis to obtain information on the different land-use types, infrastructure, population, etc., exposed to inundation in the region. An inadequate understanding of the comprehensive risk of urban flood has caused researchers to neglect vulnerability. Since urban flooding cannot be managed in isolation, an integrated approach is required (Zevenbergen et al., 2008).
Urban pluvial flood risk assessment pays more attention to larger scales, such as the catchment (Rouillard et al., 2015; Zhou et al., 2019), urban (Mebarki et al., 2012; Di Salvo et al., 2018), and regional scales (Prokić et al., 2019; Speight et al., 2017); in contrast, less attention has been given to small scales in the inner city, such as the subcatchment (Pathak et al., 2020a), community (Azizi and Meier, 2021) or street (Yin et al., 2016) scales. Therefore, the research content and scale do not meet the needs of urban pluvial flood management and prevention. The main reasons for these patterns are as follows: (1) Refined scale assessment requires more accurate data; (2) the complex surface characteristics of different natural watersheds within the city cause the current flooding mechanism to be further explored. A finer scale usually requires more sufficient and accurate data, such as data on the high-precision terrain, buildings, commercial facilities, infrastructure, the population distribution and drains network data. Traditional demographic and economic statistics used in vulnerability assessments are based on administrative divisions that are not sufficient for urban inner pluvial flood assessments, especially in relation to vulnerability. To date, drainage network data and computing power are still the main limiting factors in two-dimensional hazard hydraulic modeling in large urban areas.
The progress of big data technology provides a wealth of data sources that offer very large application potential and new opportunities for urban flood risk assessment at finer scale. At present, different types of big data are starting to be used in studies related to urban flooding. Social media or POI (point of interest) data were used for urban pluvial flood early warning (Young et al., 2021), flood risk assessment (Coletti et al., 2020), and flood monitoring and mapping (Rosser et al., 2017; Helmrich et al., 2021). POI data contains information on business, education, transportation, and medical care at an urban internal fine scale, which can break through the administrative division restrictions of traditional statistical data. Therefore, POI data can be considered a new type of data for conducting a refined vulnerability assessment.
In this paper, we presented an integrated urban pluvial flood risk assessment at a grid scale. First, limited by the municipal drainage network data, we could not establish a two-dimensional urban hydraulic model at the district level of the city; thus, a simplified hydrological model was used as a substitute to obtain hazard information. Second, spatial analysis was performed to map the exposure degree of land-use types on the basis of hazard information. Then, POI data together with traditional socioeconomic data were used to assess the vulnerability map on a finer scale with the help of statistical and spatial analysis methods. Finally, an integrated waterlogging risk (H-E-V) assessment was achieved. This study could provide a scientific reference and decision basis for urban flood mitigation and urban planning.