water Article The Influence of Urban Flooding on Residents’ Daily Travel: A Case Study of Macau with Proposed Ameliorative Strategies Kehong Li and Long Zhou * Faculty of Innovation and Design, City University of Macau, Macau 999078, China * Correspondence: [email protected]; Tel.: +853-85902862 Received: 18 July 2019; Accepted: 29 August 2019; Published: 31 August 2019 Abstract: Climate change has resulted in more extreme weather events in coastal cities, and understanding how daily life is impacted is crucial to make effective adaptation measures. Using Macau as a testbed, this research describes examines the impacts of flooding caused by storm surges on residents’ daily travel and proposes measures to ameliorate disaster risks. Spatial extents of urban floods were modelled through inundation simulations using geographical information data. An analysis of the travel distance increases from residents’ homes to common types of destinations was performed both under normal conditions and during flood events in GIS (Geographic Information System) to assess the influence of urban flooding on residents’ daily travel. The results show that one third of the land is threatened by floods in Macau. People’s average travel distance increases as the warning levels escalate, and travel distance is predicted to rise by up to 64.5%. Based on the findings, the study proposes mitigation strategies to minimize urban flooding’s impacts. It suggests that the area more densely populated is not necessarily the one requiring the deployment of preventative measures with the highest priority, as a traffic analysis is identified as the key area which demands disaster prevention measures. Keywords: urban flooding; daily travel distance; network analysis; disaster prevention and mitigation; Macau 1. Introduction Changes in global climate and atmospheric circulation frequently cause extreme weather events, such as typhoons, rainstorms, and drought [1], with the resulting harmful effects often exacerbated in cities with high urbanization rates [2]. Coastal cities are particularly subject to storm surges and floods. Research data from the Centre for Research on the Epidemiology of Disasters indicate that natural disasters related to climate change in 2016 caused global economic losses of about USD 66.5 billion, and China ranked second with total losses of about USD 13.6 billion [3]. At present, about 40 million people (0.6% of the global population or one-tenth of the total population of port cities) are currently exposed to a 1 in 100 year coastal flood event [4]. During the past decade, floods were the natural disaster that most affected people, and their intensity and frequency have been increasing [3]. Currently, flood disasters mostly occur in estuary and delta regions, and the Pearl River Delta (PRD) region has the highest risk of urban floods in China [5,6]. In the south of the PRD region, the Macau Special Administrative Region (SAR) has capitalized on its convenient geographical advantages and resources to achieve continuous population and economic growth [7]. Macau is one of the centers in the PRD area with a 100% urbanization rate and a population density (21,055/km2) ranked 1st in the world [8]. However, Macau’s current disaster prevention and mitigation systems are insufficient, and its early warning and prevention Water 2019, 11, 1825; doi:10.3390/w11091825 www.mdpi.com/journal/water Water 2019, 11, 1825 2 of 21 mechanisms need improvements [9,10]. On average, Macau suffers five-to-six urban flood disasters caused by typhoon and storm surge every year [11]. In 2017, urban floods triggered by Typhoon Hato, the most devastating to hit the PRD in the past 50 years, resulted in the death of ten people, a 1.5 billion dollar loss, and traffic paralysis for three days [8]. Public transport was suspended, and some center streets were completely impassible [12]. Consequently, the Macau government has aimed to improve the city’s capacity to prevent and defend against disasters to protect residents’ life and activities [11]. To this end, Macau urgently needs to develop a sustainable and scientific system to manage urban flooding, especially during the event of typhoon and storm surges. To support these efforts, this study aims to employ a quantitative data analysis on the impacts of urban flooding on Macau residents’ urban travel and to identify the areas that might cause the most severe traffic network problems in disasters. These areas are then prioritized for the installation of preventative and recovery measures to overcome the effects of disasters in order to minimize negative impacts. Risk prevention and adaptation measures are proposed and demonstrated at the end of this research. Factors triggering floods in cities include natural factors, such as rainfall, storm surge and seawater intrusion, and human factors, such as deforestation, drainage system blockage and improper land use. Additionally, population increases accelerate the urbanization process that converts natural lands to built-up hard surfaces with more impervious pavement, lesser infiltration rates, and higher flood peaks and runoff volumes. Floods can have a severe impact on residents’ travel. Severe floods make streets impassable, cause lane closures and reduce transportation system capacity, all of which decrease traffic connectivity and paralyze transport [13]. People have to detour to their destination, which increases their travel distance and results in more traffic congestion [14]. Measures to prevent and mitigate floods include structural measures such as the construction of dams, river dikes, and drainage networks, as well as non-structural measures such as flood forecasting and warning systems [15]. Recent studies on preventing, controlling and evaluating flooding disaster risks have mostly focused on digitalization, technicalization, and informatization [16–19]. Mark (2004) showed how urban flooding could be simulated by one-dimensional hydrodynamic modelling incorporating the interaction between the buried pipe system, the streets (with open channel flow) and the areas flooded with stagnant water. In order to visualize flood extent and impact, the modelling results were presented in the form of flood inundation maps produced in GIS (Geographic Information System) [20]. Mathews (2015) performed a GIS-based spatial analysis of U.S. population census data and siren locations to assess the disaster warning system’s protection coverage in Stillwater, Oklahoma [21]. The findings were used to improve the community’s disaster prevention and early warning capability to reduce disasters’ effects on residents’ lives and properties. Hasnat (2017) used a GIS network analysis to locate the vulnerable areas of Dhaka, the capital of Bangladesh, and vulnerability was assessed by evaluating the by disaster prevention facilities’ emergency response time and the distances to evacuation points [22]. In previous studies, most scholars used urban flooding models to find engineering solutions based on survey sample data without a comprehensive analysis of the overall population. These studies were not effectively supported by social data and could not realistically reflect urban residents’ needs in disaster situations [16]. Understanding how daily life is impacted by natural disasters is crucial to make effective adaptation measures [23]. From a planning point of view, the transport system is the backbone of cities [24], and floods’ impacts on transportation result in more travel distance that disrupts social production, logistics, and business; puts the urban environment under stress; and increases emissions [25]. Low impact development (LID) means systems and practices to manage storm water as part of green infrastructures through infiltrating, filtering, storing, and detaining runoff close to its source [26]. LID has been deemed an innovative approach to prevent flooding in previous studies [27,28]. However, the effectiveness of LID in practice varies depending on a variety of urban design characteristics and patterns [29]. Therefore, this study took a locational and directional approach using hydrological modelling, a GIS-based network analysis, and the social profile of Macau residents to analyze: (i) What are the impacts of urban flooding on Macau’s traffic network; (ii) how is people’s travel distance affected by the flood event; and (iii) where and how the prevention and adaptation Water 2019, 11, x FOR PEER REVIEW 3 of 21 Waterflood2019 levels, 11, 1825and identifies the areas which should be prioritized for the deployment of prevention3 of 21 and adaptation measures. Section 3 explains and discusses the results, and it proposes a prevention measuresand adaptation should bedesign taken, plan. in terms Section of priority 4 summarize and effectiveness.s the research Section and2 describes points out the methodsthe research and modelslimitations. applied to explore the spatial extent affected by different flood levels and identifies the areas which should be prioritized for the deployment of prevention and adaptation measures. Section3 explains2. Materials and and discusses Methods the results, and it proposes a prevention and adaptation design plan. Section4 summarizesThe spatial the researcharea and and percentage points out affected the research by urban limitations. floods on different warning levels were calculated with the coastal flood hazard model [30], Macau geographic information data, and the 2. Materials and Methods digital elevation model. The changes to the residents’ travel distance due to traffic paralysis caused by urbanThe spatialfloods were