According to the report by the Intergovernmental Panel on Climate Change (IPCC, 2007), low-lying areas, especially in densely populated areas of East and South-East Asia, have a relatively low adaptive capacity and will face a higher risk of flooding due to increased coastal flooding [1
]. The report also predicts that by 2100, more people will be flooded each year in these areas due to rising sea levels (IPCC, 2014). The population aging in the region and changes in family structure, as well as the lack of adaptation measures, will reduce people’s ability to adapt to such changes and accelerate flood vulnerability [2
]. The IPCC’s latest 1.5 Degrees report (IPCC, 2018) pointed out that it is necessary to reduce the vulnerability of floods and to accelerate adaptive capacity [3
]. Therefore, assessing vulnerability to flooding disasters is an important step in reducing flooding vulnerability and accelerating resilience. In the past, many studies on flooding vulnerability have been carried out, but most studies have not focused on social vulnerability, which is essential to reducing flooding vulnerability. Social vulnerability refers to the characteristics of a person or group in terms of their capacity to anticipate, cope with, resist, and recover from the impact of a natural hazard [4
]. It is a term used to describe the extent of the vulnerability of a particular social group, organization, or country to damages or losses when encountering a disaster [5
]. Social vulnerability is related to gender, class, race, age, and poverty [6
]. In terms of age, the social vulnerability of the young and the elderly populations is high, because these are the age groups that cannot easily seek refuge on their own during disasters. Women are at a higher risk because the distribution of low-class status and income distribution of women indicate that a large number of elderly women live by themselves. Therefore, their capacity for post-disaster recovery is poor [11
]. These social factors influence the distribution of resources and power that are necessary for disaster preparedness and countermeasures. Their requirements will be different, along with the difference in vulnerability [9
A better understanding of the level of vulnerability and how the susceptible population is distributed can be beneficial for better management of disaster risk [13
]. In the framework for disaster risk mitigation in Sendai City, it was proposed that the social vulnerability of a disaster should be recognized accurately along with the possibility of a reduction in disaster damages for the most vulnerable areas. The social vulnerability assessment is an important step in building regional robustness against flood risks [14
], and helps identify and measure local factors of vulnerability [16
]. Research on social vulnerability to disasters has gained attention in recent years [12
Social vulnerability indices have emerged as a leading tool to quantify and map the human dimensions of hazards vulnerability [20
]. From a set of seminal studies [4
], the concept of social vulnerability has been formed. Currently, the field of social vulnerability has not yet formed a special assessment framework [15
]. Social vulnerability assessment frameworks have expanded to address questions of scale [21
], temporal change [26
], specific hazards [27
], integration with ecological environment vulnerability [21
], and economic vulnerability [14
Although the research on the social vulnerability framework is in progress, there are still some problems. In terms of scale, there are relatively more studies on social vulnerability at the national, local government, and urban levels [30
]. However, there are few studies on regional differences of social vulnerabilities in meso-scales and small-scales [35
]. Therefore, it is necessary to study the social vulnerability at both the meso-scale and the small-scale to implement measures to reduce social vulnerability more accurately and appropriately.
In terms of specific hazards, disasters include small-scale, large-scale, frequent, and rare ones and hazards, etc. However, there are few studies on the social vulnerability to floods at a large-scale when considering setting the worst conditions. It is still difficult to adapt to the future trend of large-scale floods [37
]. Therefore, it is necessary to study the social vulnerability to large-scale floods by setting the conditions to reflect the worst case. The social vulnerability index still shows a good consistency to some extent regarding the index construction method. However, such consistency is likely to yield misleading conclusions [20
]. To address these issues, it is necessary to include indicators that can reflect local social vulnerability. Furthermore, the social vulnerability indicators include population density, gender discrimination, socioeconomic status, and public health conditions. These are considered to be the most important causes of vulnerability in individuals exposed to disasters and emergencies [38
]. Owing to changes in natural and socioeconomic environments, the social vulnerability has changed as well. It is, therefore, necessary to include new indicators to reflect local vulnerabilities [34
]. As far as the index weighting method is concerned, the subjective valuation method is too subjective, which would reduce the credibility of the social vulnerability assessment [33
]. Therefore, in this paper, we use the information entropy weight method as the typical objective assignment method to determine the index weight.
This study aims to improve the evaluation of social vulnerability to large flood hazards in small regions. GIS-based techniques and the information entropy weight method are employed to estimate social vulnerability quantitatively. This study refers to a comprehensive vulnerability assessment framework for the evaluation of social vulnerability. Regarding local indicators, this study refines the children’s indicators and the elderly indicators and adds some new indicators, such as isolated populations.
The study area is in Tokyo, Japan. Approximately 41% of the population and approximately 65% of the assets in Japan are concentrated in its flood prone areas. They constitute approximately 9% of the national land area, implying that the exposure to flooding is high [43
]. Recently, the vulnerability to floods has increased with an aging population, the deterioration of flood control facilities, and the diminishing of the residents’ sense of crisis. Additionally, the concentration of population and assets renders the situation worse [4
]. It also leads to changes in the social vulnerability of floods. Who is a vulnerable group now? Where are they living? It is important to know the answers to these questions. Once we can reduce social vulnerability and enhance flood prevention ability, we are able to alleviate the damage caused by floods. At present, the Japanese government’s measures to reduce social vulnerability are based on rarely targeted areas in the city. However, there are differences in the social factors for each district, so there remain many inappropriate measures taken by the city to reduce social vulnerability. Therefore, it is necessary for us to recognize and assess the social vulnerability by districts [14
]. Mapping the distribution of a district’s social vulnerabilities will be beneficial for local governments to guide the reduction of social vulnerabilities and the establishment of appropriate mitigation plans.
The weight of each index obtained by the entropy method is shown in Table 2
. According to the results of Table 2
, three indicators, namely submersion depth, population, and submersion time, are of great importance, subsequently followed by the factors of female population, the density of evacuation facilities, and the density of medical facilities. Based on the evacuation indicators, both exposure and susceptibility turn out to be significant in terms of the social vulnerability assessment.
shows the distribution of exposure to flooding in the study area. This exposure level was calculated using a combination of three indicators of submersion levels, including: Depth, submersion time, and population. Figure 3
a shows the submersion depth distribution. Notably, the depth areas between 3.7 m and 4.4 m were the dominant ones, accounting for 22.8% of the total area, which was primarily distributed in the northwest. Additionally, the depth areas between 1.2 m and 2.3 m, which accounts for 22.7% of the total area, were located in the east region of the study area. Figure 3
b presents the submersion duration distribution map. The duration hours between 133 and 546 h, accounting for 36.7% of the total area, and located in the southwest region of the study area, can be examined. Figure 3
c shows the population distribution map. A population of 40,312 was submerged, accounting for 89% of the total population. Finally, the exposure map is shown in Figure 3
d. We found very high exposure levels in the southwest that changed gradually to very low exposure in the northeast of the study area.
shows the distribution of susceptibility in the study area. Five indices are used to measure this distribution: Population of 0–9 years old, the population of 65–74 years old, the population of over 75 years old, the female population, and the isolated population. The distribution diagrams for each indicator are shown in Figure 4
a–e, respectively. As shown in Figure 4
f, the susceptibility is classified as very low, low, medium, high, and very high, and their proportions in the total area are 13%, 25%, 24%, 29%, and 9%, respectively. It is worth noting that the more susceptible areas are located in the western region of Katsushika Ward, because the population of 65 to 74 years old, the female population, and the isolated population are distributed primarily in the western region. The results also indicate that the northeast of Katsushika Ward is situated in the lowest susceptibility area.
The resilience in the study area is shown in Figure 5
, which is a combination of three indicators: The number of family personnel, the density of refuge facilities, and the density of medical facilities. The distribution maps for each indicator are shown in Figure 5
a–c. As shown in Figure 5
d, the resilience measurement is classified as five parts: Very low, low, medium, high, and very high. The number of family personnel, evacuation facilities, and medical facilities is separately concentrated in the southwestern area, the northwest and northeastern areas, and in the vicinity of the southwestern, northeast, and northwest parts of the region, respectively. The resilience distribution maps clearly show that the “very high”, “high and medium”, “low”, and ”very low“ are mainly distributed in the southeastern, northwestern, and eastern regions and near the river, respectively. The results also indicate that the northeast of Katsushika ward is situated in the lowest resilience area. Resilience in some southeastern regions is extremely low. In addition, the density of medical and evacuation facilities in the peripheral Katsushika ward is relatively low, because the density of facilities was calculated merely within the boundary of this area.
shows the distribution of social vulnerability in the study area. It is the result of a combination of three evaluation items, including the: Degree of exposure, susceptibility, and resilience. The distribution of social vulnerability is divided into five levels, including: Very low, low, medium, high, and extremely high. “High” and “very high” levels are distributed widely in the southwestern part. The moderate levels are distributed widely in the northwestern part. “Low” levels are distributed widely in the eastern part. Although the “very low” level grids are distributed widely, the area of “high” and “very high” levels occupies 36% of the area.
We found that the indicators from the depth and duration of immersion are of great significance under the exposure analysis. The western region is more at risk than the eastern one. From the perspective of immersion depth, the northwestern region is more at risk than the southwestern one. However, from the viewpoint of flood duration, the southwestern region is more at risk than the northwest. In terms of population distribution, the population in the northwestern region is more distributed, but the population in the southwestern region is more concentrated. Therefore, the focus of flood prevention measures need to be different in the southwestern and northwestern regions. Both regions should pay more attention to warnings and safe transfers at the earliest risk stage. Also, the northwestern region should focus on emergency shelter, while the southwestern region should place more emphasis on long-term evacuation and rescue. In the flood of 1947, 54,128 houses were flooded, and the Katsushika area was immersed for 20 days. The victim count reached 218,251. Ships were used for rescue and food distribution. Its occurrence, which occurred in the second year after the end of World War II, made social and economic life more unstable. From past experience, it is evident that expanding personnel and material resources are of paramount importance.
We found that the distribution of the five indicators is more prominent in the western than in the eastern region. Therefore, the western region should pay more attention to the evacuation and rescue of vulnerable people. When it comes to the susceptibility analysis, we also realized and emphasized the importance of two indicators from isolated people and people in the age range of 65–74, whose early warning and transfer are indispensable for their safety. On the other hand, the indicator from the female group would be plausibly important due to its large population. Next, the order of rescue must be seriously considered for the population over 75 years old, 65–74 years old, and 0–9 years old. The isolated populations of these groups are at greatest risk, and immediate action should be taken. In the large-scale evacuation study of floods in the National Survey of Year 2015, the risk of flooding in in the Katsushika Ward was high, and the submersion time was more than three days [59
]. Owing to the large water immersion area, the areas with depths of 1.2 m or more accounts for more than 80% of the total immersed area. When this occurs, it is especially difficult for children and the elderly to seek refuge on their own. Areas that have been immersed for more than three days covered over half of the total area, resulting in many isolated populations.
According to the resilience analysis, although there is a greater number of family members in the southwestern region, the problems are primarily focused on the difficulty of evacuation due to limited evacuation facilities. Despite the high density of medical facilities, the depth of immersion in the area is between 2.4–10.5 m. Thus, the medical facilities may also be prone to flooding, potentially resulting in the temporary suspension of medical services. Because the flooding period is 133–546 h, long-term flooding will cause problems, such as the spread of infectious diseases owing to insufficient medical care. During the floods of 1947, the southwestern region encountered difficulties in evacuation and the medical facilities were paralyzed by flooding, causing a spread of infections, owing to inadequate medical emergency services. This flood caused a significant loss of life and property. Recovery after a disaster generally takes a long time. Therefore, the southwestern region should focus on the establishment of emergency evacuation facilities, the corresponding capabilities of medical facilities, and the prevention of post-disaster infections.
In terms of the results in this study, the distribution of social vulnerability in the Katsushika area is different from the distribution of flood risk shown in previous research [61
]. In the study, the author concluded that the northwestern region of north Nakagawa in the Katsushika area is at relatively high risk, while the southwestern region is less dangerous than the eastern region. These results are different from the results of this study. This difference is because reference [61
] had a different setting of the flood model and did not consider the duration of the flooding. Even if there are reports on vulnerability analysis in the Tokyo area, they did not focus on floods, and their vulnerability analyses were on a relatively large scale [62
]. In terms of regions, according to the flood disaster map of Katsushika region, the local government is developing measures to reduce social vulnerability by using this information [63
]. The risk level in the flood disaster map of the Arakawa in the Katsushika area is ordered as follows: Eastern region < southwest region < northwest region. The government can develop measures to reduce social vulnerability by using this information. However, the situations between the north parts of the Keisei line and south parts of Nakagawa region are different from the results in this study. This is because the flood disaster map of the Arakawa in the Katsushika area is only expressed by the depth of the flood. Therefore, it is not sufficient to formulate measures to reduce social vulnerability based on this figure. This study used local indicators to achieve the spatial vulnerability representation at the district level. The classification of indicators also helped to differentiate spatially vulnerable sub-districts to floods. The number of vulnerable people was divided into the population of the 0–9, 65–75, and 75+ and more age ranges.
Compared with the previous research, the differentiation among the vulnerable populations will become more detailed and specific, which is useful for targeting corresponding priorities when disasters take place [37
]. Based on our analysis of social vulnerability, we found that social vulnerability is related primarily to the exposure rate and susceptibility. This is similar to the results of previous studies [17
]. Although the social vulnerability in “low” and “extremely low” levels is broadly distributed in this region, they are mainly localized in relatively safe eastern regions. However, social vulnerability at “high” and “extremely high” levels is concentrated in the relatively dangerous southeastern regions. Therefore, the wide distribution does not indicate that the social vulnerability to floods is low in this area. In other words, the social vulnerability of the region is significant and is primarily divided into three regions: The eastern, northwest, and southwest regions. Policies and measures to reduce the risk of flooding should be established for each of the three regions. More detailed policies and measures should be established to reduce the risk of flooding in places with high social vulnerability in the southwest region.
We should enhance the awareness of disaster prevention in the residents of the western part of the region and strengthen propaganda methods for disaster prevention awareness. The region has not experienced large-scale floods for 70 years, and its awareness of disaster prevention is low. Also, strengthening the ward’s ability to construct refugee facilities so early warnings can be established is paramount. Given the results from this study, it was found that two characteristics of “high degree of exposure” and “high susceptibility,” were considerably responsible for the vulnerability in the southwest regions of the Katsushika ward. Our results also include long durations of water immersion, as well as relatively large distributions of females, isolated populations, and people aged between 65–74. When it comes to the northwestern region, its vulnerability has been mainly influenced by the degree of exposure, susceptibility, and corresponding ability in Katsushika ward. This particular region features deep immersion, large populations involved in immersion and isolation, and lacks refuge facilities. The government is now carrying out parks construction in the southwestern region to increase its foundation to 6 m, which could be useful for rescue and refuge. However, such construction has not yet been extended to the northwestern region, even though it is also necessary [64
]. The vulnerability in the eastern region, in comparison, is relatively low, yet we found that the number of family members and the density of medical facilities are also relatively low. The construction of emergency shelter facilities should be strengthened to cope with unexpected large numbers of asylum seekers. Owing to the distinctive features of these three regions, the authors believe that such information can provide the guidelines to enact the flood-tackling strategies that are more suitable for the corresponding region. In other words, this study indicates that the evaluation criteria for measuring regional vulnerability may be discussed through the accurate analysis of the quality, quantity, and distribution of social vulnerability.
According to the indicators analysis, the depth and duration of flooding have a great impact on social vulnerability. To address these issues, protection facilities and early warnings for flooding in western regions should be enhanced to cope with prolonged flooding. Secondly, in order to weaken the vulnerability to flooding among the female population, it is necessary to strengthen their awareness of flooding protection in the western region and to enable even distribution of daily supplies after disasters. In terms of refuge and medical facilities, it is essential to enhance the construction of refuge facilities in the southwestern region and to optimize the layout of medical facilities in the northwestern and southeastern regions. For family members, it is of great importance to improve the acceptability for flooding protection information in the northwestern region and to expand social activities within the region. Regarding children and senior citizens, it is necessary to sustain the propaganda distributing information on asylum knowledge and evacuation capabilities, particularly in the areas with flooding depths of 1 m or more. Simultaneously, flood-prevention training should be regularly conducted. For example, primary and secondary schools and welfare institutions can help carry out public outreach for refuge knowledge and flood prevention information. In terms of isolated populations, it is necessary to increase accessible refuge facilities in the northwest region, as well as to improve the capacity for materials’ storage and rescue. Moreover, it is also meaningful to expand emergency shelters by connecting to private facilities, to increase the storage of lifeboats, and to organize more self-rescue trainings.