1. Introduction
Japan is particularly vulnerable to flooding due to its steep geography and humid climate characterized by torrential rains and typhoons [
1]. Approximately 50% of the total population in Japan and approximately 75% of its assets are located in flood-vulnerable areas [
2]. Moreover, the vulnerable areas are primarily in the alluvial plains [
3]. The number of floods, and enhanced damage due to flooding, have increased since 2004 [
1]. Several local heavy rainfalls in Japan have been documented by Japan Meteorology Agency (JMA) [
4] (e.g., in recent years: 3 July 2006 in Kumamoto; 11–17 July 2007 in Kagoshima, Miyazaki, Kumamoto; 30 August 2008 in Aichi Prefecture and around Chubu region; 10 September 2015 in Ibaraki Prefecture and around Kanto region). In Aichi Prefecture, the number of days of heavy rainfall, i.e., days with hourly precipitation of ≥50 mm/h and ≥100 mm/h, has increased by 60% and 110.11% since 1979 [
4]. Referring to the most severe scenario of the Representative Concentration Pathways, RCP8.5, the extreme rainfall will increase by 25%–35% in 2071–2100 [
5]. That condition triggers the rapid accumulation of runoff waters which causes flooding. In addition, the potential for flood casualties and damages is also increasing in many regions due to social and economic development, such as urbanization which puts pressure on land use change [
6,
7]. The population growth, especially in urban areas, has been increasing in the flood-prone areas [
8]. Furthermore, the United Nations defined flooding as the most frequent and greatest hazard [
9], and high population areas are relatively high-risk for natural disasters [
7,
9,
10]. Thus, we chose the urban area as a case study in Aichi Prefecture.
Flood-risk management is commonly divided into flood-risk assessment and flood-risk mitigation [
11]. One of the strategies of flood-risk management against flood impact at the regional scale is the identification of vulnerable areas to provide early warning, facilitate quick response and decrease the impact of possible flood events. Recently, the integration of remote sensing technology (i.e., a method to extract the up-to-date information from satellites [
12,
13]), Geography Information System (GIS) (i.e., data integration [
13]) and field surveys were conducted to assess the impact of flood damage by vulnerability mapping.
One of the approaches in determining the areas vulnerable to flood disaster is spatial multi-criteria analysis through Analytical Hierarchy Process (AHP), introduced by Saaty; this method selects the required criteria by ranking the parameters and combines qualitative and quantitative factors [
14,
15,
16]. The AHP method applies to the decision-making process. Some studies employed AHP for hazard assessment such as floods [
6,
17,
18,
19,
20], tsunamis [
21,
22,
23], landslides [
20,
24,
25,
26,
27,
28,
29], droughts [
30] and seismic hazards [
19]. In flood hazard assessment, Ouma and Tateishi (2014) integrated AHP and GIS to predict flood vulnerability by using six parameters: rainfall, drainage, elevation, slope, soil and land use [
6]. According to their AHP calculation, the soil was the weightiest parameter. In addition, the consistency ratio (CR) of their study was 9%. CR is used to conclude whether the evaluation is sufficiently consistent (i.e., the rational value of CR is ≤10% or ≤0.1) [
16]. Siddayao et al. (2014) investigated population density, distance from the river bank and site elevation as AHP parameters for flood vulnerability [
17]. Distance from the river bank was the highest contributing factor to floods in AHP calculation, and CR was 3.34%. Kazakis et al. (2015) calculated the flood hazard using the AHP approach and utilizing seven parameters including rainfall intensity, slope, flow accumulation, elevation, distance from drainage network, land use and geology [
18]. The flow accumulation was the highest influencing parameter to floods, and CR was 8% [
18]. Bathrellos et al. (2016) investigated urban hazard assessment with the AHP procedure and utilized six parameters (i.e., slope, elevation, distance from channel stream, distance from totally covered streams, hydro-lithological formation and land cover); the highest parameter was land cover and CR was 4% [
19]. The advantage of AHP is that it is designed to solve complex problems involving multiple criteria. In addition, it is also designed to handle situations in which the subjective judgments of individuals constitute an important part of the decision process. However, the limitation of AHP is insufficient ability to define the uncertainty because the model predicts the estimates only without estimation error of each output level [
20,
31].
This study deals with the first element of flood-risk management, i.e., the definition of flood hazard areas in a specific region. The objective is to identify flood hazard zones, where mitigation assessment should be undertaken for urban development. Flood hazard assessment can support decision-makers and governments for urban developing. Bathrellos et al. studied hazard assessment (i.e., flood, landslide, and seismic hazard assessment) as a component to measure suitability for urban development [
20] and proposed natural hazard assessment as a method for determining the suitability of urban growth and light industry development [
32]. Additionally, some researchers studied flood-risk assessment as a prominent component of urban planning [
33,
34,
35]. Thus, a spatial, multi-criteria index has been proposed to characterize such areas by integrating the technology of remote sensing and GIS to measure the physical parameters of flood vulnerability. The physical parameters of flood vulnerability were rainfall, drainage density, slope, soil and land cover. The rainfall, drainage density, slope, and land cover data were generated from satellite images; the soil was digitalized using the GIS method. All the parameters were combined by the GIS method and pairwise using AHP procedure. All data were downloaded from public open access data. The study area is located in Okazaki City, Aichi Prefecture Japan. Even though the index is tailored to the specific geography and land cover characteristics of the study area, it can be modified and applied in other areas.
2. Study Area and Flood History
The study area is located in the middle part of Aichi Prefecture Japan and has an area of 387.20 km
2 [
36] from Latitude 34°53′30″ N to 35°2′30″ N and Longitude 137°7′0″ E to 137°25′0″ E. Okazaki City is the third biggest city in Aichi Prefecture [
36] with a population of 382,846 people [
37]. It has a humid subtropical climate and an elevation ranging from 0 m to 789 m above sea level [
36]. The central city consists of alluvial plains. The eastern part of the city is a mountainous area which has an altitude of 789 m (i.e., Mt. Hongu) and the rest is lowland area which has an altitude ranging between 0 and 300 m [
37]. We selected the lowland area as the study area as shown in
Figure 1. The study area is passed by big rivers, namely the Yahagi River which flows from North to South. Thus, the land use within this area is dominated by agriculture, particularly paddy fields. Precipitation occurs throughout the year with the heaviest in the summer season (i.e., June to September) and during typhoon phenomena. The temperature range is from −1 °C to 36 °C with an average annual temperature of 17.0 °C, and the annual rainfall is about 1200 mm/year.
From 1996 to 2008, severe heavy rains and flooding due to typhoons occurred 18 times [
38]. Large flood impact occurred on 11–13 September 2000 and 30 August 2008. However, the flood in 2008 was the most severe; the rainfall intensity was about 146 mm/h at 2:00 a.m. [
4]. Around 1110 houses were flooded above floor level, and 2255 houses were flooded below floor level [
38,
39]. In addition, some areas of the neighboring city (i.e., Anjo City) were also flooded. The flood survey was conducted during the flood by the local government of Okazaki City and Ministry of Land, Infrastructure, Transportation and Tourism (MLIT) [
40]. This survey was used to verify the results of this study.
5. Conclusions
It is difficult to reduce the occurrence of natural disasters, including floods. What we can do is minimize their impact. Assessing areas vulnerable to flooding disasters is one of the parameters in creating a flood-risk map for disaster mitigation and urban planning. This study tried to assess the area that was vulnerable to flooding using integrated approaches of remote sensing, GIS, and spatial multi-criteria evaluation through the Analytical Hierarchy Process (AHP) approach. The parameters of slope, drainage density, rainfall intensity, infiltration rate, and land cover were applied to predict affected area of flooding. The AHP calculation shows that slope was the highest weight (43%) in determining vulnerability to flooding through the spatial-weighted overlay, followed by drainage density (20%), rainfall intensity (17%), infiltration rate (10%) and land cover (10%). This calculation resulted in a 0.6% consistency ratio. The slope parameter was the most important parameter because the slope influences the flow direction, runoff and soil infiltration.
There are several measurement methods for urban planning in Japan, which are applied to each area by the local government depending on local circumstances under the City Planning Law [
86]. According to the structure of City Planning System from the Ministry of Land, Infrastructure, Transportation and Tourism (MLIT), there are three components, i.e., Land Use Regulation, Urban Facilities and Urban Development Projects; one part of the Urban Development Projects is promotion for reconstruction of the disaster-stricken urban area. We proposed the result of this study as a consideration for urban planning in Okazaki City. The result of the flood-vulnerable areas shows excellent accuracy; Probability of Detection (POD—0.88), Probability of False Detection (POFD—0.28), Critical Success Index (CSI—0.44), Bias (1.9) and Area under Curve (AUC—0.95) from the Relative Operating Characteristic (ROC) graph. This method can be utilized to complement the hazard map of MLIT.
The total area of Okazaki City is approximately 95,679.2 Ha, and 10,612.07 Ha or 11.1% of the total area is highly vulnerable to flooding. From the total flood-vulnerable area in the lowland area, the land cover with the highest risk of flood was a residential area (29.6%) and agricultural area (49.5%). Those areas were close to the big river and had a slow infiltration rate, in addition to having a level to near level slope. These conditions influence the economic situation. By analysis of the building distribution and the flood-risk area, it is clear that approximately 1366.66 Ha total of construction area (i.e., offices, schools, hospitals, houses, etc.) is in the flood-prone area.
This study contributes an important approach for the effectiveness of disaster mitigation and urban planning.This method can be applied in other geographical areas, even if they have different topographical characteristics than this study area. This study aimed to evaluate the physical parameter of flood vulnerability. The use of other parameters, including river proximity, in addition to a comparison study using other sources of elevation data, will be analyzed for future work, and we will also consider the social, economic, ecological, cultural and institutional variables.