*Article* **Topographical Characteristics of Frequent Urban Pluvial Flooding Areas in Osaka and Nagoya Cities, Japan**

**Daisuke Komori 1,\*, Kota Nakaguchi 2, Ryosuke Inomata 1, Yuika Oyatsu 1, Ryohei Tachikawa <sup>1</sup> and So Kazama <sup>1</sup>**

	- **\*** Correspondence: daisuke.komori.e8@tohoku.ac.jp; Tel.: +81-22-795-5007

**Abstract:** Flooding area records have been available since 1993 in Japan; however, there have been no studies that have utilised these records to elucidate urban pluvial flooding formation mechanisms. Therefore, frequent urban pluvial flooding areas using 20 years of urban pluvial flooding area records during 1993–2012 were identified and analysed using the principal component analysis of their topographical characteristics in Osaka and Nagoya Cities, Japan. The results showed that the topographical characteristics of the frequent urban pluvial flooding areas in both cities were different, with particularly conflicting trends in principal component 1. Furthermore, the urban pluvial flooding in Osaka City could not be described solely by topographical characteristics, and the influence of anthropogenic factors such as dominant structures that may influence inundated water flows in and around frequent urban pluvial flooding areas and stormwater drainage improvements on the occurrence of urban pluvial flooding were shown to be influential. In addition, most of the frequent urban pluvial flooding areas in Nagoya City were located on almost no gradient with a slope of less than 1 degree, and thus, the mere presence of dominant structures around it would dam up the inundated water and cause urban pluvial flooding. The results of this study quantitatively showed the paradigm shift of urban pluvial flooding factors from topographical characteristics to anthropogenic characteristics by the statistical analysis of newly defined urban pluvial flooding frequency areas.

**Keywords:** frequent urban pluvial flooding; topographical characteristics; anthropogenic characteristics; urbanisation; GIS; principal component analysis

#### **1. Introduction**

Urban flooding has had a strong negative impact on many cities around the world for most of human history and certainly in recent decades [1–3]. More than half of the global population lives in urbanised areas, and the frequency as well as the intensity of hydro-meteorological extremes are on the rise [4,5]. Urban flooding is therefore likely to cause greater losses in the coming decades. For example, urban flooding and associated property damage accounted for 73% of the total damage caused by flooding in the USA from 1960 to 2016, amounting to USD 107.8 billion [6]. Of the total damage caused by flooding in Japan's three largest cities, namely Tokyo's 23 wards, Osaka City, and Nagoya City, from 2006 to 2013, 82% of the total damage was caused by urban pluvial flooding [7]. Urban pluvial flooding is one of the typical urban floodings and flood damage that occurs when rainwater is not discharged to mainstream or tributary rivers because rainfall exceeds the design capacity of a drainage facility. In recent years, severe urban pluvial flooding has been caused by the reduction of rainwater soil penetration amounts because of changes in land use and increased strong downpour occurrence frequency. Furthermore, according to the IPCC assessment report [8], the risk of urban pluvial flooding in urban areas will increase in the future as a result of increased strong downpours due to climate

**Citation:** Komori, D.; Nakaguchi, K.; Inomata, R.; Oyatsu, Y.; Tachikawa, R.; Kazama, S. Topographical Characteristics of Frequent Urban Pluvial Flooding Areas in Osaka and Nagoya Cities, Japan. *Water* **2022**, *14*, 2795. https://doi.org/10.3390/ w14182795

Academic Editor: Akira Kawamura

Received: 8 August 2022 Accepted: 6 September 2022 Published: 8 September 2022

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change. Therefore, it is important to clarify the characteristics of past urban pluvial flooding areas in large cities and urbanising cities, where damage from urban pluvial flooding will become more apparent in the future, and take efficient countermeasures against urban pluvial flooding.

Managing urban flood risk is a high priority worldwide, as suggested by a large number of cities, from all continents, taken as case studies in recent research papers dedicated to urban flood modelling [9]. In Japan, it has become mandatory for local authorities to publish urban pluvial flooding hazard maps as a measure against urban pluvial flooding. On the other hand, the spatial and temporal characteristics of flooding in urban areas are complex due to extensive changes to land use [10] that introduce micro-urban features such as buildings, roads, and drainage networks [11,12], and most inundation simulations cannot accurately simulate flooding in urban areas [13]. Thus, flooding simulations using physical models represent the flooding mechanism through known physical analyses. Therefore, if the unknown factors are mainly important for the inundation mechanism, the accuracy will be low. On the other hand, the statistical analytical approach is considered to be able to elucidate the distribution of areas where urban pluvial flooding frequently occurs (hereafter referred to as frequent urban pluvial areas) based on past flooding area records and to quantitatively evaluate their characteristics, including unknown factors that cannot be considered in flooding simulations [14].

A statistical analytical approach has been greatly developed by utilising several appropriate factors, such as bedrock geology, soil properties, land use, drainage networks, road networks, building, and precipitation. Some previous studies have identified flood mechanisms. For example, Fariza et al. (2019) used fuzzy multi-criteria decision making (FMCDM) to assess urban flood risk levels in Sidoarjo, Indonesia [15]. Sato and Hayashi (2014) used principal component analysis (PCA) to analyse the main topographic characteristics of inundation in the Musashino Plateau of Tokyo and Saitama [16]. However, no report of an earlier study describes the quantitative assessment of the universal characteristics of flood-prone areas because they analysed areas where inundation has occurred at least once [14], and most of the earlier studies were limited to case studies of one city or one case study.

Djamres et al. (2021) identified the frequent urban pluvial flooding areas using seven years of urban pluvial flooding area records during 2008–2015 and analysed, using the PCA, their topographical characteristics in Tangerang, Indonesia [14]. Results showed that frequent urban pluvial flooding areas in Tangerang emerged because of a slope in the upstream condition, the correlation between concave and flow length conditions, the correlation of the slope condition and distance to a river, and relationships among flow length in upstream characteristics and distance to a pond. Furthermore, 29% of frequent urban pluvial flooding areas had low topographical similarity because of anthropogenic factors such as changes in overland flow directions due to the change of a slope in the upstream condition by land-use change and trapping of flood water by "dominant structures" that may influence inundated water flows in and around frequent urban pluvial flooding areas. Mignot and Dewals (2022) argued that a particular impediment to progress in urban flood modelling science is that the conclusions of most studies remain genuinely site-specific formulations and that significant progress could be made by attempting to extract general knowledge from the collection of existing case studies [9]. They also suggested that this may be achieved by designing appropriate metrics for classifying and standardizing the definition of flooding scenarios, investigated processes, and effects of analysed factors [9]. As the newly defined urban pluvial flooding frequency areas contain primary factors related to flooding mechanisms, statistical analysis of frequent urban pluvial flooding areas may reveal previously unknown factors such as anthropogenic factors and their relative influence on the flooding mechanism.

In Japan, flooding area records have been available since 1993, and Kakehashi et al. (2014) [17] used these records to identify frequent river flooding areas throughout Japan and to elucidate their formation mechanisms. However, there have

been no studies that have utilised these records for urban pluvial flooding. Therefore, the objectives of this study were to identify the frequent urban pluvial flooding area using these records for 20 years during 1993–2012 in Osaka and Nagoya Cities, the two cities with the largest proportion of urban pluvial flooding damage from 2006 to 2013 [7], and to analyse the topographical characteristics of frequent urban pluvial flooding areas and their distribution at both cities by applying methods reported by Djamres et al. (2021) [14]. This study also aimed to elucidate anthropogenic characteristics of urban pluvial flooding from the views of the location of "dominant structures" and the impact of drainage system improvement.

#### **2. Materials and Methods**

#### *2.1. Study Area*

In this study, two cities, namely Osaka and Nagoya Cities in Japan, were selected as the study area. The location and detailed information of the target cities in Japan are shown in Figure 1 and Table 1. These cities were selected because their urbanised areas cover more than 90% of the city area, while they have differences in urban population, urban area population, and other factors related to the urban scale of the cities, and therefore, they are suitable for this study to compare characteristics of urban pluvial flooding areas due to differences in an urban scale. Furthermore, there were relatively more urban pluvial flooding area records than in other cities, and it was thought that more reliable analysis results could be obtained (in detail, see Section 2.2). In addition, the amount of capital expenditure on sewerage projects in each city was large, and it was considered that cities with larger investments in sewerage projects were more likely to have experienced more urban pluvial flooding in the past (Table 1).

**Figure 1.** Location of the target cities in Japan and low-lying areas in the target cities. Low-lying areas are classified by the Ministry of Land, Infrastructure, Transport, and Tourism (MLIT) as areas where the elevation is lower than the surrounding [18].



Note: <sup>1</sup> In detail, see Section 2.2.

The annual maximum one-hour rainfall data as short-time extreme rainfall causing urban pluvial flooding in the study area for 20 years during 1993–2012 in Osaka and Nagoya Cities are shown in Table 2. One-hour precipitation data from JMA AMeDAS stations (Osaka: 34◦40.9 N, 135◦31.1 E; Nagoya: 35◦10.0 N, 136◦57.9 E) were used [19]. The average annual maximum one-hour rainfall was 33.2 mm in Osaka City and 39.4 mm in Nagoya City, respectively. Above-average annual maximum one-hour rainfalls were recorded seven times in Osaka and nine times in Nagoya, with no bias towards increased annual maximum one-hour rainfall through the 20-year period 1993–2012.

**Table 2.** Annual maximum one-hour rainfall during 1993–2012 in Osaka and Nagoya Cities. The shaded values are greater than the average for each city.


#### *2.2. Identification of Frequent Urban Pluvial Flooding Areas*

An example of a flooding area record in the case of urban pluvial flooding (urban pluvial flooding area records) is shown in Figure 2. Flooding area records are produced by each municipality as part of flood damage statistical surveys. Flooding area records since 1993 are stored as image data (pdf format) by MLIT. In this study, a total of 457 urban pluvial flooding area records for 20 years from 1993 to 2012 in Osaka and Nagoya Cities were obtained from MLIT, and frequent urban pluvial flooding areas were identified.

To confirm the reliability of urban pluvial flooding area records in the study area, these records were compared with the presence or absence of flood damage caused by urban pluvial flooding in the flood damage statistics. It was confirmed that 89.5% of urban pluvial flooding in Osaka City and 97.4% in Nagoya City were correctly recorded as the urban pluvial flooding area records. Here, these ratios were calculated by dividing the number of urban pluvial flooding area records by the number of urban pluvial flooding events recorded in the flood statistics. The reason why the flood pluvial flooding area record did not reach 100% might be that the flooding area record has been allowed not to record cases where the flooded area was less than 1000 m2, and the number of flooded houses was less than 10 [17]. Namely, it was possible that although urban pluvial flooding damage was recorded in the flood damage statistics, flooding area was not recorded in the urban pluvial flooding area record.

**Figure 2.** An example urban pluvial flooding area record (in Japanese). The seven shaded areas indicate the area where urban pluvial flooding occurred by Typhoon No. 17 on 22 September 1996. The scale of the map is 1/2500.

These urban pluvial flooding area records for the 20 years from 1993 to 2012 in Osaka and Nagoya Cities were input into GIS, and vector data were created for each year for the identification of frequent urban pluvial flooding areas. The areas where urban pluvial flooding occurred in Osaka and Nagoya Cities each year were converted into raster data of 10 m, 30 m, 50 m, 100 m, 200 m, and 400 m meshes. If even a small amount of urban pluvial flooding occurred within a mesh, that mesh was regarded as having been subject to urban pluvial flooding. The total years of urban pluvial flooding that occurred in each mesh over the 20 years from 1993 to 2012 were calculated by adding up the raster data for each year.

### *2.3. Topographical Characteristics of Frequent Urban Pluvial Flooding Area*
