In recent years, large-scale disasters caused by floods have occurred in Japan, such as the heavy rain in western Japan in July 2018, Typhoon Hagibis in 2019 (2019 East Japan Typhoon), and the flooding of Kumagawa River in 2020 (heavy rains in July 2020).

Large-scale flooding causes not only direct damage, such as loss of human life and damage to buildings and infrastructure, but also indirect damage (economic dam- age) due to the closure of lifelines such as water, electricity, roads, and railways; both impact the affected region’s resilience. Improving disaster resilience has been included in the government’s National Resilience Goals as “minimizing damage to lifelines, fuel supply related facilities, transportation networks, etc., and restoring them as quickly as possible” as a part of “initiatives for building national resilience and preventing and mitigating disasters” [1]. Government initiatives will improve the disaster resilience of lifeline facilities, but it will take a substantial time before it bears fruit. Meanwhile, it is important to understand the current level of disaster resilience and consider non-structural measures accordingly to prepare for the next disastrous event.

Assessing current disaster resilience requires under- standing how each building or lifeline structure is dam- aged in a disaster and the recovery time. There are various kinds of lifelines, such as electricity, gas, water supply, sewerage, road, and railway. In this study, we decided to focus on water supply related facilities because many facilities of water supply systems were damaged by recent flood disasters in Japan.

When a major earthquake occurs, the Japan Society of Civil Engineers (JSCE) and other organizations conduct a large-scale damage reconnaissance survey and publish survey reports [2–4]. However, there have been few re- ports on typhoons and flood disasters. The only research paper on flood damage to water supply related facilities was a study by Takanishi et al. [5]. To the best of our knowledge, this is the only study on the subject, and there are only a few other individual survey results. For ex- ample, Ito et al. summarized the results of a flood reconnaissance survey conducted in Joso City following the 2015 Kanto-Tohoku heavy rain, in which they reported the damage and recovery of the lifelines of electricity, water supply, sewerage, telecommunications, roads, and railways [6]. Sugiura and Hirayama reported on the water damage risk of water supply related facilities during the 2018 heavy rain disaster in western Japan [7]. Describes the flood damage during the 2019 East Japan Typhoon, focusing on the damage and recovery of electricity and gas, but does not mention damage to the water supply related facilities [8]. It is similar in foreign countries, as only a few reports of damage caused by Hurricane Katrina in 2005 [9–11] and flooding in Wisconsin and Iowa in 2008 [12] are available. Therefore, even if all the contents of these papers and reports are used, there is not enough data to model lifeline flood damage and recovery. In addition, the Hazus technical manual by the Federal Emergency Management Agency (FEMA) [13, 14] also provides damage and recovery functions for earthquake damage based on the results of a reasonable amount of re- search. However, regarding flooding, although the dam- age functions of water supply related facilities are shown, references or studies on which they are based are not pro- vided, and it is presumed that they are based on engineering judgment rather than on actual damage data. In addi tion, there is no mention of a recovery model.

As shown above, comprehensive studies on the dam- age and recovery of water supply related facilities due to floods have never been conducted, not only in Japan but also worldwide. This is because of difficulty in collecting sufficient data from many facilities, and thus, there have been reports of damage and recovery of an individual facility, but not more. However, the occurrence of several severe flooding disasters in recent years in Japan has made it possible to conduct such a comprehensive study.

In general, there are analytical approaches, experimental approaches, and approaches based on case studies for such research, but in the case of complex systems composed of multiple facilities/components such as life- lines, it is not easy to take analytical or experimental approaches. Based on case studies, it is important to understand which facilities/components are critical to flood damage or recovery, what kind of damage was observed in what component, and which of the various processes are critical for recovery. Therefore, here, flood damage to water supply related facilities and their recovery in re- cent years were investigated not only at the facility level, but also at the component level through a questionnaire survey.

Questionnaire Survey

The flood disasters and the number of facilities covered here are summarized in Table 1, which shows 61 facilities that were affected by six flooding events occurring in Japan after 2015. We conducted a questionnaire survey of 21 municipalities. We selected water supply related facilities listed as flooded facilities in the report of the Disaster Prevention Information by the Cabinet Office [15]. The 2015 and 2018 disaster surveys were conducted in 2018, and the 2019 and later disaster surveys were conducted in 2020. Regarding the July 2020 heavy rain, the Cabinet Office’s report did not publish any information on dam- age to water supply related facilities. However, we contacted four municipalities where severe flooding occurred by phone to inquire about the damage to water supply related facilities and conducted a survey in one city that indicated that some facilities suffered damage.

The questionnaire mainly asked about the characteristics of flood damage, maximum water depth from the ground level at the site, vulnerable components of the facility, recovery process, and key factors to improve recov ery time.

Table 1: Flood disasters and number of facilities covered in this study.

WTP: water treatment plant, Intake: intake facility, and Others: pumping facility, distribution facility, storage facility, or facility for small water supply system (small WSS).

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