1. Introduction
In July 2018, wet air from the south generated heavy rainfall in western Japan. In the city of Seiyo, rainfall registered 347 millimeters per 24 h (JMA, 2018) [
1]. Inundation in the Hijikawa River Basin of Ehime Prefecture killed nine people (Ehime Prefecture, 2018) [
2]. The inundation began soon after a discharge operation of two dams on the Hijikawa River.
Communication on the operation of a dam is crucial in evacuating residents before downstream flooding occurs because the release of a dam suddenly raises the river level.
This paper examines the functions of and problems relating to information provided by dam operators for the evacuation of residents during the July 2018 flooding of the Hijikawa River. We investigate what types of warnings were issued by dam operators and emergency managers of local government, in which channels those warnings were sent, and how residents reacted to the warnings.
The population’s response to warnings is a main theme in the field of the sociology of disasters and has been well researched (e.g., Dabek, 1986 [
3]; Perry, 1994 [
4]). Quarantellit (1982) [
5], for example, pointed out three elements of the warning process: assessment, dissemination, and response. Furthermore, Mileti and Sorensen (1990) [
6] conceptualized the warning response process as hearing, understanding, believing, personalizing, deciding and responding, and confirming. Moreover, Sorensen and Sorensen (2006) [
7] summarized the results of research conducted in the United States on factors that affect the responses of people. They cited the residents’ knowledge of hazards, personal warning channels, frequency of warnings, message consistency, message certainty, and source credibility as factors that promote evacuation. Additionally, several empirical studies were conducted in Europe (e.g., Parker et al. 2009) [
8]. Those studies, like the present study, are interested in the effects of warnings and we can learn much from them.
However, warnings can now be sent through newly emerging media. The Public Warning System (PWS), in the case of mobile phones, allows warnings to be sent directly and with geo-targeting. In Japan, the PWS (which began as the Cell Broadcast Service) has been used to send disaster warnings since 2007. Although the PWS has been experimentally and conceptually studied (e.g., Jagtman, 2010 [
9]; Beanet et al., 2015 [
10]; National Academies of Sciences, 2018 [
11]), the role of the PWS in actual disaster situations has not been sufficiently investigated.
Meanwhile, although case studies of dam failures have been investigated (e.g., Wayne et al., 2008 [
12]), few publications have analyzed the warnings of discharges of dams (e.g., Katada and Kuwasawa, 2009 [
13]). There is some literature, such as dam operation manuals and textbooks (e.g., Water Resources Environment Center [
14], 2004; United Nations, 2008 [
15]). There is also a guidebook (Mileti and Sorensen, 2015 [
16]) for dam operators that is based on studies of the sociology of disasters. One report (Yoshii, et al. 2007 [
17]) describes the attitude of emergency managers and residents toward warnings of dam discharge.
Moreover, floods due to dam discharge are similar to flash floods in that the river level rises rapidly. Gruntfest (1977 [
18]), for example, conducted a sociological study on the relationship between warnings and evacuations in the event of flash floods. Furthermore, with development of weather forecasting technology, advanced flash-flood early-warning systems have been developed (Carpenter et al., 1999 [
19]; Georgakakos, et al., 2006 [
20]; Liu, et al. 2018 [
21]). However, dam discharge floods differ from flash floods in that warnings are issued by the dam operators who trigger the floods.
It is therefore important to investigate the residents’ reactions toward warnings of the 2018 flooding of the Hijikawa River as a case study of an actual disaster.
4. Discussion
The survey results show the function of information provided by dam operators as follows. First, the discharge warning issued by the dam operators had no effect, because few people heard the warning and even those who heard it were not encouraged to evacuate. Second, the evacuation instructions provided by the local government were received by half the residents, and few residents who heard the instructions felt a sense of danger. However, the evacuation rate of those who heard these instructions was higher. Third, the most effective channel for the dissemination of evacuation instructions was route alerting by the volunteer fire corps. Therefore, information from the dam operators had an indirect effect in promoting evacuation through the route alerting of the volunteer fire corps. Fourth, the PWS was useful in disseminating evacuation instructions where it was used.
We found problems with the warning process in the case study. According to Mileti and Sorensen (2015) [
16], the warning and protective action process can be divided into the three time periods: warning delay time (issuing), warning diffusion time, and protective action initiation time. In the case study, there were problems in all these stages. In the first stage, the delay in issuing evacuation instructions shortened the time for route alerting. The issuing of evacuation instructions took 2 h and 40 min in Seiyo and 2 h and 20 min in Ozu after notification of the FCOEF. In the second stage, the channels used to transmit warnings did not work efficiently. In the third stage, residents did not feel a sense of danger when hearing a warning. This is a problem relating to the content of the message.
We propose the following solutions to the above problems using knowledge of disaster sociology such as that reported by Milleti and Sorensen (2015) [
16]. For the first stage, Mileti and Sorensen (2015) [
16] recommended that an emergency manager in local government makes a public protective action list, classifies flood threat levels, and establishes thresholds. For example, we can create danger thresholds according to the notification of the FCOEF and the predicted amount of discharge flow. In addition, we can create warning levels according to these thresholds and determine the evacuation area according to the warning level. These measures can prevent delays in alarm announcements.
For the second stage, the effective use of communication channels needs to be researched. We found an example of the effectiveness of the volunteer fire corps in the free-answer column of the questionnaire.
“I suspected that my house (would be flooded), but I was evacuated early because a firefighter let me know. I thought I wouldn’t evacuate because my home was on the third floor, but I thought I shouldn’t bother neighbors because I was old, so I did what the firefighters said.”
(A resident in Seiyo)
Communication through the volunteer fire corps has some effectiveness. This is the so-called “persuasion without conviction” effect (Lazarsfeld et al. 1948 [
29]) of personal communication. In other words, one can be persuaded without agreeing to the reason. However, this communication has its limits because it takes time to visit each house, and the area that can be covered is therefore limited. In the considered flood event, 77 volunteer firefighters visited 912 houses and called for evacuation in Seiyo (Ehime Newspaper 14 September 2018 [
30]). This would be an upper limit of what is possible. Furthermore, it is difficult to use this channel in urban areas because there are few volunteer fire corps members.
A city’s disaster prevention radio system (i.e., indoor receivers and loudspeakers) has a certain effectiveness but is adversely affected by the noise of rain because it uses voice. Meanwhile, the PWS will be effective during heavy rain because people carry their mobile phones with them. However, Ozu used the PWS late and Seiyo did not use it at all, because officials in Ozu forgot to send warnings via the PWS and officials in Seiyo were worried about an inundation of inquiries from citizens (JASDIS, 2019 [
31]).
There are many channels through which a warning can be disseminated. However, each channel has advantages and disadvantages. Hence, we compare major channels according to four technological characteristics: ability to force notification, ease with which a notification is heard in heavy rain, coverage area, and ease in using the channel (
Table 7). First, the ability to force notification is high for the DRS and mobile phone, because a warning is automatically received even when residents do not have any intention to search for information. Second, the sound of heavy rain does not greatly interfere with hearing a notification via indoor speakers and mobile phones. Third, notification through route alerting only reaches some parts of a city, while the DRS, PWS, and e-mail from local government cover the entire city, and television and mobile applications cover the whole country. Fourth, the DRS and television do not require complicated operations to use, whereas e-mail from local government requires registration and disaster-prevention mobile applications need to be downloaded. In particular, at night during rain as in the case of the considered flood, it is difficult to disseminate warnings. Warnings should therefore be disseminated through all media channels to ensure that all residents hear them.
For the third stage, an effective message is needed. In general, residents do not evacuate immediately when they hear warnings, and it is thus necessary to send appropriate messages frequently from an early stage (Mileti and Sorensen, 2015 [
16]). A warning message requires five elements: source, threat (i.e., the possible event and its impact), location, guidance (i.e., the protective action to take), and expiration time (Mileti and Sorensen, 2015 [
16]). In addition, the specificity (e.g., a place name) and clarity (e.g., a message free from jargon such as “10,000 m
3/s flow” or “spillway”) are most important in the style of the message (Mileti and Sorensen, 2015 [
16]).
Table 8 presents example messages in the case of controlled dam discharge.
Compared with the examples given in
Table 8, the messages issued in the case study were insufficient in terms of both specificity and clarity. For example, in the dam operator’s warning in
Table 2, the location and possibility of flooding are ambiguous in the expression “the water level of the downstream river will rise rapidly”. In addition, the action to be taken is not concrete in the expression “take certain precautions”. In the evacuation instructions of the local government in
Table 3, the cause, possibility of inundation, inundation depth, and time are ambiguous in the message “Because the river has reached the level where the river is likely to flood”. Moreover, the location is not specific in the expression “As the water of the river will reach the highest level ever, it is supposed that land that has never been inundated before can be inundated.” These inadequate messages did not generate a sense of crisis in residents. It is therefore necessary to improve the specificity and clarity of the warning message. However, to do so, it is essential to conduct a flood simulation that matches the discharge volume of the dam.
Certainly, it is not easy to simulate the flooded area accurately. However, in the case of a dam, the discharge flow is predicted several hours before. Therefore, if simulations corresponding to the discharge were conducted, they would be used effectively, especially in areas immediately downstream of the dam where dam discharge directly affects the river water level.
Another problem was the low frequency of message dissemination. Here, each dam operator sent the discharge information of the dam only once, while the city of Seiyo sent the evacuation instructions three times but it did not send anything between the first FCOEF notification and evacuation instructions. The low frequency at which messages were sent resulted in the residents not listening to the messages and not feeling a sense of crisis. It is necessary to send messages more frequently to encourage residents to evacuate.
Furthermore, the sender’s credibility is important in promoting evacuation (Mileti and Sorensen, 1990 [
6], Quarantelli, 1980 [
32], Stallings, 1984 [
33]). Although the credibility of the dam operator before the flood was not clear, 82.3% of respondents answered that “the discharge operation of the dam was inappropriate” and 68.4% of respondents answered that “dam information was insufficient” in our questionnaire survey. Therefore, the credibility of the dam operator is presently low.
The effectiveness of the volunteer fire corps revealed in this study is consistent with the results of previous sociological studies. It was also as expected that the PWS played a certain role in the dissemination of warnings. Meanwhile, it was a new discovery that direct public information provided by dam operators to residents was ineffective.
The present study has its limitations. For example, we could not reveal why the transmission capability of the PWS and Indoor receiver of DRS did not work completely. Moreover, this case study has geographical limitation that only covers the rural area where the volunteer fire corps are relatively active. Therefore, further empirical studies on the effective use of dam discharge information and new dissemination channels are needed.