Date Published: June 6, 2019
Publisher: Public Library of Science
Author(s): Ryo Horiike, Hisao Nakai, Tomoya Itatani, Fumie Shirai, Kaoru Konishi, Michio Murakami.
The Nankai Trough in Japan has been identified as a geological feature that could cause extensive damage in the event of a major earthquake. In this study, we investigated the impact of effective guidance signs for hearing-impaired individuals requiring special care when escaping to a tsunami evacuation building (emergency evacuation location) using geographical information system (GIS) and viewshed analysis. We selected an area we considered would suffer severe damage following a major earthquake and tsunami; we identified difficulties in the provision of escape routes. Using GIS, we determined the time required for escape and tsunami arrival time if effective signs were installed; we undertook such analysis using the height data of buildings in the target area. With effectively installed guidance signs, the average required evacuation time was 36.88 minutes; without such signs (which is currently the case in the target area), the average time was 47.10 minutes: that would result in citizens getting caught by the tsunami. Installing effective guidance signs would allow hearing-impaired individuals to escape to an evacuation building before being hit by the tsunami.
In Japan, the probability of an earthquake occurring in the Nankai Trough (extending from offshore Shizuoka Prefecture to Shikoku and Kyushu islands) over the next 30 years has been estimated as 70%–80% (Fig 1) . In the event of such an earthquake, the height of the subsequent tsunami in Kochi Prefecture (on the Pacific Ocean) has been calculated as 34 m, the highest in the whole country. Nationwide, the projected death toll would be49,000 people, of whom 37,000 (about 75%) would die as a result of the tsunami [2,3]. Japan is completely surrounded by sea, and so the tsunami could strike anywhere along its coastline.
For our analysis, we collected data for June to October 2018.
The area around our targeted tsunami evacuation building was 300 m long and 200 m wide. That area had two streets running north to south and four streets running east to west; they intersected at eight locations (Fig 2). The targeted building was situated on a street in the southern part of the area, and its height was 49 m. There were two buildings on a street north of the targeted building: they were both 45.5 m high, but they were not designated as tsunami evacuation structures. The evacuation sign in the area was at the south-western end of the tsunami evacuation building: it was the only sign identifying the building as such, and its height was 2.52 m. Without any other structures, the visible area with a 20-m radius centred on that sign was a maximum 1,256 m2; however, the visibility was obstructed, mainly by other buildings. Using viewshed analysis, we determined that the visible area centred on the tsunami evacuation building sign was 653.5 m2—just 52.1% of the maximum area.
In this study, we examined the effect of continuous guidance signs to tsunami evacuation buildings for hearing-impaired people. We found that with the current absence of such signs in the event of a tsunami, those individuals would have to escape via non-tsunami evacuation buildings. However, with a set of continuous signs, the visible region of guidance signs would extend to the tsunami evacuation building, allowing hearing-impaired people to escape. This study determined that in addition to helping individuals with hearing disabilities, seamless guidance signs would assist foreign visitors in recognizing the tsunami evacuation building from a radius of 20 m under any circumstances. One investigation found that to be effective in the event of a tsunami striking at night, signs for evacuation guidance and tsunami evacuation buildings should have a power-storing function through solar energy . Solar-powered guidance signs are necessary owing to the power breakdowns and disruptions in communication networks that frequently accompany a major earthquake.
We found in this study that the visible area of guidance signs was obstructed by nearby structures: sometimes, such obstruction reduced the visibility by about half. Thus, we undertook viewshed analysis using building data around the tsunami evacuation building. Following a tsunami disaster, visual information plays a vital role. The time required for victims to escape was delayed when they stopped at a non-tsunami evacuation building with the same height as the actual evacuation building: that would result in them being caught by the tsunami. It is necessary to install effective guidance signs for evacuation of the hearing impaired in the event of a tsunami.