Walking along rivers in Osaka such as the Yamato River or the Yodogawa, I was often surprised by the height of the flood-protection barriers running along their length. The rivers themselves are wide and shallow, making their way westward across the flat plain from the mountains of Nara to Osaka Bay. The channel is hundreds of metres wide, and the dykes are 7 or 8 metres high. As you can see from the pictures below, it is simply inconceivable that any amount of rain could cause the water level to rise enough to necessitate such high earthworks.
This is the bank of the Yodogawa. The area between the dykes is used for recreation, such as baseball and tennis, picnics, kite-flying and so on. Nothing is built there.
This is the Yamato River, near where we used to live.
What I had failed to understand was that the dykes are there to protect from a different threat: not from water coming down the river from the mountains, but from water rushing UP the river from the sea.
The city of Osaka is very vulnerable to flooding. It is built on a flat plain, much of which is below sea level at high tide, and which is crossed by numerous rivers. The development of the city has seen the coastline move steadily west as more and more land has been reclaimed from the bay. Meanwhile, extraction of groundwater has caused the land to subside.
Over the centuries, Osaka has experienced several catastrophic floods leading to massive destruction and loss of life. These floods are caused by tsunami (associated with offshore earthquakes; most recently in 1854) or by storm surges (associated with typhoons; most recently in 1934).
On a visit to Tempozan last August, we saw barrier walls encircling the low-lying area (which was reclaimed from the sea about 100 years ago). In the event of a tsunami or storm surge warning, huge metal gates slide into place, completing the barrier and preventing the water from entering the protected area.
The sign says that if you feel a large earthquake, you should hurry into the protected area inside the dykes.
In the event of a major Nankai or Tounankai earthquake, a bay-enhanced tsunami would arrive at the southern end of Osaka prefecture (Misaki-cho) after about 1 hour, and at the city and port of Osaka itself after about 2 hours. That is the amount of warning that would be available to the populace (to move away from the shore and into high and sturdy buildings) and to the city authorities (who operate an impressive flood-prevention infrastructure including tidal barriers at the mouths of around 200 rivers and streams, pumping stations and warning systems).
Last month, I visited the Tsunami/Storm Surge Disaster Prevention Station.
This facility is manned 24 hours a day, every day.
They are aware that in the event of a tsunami coming up the bay, there is zero tolerance for error. If even one of the flood barriers fails to close, the system will have failed and the city will be flooded. The terrible scenes from March 2011 in north-east Japan are still very fresh in the minds of Japanese people, and earlier flooding events in Osaka are also remembered.
The flood prevention station has an exhibition area with displays that explain the vulnerability of the city, the causes of tsunami and storm surges, past flooding events, and what you should do if a tsunami were to come.
The highlight of the exhibition is a movie theatre that shakes and rumbles to simulate the effect of being in a large earthquake and tsunami event. On that day, I was the only visitor, and so they ran the film especially for me. I sat on a stool in the middle of an otherwise empty cube-shaped room, and the video was projected onto the front and side walls as well as ceiling and floor to create quite an immersive experience. It was genuinely scary; the film had simulated images of walls of water rushing inexorably towards me in narrow city streets. I left feeling shaken in more ways than one.
In the film, we saw prefectural employees trying and failing to get one of the barrier gates closed, before abandoning their post and running for their lives in the face of the massive wave. It was this failure that allowed the city to be inundated. I thought this aspect was a bit bizarre; I would have thought the authorities would have tried to present a more reassuring picture of their own competence and dedication to protecting us all.
I read elsewhere an interesting essay about how people become complacent about a danger, if it happens at intervals of more than a century or so. Even where written records exist, what matters is whether the previous event is within living memory. In the area affected by the 2011 disaster, there were coastal towns which had been destroyed by flooding in previous centuries. In one case, the villagers had decided to relocate the whole village (a fishing village) inland, to higher ground. But over time, the inconvenience of being further from the coast outweighed the increasingly remote memory of disaster, and people moved back to the shore. In another case, a seawall was built to protect a town . However, eventually houses were built on the seaward side of the wall.
While the results are tragic, it is not because of stupidity. Are the people who live in Puyallup, in the path of lahars from a potential eruption of Mount Rainier, stupid? The people of Naples? Those who make their home in Sacramento, aware of the risk of catastrophic flooding? People who live in Seoul, in the shadow of DPRK artillery? It is just not in our human nature to worry about a high-impact low-probability risk that has not occurred in our lifetime, or in that of our parents or grandparents, nor most of the time would it be useful to worry. Our day-to-day lives are dominated by more immediate concerns.
Note on the word of the day
津波 tsunami means “harbour wave”. 津 tsu means “harbour” and nami means “wave”. Both characters include the suihen “water” radical; the three strokes on the left side that indicate that the meaning of the character has some connection with water.