Sea defences in Japan

July 13, 2006

hokusai

My neighbours, friends and ‘Matsushima Grandparents’ to my children, Mr and Mrs Miura told us the other night about two flooding events in my town which I thought I would relate here. (The Miuras are in their 60’s).

The first was in 1960 when the tsunami caused by the magnitude-9.5 Great Chilean Earthquake (the strongest earthquake ever recorded) hit Japan. It spread across the entire Pacific Ocean, with waves measuring up to 25 metres high.

When the tsunami hit Onagawa (link), Japan, almost 22 hours after the quake, the wave height was 3 m above high tide. In Matsushima the tsunami washed up the Takagi River and totally inundated the area where I live – near Matsushima Station on the Tohoku train line. This area is very low lying, perhaps less than 1m above sea level and at the time was mostly covered in rice fields. It sounded from Miura-san’s description to be more like a huge swell than a very powerful and destructive tsunami although this may be the result of the islands that afford Matsushima town some protection. You can see the map of the bay here.

The second flooding event was in 1978 when very heavy rains washed down the river and flooded the whole area of ‘Takagi’: the main residential area of Matsushima town. The water was about 1m deep throughout town and many buildings were heavy damaged. The floods also destroyed the river bed, washing tones of mud over what used to be a stoney river bed. Miura-san explained that he and all the towns children used to jump off the bridges and swim in the crystal clear river. The stony river bed and clear water also used to supply the town with shellfish but the shellfish beds were destroyed.

I was surprised that such a river flood would happen so close to the sea, but perhaps the flooded river combined with a high tide to cause the problems.

My town now has concrete walls surrounding much of its sea front. The river bank is also concreted and they are busy raising the river walls by about a meter. The walls have steel gates at intervals which appear to be on a timer mechanism that allows them to be remotely closed. I assumed that these were tsunami defences and this was confirmed by Miura-san.

I find the walls ugly and the get in the way of my enjoyment of the river and the sea. I also felt instinctively that they seemed to be a waste of resources and wanted to do more research on their effectiveness. The following is an extract from Wikipedia’s entry on Tsunamis.

While it is not possible to prevent a tsunami, in some particularly tsunami-prone countries some measures have been taken to reduce the damage caused on shore. Japan has implemented an extensive programme of building tsunami walls of up to 4.5 m (13.5 ft) high in front of populated coastal areas. Other localities have built floodgates and channels to redirect the water from incoming tsunamis. However, their effectiveness has been questioned, as tsunamis are often higher than the barriers. For instance, the tsunami which hit the island of Hokkaido on July 12, 1993 created waves as much as 30 m (100 ft) tall – as high as a 10-story building. The port town of Aonae was completely surrounded by a tsunami wall, but the waves washed right over the wall and destroyed all the wood-framed structures in the area. The wall may have succeeded in slowing down and moderating the height of the tsunami but it did not prevent major destruction and loss of life.

 

The effects of a tsunami can be mitigated by natural factors such as tree cover on the shoreline. Some locations in the path of the 2004 Indian Ocean tsunami escaped almost unscathed as a result of the tsunami’s energy being sapped by a belt of trees such as coconut palms and mangroves. In one striking example, the village of Naluvedapathy in India’s Tamil Nadu region suffered minimal damage and few deaths as the wave broke up on a forest of 80,244 trees planted along the shoreline in 2002 in a bid to enter the Guinness Book of Records. [6] Environmentalists have suggested tree planting along stretches of sea coast which are prone to tsunami risks. While it would take some years for the trees to grow to a useful size, such plantations could offer a much cheaper and longer-lasting means of tsunami mitigation than the costly and environmentally destructive method of erecting artificial barriers.

 

I’m not suggesting that Japan should plant hundreds of trees on its sea front because the problem with this solution in Japan is a lack of flat land for such planting. However I do still question the effectiveness of these huge ugly concrete walls (different by the way from the terapods which exist as wave breakers).

Lastly reading about tsunamis led me to ‘megatsunamis‘. There was one very astounding ‘event’ at Lituya Bay, Alaska on July 9, 1958.

A 7.5-magnitude earthquake occurred along the Fairweather fault with an epicenter near Lituya Bay. It generated a tsunami that washed away trees up to 520m above mean sea level. During the last 150 years, five giant waves have occurred in Lituya Bay. The previous event occurred on October 27, 1936, it washed out trees to a maximum altitude of 150m and was not associated with an earthquake.

The July 9, 1958 earthquake occurred at about 10:15pm which is still daylight in Alaska. The weather was clear and the tide was ebbing at about plus 1.5 meters. Bill and Vivian Swanson were on their boat anchored in Anchorage Cove near the western side of the entrance of Lituya Bay. Their astounding observation was as follows:

“With the first jolt, I tumbled out of the bunk and looked toward the head of the bay where all the noise was coming from. The mountains were shaking something awful, with slide of rock and snow, but what I noticed mostly was the glacier, the north glacier, the one they call Lituya Glacier. I know you can’t ordinarily see that glacier from where I was anchored. People shake their heads when I tell them I saw it that night. I can’t help it if they don’t believe me. I know the glacier is hidden by the point when you’re in Anchorage Cove, but I know what I saw that night, too.

The glacier had risen in the air and moved forward so it was in sight. It must have risen several hundred feet. I don’t mean it was just hanging in the air. It seems to be solid, but it was jumping and shaking like crazy. Big chunks of ice were falling off the face of it and down into the water. That was six miles away and they still looked like big chunks.

They came off the glacier like a big load of rocks spilling out of a dump truck. That went on for a little while—it’s hard to tell just how long—and then suddenly the glacier dropped back out of sight and there was a big wall of water going over the point. The wave started for us right after that and I was too busy to tell what else was happening up there.’’

Howard Uhlrich and his son were in another boat further up the bay:

Dashing to the deck, Uhlrich beheld the writhing and twisting of the high peaks and the clouds of dust and flying snow about their summits. Petrified, he watched for 2 minutes or more until his attention was attracted to a new sight. There was a gigantic wall of water which he thought to be 1,800 feet high erupting against the western mountain, then coming down the bay, cutting a swath through the trees on the summit of Cenotaph Island, backlashing against the eastern shore up to a height of 500 feet, then heading for the Edrie (his boat), now a wall of water 50 feet high.

Suddenly he realized he had to move. Cursing himself for delaying, he got a life jacket on his son, then somehow got the engine going, but he was unable to heave the anchor in time. Just before the water struck he veered the chain to its end, hoping to slip it, at the same time maneuvering the Edrie to face the wave. As she lifted to the swell the chain tightened and snapped, its short end whipping up and winding around the pilothouse. The boat was swept, completely out of control over what had been dry land a moment before. By now Uhlrich remembered his radio. Shouting into it, he made the international voice-radio distress call, “Mayday, Mayday – Edrie in Lituya Bay – all hell broke loose – I think we’ve had it – goodby!” The wave, however, changed course and bounced off the shore, allowing Uhlrich, with strenuous efforts and certainly with superb seamanship, to get his boat under a kind of control. He now began devoting himself to evading huge chunks of churning ice, any one of which could have made kindling wood of the Edrie.

 

 

Read the full details and scientific discussion on what actually happened here (a pdf file) and another account here. Here is a quirky review of the incident related to surfing.

Interestingly though, reading the accounts,

“Uhlrich beheld the writhing and twisting of the high peaks…”

sounds much more plausible that the indigenous people have gotten it right:

The Tlingit natives believe that a powerful spirit, Kah Lituya, lives in deep ocean caverns near the entrance to the bay. He resents being disturbed and rises up to shake the mountains and wash away intruders with giant waves.

Edinburgh drowned by a megatsunami.

tsunami vision

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