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Mar 31 2011

Sea walls were no match for this tsunami

Written by Nippon Sekai   
Thursday, 31 March 2011
NHK World aired a video detailing how Japan's maze of 10 meter high "anti-tsunami" sea walls ultimately failed to save their respective communities from destruction.

Part of the Taro sea wall

Japan has been constructing these giant sea walls for decades in order to protect its coastal communities from the devastating tsunamis that have slammed the country throughout its history. They now cover 40% of the nations coastlines. One of its most known sea wall was the one at Taro (田老町) as it was featured in National Geographic and Discovery Channel disaster related documentaries.

The community there prided itself on the civil engineering of the wall; which stands at 10 meters high and runs for around 1.6km. The village has been destroyed by successive tsunamis generated by the 1896 Meiji-Sanriku and the 1933 Showa-Sanriku earthquakes. Vowing to rebuild each time, the anti-tsunami sea wall was constructed.

These sea walls have been a hot political topic as its detractors have said they were a major waste of money (with money being kicked back to construction firms and suppliers who lobbied those politicians who pushed for them) and equally an eye sore. Some fisherman also were not in favor of them since it prevented them from being able to easily see the ocean from their homes. Regardless, these major construction projects persisted as the walls have proven to be useful against smaller tsunamis as well as storm surge from typhoons.

The massive sea wall in Kamaishi City had all of its sections finally completed in 2010 while the sea wall under construction at Kuji City in Iwate Prefecture was supposed to be completed soon.

These sea walls proved to be no match for the massive tsunami generated by the March 11th M9.0 earthquake. The massive flood gates of the Taro sea wall were ripped open and the tsunami topped the wall by at least 5 meters. The same thing happened at Kamaishi City, Miyako City, and Ofunato (which saw some of the highest wave heights in its narrow V-shaped bay). The result was not much different this time as these towns saw their communities destroyed once again. Did these walls give its residents a false sense of security? Perhaps for some who did put their entire faith in these walls and failed to heed the early warnings until it was too late. But others also argue that it provided an extra margin of time for citizens to evacuate and also stopped some of its full force energy which would have caused it to climb higher in the back of these coastal valleys. Still, the negative effects could easily be seen in the numerous videos of how these walls also ended up trapping the water and allowed it to slosh around inside like water in a bowl.

The sea wall at Taro, in better times (top) and after the March 11th tsunami (bottom)


 

NHK World video: Model town destroyed by the tsunami

Ch4 video: Sea walls fail Japanese community

 


Personal thoughts: I believe walls like these do have an ability to reduce some of the power of a tsunami but as nature has proven over and over again, all we can do is engineer solutions to buy some time so to speak. With smaller events, our engineered solutions can absorb the impact (i.e. smaller tsunamis can be stopped as there is not enough volume whereas with earthquakes, buildings can sway on their shock absorbers) but with the rarer larger magnitude events, they probably are going to succumb to the forces involved. With massive tsunamis such as this one, the volume of water displaced is the issue. IMHO, these walls could have been 10 meters higher but the local geographies of these areas (bays and harbors) coupled with the volume of water would have resulted in the water just continuing to rise in these bays as the successive tsunami waves pushed and piled up on top of the ones in the front. Additionally, the earthquake caused the land to subside lowering the height of these walls while also allowing all of that displaced water to easily fill these lowered areas. Going forward, it is clear the government and communities are going to have to reevaluate their disaster plans and evacuation policies. Sea walls like these also need to be looked at as a temporary barrier meant to buy evacuation time, not as a stop it all barrier. Thus they will need to look at whether it is economically feasible to continue to build them or whether or not the money would be better spent putting into place better evacuation centers and providing better education which can be easily supplemented by the wealth of data and information which will come out of this disaster.

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3.26 Copyright (C) 2008 Compojoom.com / Copyright (C) 2007 Alain Georgette / Copyright (C) 2006 Frantisek Hliva. All rights reserved."

 
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