Following the terrible damage inflicted in March last year by tsunami washing upstream along river courses, disaster-hit Miyagi Prefecture has decided to build protective embankments along its rivers opening onto the sea.
The prefecture had been considering two common anti-tsunami damage strategies for coastal rivers: embankments and sluice gates near river mouths. The prefecture opted for embankments due to the deaths of a number of emergency workers in last year’s tsunami when they went to close existing sluice gates, while areas around river mouths near gates that had been shut suffered greater damage.
According to the Miyagi official in charge of the project, about half of the about 30 significant grade-2 rivers in the prefecture are currently defended by sluice gates. However, the prefecture will build protective embankments along all these rivers, except for one that is administered jointly with several municipalities. A detailed work plan will be drawn up, and construction on the embankments is expected by fiscal 2015 at a total cost of tens of billions of yen. The Ministry of Land, Infrastructure, Transport and Tourism, meanwhile, will build embankments along four rivers in Miyagi designated as major grade-1 water courses by the central government.
Among the advantages of the embankments cited were that, unlike sluice gates, they do not require manual operation, and they are cheap to build. The one major disadvantage cited was that the embankments will eat up land that could otherwise be used for industry or farming.
The prefectural town of Minamisanriku — which was nearly wiped off the map by the tsunami, and where three rivers flow through its Shizugawa district — initially rejected a proposal to build embankments that would reach up to 8.7 meters, saying they would effectively cut the city center into four parts. The district eventually approved the construction of the embankments after consultations with the prefecture showed that built-up areas could be raised to almost the same height as the embankments. The district revised its land-use plans to allow the embankments in February.
The infrastructure ministry instructed all the tsunami-hit prefectures to build sluice gates and embankments to prevent tsunami from doing damage along river courses. Fukushima Prefecture has said it plans to build embankments along 17 of its rivers, but has not yet decided on how it will address the 12 rivers running through the nuclear disaster exclusion zone. Iwate Prefecture has decided to install sluice gates on four of its rivers, and is still considering which method it will use for its remaining waterways.