The Japanese government is busy planning to build a series of cement sea walls nearly five stories high along its coastline, saying it's part of an effort to block future tsunamis.

A 2011 tsunami triggered by a magnitude 9.0 earthquake devastated the northern part of Japan and resulted in the meltdown of three of six reactors at the Fukushima Daiichi nuclear power plant.

Nearly 18,500 people died or went missing.

Authorities said many of them ignored early warnings to leave low-lying areas, and they want to avoid that happening again.The plans call for a 250-mile chain of sea walls - in some places they would be as tall as five stories high - and at a total cost of $6.8 billion.

Critics of all the construction say it will damage marine life and negatively affect the scenery, and it would do nothing to help people who need to move to higher ground when a tsunami hits.

They point to one example, a 24-foot wall that slowed the water a bit in the southern town of Iwanumalguchi, but it wasn't enough to stop the water altogether, and the tsunami swept up to three feet inland, taking everything in its path.

Town authorities say they have no plans to make that wall any higher. "We don't need the sea wall to be higher. What we do need is for everyone to evacuate," Mayor Tsuneaki Iguchi told Reuters.

"The safest thing is for people to live on higher ground and for people's homes and their workplaces to be in separate locations. If we do that, we don't need to have a 'Great Wall,'" he said.