The July 2021 Atami, Japan Landslide: “A Man-Made Calamity”
“This is hell,” a survivor said after a steep slope failed in the idyllic resort town of Atami, Japan, sending a torrent of mud careening into houses and people. Ten are dead, seventeen still missing as of this writing.
Landslides are a grim fact of life in many areas with steep topography. Gravity works. Things get loose and come down. Travel into the mountains basically anywhere, and you’ll see the evidence: streaks of bare earth where trees have been swept away, cascades of boulders marking areas where rock failed under the influence of weathering and physics. Sometimes earthquakes shake things until they topple. It seems like the most natural thing in the world.
But not this time. Nothing about this tragedy was natural.
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Atami has been a beloved retreat since the 5th century CE. Plate tectonics molded it into a place of breathtaking beauty and hot springs.
Atami from above. Credit: Matsukin (CC BY 2.0)
The peninsula it sits on, Izu, began its existence as a collection of seamounts and volcanic islands erupted in the Miocene, which began colliding with the Japan Arc about one million years ago. Volcanoes continued erupting as the peninsula uplifted, building edifices such as Mount Atami. Volcanism stopped around 200,000 years ago; part of majestic Mount Atami eroded away, and the sea drowned its crater, leaving a magnificent curving coastline backstopped by dramatic steep slopes with gorgeous ocean views. When humans arrived, they discovered that volcanoes and tectonics had gifted them with hot water springs, inspiring them to name their settlement “hot sea.”
Its proximity to Tokyo and natural gifts made it an ideal resort town. It saw rapid growth when the Tanna Tunnel allowed a railway to reach it in 1934. Now just an easy train ride from Tokyo, the city swelled to 40,000 residents, and numerous visitors flocked to drink in its delights. Existing resorts were expanded and new ones built. Development stretched up into the precipitous mountain slopes. Such terrain is already delicately balanced: disturbing it is courting disaster. Landslides were inevitable, and they did happen from time to time. But Japanese folks are used to living with a temperamental island. When heavy rainfall caused the land along Highway 135 to slip in 2003, officials quickly recognized the dangers, hired engineers to implement mitigation measures, and the slide was arrested without substantial damage to infrastructure.
But in 2006, an Odawara real estate company purchased 116 hectares of forested ridges and rough terrain above the city. They applied to develop one of those hectares into a new residential area. And the steps toward a tragedy began.
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When will we learn to place people above profits?
We know that clearing trees from steep slopes increases landslide risk. We’ve known this for a very long time. In 1991, William R. Meehan put some solid numbers on the problem:
The frequency of mass erosion [landslides, debris flows, earthflows, etc] is strongly linked to the type and intensity of land treatment in the basin. Although most mass movements are associated with roads and their drainage systems, many originate on open slopes after logging has raised soil water tables and decreased root strength.” [Meehan, 1991 page 194] He also stated that: “The increase in mass movement due to clear cutting varies widely, ranging from 2-4 times in Oregon and Washington…to 31 times in the Queen Charlotte Islands… An increase of 6.6 times…is probably closer to the norm.
If you’re going to cut down a forest and muck about with the natural drainage in an area prone to landslides, you have to do it with an eye to landslide mitigation. You can’t just go in, chop trees, shove soil around, and call it a day. Yet, that’s exactly what the unnamed company did.
They cut down the forest. They trucked in sand and soil and started filling in the mountain valley. They also cheated with their fill, and tossed in wood chips, tile, and other industrial waste instead of disposing of it properly. The city government got wind of what they were up to, and told them to fix the issue. Unauthorized debris continued to be dumped in the fill site. Officials told them to fix it again. And again. The company, probably for reasons of greed, chose not to cooperate. And then, in 2011, they abandoned the project altogether, leaving deforested slopes and sloppy valley fill behind.
A Tokyo-based holding company soon purchased the site, and happily obliged the city officials when told they needed to properly fill in the site. They did as they were told, packing thousands of square meters of dirt left over from previous activities into the fill area. The plan they submitted said they would use 36,000 cubic meters of sand and soil, packed to a height of 15 meters. But in the decade from 2009 to 2019, an estimated 56,000 cubic meters of dirt ended up crammed into the upstream area of the valley, rising to as much as 50 meters in height. And as far as we know, no one was paying close attention to how well the work was done. No one was closely monitoring the site to see what changes to the forest cover and drainage area would do. No one was ensuring the fill itself had proper stabilization and drainage. Despite Japan’s keen awareness of hazards and expertise in mitigating them, no officials seem to have sounded the urgent alarm about this fill site.
We know that water-saturated soils are prone to sliding. We know that water will penetrate soils used to fill in ravines and valleys and stream beds. We ignore that knowledge at our peril. It’s only a matter of time before things go awry. Atami was living beneath an impending disaster.
But for years, nothing happened. For years, the ominous pile of dirt loomed over Atami, clearly visible in satellite images of the area.
Landfill area that became the Atami 2021 Mudslide, from 2011-2020. Credit: Dana Hunter/Google Earth.
It looked raw, but solid. No one took any further steps to secure it.
And then, in early July 2021, fed by anthropogenic climate change, the summer rains fell like a burst dam.
Torrential downpours soaked Atami and its environs, in some locations delivering a month’s worth of water in 24 hours. 12.4 inches of rain fell on the city in a mere 48 hours. City officials sounded a warning: the rains were causing “life-threatening conditions.” Atami was in trouble.
In the valley high above the city, above houses and roads full of people, the over-saturated fill lost its battle with gravity and began to slide.
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[…] up in the hills above Atami, rainwater finished infiltrating the nearly 56,000 cubic meters of soil and debris that had been pack…. Unable to drain, the water instead saturated the fill, putting intense pressure on unconsolidated […]