{"id":67070,"date":"2026-09-17T08:00:48","date_gmt":"2026-09-16T23:00:48","guid":{"rendered":"https:\/\/norimen.net\/?p=67070"},"modified":"2026-09-16T19:09:28","modified_gmt":"2026-09-16T10:09:28","slug":"risk-of-landslides-after-heavy-rain","status":"publish","type":"post","link":"https:\/\/norimen.net\/en\/mountain-saturation-risk-after-heavy-rain\/","title":{"rendered":"Stay Alert in the Mountains for 1\u20132 Weeks After Heavy Rain | An Article for the General Public on \u201cSoil Doesn\u2019t Dry Out Easily\u201d\u2014What You Need to Know Before Silver Week"},"content":{"rendered":"

Hello, everyone.<\/p>\n

This is Enta.<\/p>\n

It's been raining on and off for the past few days, hasn't it?<\/p>\n

Plus, Silver Week starts this weekend (in 2026, it\u2019s a five-day holiday from September 19 to 23). I bet there are quite a few people thinking, \u201cIt\u2019s been a while since I\u2019ve had a long weekend\u2014maybe I\u2019ll head to the mountains.\u201d<\/p>\n

Oh, slope maintenance is just a regular job! ...<\/p>\n

But that's beside the point.<\/p>\n

This time, I\u2019d like to share a story that I feel compelled to write about precisely because of the timing.<\/p>\n

To get straight to the point,Even after the heavy rain stops and the weather clears up, the mountains don't dry out easily.<\/strong><\/p>\n

In particular, for about one to two weeks after heavy rain, the mountains may remain saturated with water, and from a technical standpoint, this is a period that requires caution.<\/p>\n

Even after the heavy rain stops, the mountains still hold onto the water.<\/h3>\n

As I mentioned in a previous post on this blog, as a river swelled by rain begins to recede, some of the water flows over the ground and recedes immediately, while other water seeps into the ground and gradually emerges.<\/p>\n

Although the water level above ground stabilizes relatively quickly, the water underground doesn't drain away that easily.<\/p>\n

This isn't just true of rivers; the same applies to the mountain slopes themselves.<\/p>\n

The Nagano Prefecture Soil and Water Conservation Division\u2019s website states, \u201cDuring heavy rains, rainwater accumulates in the soil, but it takes time for it to drain away, so there is a risk of landslides even after the rain has stopped.\u201d<\/p>\n

During the 2018 torrential rains in western Japan, the Landslide Prevention Public Relations Center issued a warning in response to an interview with the Kobe Shimbun, stating, \u201cSince landslides are caused by rainwater seeping into the ground, there is a risk of them occurring even after the rain has stopped, so caution against landslides is necessary for several days.\u201d<\/p>\n

If it turns into a larger-scale \u201cdeep collapse,\u201d the situation will drag on even longer.<\/p>\n

According to a report from the Ministry of Land, Infrastructure, Transport and Tourism, deep-seated landslides not only \u201coccur in conjunction with heavy rainfall\u201d but \u201cmay also occur after the peak of rainfall has passed and the rain has stopped,\u201d and,<\/p>\n

During Typhoon No. 12 in 2011, total rainfall exceeded 1,000 mm across a wide area of the Kii Peninsula, resulting in 72 cases of deep-seated landslides and 17 instances of river channel blockages (natural dams).
\n(From a news article)<\/p>\n

\"Large-Scale<\/p>\n

*Please note that the timeframe of \u201c1 to 2 weeks after heavy rain\u201d is a rough estimate I have derived by comparing my on-site experience with various reference materials; it is not a specific number of days set by any particular government agency. Please consider it merely as a general trend based on industry data.<\/strong><\/p>\n

The graph below conceptually illustrates the number of days elapsed since the peak of heavy rainfall and the state of the natural ground as it retains water.<\/p>\n

The key point I want to convey is that even if the visible rise in water level subsides quickly, it takes much longer for the entire ground to return to its normal moisture content.<\/p>\n

I compiled various data points into a graph, and it's not too far off.<\/p>\n

\"An<\/p>\n

Why Are Slopes Along Rivers Dangerous?<\/h3>\n

\"Mountains near rivers are prone to landslides\"\u2014that\u2019s something you really come to understand firsthand when you\u2019re working in the field.<\/p>\n

There are several overlapping reasons.<\/p>\n

First, when the river\u2019s water level rises, the groundwater level on the slopes adjacent to the river is pushed up as well (this is the phenomenon known as \u201cbank storage,\u201d where river water seeps into the land on the inland and uphill sides of the levee).<\/p>\n

Furthermore, when the current of a river\u2014whose water level is rising\u2014erodes the base of the slope (the toe), a phenomenon known as scouring occurs, causing the soil that supported the slope from below to be lost, thereby destabilizing it.<\/p>\n

As a slope engineer, you\u2019ve probably seen cases at riverside job sites where \u201cthe slope itself seems fine, but only the toe is eroded,\u201d right?<\/p>\n

It's hard to see from above, but if you look closely at the riverbank slope, you'll see it clearly.<\/p>\n

*Although it is said that capillary action (the upward movement of water through pores) in soil containing fine particles also influences the relationship between slope moisture content and river water levels, there are significant individual variations depending on soil type and topography, so this cannot be generalized or stated with certainty. This explanation is intended to describe a general trend only.<\/p>\n

\"Changes<\/p>\n

When a slight tremor occurs in saturated soil,<\/h3>\n

Another thing to be mindful of is the combination with earthquakes.<\/p>\n

The strength of soil (shear strength) tends to decrease as it absorbs more water.<\/p>\n

When a slope is barely holding together (technically speaking, when the safety factor is close to 1), even a slight tremor can trigger a collapse.<\/p>\n

In an emergency public notice titled \u201cBe Aware of Landslides After an Earthquake,\u201d the Ministry of Land, Infrastructure, Transport and Tourism\u2019s Landslide Control Division cites the case of the Tateno district (Minami-Aso Village, Kumamoto Prefecture) as an example following the Kumamoto Earthquake, where landslides worsened due to rainfall falling on ground that had been loosened by the earthquake.<\/p>\n

In this case, the sequence is \u201cearthquake \u2192 rainfall,\u201d but to put it another way, this means that \u201cwhen another stimulus is applied to ground that has already lost some of its strength, it can trigger a collapse.\u201d<\/p>\n

In a situation like this one, where \u201cthe ground has already become saturated due to heavy rain,\u201d even a small tremor of around Seismic Intensity 1 could trigger a landslide\u2014the possibility isn\u2019t zero.<\/p>\n

Whether a slope will actually collapse from a magnitude 1 tremor depends entirely on the slope\u2019s geology, gradient, and moisture content. The point I\u2019m trying to make here is not a definitive statement that \u201cit will definitely collapse,\u201d but rather a general principle of mechanics: \u201cWhen the safety factor is at its absolute limit, even a disturbance that would normally be insignificant can serve as a trigger.\u201d<\/p>\n

\"The<\/p>\n

To be honest, if I were to flat-out say, \u201cIt\u2019ll collapse even with a seismic intensity of 1,\u201d I\u2019d probably get criticized for being too alarmist, but from our perspective as people working on-site, slopes with a safety margin that\u2019s already stretched to the limit really are a matter of a hair\u2019s breadth.<\/p>\n

Even a tremor that would normally be nothing to worry about becomes a different story when the ground has completely absorbed all the water.<\/p>\n

I hope that anyone heading into the mountains during Silver Week will take a moment to think not just, \u201cIt\u2019s sunny, so it\u2019ll be fine,\u201d but also, \u201cHow many days have passed since that heavy rain? Isn\u2019t the ground still saturated with water?\u201d<\/p>\n

 <\/p>\n

See you later.<\/p>\n

The mountains are already saturated. A slope collapse could occur at any moment | Heavy Rain Warning<\/a><\/p><\/blockquote>\n