Photo Credit: NASA.org
Heavy rain hit the Pacific Northwest last weekend. Lots and lots and lots of rain fell and authorities are warning people to beware. Here is the mechanics of a mudslide. Heavy rain saturates the soil, it liquifies and down it comes.
And it comes down hard.
So authorities in all impacted states are telling people to be careful and pay attention.
On Saturday, March 22nd last year, mud and water flowed downhill at super speed and slammed into a neighborhood near Oso, Washington. It quickly destroyed 49 homes and other structures on its way to the North Fork of the Stillaguamish River. The slide left a debris field of almost a square mile.
The event killed 43 people.
The U.S. Geological Survey recently released its findings of the cause of the hill’s collapse. USGS scientist Richard Iverson and his team say too much water is the cause. They looked at readings at the Darrington Ranger station for rainfall totals for the 180 day span before the slide and found that time period to be 91% wetter than the average for the same time period over the last 86-years.
Ironically, the slide happened on a dry day and Iverson and his group conclude if the hill had been just a tiny bit drier on that day, the force of the slide would not have been as great. In fact, the slide might not have liquified at all. It is the water — Iverson said — that gave the slide it’s power.
“That is the big takeaway, that the mobility of the landslide was very large, not necessarily unprecedented, but very large. The model results indicate that, for example, had the water content been about 5 percent smaller, that it might have been far less mobile,” he said.
Looking at a drier hill, Iverson said, “That alternative case we simulated only travels about 100 meters. In that case the landslide would have crossed the river but wouldn't have done much more than that.”
Source link: heraldnet.com