A view from above shows a wicked scar – what remains after thousands of pounds of mud and trees tumbled into the path of a commuter train Monday evening.
Local geologists anticipated local trouble spots for landslides ahead of the derailment, and the risk continues in many Bay Area communities as long as El Nino rains keep falling.
“We’re looking at a year where we could see some failures in March if the next round of storms is powerful,” geologist Jonathan Stock told NBC Bay Area just hours before the Niles Canyon landslide derailed the ACE train.
We asked his colleague at the U.S. Geological Survey whether the agency could have anticipated the slide.
“We can’t make that one-to-one connection yet,” said Brian Collins, a civil engineer.
Collins said the past two storms over the weekend created near-saturated conditions. Also, the pore water pressure at the closest monitoring station to the derailment site was elevated.
“That’s the kind of pressure you feel at the bottom of a swimming pool that presses on your ears. That’s the kind of pressure that causes landslides,” Stock said.
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The combination of saturation, pressure and continuing storms means slopes around the Bay Area are prime for sliding.
“But there are times when pressure builds and nothing happens,” Collins said.
Geologists working on the USGS’s Landslide Initiation Project have been monitoring four sites in the Bay Area – Marin County, San Bruno, Pacifica and Castro Valley – since 2009.
The Castro Valley monitor showed elevated pressure levels at the time of the derailment, according to Collins.
Collins says if pressure levels stay elevated, they make recommendations to the National Weather Service to issue warnings. This time, however, the monitoring station was too far from the Niles Canyon slope to detect a slide. Collins says the Castro Valley slope has similar attributes to the Niles Canyon slope; however, he can’t be sure of the exact conditions.
Usually, there needs to be about a half inch of rain per hour in order to cause already-saturated soils to slide; however, Niles Canyon saw about a tenth of an inch per hour at the time of the derailment, according to the National Weather Service.
While Collins says geologists can’t predict mudslides, he says to expect them in the Bay Area this season.
As the rain falls, scientists continue to learn more from the monitoring sites. The state has been in drought for much of the time the project has been up and running.
“These days, it will be wet enough so we can learn something to make a difference,” Collins said.
