Monday, April 14, 2014

Perspective on Rain and Oso/Hazel Landslide

Rain gets a bum wrap when it comes to landslides. Heavy rain caused the landslide is a common statement. Record breaking rain caused landslide or landslides. Yes, there is a correlation of when landslides happen and rain events, but you can not blame the rain for making steep mountain slopes, steep shoreline bluffs or steep river valley bluffs. The blame the rain gets added to by forestry types as well. Any suggestion that a landslide was caused by logging leads to finger pointing at rain.

Rain has taken a fair bit of blame for the Oso/Hazel Landslide. And much as been made of record breaking rain. So it was time to check those rain fall records.

The two stations with long term records are Arlington and Darrington. Oso is located roughly half way between these two stations. Darrington has records going back to 1911 and Arlington's goes back to 1922. Finney Creek is a remote station with too short of a record to be much use for comparison. Darrington being more deeply in the mountains is far wetter - roughly twice as wet as Arlington.


The 19.30 inches of rain for March 2014 was the wettest March ever. The mean for March is 8.61. So headlines are record rain causes landslide. Twice normal rainfall triggers landslide. However, slow down and look at some other months. Like January 1953 when 31.22 inches fell. Or the infamous winter of 1933 when 30.42 inches fell in December. All in all the March record has been exceeded in other months at Darrington 17 times. 


The March 2014 rainfall of 8.70 inches at Arlington fell short of the March record of 9.23 inches. The March 2014 rainfall amount has been exceeded 20 times in other months.  


The Wizard said...

Thank you Dan, one thing I like to do is make a three day storm precipitation cumulative graph for the peak annual discharges, plot them and look for a break in slope. I have posted a new geologic GIS database with the extent of the LIDAR analysis by R. A. Haugerud (2014) with some additional deposit and slope perspective.
E. Martinez

Anonymous said...

Dan: I think your observation that other factors that cause landslides (other than heavy rainfall), are often overlooked, is a good one. Part of the problem may be that a distinction is sometimes not made between a relatively short term event like an earthquake or heavy rain storm that can be defined as a triggering event and those you mention which can be part of the overall cause of the failure.

As you pointed out, there have been other heavy rainfall events in the past in the Darrington/Arlington areas. If those events didn't trigger landslides then it seems reasonable to assume that the necessary contributing causes we're not in place.

Still, attention must be given to forecasted heavy rainfall events to allow for any timely mitigation efforts that could be taken. Even if only a small percentage of heavy rainfall events over time triggers a landslide in an area of great concern like the Oso area an awareness of the correlation with landslides you mention could help save lives and prevent disasters.

Doug McKeever said...

Dan, I am glad you commented on this all-tto-common assertion that "heavy rains caused the slide." As I tell my students, if heavy rain is THE cause, then logically every patch of earth that was rained upon would be sliding. So it should be obvious that rain can be a trigger but is never THE cause. Mass movement always has multiple causes, as all geoscientists know, but with pervasive statements in mass media shouting "Record rains are to blame" policy makers and homebuyers are too prone to overlook obvious major long-term internal and external contributing factors for landslides as at Oso, such as weak earth materials, steep topography, and continuing erosion and oversteepening at the slope base by a river.