The United States Geological Survey (USGS) has developed a landslide warning system for the bluffs in the Seattle area based on historic records of landslides combined with rainfall. In general, the Puget Sound area does not get very intense rain fall events that often. But the area does get periods of extended wet weather with lots of small rain fall events that can be accumulative. Hence, one of the key predictors is combining rain fall accumulation as presented in the chart below.
By way of example: if 3 inches of rain feel over a 15 day period and then a big storm came in and 3 more inches fell over a 3 day period, the threshold would be exceeded and landslides in the area should be expected. It does not mean every slope will be unravelling, but it does mean that slopes considered at risk are much more likely to fail during these types of events.
The other predictor is a rain fall intensity over time approach.
Relative to many locations around the country and even in areas of Washington State 0.1 inches of rain per hour is not very intense, but if it rains at 0.1 inches per hour for 30 hours there is a very likely risk that some slopes sensitive to rain fall will fail. Again, this is not for any given slope. And I suspect this pattern is not a stand alone event as most of the time in western Washington when an intense rain fall takes place there have been previous storm events already making slopes wet.
One can go on line and check out the rainfall monitoring using the above threshold tools http://landslides.usgs.gov/monitoring/seattle/rtd/plot.php. Here is the plot of the 3-day and 15-day rainfall accumulation totals for January 10. The Seattle-Tacoma Station is just below the threshold.
In addition, the monitoring site has an antecedent water index plot that serves as a warning for when the landslide risk is primed and at risk of being bumped over the two limits as presented above.
By way of example: if 3 inches of rain feel over a 15 day period and then a big storm came in and 3 more inches fell over a 3 day period, the threshold would be exceeded and landslides in the area should be expected. It does not mean every slope will be unravelling, but it does mean that slopes considered at risk are much more likely to fail during these types of events.
The other predictor is a rain fall intensity over time approach.
Relative to many locations around the country and even in areas of Washington State 0.1 inches of rain per hour is not very intense, but if it rains at 0.1 inches per hour for 30 hours there is a very likely risk that some slopes sensitive to rain fall will fail. Again, this is not for any given slope. And I suspect this pattern is not a stand alone event as most of the time in western Washington when an intense rain fall takes place there have been previous storm events already making slopes wet.
One can go on line and check out the rainfall monitoring using the above threshold tools http://landslides.usgs.gov/monitoring/seattle/rtd/plot.php. Here is the plot of the 3-day and 15-day rainfall accumulation totals for January 10. The Seattle-Tacoma Station is just below the threshold.
In addition, the monitoring site has an antecedent water index plot that serves as a warning for when the landslide risk is primed and at risk of being bumped over the two limits as presented above.
As can be seen by the plot the stations had antecedent water indexes above the warning threshold in late December and got pushed back above the threshold from the heavy rain on January 9.
With drying conditions expected landslide events should taper off next week, but the area has had a long spell of wet weather where the antecedent wetness has been frequently above the warning level when storms have arrived.
Since Thanksgiving there have been 73 landslide closures on the rail line between Everett and Seattle. This stretch of rail passes along miles of high steep bluffs many of which are susceptible to landslides from just the type of rain fall accumulations presented above. With 27 days of measurable rain in Seattle during the month of December (an all time record), the USGS warning system is getting a good test sample.
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