Wednesday, November 25, 2015

Weathering another storm

Main track at the Snoqualmie Depot
during peak rainfall. 
November in Western Washington is synonymous with severe winter storms.  High winds, heavy precipitation, and power outages occur nearly every November and this year was no exception.  Fortunately, no significant damage occurred, but it was a nail-bitter because this was the first major storm event since completion of the PSE power plant reconstruction, and combined with the 2005-era Army Corps of Engineers section 205 river widening project.

Snoqualmie River at Snoqualmie
approaching peak flow.  Sandy Cove
Park is completely flooded.
The first indications of a threat usually arrive in the form of a flood warning.  A stage three flood is fairly common in Snoqualmie with several occurrences per year, while the more severe stage four floods occur every few years.  The National Oceanic and Atmospheric Administration ("NOAA") predicts peak water flow using computer models based on data from prior storms.  There is some margin for error, but the model updates as the storm progresses. 

South Fork of the Snoqualmie River
at peak flow.  Under normal flow,
there is approximately 20 feet of
headway under Bridge 35.
Flooding is one of the greatest threats to both the City of Snoqualmie and the Northwest Railway Museum.  Since 1948 there have been more than two dozen Presidential disaster declarations, and track and/or bridge damage has occurred in every event.  Tracking historical river peaks, there has generally been track damage at or above 43,000 cubic feet per minute as measured at Snoqualmie.  November 17, 2015's flood was projected at more than 48,000 cubic feet per second.  Actual peak flow was almost 49,000 cubic feet per second, yet there was no track or bridge damage.

The Museum is fortunate the flood reduction measures have worked in favor of preserving track and bridges, yet concerns remain about the ever-increasing frequency and severity of high rainfall events.  And, as with all high water events, all the affected bridges were inspected for damage and were found safe and suitable for service. Though no damage occurred, significant resources were utilized monitoring conditions and preparing for potential impacts.  Hopefully, snow levels will drop and the threat of further high rainfall events will diminish!                          (In the interest of full disclosure, the flood reduction measures have been controversial below Snoqualmie Falls and many think that flood impacts have now in creased in Fall City, Carnation, Duvall, and all points in between.)

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