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Sep 29 2011

Hurricane Irene and Schoharie Creek

The northeast U.S. has gotten drenched this year.  The two most recent events have been Hurricane Irene and Tropical Storm Lee.  The Schoharie Creek Valley in central New York saw record flows from Irene, and the communities along the valley were devastated.

In the Schoharie valley, the creek flooded with such force that tree branches stuck to power lines, cornstalks lay broken in puddled fields and trees simply snapped.

It rushed over the Gilboa Dam , which officials said is holding, before washing out roads. People fled their homes from water they said rose like never before.

The Greene County community of Prattsville was heavily damaged: Mobile homes were washed from their foundations, and one blocked the region’s arterial road.

Read more: “And the rivers roared” (Times Union)

The above news link includes several photos of the flooding, and below are a few videos that show some of the damage.

 

 

This valley is no stranger to flooding having experienced several large floods in the last 30 years, and could possibly continue to see increased flooding.  There are not official numbers out yet about the statistical level of this event.  Several of the gages along Schoharie Creek were also destroyed or reached levels beyond their limits, so the peak flow is yet to be determined.  Based on a quick look at the gage near Burtonsville, and the FEMA FIS report,  it looks like the flood was in the ballpark of the 400 year flood event.

Historic Bridge Failure

One note of particular interest is the history of this creek related to bridge scour.  The I-90 bridge over Schoharie Creek collapsed on the morning of April 5, 1987 during spring flooding (complete story).  The cause of the collapse was the scour around one of the main piers.  This bridge collapse was as substantial in impact to that of the I-35W bridge failure in that it set in motion some changes in how bridges were designed and inspected related to hydraulic effects.  The resulting research lead to the publication of Federal Highway Administration hydraulic engineering manuals; HEC-18, HEC-20, and HEC-23.

The flow during the 1987 event peaked closed to 65,000 cfs.  The peak flow from Irene will likely be close to 120,000 cfs.  Below is a video of a fly over of Schoharie Creek during the Irene flooding.  The new I-90 bridge can be seen at 0:55 in the video.  Note the large amount of overbank flow coming in from the top.  This flow pattern was a large part of the scour that caused the collapse in 1984.  This section of roadway was closed during the Irene flooding for safety.

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