Wet winter could be Howard Hanson Dam's first big test

Glenn Farley reports.

SEATTLE – With more than 10 inches of rain, October not only set a rain record for the month, but was also recorded as the 13th wettest of any month at Sea-Tac Airport.

November has started out fairly dry, but November is historically the wettest month of the year followed by December.

If wet weather continues, it could mean trouble for the Howard Hanson Dam on the Green River.

In January 2009, a storm combined with a warm up and melting snow, sent the level of water behind the Howard Hanson Dam to its highest level ever. It was six feet above the previous 1996 record.  It was also the storm that created weaknesses in the earthen dam that sent community leaders in the Green River Valley scrambling to raise levees and worried residents for years while repairs were made.

"There's always a chance you'll discover something, and that's what happened in 2009," said Dan Johnson, now the Operations Project Manager for both the Howard Hanson and Mud Mountain Dams in the Cascade Mountains. Both dams were built for flood control on different rivers. 

Repairs were made. First there was a curtain of grout. A cement-like substance was pumped into what was known as the "right abutment," the remains of an ancient landslide forming the right side of the dam. 

What Army Corps engineers found was that under the pressure of the high pool, the abutment was being compromised as fine materials like sands, powdered rock, and soil were being washed out.  Noticeable depressions were forming above. 

One of the last techniques used to protect that part of the dam was the installation of pumps because the grout curtain can't do it all.

"As the water level rises in the dam and the right abutment, we can pump that water level down and keep the higher levels of the dam dry, so we don't have any risks," said Johnson.

In the valley below, the extra flood protection has been taken down, and it's business as usual in the neighborhoods and business parks that now dominate a valley that saw repeated and devastating flood events in the decades before the December 1961 opening of Howard Hanson Dam.  But if the weather provides a test like it did in 2009, the Army Corps of Engineers says the dam is ready for that and a lot more. 

"We're very ready for November and December," said Johnson.  "Even January and February and until March you have a chance of a big storm."  

Copyright 2016 KING


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