King County prepares major plan to improve Green River levees

There’s a whole lot of time, work and money going into a plan to improve the Green River levees that protect the cities of Kent, Auburn, Tukwila and Renton from flooding.

King County Flood Control District officials expect to complete this summer the formation of the Green River System-Wide Improvement Framework (SWIF) designed to improve flood protection as well as save salmon and enhance open space, recreation and public access.

“It’s an important conversation,” said Kent City Council President Dana Ralph after hearing a SWIF presentation on Feb. 3 at a council workshop. “We are very fortunate to have that asset in the middle of the city and we need to make sure we are doing everything to protect it and protect the businesses that are along the river at the same time.”

Jennifer Knauer, SWIF project manager for the county, told the council that the plan focuses on the lower Green River from Auburn (near Highway 18) to Tukwila, a stretch of about 21 miles that includes about a dozen levees that protect more than 100,000 jobs and $7.3 billion worth of structures and contents.

The U.S. Army Corps of Engineers recently approved the SWIF approach for levees across the nation. County officials will submit the Green River plan this summer to the Corps for approval. It is one of about three dozen SWIF projects nationwide. County staff began work on the plan more than two years ago.

The SWIF is scheduled to go before the King County Flood Control District executive committee on Feb. 17 for approval and then to the full flood district board in March. The plan is expected to be a 200-page document and the Corps could send it back to the county if it doesn’t meet Corps requirements.

“We have been told by the Corps that the Green River SWIF is the most ambitious in the nation and it’s ambitious because of its complexity,” Knauer said. “There are no other SWIFs that are trying to tackle the complexity of land use, ecological, socio-economic sensitive issues within a major river and its floodplain through the SWIF process. So all eyes are on this project and its outcome.”

The formation of the Green River plan is funded by the King County Flood Control District and a $300,000 grant from the state of Washington through Puget Sound Partnership.

The flood district provides about $10 million per year for Green River projects through a countywide property tax of 10 cents per $1,000 assessed valuation that brings in about $50 million per year for river projects across King County, including the Snoqualmie, Cedar and White rivers.

“We know our capital program for SWIF will likely exceed $300 million,” Knauer said about projects to rebuild Green River levees, install flood walls or build levee setbacks.

Several levee projects are already completed or underway in Kent, Auburn and Tukwila. State and federal funds have helped pay for those projects. But more work is needed to bring levees up to the SWIF goal of a 500-year flood protection. Levees at that standard could handle a river flow of 18,000 cubic feet per second (cfs) and include 3 feet of freeboard or height above the water.

Hanson Dam impact

Flooding occurred often in the Green River Valley from the 1890s to 1960 before Congress funded the building of the Howard Hanson Dam to protect mainly agricultural lands. Now many homes and businesses exist along the river as well.

“We haven’t experienced a flood of this magnitude (500 year) since the dam was built (in 1961) but we do know there have been very large magnitude floods in Western Washington and all of the indicators are with shifting climate trends that we need to be prepared to protect our vital economic assets should there be a very significant event in the future,” Knauer said about the reasons for the higher protection.

The largest Green River flows since the dam began operating is 12,400 cfs, said Lorin Reinelt, managing engineer for the county Water and Land Resources Division. He said three events since 1950 included flows of larger than 12,000 cfs.

But large 500-year flood events occurred in 2007 in the Chehalis River and in 1996 in the Nisqually River, Reinelt said about the largest river flows in the last 20 years in Western Washington.

Knauer said the Hanson Dam was initially built to provide a 500-year level of protection but the Army Corps reported it’s only providing a 140-year level of protection. She said people have asked her why not improve the Hanson Dam rather than spend money on the levees.

A Corps study indicated that the complexity and cost to improve the dam makes that option very challenging and uncertain. The dam is an earthen embankment dam. Crews repaired the dam a few years ago for about $40 million with additional drains and a drain tunnel after 2009 flooding damaged an abutment to the dam, but the Corps estimated costs at nearly $500 million to install a concrete cutoff curtain as a more permanent fixture.

Favorable response

Ralph, the council president, likes the direction of the improvement plan.

“The goal of the SWIF process and the outcomes we are looking for is dealing with the Green River in a holistic way,” Ralph said. “There are multiple things that are happening there including flood protection, which is the primary focus of the systemwide improvement, and how do we protect the assets that are in place along the river and at the same time acknowledge that there is habitat and endangered species in the river so how are we working toward making sure the temperatures in the river are sustainable and that recreational assets along the river are available to our residents so they can access it.”

Councilwoman Deborah Ranniger, who served on a Water Resource Inventory Areas (WRIA) committee that helps to improve salmon habitat for the Green River, said she likes how the SWIF looks at more issues than simply flood protection.

“It was clear to me the folks involved in the resource have worked with WRIA 9 and the Muckleshoot tribe and other folks that are very interested and concerned about habitat protection and environmental protection,” Ranniger said after the county presentation. “I think the projects being planned make a lot of sense. I noticed one of the top priorities was shade protection and that historically has been a big challenge to make happen along the Green River. So we are making slow but steady progress.”