It’s hard to imagine a busier region than Washington’s Green River Valley. The industrialized estuary south of downtown Seattle straddles the Green River from its mouth, where it is known as the Duwamish Waterway and empties into Elliott Bay, and meanders miles upstream to embrace the towns of Auburn, Kent, Renton, and Tukwila. The valley is home not only to 170,000 residents, but also to the Seattle-Tacoma International Airport and sprawling corporate complexes and distribution centers belonging to companies such as Boeing, Microsoft, Starbucks, and REI.

The area has been protected from flood damage for so long now – nearly half a century – that most of its residents cannot remember the days of recurrent floods – including the December 1959 cataclysm that submerged buildings up to their second floors and washed irreplaceable topsoil into Puget Sound.

By then, the effort to tame the Green River was already under way about 30 miles east of Auburn, at Eagle Gorge, where the U.S. Army Corps of Engineers’ (USACE) Seattle District was constructing what would become the Howard A. Hanson Dam, a flood control and water storage project that became operational Christmas Day, 1961. With major flooding under control, the Green River Valley became more attractive to industry, and by the fall of 1996, the dam had prevented flood damages amounting to more than $694 million. Today, officials estimate that the property damage alone from a severe flood could total $3.77 billion in the Green River Valley.

“Flood Control” Is Not Forever
The 21st century has introduced an era when the USACE has been changing its approach to managing flood risk. The Corps is responsible for the safety of a portfolio of dams that are increasingly either approaching or already past their intended service lives. Nobody in the Corps uses the term “flood control” anymore – because, really, who can control a flood – and nobody pretends that a dam, especially one at 50 years old, permanently erases the flood risk for a given watershed.

In January 2009, the residents of the Green River Valley were awakened to the 21st century reality of flood risk when record-high rains, spilling into the reservoir at rates up to 30,500 cubic feet per second, raised the Howard Hanson Dam’s flood pool storage to a record 1,189 feet above sea level, just under the dam’s authorized maximum of 1,206. Because the lower Green River Valley was already experiencing high tributary inflows, the Corps cut the dam’s outflow to zero in order to prevent flooding.

After holding a record level of water behind the dam, the Corps became concerned about several discoveries: two depressions in the dam’s right abutment (the valley wall against which the dam is constructed); increased water levels in monitoring wells; and the increased appearance of sediment-laden waters in the drainage tunnel beneath the abutment. Subsequent USACE studies of the depressions have discovered the likely causes of the seepage and erosion, but also compelled the organization to place temporary restrictions on the pool’s elevation. While the dam itself is not in danger of failing, the inability to use its full storage capacity has raised the flood risk for those living and working in the Green River Valley.

The Corps responded immediately both to the engineering problem posed by the abutment and to the elevated flood risk to the people within the valley. In the summer of 2009, it awarded contracts to install a 475-feet-long, 150-feet-deep seepage barrier – known as a grout curtain – within the abutment’s inner flank, and for additional drainage tunnels to safely control and manage seepage entering the right abutment from the reservoir. Funded by the American Recovery and Reinvestment Act, this first round of interim work, totaling about $15 million, was completed in February 2010, and lowered the risk of catastrophic flooding from a 1-in-3 chance to 1-in-32 – still an unacceptable level of risk, but a dramatic improvement.

Meanwhile, USACE launched an aggressive public outreach and information campaign that included more than 100 public meetings, as well as conducting tabletop exercises and distributing inundation maps for several different flooding scenarios, as well as the distribution of risk-reduction equipment such as ring dikes, sandbags, Supersaks, and Hesco barriers – large, square baskets filled with material and stacked high. Boeing alone, over the next several months and at its own expense, constructed about $25 million worth of ring barriers – in some spots, 16 to 20 feet high – around its two key facilities in Kent.

A New Level of Public Involvement
According to Lt. Col. James Rollins, deputy commander of the Corps’ Seattle District, the tabletop exercises and inundation maps were a mere starting point for engaging communities in the discussions and decision-making regarding the risks of – and appropriate responses to – flooding in the Green River Valley.

Over the next several months, the U.S. Department of Homeland Security, USACE, the Federal Emergency Management Agency (FEMA), and public and private stakeholders from the Green River Valley in Washington have been collaborating to conduct the 2010 Dams Sector Exercise Series – Green River Valley (DSES-10) to address regional disaster resilience issues.

The DSES-10 effort focuses on the analysis of short- and long-term regional impacts resulting from a flooding scenario affecting the King County communities of Auburn, Kent, Renton, and Tukwila.

“The primary goals of this collaborative effort are to achieve a greater understanding of the potential impacts associated with significant flooding events along the Green River Valley, identify critical infrastructure interdependencies that influence local and regional disruptions, and assist public and private stakeholders in improving recovery strategies and business continuity plans,” said Yazmin Seda-Sanabria, senior program manager for USACE’s Critical Infrastructure Protection & Resilience (CIPR) Program.

She said the DSES effort enhances regional resilience and promotes robust partnerships at the local and regional levels.

Each year, the DSES program collaborates with public and private stakeholders within a region to help identify, analyze, assess, and enhance regional preparedness and disaster resilience using a series of multi-jurisdictional discussion-based activities (workshops, seminars, tabletops, etc). For any given region, the DSES collaborative process is based on a particular scenario that serves as the triggering event to analyze impacts, disruptions, critical interdependencies, and stakeholder roles and responsibilities. Implemented through a series of discussion-based activities (meetings, seminars, workshops, etc), complemented by data-gathering, information assessment, and analysis efforts. A systematic process is followed to consolidate findings, and support a framework to inform future resource requirements and investment justifications aligned with federal grant programs.

“This year, the DSES efforts are focused on the Green River Valley, taking advantage of the multiple activities, capabilities, and proactive engagement demonstrated by Green River Valley stakeholders regarding flood-risk planning and management as a result of the operational conditions at Howard Hanson Dam,” said Seda-Sanabria.

The DSES-10 Initial Planning Workshop was conducted April 28, 2010, in Seattle. More than 150 representatives from Green River Valley local government, private entities, non-profit organizations, and Washington and federal government agencies participated at this event. The workshop provided an effective forum to discuss the multiple aspects of the DSES-10 effort: the Regional Baseline Assessment, the Regional Consequence Assessment, and a Regional Resilience Strategy.

The second major event, the DSES-10 Regional Baseline Assessment Workshop, was conducted June 30 in Seattle. More than 60 representatives from Green River Valley public and private stakeholders participated at this event. The workshop served as a working session to review and evaluate the ongoing regional baseline assessment data-gathering activities focused on identifying and characterizing infrastructure interdependencies, supply chain dependencies, preparedness, and public- and private-sector business continuity capabilities that may serve as effective drivers to enhance regional resilience. The findings from this regional analysis are being summarized as part of a Regional Baseline Assessment Report that was completed in September 2010.

“The granularity of the DSES,” said Rollins, “is far greater than what you would be able to do in a tabletop exercise. A tabletop, for all practical purposes, is meant to synchronize community response, synchronize the message, and it’s meant to be the first step in preparing for any potential crisis. Now the DSES really drives it into second- and third-order effects. It goes all the way through to recovery. So DSES-10 is a more Herculean task than some of the preparations that we’ve done to date.”

A series of regional stakeholders’ interviews were conducted throughout July as part of a voluntary process aimed at eliciting additional information that could be of assistance in filling knowledge gaps that could be relevant to the dependencies’ and interdependencies’ analysis in the region. According to Rollins, this report should offer a big-picture look at what’s at risk in the Green River Valley: “Basically, DSES-10 will identify and consolidate regional baseline information as it relates to economic structure, industrial development, [and] the economic and social landscape.”

“The most recent DSES-10 event [Regional Consequence Assessment Workshop] took place on Oct. 21, 2010, in Seattle. This workshop focused on the regional consequence assessment of direct and indirect consequences, to include the evaluation of short-term and long-term impacts and critical infrastructure dependencies and interdependencies associated with the selected regional disruption flooding scenario,” said Seda-Sanabria.

Findings from this area will be consolidated into a Regional Consequence Assessment Report. This information will support the development of a Regional Resilience Strategy as the final outcome of DSES-10, which is intended to assist public and private stakeholders in identifying and prioritizing challenges and potential solutions to enhance regional resilience.

“This is where it really gets tricky,” said Rollins, “because then you’ve got a facilitator trying to get all these separate entities to take on improvement activities so that they can become self-actualized, shall I say, to solving some of their own problems, as well as getting people that serve them to get ready. So FEMA is going to have some things to do. The Corps is going to have some things to do as a result of this [strategy], just as the [cities] of Kent and Renton and Tukwila and related entities. We’ve all got to come away with a list of things that we can do now in order to improve our regional resilience.”

 

Understanding Residual Risk
On July 29, President Barack Obama signed into law an emergency appropriation of $44 million to fund further interim repairs to the Howard Hanson Dam: extensions and enhancements to the existing drainage system, along with interim improvements to strengthen and stabilize the dam’s spillway. Given the improved seepage rate already introduced by the new grout curtain, it’s possible that this next round of repairs will enable the dam’s operators to fill the flood storage pool to capacity without increasing the flood risk. A report addressing the question of whether this next interim fix – due to be completed in early 2011 – will be a “permanent” or long-term solution is scheduled for completion in November 2010.

“Permanent” is a word Rollins discourages; there is no such term in the Corps’ risk-based approach to dam safety and flood protection. One of the successes of the DSES, he says, is that it helps communities understand the concept of residual risk. “Dams like this do such a good job of raising the odds of flooding that it takes it out of people’s consciousness,” he said. “I mean, if it wasn’t for some of these signs of strain that we had in 2009, people would have never realized: ‘Oh, geez there is a dam up on the Green River.’ They just kind of took it for granted that the Green never floods. … So now that we’ve built their level of awareness up, the next logical step is to say: ‘OK, folks, this structure will take care of the up to 1-in-146 event.’ But if we are unlucky, like Nashville was this year, and get a 1-in-500 event, you’re going to flood, and there’s nothing that anybody can do about it because you’ve simply exceeded the design capacity of the dam. With weather events the way they are, that potentiality does exist. So does the DSES only look at a dam failure scenario? It shouldn’t. It should look at flooding as a consequence of a floodplain. And therefore there is some chance that that could happen with the dam acting at its full operational capability.”

Given the recent structural improvements to the Howard Hanson Dam, chances are that the people of the Green River Valley will not have to mobilize their resilience strategy anytime in the near future – but the awareness and capabilities the DSES has awakened in the communities of the area, Rollins said, demonstrate that it ought to be enacted in more floodplains around the country, especially those with dam safety issues.

“I think [the DSES] is absolutely a vital part of preparedness,” he said. “When we start planning, we think about how to get the sandbags, how to get them filled, how to get people food and water, but we don’t think about second- and third-order effects. ‘What are we going to do when the risk is highest in order to maintain business continuity?’ ‘What are we going to do after the floodwaters recede and I’ve got this big mucky mess in my front yard?’ ‘What if my trucks can’t get into my warehouse, or my whole lower level of my residence is now growing black mold?’ It takes people past that initial response and into the long-term planning, asking those questions of where to turn for resources in order to recover from this,” Rollins said.

This article first appeared in U.S. Army Corps of Engineers: Building Strong, 2010-2011 Edition.