WALLA WALLA, Wash. – U.S. Army Corps of Engineers levee safety officials and levee managers from Walla Walla County’s Mill Creek Flood Control Zone District met today with Walla Walla County commissioners to inform them of a recent Corps screening-level risk assessment of the Mill Creek Levee System.
The Mill Creek Levee System’s latest annual inspection meets levee-safety criteria required to participate in the Corps’ Levee Safety Program. Inspections evaluate the condition of the structure and maintenance activities. Inspections do not examine what could happen if a flood occurred which exceeded the levees’ capabilities.
The Levee Safety Action Classification (LSAC) is a new method of evaluating levees according to risk. The Corps is conducting risk assessments of all levee systems in its Levee Safety Program nationwide to better understand the risks to communities, and advise and assist levee managers. The initial screening-level risk assessments result in a LSAC assignment for all levees. This way of classifying levees allows for a common comparison or prioritization according to risk. Levee screening and LSAC supports the Corps’ levee safety mission as outlined by the National Levee Safety Act, Title IX, Section 9004 of the Water Resources Development Act of 2007. The program evaluates the risks and potential consequences of levee failure during extreme flow conditions, and what that could mean for those who live and work in leveed areas.
Levee screening supports the following principles:
¬ Life safety is paramount.
¬ Flood risk reduction infrastructure reduces risk; it does not eliminate risk -- know your risk.
¬ Living with flood risk reduction infrastructure is a shared responsibility -- know your role.
¬ Take appropriate actions to reduce your risk.
¬ Flood risk is dynamic and changes over time.
“It is important for people to know how levees are expected to perform during extreme conditions and what the potential consequences of failure would be – in other words, to place levee systems in a risk-informed context,” said Herb Bessey, Corps Levee Safety Program manager at the Walla Walla District. “The Corps is doing screening-level risk assessments on the levees it manages, too.”
Bessey noted that the new LSAC screenings do not replace Levee Safety Program inspections nor constitute a levee system evaluation for the FEMA-managed National Flood Insurance Program. The screening emphasizes the risks associated with each levee system’s performance and consequences of a levee failure --as it presently exists from the perspective of someone within the leveed area.
The Mill Creek Levee System is a 6.8-mile-long combination of levee segments individually maintained by different managing agencies -- the Corps (1 mile) and the county’s Mill Creek Flood Control Zone District (5.8 miles). The levees, in combination with Corps operation of the Mill Creek Project (dam and storage reservoir), reduce the risk of flooding to the cities of College Place and Walla Walla, Washington, and adjacent areas, but does not eliminate all risk. Portions of the channel were constructed in the 1930s, though most of the leveed channel was completed in the early 1940s. The levee channel, including weirs and the concrete channel, was fully rehabilitated in the 1980s. The channel has performed well during several large flood events. However, over time, continued safe operation will require increasing amounts of maintenance. The Corps is helping Walla Walla County identify risks to inform maintenance decisions, allow for better emergency planning, improve public awareness and reduce risk.
Additional information about the Mill Creek Levee System is available on the National Levee Database online http://nld.usace.army.mil.
Defining Risk
To define the level of risk associated with levees, the Corps gathered existing information to evaluate several factors:
Performance - How will the levee perform during extreme flood conditions?
Probability of Event - How likely will that flood-hazard occur?
Consequences - What are the likely effects should levee failure occur during those flood conditions?
These three factors, discussed in detail below, are combined to provide a complete view of the risk associated with each levee system.
Performance - Annual and periodic levee safety inspections assess the physical condition of the structure. These levee inspection reports, design and construction records, a limited engineering assessment and other studies, water-modeling data, and observations provided information about levee system performance. The primary performance risk factor identified for the Mill Creek Levee System is progressive embankment erosion during an extreme flood event. High flows have the potential to undermine and damage weirs, causing excessive erosion at isolated locations along the base of the levee. Weir movement or collapse during an extreme event can damage the armor stone along the base of the levee and this can result in progressive erosion that can lead to failure.
Probability of Event - Historical records of precipitation, snow pack, temperature; water-management data and predictive modeling were analyzed to estimate the likelihood of an extreme flood.
Potential Consequences of Levee Failure - Using records regarding damage and population affected during the Flood of 1931, and analyzing current data about property values and population densities within the leveed area, Corps economists estimated the following potential life-safety, economic and environmental consequences for the Mill Creek Levee System:
|
Left Bank Failure
Population at Risk: 19,000
Number of Structures: 8,500
Estimated Damages: $800 million
|
Right Bank Failure
Population at Risk: 2,700
Number of Structures: 1,100
Estimated Damages: $400 million
|
Total of Both Systems’ Failure
Population at Risk: 21,700
Number of Structures: 9,600
Estimated Damages: $1.2 billion
|
“Most important of all -- there is potential life-loss if a levee failure occurs during extreme flow conditions,” said Bessey. “The reality is that Mother Nature can’t be controlled. Someday there will be a flow event that will exceed the capability of this levee system. That’s why we’re examining risk and raising community awareness about those risks -- our top priority is to avoid loss of life.”
The Corps’ Walla Walla District performed initial screening-level risk assessments on 128 systems in its Levee Safety Program. Mill Creek’s levee system was the first to complete higher-headquarters review and approval of an LSAC ranking. The potential consequences of life-loss weighed heavily in the Mill Creek Levee System’s screening-level risk assessment, said Bessey. Risk assessments were reviewed in comparison with all Corps-evaluated levees. Across the nation there are approximately 2,200 levee systems, or 14,700 miles of levee in the Corps’ Levee Safety Program.
“LSAC is about helping levee sponsors and communities better understand the benefits and risks associated with living and working within a leveed area,” said Bessey.
Risk assessments are not report cards, he added. They are tools that can help levee managers prioritize risk-reducing actions in a time of constrained resources. The Corps plans to use risk assessments levee-safety activities associated with the federally owned and operated portion of the system. The Corps will also encourage the non-federal sponsor of the system, Walla Walla County, to do the same.
Building upon a foundation of shared responsibility in developing and implementing flood risk management solutions for levee systems, the Corps continues working with Walla Walla County to find ways to reduce levee risk. A detailed, more in-depth assessment of levee system is planned to occur in 2018. Also, the county has requested the Corps conduct a Flood Risk Management General Investigation Feasibility Study of Mill Creek. A request for funding has been submitted for consideration in future budgets.
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