WEST RICHLAND, Wash. – U.S. Army Corps of Engineers levee safety officials and levee managers from Benton County Diking District No. 1 met Wednesday with West Richland and Benton County officials to inform them of a recent Corps screening-level risk assessment of the West Richland Levee on the Yakima River.
The West Richland Levee’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. Levee risk screenings examine what could happen if a flood occurred which exceeded the levees’ capabilities.
The Levee Safety Action Classification (LSAC) program 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, such as river conditions that exceed the height of the levee, 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 West Richland Levee was constructed in 1962 and extends along the inside of a meander curve on the Yakima River, about 7 miles upstream of the confluence with the Columbia River. The levee is about 5,800 feet long, extending downstream from the Van Giesen Street Bridge. Levee height varies up to 22 feet. The leveed area includes residential development in West Richland, Washington.
The levee is relatively tall and is constructed of local materials. It lacks a low-permeability fill zone or seepage cutoff wall that would reduce seepage. The drainage system along the landside toe discharges into the ditch that feeds the pumping plant. It collects only surface runoff and will do little to reduce the risk of land-side sand boils. The Corps’ risk-assessment team believes an extreme event would probably clog or overwhelm the drain. Past performance and a reported sand boil confirm that seepage through the levee prism and through foundation soils are risks. During an extreme flooding event, land-side piping and boils should be expected along most of the property immediately behind the levee. The risk of levee failure during an extreme flooding event because of seepage and piping in foundation soils is considered high.
The Corps is helping West Richland Levee System sponsors identify risks to inform maintenance decisions, allow for better emergency planning, improve public awareness and reduce risk. Additional information about the West Richland Levee System is available on the National Levee Database online http://nld.usace.army.mil.
To define the level of risk associated with levees, the Corps gathered existing information to evaluate several factors:
Probability of Event - How likely will that flood-hazard occur?
Performance - How will the levee perform during extreme flood conditions?
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.
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.
Performance - Annual and periodic levee safety inspections assess the physical condition of the structure in accordance with its design criteria. LSAC risk assessments rely on the inspection reports, design and construction records, river stage and flow velocity estimates, and observations of past performance. The assessment process identifies different ways the levee could fail during a flood, and assigns each a likelihood or potential. The mechanism of failure that ranks as most likely is identified as the “risk-driver failure mode.” Based on screening level assessment, the risk drivers for the West Richland Levee system is embankment and foundation seepage and piping. Embankment stability and embankment erosion during an extreme flood event are also potential mechanisms of failure.
Potential Consequences of Levee Failure -
Corps economists analyzed current data about property values and population densities within the leveed area, and estimated the following potential life-safety, economic and environmental consequences for the West Richland
Levee Failure Potential Consequences
Population at Risk: 118
Number of Structures: 40
Estimated Damages: $9.6 million
“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. West Richland’s levee system was the second to complete higher-headquarters review and approval of an LSAC ranking. The potential consequences of life-loss
was a factor in the West Richland Levee’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.
“The 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. LSAC risk assessments do not affect Levee Safety Program inspection results -- a Levee Safety Program periodic inspection evaluates the condition of a levee in accordance with its design criteria, while LSAC risk screenings examine what would likely occur during a flood event that exceeded the levee’s capability.
Building on a foundation of shared responsibility in developing and implementing flood risk management solutions for levee systems, the Corps continues working with the Benton County Diking District No. 1 to find ways to reduce flood risk.