Dworshak Dam recently given improved safety rating
WALLA WALLA, Wash. – The Walla Walla District of the U.S. Army Corps of Engineers is continuing its ongoing effort to improve safety at its dams and dam-related levees throughout the Snake River basin, at McNary Lock and Dam/Lake Wallula on the Columbia River, and at Mill Creek in the Walla Walla River Basin.
The objective of the Corps’ Dam Safety Program is to maintain public safety, make Corps dams safer and minimize risks. What’s new in recent years is how the Corps assesses its dams and incorporates risk management concepts into dam safety management, routine activities and programming decisions. Hurricane Katrina in 2005 and the resulting flood damage in the New Orleans area gave rise to a new era of public safety requirements concerning dams and levees. This changed how the Corps and other federal agencies do business. It changed how the nation invests in infrastructure challenges across the country.
“Public Safety will always be our highest priority,” says Lt. Col. David Caldwell, Walla Walla District Commander. “While we can’t completely eliminate risk, we are able to understand it and mitigate for that risk. That’s where we’re currently placing our efforts."
Since 2007, the Corps has used a risk-informed process to prioritize addressing dam safety deficiencies on a nationwide basis. All Corps dams and “appurtenant” or dam-related levees were initially screened and assessed for dam safety issues and deficiencies and their potential risk to the public. After screening and assessment, the Corps initially categorized each dam or dam-related levee into one of five Dam Safety Action Classes (DSAC) based on individual dam safety risk, with DSAC-I “Urgent and Compelling” being the highest-risk structure and DSAC-V “Normal” being the lowest-risk structure. The safety classifications are assigned by a Corps headquarters “Senior Oversight Group.” These initial screenings were based on a brief records-only review so the Corps could quickly assess the scope of the challenges nation-wide and start prioritizing studies and repairs.
Additionally, when these initial safety classifications were assigned to each dam, the Walla Walla District developed and began implementing “interim risk reduction measures” (IRRMs) at each of the District’s dams and appurtenant levees to reduce risk of damage to property and help prevent loss of life. IRRMs include both short-term and ongoing efforts to reduce public risk while long-term solutions are pursued. They can be structural or non-structural.
As a result of Dam Safety Action Classification program efforts in recent years, the Corps has performed in-depth studies to obtain a better understanding of risks and conditions at its dams. In some cases, new observations were made of symptoms of potentially serious problems. In other cases, the Corps learned original design and construction methods do not meet current safety standards. DSAC ratings are reviewed during routine periodic assessments and during special studies, during which dams are more closely reviewed and assessed.
In May 2012, Dworshak Dam and Reservoir near Orofino, Idaho, received a safer and improved DSAC-III “High Priority” safety rating from Corps headquarters after a rigorous study and review by national safety experts. Dworshak was initially classified in October 2007 as DSAC-II “Urgent” because of engineering unknowns related to structural stability and foundation seepage of concrete gravity sections.
The improved “High Priority” rating for Dworshak is based on completion of a detailed “Phase I Issue Evaluation Study,” which took a closer look at the dam. The improved safety classification is based on
1) a confirmation of the robustness of dam design and historical performance of the structure, and
2) that potentially significant failure modes are a result of rare seismic events.
Dam safety action classifications are based on a calculation of each project’s probability of failure and the consequences of failure. Probability of dam failure is based on an analysis of potential deficiencies and potential failure modes of project components. Potential consequences of failure are impacts to downstream life, property, resources (such as hydropower generation), economies and environment. Improved understanding of risks enable the Corps to better address them through structural and procedural risk reduction measures. The dam safety classifications also assist the U.S. Congress and the Corps in prioritizing funding for dam safety improvements.
Other Walla Walla District dams and dam-related levees and their current dam safety action ratings descriptions are:
- Ice Harbor Lock and Dam, Burbank, Wash. – ”High Priority”
- Lower Monumental Lock and Dam, Kahlotus, Wash. – ”High Priority”
- Little Goose Lock and Dam, Starbuck, Wash. – ”High Priority”
- Lower Granite Lock and Dam, Pomeroy, Wash. – ”High Priority”
- Lucky Peak Dam and Lake, Boise, Idaho – ”High Priority”
- McNary Lock and Dam, Umatilla, Ore. – ”High Priority”
- McNary Levee System in Kennewick-Pasco-Richland, Wash. – ”High Priority”
- Mill Creek Diversion Dam, Walla Walla, Wash. – ”Urgent”
- Mill Creek Storage Dam, Walla Walla, Wash. – ”High Priority”
The Walla Walla District held a series of open houses, public meetings, and meetings during the past four years with Washington, Oregon and Idaho officials in the vicinity of each of its dams to keep the public and officials informed. Dam safety efforts, classifications and potential flood impacts have been explained at these events.
“We’re actively pursuing risk mitigation measures to enhance public safety,” says Caldwell. “We’ll continue to communicate with the public and officials about our dam safety efforts as we implement risk reduction measures. It’s an ongoing process.”
Details about each Walla Walla District dam or dam-related levee system are available at the District website at http://www.nww.usace.army.mil/.
Release no. 12-60