By Jimmy Waddle, P.E., M.SAME, and Linda Adcock, M.SAME
While many think of “dams” as the large federal dams mostly built between 1930 and 1970, the vast majority across the United States are small, privately owned structures that may have changed ownership multiple times. When these dams were built, no large populations existed downstream, but over time residential and commercial areas have grown and developed, pushing dams into the high-hazard category (meaning loss of human life is likely should one fail).
As of 2017, of the 90,580 dams in the United States, nearly 15,500 of them were rated as high-hazard-potential dams by the American Society of Civil Engineers. For comparison, of these 90,580 dams, two of the principal federal dam operators, the U.S. Army Corps of Engineers and the Bureau of Reclamation, operate and oversee approximately 700 and 340 dams, respectively. The eight federal agencies with authority for dams in the country are responsible for only five percent of the dams nationwide. Federal stakeholders may operate highly visible landmarks like Hoover Dam, Grand Coulee Dam, and Bonneville Dam, but they are not the prominent party responsible for the majority of risk along America’s waterways.
Dam operators today must contend with an increase in heavy rainfall events, competing operational interests, and limited funding. State dam safety programs are working to provide dam owners with information and resources for proper inspections, investigations, and analysis; however, more focus is needed to reverse this trend.
Additionally, struggles with water rights and water supply are going to become more the norm in highly populated areas as the number of people continues to grow, but the amount of available water does not.
CHANGES TO RISK LANDSCAPE
The average design life of a dam is typically between 50 and 70 years—after 50 years, the maintenance and rehabilitation costs to keep the structure acceptable increase significantly. In the United States, the average age of a dam is 56 years. While complete dam failure is the most catastrophic risk presented by an aging dam, it is not the only one. Flood flow, seepage, and piping all pose severe risks if not properly monitored and corrected, especially as extreme weather events increase in frequency.
Cities manage floodplain development based on historical data, which can potentially leave operators unprepared for statistical outliers. In May 2010, intense rainfall over the Nashville area caught people by surprise. Over a 36-hour period, 13.57-in of rain fell on the Music City. Over 10,000 people were displaced from their homes and about 11,000 properties were damaged or destroyed in the flood. Private property damage amounted to more than $2 billion. This is not an isolated example.
While complete dam failure is the most catastrophic risk presented by an aging dam, it is not the only one. Flood ﬂow, seepage, and piping all pose severe risks if not properly monitored and corrected, especially as extreme weather events increase in frequency.
Extreme rainfall events can also illuminate problems with dam spillways. In 2017, the main spillway of the tallest U.S. dam, the 770-ft Oroville Dam in California, began to erode after a season of heavy rainfall. A secondary, emergency spillway was used for the first time in the dam’s 50-plus years to compensate. But this second spillway also began to erode, so more than 180,000 people were evacuated from downstream while engineers worked to lower the lake and keep both spillways from causing a catastrophic failure.
Beside what’s termed hazard “creep” (development near and downstream of dams), other dilemmas include seepage and piping of earthen dams.
Seepage is one of the most common concerns for dam owners. As water elevations rise and more pressure is applied to the structure, hydraulic forces can worsen seepage through foundations or existing material underneath a dam. When engineers built dams and levees decades ago, they did not have the engineering and construction technology and methods available now. They did not fully understand the causes and long-term effects of water pressure along with the criticality of the interface between existing material/foundation and the new embankment or concrete structure.
Another complexity facing dam operators is the issue of competing interests. Many dams across the United States originally were built for multiple purposes, and that often has led to differing operational and long-term goals. For example, Center Hill Dam in Middle Tennessee was originally built for flood damage reduction and hydropower. Over time, however, Center Hill Lake has grown a large recreational component and water supply role, which many users assume is its primary purpose.
The competing interests of dams must have a give and take, such as when flooding necessitates holding water in reservoirs to prevent more flooding downstream, or when water must be passed in order to create storage for upcoming rainfall. Protection of human life has to take precedence over recreational and other uses.
Material moving through or underneath a dam or foundation, called “piping,” can also create a void that could cause dam failure, such as in 1976 at the Teton Dam in Idaho. In that instance, disaster was almost immediate. Snow melt from the mountains filled the dam faster than expected, and issues with the geology and preparation of the dam abutment caused a seepage and piping failure. The 305-ft earthen dam, which was built for water supply, hydropower, and irrigation, failed on the first filling, killing 11 people and 13,000 cattle, and destroying thousands of homes and businesses. It was never rebuilt and the estimated damage ranged from several hundred million to over a billion dollars.
To control seepage and prevent piping, adequate design and long-term monitoring are key. Instrumentation is often necessary to determine when seepage is becoming harmful and the dam owner should take action. Seepage and piping are insidious because this internal erosion can be happening deep in the foundation of a dam without warning, and earthen embankment dams have failed very quickly because of seepage and piping. Relief wells, drainage systems, and other means can assist in controlling seepage before it causes failure.
INCREASING THE SAFETY PROFILE
A critical requirement to ensure greater dam safety in the years to come is funding. Federal and state governments need to make not only bridges and road infrastructure a priority target for funding, but dams and levees as well. Funding needs to be appropriated to maintain all current dam and levee safety programs and to operate, maintain, and rehabilitate the dams and levees that have outlived their design life.
Funding needs to be appropriated to maintain all current dam and levee safety programs and to operate, maintain, and rehabilitate the dams and levees that have outlived their design life.
Unfortunately, it often takes a disaster to bring attention, such as when the Kelly Barnes Dam in Georgia failed and sent flood waters over the 186-ft Toccoa Falls, killing 39 people. Jimmy Carter was Georgia governor at the time. After he was elected president, the United States established Federal Guidelines for Dam Safety in 1979, and Carter signed a Presidential Executive Order that same year that created the Federal Emergency Management Agency and put it in charge of dam safety.
It does not need to happen that way, though. Positive change does not have to come about following tragedy. Dam owners should have the necessary resources and funding available. With additional support from state legislatures and prioritized funding sources, owners will be able to address ways to reduce the hazard presented by aging dams and assure that these critical pieces of infrastructure continue to deliver reliable water and utilities to the populations they serve.
The public needs to understand that these structures could fail. They could become overstressed. They could overtop. Dams and levees mitigate risk—but they do not eliminate it.
Jimmy Waddle, P.E., M.SAME, is Vice President, Business Development and Senior Dam Safety Engineer, and Linda Adcock, M.SAME, is Project Manager, K.S. Ware and Associates. They can be reached at firstname.lastname@example.org; and email@example.com.
[This article first published in the July-Aug 2020 issue of The Military Engineer.]