By Michael F. Rogers
Dam safety is an aging concern of the profession, making it critical for us to keep this topic at the forefront of conflicting priorities for owners, regulators and the worldwide engineering community. Lessons are being learned around the globe, some tragically, that must be communicated and shared to maintain our highest priorities of protecting and serving the public with some of the most important infrastructure facilities in the world. With even the smallest dam comes great responsibility to prioritize safety, minimize risk and never become complacent, recognizing that all man-made structures have a useful service life that must be understood and respected, even to decommissioning when that service life is complete.
Dealing with El Nino
The current “monster” El Nino weather pattern in the Pacific Ocean has serious implications for dams in North America, and for the state of California in particular. Most of the western U.S. has been experiencing drought conditions over the past four years, with California suffering the most significant impacts. Most of the state has been classified as experiencing “extreme” to “exceptional” drought since 2014. Recent large storms have begun to make a dent in the state’s water deficit, but still reservoirs remain far from filled.
Being confident this weather would bring significant drought relief over the winter of 2015-2016, we did a lot of work in 2015 that involved looking at all aspects of dam safety for several important dams in southern California. It was vital to determine what owners of reservoirs and dams needed to do to get ready for possible sudden increases in water inflow. There was a significant push to make sure the dams were ready for the anticipated extreme operating change in terms of both equipment (such as the radial gates and low level outlets) and instrumentation.
This anticipated weather pattern provides a great opportunity to get a good assessment of the performance of dams and dam safety equipment. Some reservoirs hadn’t experienced water levels this low since they were initially filled, decades earlier, and there was potential that these reservoirs would be completely filled in just one season. With that anticipated large range of reservoir levels, it was important to verify the instrumentation was ready to go (e.g. verifying piezometers were performing correctly and the potential effects of uplift on concrete dams).
|Bear Valley Dam is a 92-foot-high multiple barrel arch dam in southern California. Work to ensure the spillway gates were ready to operate was required in the event of a large rainfall event from El Nino.|
We performed this type of work at Bear Valley Dam. Located on Bear Creek, a tributary of the Santa Ana River, Bear Valley Dam forms the famous Big Bear Lake in the San Bernadino Mountains of southern California. Last fall, the water level in this reservoir was 13 feet down from normal pool, the lowest the reservoir has been in more than a decade. With a large watershed (38 square miles) relative to the available storage in the reservoir, there was potential that water levels could reach the top of the dam with just one or two large storms. Thus we worked with the dam owner, Big Bear Municipal Water District (BBMWD), to make sure their spillway gates were ready and they had an operational plan ready to implement.
To provide context as to why this is important and relevant, a dam failed in the Midwest last year because one of its radial gates didn’t operate when storms came. As a result, the embankment dam got overtopped and failed. This was a situation where the dam would probably have survived the large flooding event if the owner had been able to operate that gate.
At dams where gates have sat a long time without operating, it is vital to make sure they are ready to go before major storm events. Dams like Shasta and Oroville in northern California are examples of situations where large gates have sat idle for five to eight years. The California Department of Water Resources and U.S. Department of Interior’s Bureau of Reclamation spent considerable time this last fall exercising those gates and making sure they were ready to go. As a result, the risks to the projects have been minimized by addressing theoretical potential failure modes (PFM) association with non- or mis-operation of critical spillway gates.
The Big Bear project provides another dam safety lesson in today’s market. Water is a valuable commodity in California, so it can’t be wasted. At Big Bear Lake, the local economy depends on tourist recreation that comes from high lake levels in the summer. Thus BBMWD must always gauge how much the reservoir level will rise when these rain storms occur across their watershed, in an attempt to balance water releases and storage while avoiding overtopping the dam.
Determining the hydrology of watersheds is difficult because you are trying to make educated guesses: With a particular amount of rainfall or snow/snowmelt, how much will run off and how much will be absorbed? What impact will that have on the reservoir level? Typically there is not a lot of instrumentation installed around watersheds and this makes it difficult to tie precipitation events to rising water levels in the reservoir. BBMWD took this opportunity to augment its database of weather data using regional data from the nearby airport and ski parks. When faced with predicted large storms, they need to draw the reservoir down to prevent overtopping the dam. District General Manager Mike Stephenson says, “That’s a delicate process. You don’t want to release any more water than you have to because it is so valuable. But you also have to balance that [keeping water in the reservoir] with keeping the dam safe.” With the right instrumentation in the right location, you can better hone in on best answers for operating the dam and reservoir in these extreme situations.
With regard to “cool” newer things that are going on in the practice of dam safety, the idea of risk assessment and probable maximum floods (PMF) for high-hazard dams is an important aspect. We see more dam owners wanting to use risk assessment concepts that utilize extreme hydrologic and seismic events (i.e., PMFs and maximum credible earthquakes) to prioritize dam safety budgets and focus dam safety improvements on those projects where they can maximize the return on their investments for protection of the dam and residents downstream. A lot of work is being done in the industry to identify good assessments of risk for these projects, especially for owners with multiple projects or a portfolio of dams. Examples of this include Southern California Edison, Reclamation, the U.S. Army Corps of Engineers, and Pacific Gas & Electric for their west coast inventory of high-hazard projects.
Owners of these high-hazard projects will tell you there is always a need for regular updates and maintenance. When looking at dam safety, we are trying to find those projects that have the most immediate need for reducing risk by investing money in a targeted fashion to address the highest risk-effective areas. With regard to what most owners are looking for these days in projects to take on, they want to know how they can reduce overall project and portfolio risks to industry-acceptable levels. I believe that the dams profession – owners, regulators and engineering consultants – is doing a very good job in this respect. A lot of that work is being led by the federal government, primarily the Corps and Reclamation with their risk assessment methodologies. Overall, this work is making the industry more robust with regard to risk-informed decision making, thus leading to safer projects and safer populations who are at risk downstream of dams.
|This upstream view of Bear Valley Dam shows the level of the reservoir, which was 13 feet down from normal pool last fall, the lowest the reservoir has been in more than a decade.|
In my role at the International Commission on Large Dams (ICOLD), I’m seeing a lot of international interest in this approach, particularly the risk assessment using a PFM approach to dam safety to increase the safety of projects all around the world. The PFM approach of looking broadly at project-specific characteristics and situations when prioritizing dam safety investments will likely be the worldwide standard for the current and next generations of dam professionals.
The average age of our more than 84,000 dams in the U.S. is about 55 years. Any person 55 years old (or older) will tell you that at this age, you need to get regular check-ups for your health. Dams are no different. However, owners of a lot of these projects haven’t taken a fresh look at dam safety, including hydrology, since the 1970s or 1980s. We know watersheds change, along with our science of prediction for large precipitation events. The focus in previous updates to PMF studies had been the probable maximum precipitation part, the meteorology part, and not so much the changing watershed part. Updating the watershed characteristics for development, for runoff, and actually putting some measuring gages in the watershed area around the reservoir for validation of assumptions, which can be connected by remote telemetry, allows you to get that data in real time to control centers for assessment and monitoring. That data can be used to predict what’s going to happen in the reservoir, allowing an optimized operation for safety of the dam and downstream residents and property.
Speaking of my international work, both with MWH and ICOLD, I have seen some innovative applications of proper watershed monitoring in China. China Three Gorges Corporation (CTGC) is building projects in China and around the world, and part of the work CTGC is doing on big rivers with cascades of dams and hydro plants is getting good instrumentation installed in the upper reaches of watersheds so they can predict flows and are prepared in real time for these flooding situations. Mountains and watersheds in China are almost completely fed by runoff. They need to know, to a high degree of accuracy, what’s coming down the river along a particular cascade. I’ve seen some of the state-of-the-practice work being done by CTGC in this regard, including work in ICOLD on a new bulletin by the Technical Committee on Integrated Operation of Hydropower Stations and Reservoirs.
The Chinese dam engineers and builders are in a great position, and we’re in a position to learn from them, because of the sheer number and size of dams and hydro projects they are building. Hydro developers from China, including CTGC, are leading developers of large projects in Africa and South America. We are talking about a lot of technology being developed for large projects in China and bringing this to projects around the world.
Dealing with aging structures
A common question from dam owner clients is about the remaining useful service life of their structures and equipment. Many projects in the U.S. were built 50 to 100 years ago and have surpassed their design life. Owners ask, “If I have a concrete or embankment dam, what do I need to do to get another 50 to 100 years out of my project?” For well-maintained projects, not a lot needs to be done. It’s a bit like an older model car or older house, or an aging person. You have to do some routine maintenance (or exercise), and every so often you have to do major overhauls to stay healthy and productive.
A lot of U.S. projects are ready for a major overhaul, or at least a significant check-up or review of high-risk aspects of structures. And by a major overhaul I say for concrete dams, looking at deterioration over time, particularly in areas susceptible to freeze-thaw damage and how much of the concrete remains to carry original design loads. They may need an overhaul to remove deteriorated concrete, bridges or spillway piers. Thinner concrete structures are usually more susceptible to material losses through deterioration. In some cases, projects may be ready for an overhaul to completely replace them or remove them completely from service. Over the next decade or so, owners will need to decide if the cost and benefit of maintaining an aging dam are worth the investment to continue periodic refurbishments, possibly replace all or a portion of the structure, or decommission the project entirely.
One of the big disservices we could do as a profession is try to keep old projects, those that are reaching the end of their design life, in operation through minimal investments and not properly assessing the overall structural condition and risks of keeping those projects in service after their design life. This approach increases risk to the project, environment and downstream population and property as PFMs change and escalate with time. The ultimate risk facing many projects is structural failure, either limited or catastrophic. As time goes on, that risk of failure will slowly increase and must be met with increasing investment and vigilance by owners.
Many of the world’s dams have reached an unacceptable level in terms of risk and, unfortunately, we regularly see news reports of dam failures. As a profession, we should strive for the highest possible safety standards that minimize risk to our worldwide inventory of dams. We as owners, regulators and the engineers who are looked to for proper assessments need to be cognizant of our professional duty to keep the public safe, including replacement or removal of dams at the end of their design service lives.
Mike Rogers, PE, PMP, is vice president, principal civil engineer and senior project manager with MWH. He also is chair of the U.S. Society on Dams’ Technical Committee on Concrete Dams and vice president for the Americas Region of the International Commission on Large Dams.More HR Current Issue Articles
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