Power dams, climate change, and extreme events
Hoping for the best, planning for the worst
Most of the extreme heat we’re experiencing is a matter of natural variation. Records that go back nearly two centuries and measurements from things like tree rings and isotope variation in ice cores confirm that there’s a strong, cyclic component to temperature. It’s a safe bet that, right now, any global warming effects are really secondary to these normal cycles of heating and cooling. It’ll probably continue that way for the next 20 years or so, at least. Still, the overall trend in temperature is gradually, but decidedly, up. And, at least on average, higher than any natural fluctuation could account for.
So, what do warmer temperatures mean for rainfall and water levels? We can’t say for sure. What is certain is that our previous records are no longer reliable predictors. So if the variation, either higher or lower, is much more than we anticipate, how will it impact the future of our hydroelectric dams, massive structures that depend on a specific range of water flow to produce power, safely and reliably?
In war and engineering, the cardinal rule is to protect your flank. With dams, that means adequately shielding them from extreme climate events, like floods, hurricanes, landslides, and glacial outbursts.
Dam life and the concept of risk
The Proserpina Dam in Merida, Spain, was built in the first or second century AD. It’s still used by local farmers, pooling and directing water to irrigate their crops. These days, with dams designed to generate electricity, planning for a one-hundred-year lifespan is more the norm.
With infrastructure, lifespan estimates are heavily based on risk factors and their mitigation. We engineers and the financiers and insurers of big projects like power dams are very familiar with these concepts. Risk is all about uncertainty—probability and consequence. An event that’s both highly probable and likely to have severe consequences is the very definition of high risk.
Risk management is all about the prudent deployment of capital. For big, expensive infrastructure projects like power dams, we can’t design or plan for something that may or may not happen a hundred years from now. The possibilities are just too many and too varied to predict with any kind of accuracy. The principle of disproportionality—and common sense—tells us that if we spend, say, ten times more to address the possibility of a worst-case scenario than it would cost to repair any damage that the uncertain event might actually cause, we’re playing a losing game. A much better strategy is to “prethink” what may happen, and design some kind of retrofit plan into the structure. That way, it can be added or adjusted to mitigate damage or harmful effects, if and when the need arises.
One person’s risk is another’s opportunity
In the next hundred years, glacial melt is going to be both a risk and an opportunity for power dams. In some cases, the spillways we design today or have designed in the past may not be able to manage the water flows if glacial melt continues and increases. So we need to monetize the risk of that happening and decide what to do. Designing a dam to be retrofitted easily with a labyrinth weir, for example, might be the economical solution that’s needed.
On the other hand, more water carries the potential for more generation. So, with varying turbine capacities or even the appropriate turbine selection, it might be possible to keep operating if there’s a significant increase in the amount of water feeding it.
In Iceland, for instance, my colleagues and I documented a study for a hydroelectric developer who was considering a glacier-fed facility, a project that was the subject of a presentation we made recently at the H. G. Acres seminar. In this case, it was recommended the plant be “over-installed” compared to what historical flows would predict, as water from thawing glaciers was accelerating and expected to continue in the future. By selecting variable-installed-capacity turbines, the facility may be able to continue operating at high efficiency when flows begin to subside, too.
Warmer temperatures may also mean drought. If that happens, excess water may be absorbed into the ground, reducing the chances of flood. There may be less water flowing through dams, and that could mean less runoff, smaller floods, and less power production.
More frequent, violent storms may occur if certain climate change predictions come about. These can pose risks to safety—of the dam structures themselves and the people working there. Often, some relatively minor modifications to plans and strategies can go a long way to mitigating the damage these events can cause.
Wanted: more clean, abundant power
Developing regions, like China, South America, and northern India have the most to gain from new hydroelectric projects. But rivers in these areas, especially immature ones in volcanic or tectonically active regions, tend to have high sediment yields. These flows can move dramatically, reshaping land forms and the river system itself, posing serious risks to dams in their paths. So can landslides and changes in land use, such as deforestation and transportation. Fine earth materials get pulled into the flow and may build up close to the dam site, preventing the movement of water. Permafrost in the far north may be compromised by warming, too. Its thaw has implications for total differential settlement and raises the potential for hydraulic cracking at the core of embankment dams.
Whatever their risk factors and frailties, hydroelectric dams clearly have to be protected and maintained in shifting climates. With a near-limitless supply of water to power turbines, and infrastructure that can be built to last for decades—even centuries—they are still the greenest, cleanest life cycle source of energy we have on Earth. And they probably always will be, if we’re willing to give some thought to the possible effects of climate change and prudently manage the risks through adaptive and innovative design.
About the author
C. Richard Donnelly
Principal Consultant, Waterpower
With a graduate degree in geotechnical/civil engineering and more than 38 years’ experience in designing and constructing dams, underground structures, and excavations all over the world, Richard Donnelly is Hatch’s principal consultant for water power. He has over a hundred technical papers to his credit, and has received national awards for dam engineering, dam safety, risk-informed dam safety assessments, and dam decommissioning. In 2013, Donnelly was awarded the Professional Engineers of Ontario's medal for excellence in engineering.