The near failure of a century-old dam in Minnesota has drawn renewed attention to the vulnerability of the US’s ageing water infrastructure. Thousands of dams are in poor condition, and many were designed based on estimates of extreme rain that may now be outdated as climate change drives stronger precipitation in some parts of the country.
“The state of the infrastructure is such that it’s not able to resist even minor events in many cases,” says at Arizona State University.
Of the country’s more than 90,000 dams, around 25,000 are considered “hazardous” – they are large enough to pose a significant danger to people if they were to fail. Within this category, the conditions of more than 7000 dams were unknown, according to the . And more than 4000 were deemed to be in poor or unsatisfactory condition at their last inspection.
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One such dam is the 114-year-old Rapidan Dam in southeastern Minnesota. On 24 June, following days of heavy rain that caused widespread flooding in the US Midwest, the swollen Blue Earth river jumped its banks at the dam. As water poured around a support structure, county officials warned that the dam was in “imminent failure condition” if the floodwater eroded the rock keeping the concrete structure in place.
A day later, the flow of water had lessened, and the main structure of the dam appeared to be stable. But experts say the close call illustrates how a combination of unusual precipitation and dams in disrepair can lead to disaster. “You’ve got the health of the dam, and then you’ve got the hazard,” says at Princeton University.
Even if dams are in good condition, they may still be vulnerable to extreme events that weren’t accounted for by the engineering standards that have long been used to design US dams. “The existing procedures reflect remarkable engineering judgement”, says Smith, but they relied on an understanding of precipitation that is now outdated.
For nearly a century, the most hazardous dams – and other infrastructure like – have been designed to withstand the most extreme precipitation events deemed possible in a given area, based on an calculation of “probable maximum precipitation”, or PMP. PMP is usually determined using a statistical estimate from storms in the historical record. It is used to design safety mechanisms such as spillways, which serve as emergency release valves to prevent water from running over the top of a dam.
But in a recent National Academy of Sciences , Smith and his colleagues found the way these PMP estimates are calculated is based on flawed science and may be inadequate to capture extreme precipitation hazards in a changing climate. The nearly 200-page report provides detailed recommendations for how to revise estimates of extreme precipitation. Using physical climate models rather than records of past storms, for instance, would ensure estimates reflect our best understanding of how the atmosphere works, as well as the influence of a changing climate, says Smith.
Around 15 states have already carried out such reassessments of extreme precipitation hazards and begun to update affected infrastructure, says at the Association of State Dam Safety Officials. But “there’s a lot that hasn’t been done”, he says.
Dams that are in disrepair or operated incorrectly can also fail as a result of less extreme precipitation, says Lall. “If your infrastructure is ready to fail, the fact that you designed it for a thousand or ten-thousand-year event that you miscalculated is kind of irrelevant.”
That appears to have been the case at the Rapidan Dam. The precipitation that drove the near failure of the dam seems not to have exceeded the PMP estimates for the area, says at Applied Weather Associates, a consultancy in Colorado. But debris in the river may have plugged the spillway, leaving excess water nowhere to go but around the dam.
In fact, in an of more than 600 dam failures in the US since 2000, Lall and his colleague Jeongwoo Hwang at North Carolina State University found most of them were not associated with particularly extreme rainfall. Rather, the study, which has not yet been peer-reviewed, found they more often occurred during a moderate rainfall event following a stretch of unusually wet days. Such sustained wet weather can soak the soil, leading to more runoff. Dam operators hoping to save water may also have kept water levels too high to adequately respond to flooding in some cases, says Lall.
“This is happening because we are not paying attention,” he says.
