The water outage was caused by a rare natural phenomenon known as frazil ice, which led to the freezing of the protective grating in the canal that transports water from Lake Ülemiste to the city's water network.

The last time this type of ice stopped the flow of water to the city was on December 31, 1987, said Deputy Mayor of Tallinn Tiit Terik (Center) on Wednesday.

Janek Laanearu, tenured associate professor at TalTech University's department of civil engineering and architecture, stressed that freezing is a common occurrence in Estonia, but frazil ice differs from regular ice.

Frazil ice consists of tiny, needle-like or disc-shaped ice crystals that form in supercooled, turbulent water and remain suspended, rather than forming a solid sheet immediately. 

The phenomenon is known for clogging water intakes as the crystals accumulate on grates and screens. Blockages can affect water supply systems, hydroelectric power plants, nuclear power plants, and vessels operating in cold waters, causing unexpected shutdowns or even structural damage.

Laanearu said when infrastructure is designed, all possible scenarios must be taken into account.

"If you overlook something in the design, like leaving out a single beam, the building can collapse and people can die. Water systems are built to last 50–100 years, and all possible events during that time must be accounted for. That is a fundamental principle of construction," he told ERR.

The associate professor does not know exactly what happened in Lake Ülemiste and the reservoir, but he believes it was a combination of unfortunate factors.

"The temperature may have dropped quickly, the water quality might not have been exactly as expected, and it is possible that certain structural solutions used promoted frazil ice formation. For example, with metal grates, heat transfers out of the water faster via the metal. If one thing goes wrong, you can manage. But if two things go wrong at once, then it becomes unmanageable," he said.

Technology to mitigate frazil ice risks exists but the main factors influencing their adoption are cost and frequency of occurrences, said Ivar Annus, also a tenured associate professor at the same institute.

"If a similar situation has only happened a few times in Tallinn over the past half-century, then we have to consider whether a major investment is justified. Technically, solutions are available, but it's uncertain whether they are economically viable," he told ERR.

CEO of water company Tallinna Vesi Aleksandr Timofejev told Vikerraadio's "Uudis+" that the company is studying which solutions are used in other countries. An earlier attempt to heat the grates at Tallinn's water treatment plant to prevent ice formation did not work, he said.

The reason is likely because the treatment plant draws about 4,000 cubic meters of water per hour.

"Imagine such a large flow moving continuously at a temperature of about zero degrees. In principle, the grates function like a car radiator, water flowing through them cools them down very quickly," he said.

Timofejev said the lake, which holds 28 million cubic meters of water, is extremely difficult to heat and would require an enormous amount of thermal energy.

"We've looked into different technologies. The internet has plenty of information — there are ultrasonic solutions and other tools. But whether they work under all conditions is a question we need to answer. We are working on it. I certainly cannot say right now what the investment would look like, how it would affect water prices, and whether such investments would be reasonable," he said.

Laanearu stressed it is likely something will always be missed in the construction process.

"Systems are built, then renovated and upgraded. It cannot be said that frazil ice was completely unconsidered, but in some sense, this was still an unforeseen situation," he said.

However, he pointed out that critical infrastructure must also have emergency alternatives.

"If there's one main intake point for water, there should also be an alternative source to feed into the same system if necessary. It seems that this has not been developed adequately. The solution would be a loop network — if one part fails, water can come from elsewhere and the service doesn't stop," Laanearu explained.

The associate professor said similar situations may become more frequent in the future as the weather is expected to become more variable.

"In our climate zone, is it common for temperatures to fluctuate around zero. Climate and environmental changes directly impact infrastructure design, and that infrastructure needs to be climate-resilient," he said.

Laanearu believes lessons must be learned from the incident.

"The Ülemiste water treatment plant exists to provide a service. If that service fails, it cannot just be treated as a temporary inconvenience. Water is vital and must be continuously available," he stressed.

He said normal conditions were restored relatively quickly, which shows that the emergency response was effective.

"But now there is a lot of serious thinking to do to ensure this does not happen again. The worst thing would be if it happened a second time. That would be unforgivable. The first time may have been a coincidence of bad factors — the second time would no longer be a coincidence," the expert said.

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