Pärnu rainfall reached once-in-a-century intensity, expert says

ERR reports that Pärnu received so much rain within a few hours that the event could be described as a once-in-a-century rainfall, according to Ottar Tamm, a junior professor at the Estonian University of Life Sciences.

Less than 24 hours brought 122 millimetres of rain to the city. Estonia’s absolute rainfall record was set on the island of Saaremaa, where 148 millimetres fell in one day.

On natural ground, the surface can generally absorb and disperse such a volume of water. In cities, however, the large amount of paved surface makes that more difficult.

Pärnu received 33 millimetres in one hour, an amount that statistically occurs once every seven years. Two-hour rainfall reached 56 millimetres, which occurs once every 22 years, while the six-hour total corresponds to an event expected once every 77 years.

“These are absolute values, but there is a lot of variation. In reality, it can quite freely be called a once-in-a-century rain,” Tamm said on the programme “Uudis+”.

He said a warming atmosphere can hold more moisture, leading to heavier downpours globally. Such rainfall is not normal, Tamm said, but people should expect more flooding and prepare for it.

Widening stormwater pipes or drainage ditches would not be a physically feasible or sensible long-term solution, he said. Underground space in modern settlements is limited by dense networks of electricity cables, communications lines and heating pipes.

Building ever-larger stormwater pipes would also be too expensive for cities. During exceptional rainfall, it would be considerably wiser to direct water deliberately into a river or the sea in order to minimise damage.

Tamm also recommended above-ground and nature-based solutions. Instead of always trying to remove or infiltrate water as quickly as possible, rainwater could be retained before it reaches the pipe network managed by the water utility. This would require rethinking the role of streets and courtyards.