Record rainfall and drought make stormwater management more important

August was the rainiest month in Luleå since records began in 1930. The heavy rainfall raises the question of how our cities will cope with a climate characterized by greater variability. September 24 is Stormwater Day. Several projects are underway at Luleå University of Technology aimed at helping society manage heavy rainfall and harness stormwater as a resource.

Luleå University of Technology is home to Sweden's largest research group in Urban Water Engineering. Its research includes how cities can manage stormwater and other water-related challenges. In August, 261.9 millimeters of rain fell in Luleå. The previous monthly record was 196.6 millimeters, set in July 2020. The normal annual precipitation is about 600 millimeters, which means that nearly half of the normal annual precipitation fell in a single month. On the night of September 21 through 1:00 p.m., 59.6 mm of rain fell in Luleå, which was the highest amounts of precipitation ever recorded in a single day.

The heavy rainfall is not just a local problem in Luleå. In Norrbotten in the month of august, according to measurements from the Swedish Meteorological and Hydrological Institute (SMHI), there was nearly 400 percent more rainfall than usual, which has caused major flooding in Boden and problems for farmers throughout the county.

“As long as the drainage system can handle the water, we don’t notice much of a difference. But when capacity is insufficient, it can cause flooding and damage to buildings and infrastructure,” says Ico Broekhuizen, associate professor of Urban Water Engineering at Luleå University of Technology.

One millimeter of rain is equivalent to one liter of water per square meter. Luleå University of Technology’s campus covers approximately 280,000 square meters, which means that one millimeter of rain is equivalent to 280 cubic meters of water across the area. On August 17, the university’s rain gauges recorded a peak of six millimeters in a quarter of an hour—equivalent to 1,680 cubic meters of water across the campus in just 15 minutes.

Wet in the north and dry in the south

August also illustrates just how great the differences can be between different parts of the country. While Norrbotten experienced a drastic increase in precipitation, parts of southern Sweden received only between 25 and 75 percent of the normal amount.

A changing climate can lead to greater fluctuations between heavy rainfall and periods of drought. A single weather event cannot in itself be explained by climate change, but the heavy rains are consistent with the long-term trends that researchers are observing.

“When more rain falls in a shorter period of time, the strain on stormwater systems increases. That is why we must plan for rainfall that is heavier than what the systems were originally designed to handle,” says Ico Broekhuizen.

Digging up streets and replacing pipes is costly and involves major disruptions to the urban environment. Municipalities therefore need to be able to determine where the pipe network must be rebuilt and where water can be managed in other ways.

“We need to be able to assess when it’s necessary to dig up and replace pipes and when other measures are more appropriate. To make that judgment, municipalities need to know where problems might arise and what the consequences of different solutions will be,” says Ico Broekhuizen.

Stormwater can be a resource during droughts

When it rains heavily, stormwater management is often about diverting water and preventing flooding. During dry periods, however, that same water can instead become a valuable resource.

As part of the CUREE research project, doctoral student Nils Brattgård is investigating how water from urban surfaces, such as roofs and parking lots, can be collected and used. For households, this could involve, for example, irrigation and toilet flushing.

“We don’t need to use drinking-water-quality water for everything. By collecting water locally, we can reduce the strain on drinking water production and use the right water quality for the right purposes,” says Ico Broekhuizen.

The possible uses depend, among other things, on the water’s quality, how long it can be stored, and what treatment is required.

“We must plan for both too much and too little water. It’s about protecting cities from flooding, but also about harnessing stormwater and reducing the strain on other water resources. These issues need to be addressed together,” says Ico Broekhuizen.