6 September 2018

Researcher: Cloudburst runoff should be used for extreme drought

Urban planning

This summer's drought has demonstrated that diverting heavy runoff away from cities can be problematic. Seepage and drainage considerations must be considered hand in hand so that rainwater can permeate soil and be available to vegetation during periods of drought, according to Professor Marina Bergen Jensen of the University of Copenhagen’s Department of Geosciences and Natural Resources Management.

This summer's drought has been replaced by more typical and much milder Danish summer weather, including a few cloudbursts the likes of which have been experienced during recent years. Ever since torrential cloudbursts flooded the capital in 2011, there has been a focus on planning cities and engineering solutions that divert water away from cellars, roads and other vulnerable areas. However, this one- dimensional focus on steering water away has been misguided according to Professor Marina Bergen Jensen of UCPH’s Department of Geosciences and Natural Resource Management.

"This summer's drought demonstrates that we must prepare ourselves for extreme weather on both ends of the spectrum. That is to say, solutions that work with nature — whether there is too much or too little rain,” she says.

According to Professor Jensen, the system for securing Danish cities in the event of cloudbursts and torrential rain focuses only on extreme rainfall. But after the past summer, she hopes that greater attention will be paid to the fact that it is also wise and relevant to be equipped for extreme drought.

"The trick is to combine seepage with drainage. In this way, soil would be able to soak up water, as long as there is space. Water that cannot be absorbed into soil should be drained off," explains the researcher, who generally believes that the current storm water runoff solutions implemented in Danish cities are not yet strong enough.

"The better we can understand the natural foundations of different cities and the dynamics related to their groundwater levels – the more we can achieve robust solutions," she adds.

Danish municipalities are generally quite advanced in terms of devising climate change adaptations. They work with newer, greener solutions, as well as with traditional ones. However, a discussion of how these solutions connect with the natural foundations of cities has been missing. Hydrologic balances, water quality and the importance of water for nature, are a few of the things that have yet to be adequately addressed.

"China has launched a program in which 30 cities are to be transformed into ‘sponge cities'. The idea is that runoff should percolate and evaporate within city limits. In contrast with Danish climate adaptation planning, quantitative targets have been set up for how much annual rainfall should be captured, so as to maintain a defined hydrologic balance," Jensen says, adding that the Chinese efforts are being followed closely by her research team.