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Heavy soil sealing combined with insufficient retention and infiltration (seepage) potentially increases the risk of pollution and flooding in cities during heavy rainfall. Sustainable, decentralized urban drainage systems can reduce this risk to a certain extent, according to calculations made by researchers using the city of Graz as an example.

The team was led by Dirk Muschalla and Albert König from the Institute of Urban Water Management and Landscape Water Engineering at Graz University of Technology (TU Graz). The bundle of feasible measures they investigated would provide a distributed retention and infiltration potential of around 500,000 cubic meters of water with simultaneous greening of the city.

Measures on public land alone already offer lots of potential

"We are aware that we cannot rebuild the entire city. That is why we have only taken into account feasible measures on public land and the effects of the current Graz building regulations on rainwater retention on private land," explains Muschalla. "However, the measures taken on public land in Graz alone already offer the potential for a retention volume of around 190,000 cubic meters of water."

The new building regulations for private new-build projects in Graz require proof that the water from a rainfall event occurring once every 30 years—i.e., precipitation of a magnitude that statistically only occurs once every 30 years—must be able to be retained on the property in question.

Based on the calculations of the research team in Graz, this measure alone could result in a decentralized water retention volume of around 296,000 cubic meters of water. As the building regulations only affect future developments, their full impact through private land will materialize over a longer time horizon of 30 to 50 years.

Detailed analysis of suitable areas

The researchers investigated several measures for public spaces: the unsealing of car parks to create parking spaces that allow water to seep away, the installation of tree pits, and the expansion of roadside greenery to create seepage and retention basins so that water is not held back at the curb but can be treated and infiltrated in the existing green strip.

The researchers took a particularly close look at tree pits, as they are quite complex and pose special opportunities in terms of rainwater management. The tree pit infiltration systems, built according to the Stockholm system, are located under footpaths and car parks and consist of a framework of coarse gravel with stones up to 15 centimeters in diameter. There is substrate for the tree between the stones. This provides the tree with root space, air and water while the surface remains usable.

Tree pits are particularly advantageous because they both relieve the burden on the sewage system and contribute to the mitigation of the urban heat island effect. A comprehensive spatial analysis of suitable areas in Graz showed a potential storage volume of 65,000 cubic meters of water just for this measure alone.

Sewer system not negatively affected

"Of course, the implementation of such measures cannot happen overnight, but the potential to better protect a city like Graz against heavy rainfall events with Blue-Green infrastructure is very high," says Albert König. "High flow rates will become less frequent and flush sediments less regularly. We also investigated this effect to make sure that it won't negatively impact the sewer network. Our investigations have shown that there are no significant disadvantages to be expected."

The deputy mayor of Graz, Judith Schwentner, explains, "The findings of this research project emphasize that we need green spaces, and trees in particular, not only for the design of our living space in the city. They also make a significant contribution to urban drainage and local flood protection. This encourages us to consistently continue on the path we have already taken to develop Graz into a 'sponge city' and to further expand our pioneering role in Europe in this area."