Impluvium and multifunctional tree planting pits for clean-water sponge cities

Soil artificialization goes together with increasing impermeable surface areas. The increased surface runoff and flood hazard is as a major threat in large cities. Moreover, runoff waters transport pollutants to downstream surface waters. A key strategy for sponge cities is to design permeable soils allowing for rain water infiltration, which should go together with runoff water depuration. High quality permeable soils are also necessary to secure and increase the tree cover in cities, which is highly recommended both to fight against heat waves, to provide a comfortable environment and contribute to urban biodiversity. We present a combined solution to these different goals.

To this end, a highly permeable and fertile growing media was designed based on recycled manure and partly pyrolyzed organic matter. The depuration properties of this substrate were quantified for road runoff pollutants (soluble and micro particle forms) and commonly used pesticides. About 100% removal of these pollutants were observed based on breakthrough experiments on Technosol columns, with outflow quality matching the Swiss Water Protection Agency requirements. The fertility of the substrate was assessed in greenhouse pot experiments. Faster plant growth than with conventional horticulture production methods was observed, though no fertilizer was applied to the growing media.

The growing media, named TP70, was then mixed with 70% stones (100-150 mm size) to obtain a Technosol (TP-P) with high bearing capacity such as required for urban use. An experimental 1m large tree plantation pit was built in Lausanne city using a 60 cm layer of the Technosol, covered with 20 cm of stone ballast. The pit was installed under the walkway of a 4% slope street. Runoff water was injected into the ballast via collectors and infiltrated into the Technosol porosity before drainage at the low-end of the pit.

The volume of the pit was designed to comply with the local regulation on runoff water regulation, namely offering a fast-drainage porosity volume larger than the amount 10-year return-time rainfall on the corresponding watershed, with full drainage in less than 4 hours. It was planted with trees and offered an available water volume corresponding to 9 days of maximum evapotranspiration in the experimented case. Both tree growth and hydrological functions of the pit offered high performance. Based on this first experiment, an eco-district was fully equipped with these impluvium pits, thus infiltrating all the rain waters on the district, and Lausanne city is now extending the technology to the city.