In-situ evaluation of sponge-city-type sites for urban trees to tackle flooding and heat islands

Trees in urban environment face plenty of problems that hamper vital and long-standing growth, which would be essential to counteract urban heat island effect. The major issue is a tremendously reduced volume of appropriate rooting space due to impervious surface and highly condensed underground in the immediate surrounding. Sponge city substrate based on the model of Stockholm promises to provide conditions suitable for root growth even underneath sealed surfaces. This innovative type of substructure construction method consists of unconsolidated fine substrate flushed into the voids of edged stones that serve as load-bearing structure. If well-designed in a function-oriented manner, the volume of sponge city substrate is able to serve as an underground retention basin saving soil water for transpiration and enabling excess water to infiltrate further into the groundwater. To support the creation of such highly functional substrate-pore systems, knowledge about the effects of different materials and methods on the hydrological functions is needed.

In Austria several projects using sponge city for urban tree planting have been implemented in recent years in various cities and municipalities. In order to increase the understanding of the system in hydrological, soil physical and implementational terms and to enable improvements and identification of reasons for malfunction, research is performed at laboratory, lysimeter and field scale. The latest monitoring project has been built in a small street in Graz, where both sides next to the street have been excavated and rebuilt with sponge city substrate. Two different substrate types have been used and 9 trees have been newly planted. The closest monitored part consists of about 100 m³ sponge city substrate, 4 trees and various types of surface design and usage including parking space, perennial plantings and a seepage basin with topsoil passage for purification of street water. Sensors measuring matric potential, volumetric water content, electrical conductivity, soil temperature, sap flow and water inflow from roof and street deliver the basic data to calculate the full water balance within this area and set up a water balance model offering the opportunity to assess the impact and potentialities of sponge city substrate in various temporal and spatial scenarios.

Coupling data from sponge city lysimeters, laboratory experiments and other field monitoring sites an estimation of ecosystem services accomplished by this innovative construction type will be attempted. Focus will be put on retention behaviour for heavy rainfall, plant water availability as well as tree vitality, growth and transpiration.