Hydrologic characterization of sponge-city systems for urban trees based on monitoring and modelling

Sponge-city sites for urban trees based on the model of Stockholm promise to improve the chances of trees surrounded by sealed or condensed surfaces for root growth and therefore vital tree development tremendously. The system furthermore helps to conquer the urban heat island effect, aids in stormwater management as an underground retention basin saving soil water for transpiration and hence supports the ambition to approach a (more) natural hydrologic cycle. The actual capacity of the system to fulfil these services is determined by a variety of design criteria which need to be optimized based on detailed knowledge about the hydrological functionality of the different elements of the system. The aim of several monitoring and modelling studies in Austria is to gain such knowledge to ensure a proper performance of the system in terms of tree growth and rainwater retention.

First of all, the substructure construction consists of unconsolidated fine substrate flushed into the voids of edged stones that serve as load-bearing structure, assuring root-favouring pore distribution. Technical components like an inlet and surface water distribution system accompany the substrate. Further influencing parameters are e.g. properties of the existing substrate underneath, design of urban surface as well as origin and treatment of fed surface water. Based on monitoring sites in the shape of lysimeters, field scale projects in real urban settings, and laboratory experiments, modelling approaches for the hydrological functionality of the sponge-city systems are generated. Crucial system elements and target values for design properties are derived from these simulations. In the longer term, the results should serve as a supportive planning tool for engineering projects in urban environments.