There are now a wide range of options for implementing sponge city projects. With regard to the different methods and structural designs, the topic of “Planting in urban areas” will be illustrated using a few realized examples.

Text Georg Zeleny

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The term “sponge city” is now a frequently used buzzword and basically describes an urban planning concept that aims to collect and store as much rainwater and surface water as possible locally and therefore decentrally, instead of draining it directly into the sewer system. This is intended to prevent or mitigate flooding during heavy rainfall events. The aim is to improve the urban climate and promote the health of urban trees and the resilience of entire urban ecosystems, which is an important climate change adaptation measure. In addition, urban plants as “green infrastructure” contribute to improving urban hygiene and the microclimate.

There are now many different design options – from the ground surface to trellis areas and façade greening through to roofs. For this reason, this article will only deal with design types in the area of the ground surface. In order for the “sponge city” to develop its effect and live up to its name, technical framework conditions and economic factors must be taken into account when planning and implementing this green infrastructure. Based on installations that have already been completed, it will be shown that there are now different types of realization and structural implementation.

What must always be taken into account and is essential for professional planning is data on the existing subsoil or soil and its properties, knowledge of the existing installations and neighboring buildings as well as the type and extent of the areas to be drained. Furthermore, numerous regulations, such as ÖWAV regulation sheet 45 [1], ÖNORM B 2506 parts 1 to 3 [2], the Chemical Groundwater Quality Target Ordinance [3] and many others must also be observed.

Different sponge city construction methods

Stockholm system

One way of implementing this is the use of structural soils (very coarse-grained base layers with a relatively large surface area and fine substrate slurried in between using additives such as biochar) as a root space, which is usually combined with the installation of pipes for the aeration and irrigation of the planting areas. This type of construction is commonly referred to as the Stockholm system and is a system developed in Sweden that has been adapted for Austria and has been used in Graz since 2017 and in the town center of Amstetten (Lower Austria) since 2023.

The Stockholm system has already been implemented in several projects in Graz, for example in EggenbergerAllee, KaiserJosefPlatz and Köflachergasse. The surrounding surfaces outside the immediate planting areas of the trees can be equipped with asphalt or paving for bicycle and vehicle traffic as well as for other uses, as the trees can be supplied with water via the built-in irrigation and ventilation pipes. If the surfaces of the project area need to be paved to a large extent due to system or usage requirements, this design may be advantageous. Thanks to the technical equipment, these systems can also be installed in rather unfavorable locations. It should be noted that with this type of design, surface water may only be discharged directly from areas with very low levels of contamination (class F1 according to ÖWAV RB 45).

The underground discharge and distribution of roof water is possible and sensible in principle, but legal aspects must be taken into account if the roof water comes from non-public areas. Due to the limited water storage possibilities, the supply of water at short periodic intervals is necessary, but can be implemented in urban areas using suitable irrigation systems. The root space created by structural soils can be covered with asphalt or paved surfaces, which can be advantageous in very densely built-up and used central locations. A combination with soakaway beds is also possible.

StreetTREE©

StreetTREE© is another sponge city construction method where space is limited and fixtures cannot be installed. This innovative approach allows trees to be planted independently of existing fixtures, provides the tree with sufficient root volume and forms a possible component of a sponge city. This construction method is intended to enable the planting of trees in the existing city and thus make a valuable contribution to climate change adaptation.

A trough with substrate is created in the soil as a retention basin and water reservoir for the tree. Above ground, the young tree is placed in a planting ring, creating an extension of the root zone. The stored water from the substrate is pumped to the tree, which is then watered on site. The planting ring is made of plastic or concrete. The entire system is the size of a parking lot and is primarily intended for existing street areas, as the root space can be extended by the open planting ring at the bottom. A substrate with good water storage capacity is used in the planting ring and the area below. Sealed surfaces are used to provide water and nutrients for the tree, thus compensating for the limited root space. Water is not lost through the trough. A negative impact on the groundwater is also not to be assumed, as when the trough is full, it overflows into the sewer – a building block for decentralized and sustainable rainwater management. In the municipality of Wieselburg (Lower Austria), a street was equipped with this system developed locally and scientifically monitored over a period of three years.

DrainGarden©

Another approach is to create greened or partially greened areas – so-called “green infrastructure” – or at least areas with infiltration capacity within residential areas. These areas allow the establishment of effective rainwater management measures, i.e. the decentralized absorption and storage of rainwater. If rainwater from sidewalks, driveways, roofs and fire lanes, i.e. from unpolluted areas, can be discharged directly, additional care must be taken when draining residential streets, public squares or parking lots to ensure that the substrate in the green areas also enables reliable and verifiable purification of the rainwater so that no contamination is introduced into the groundwater. Planting trees, bushes, shrubs and other plants in the substrate is an integral part of such systems, as as much water as possible should be evaporated. The required volumes of the substrate bodies and the necessary root spaces for trees usually correlate well and can therefore be put to multiple uses.

This type of design can be implemented with appropriately produced substrate mixtures such as the DrainGarden© system. This construction method has been developed in recent years by Austrian companies with scientific support from domestic university institutes and has been implemented in dozens of projects since 2013.

In the course of project planning, the areas to be drained must be defined in terms of size and type of area and the properties of the existing soil, in particular its permeability, must be investigated. In order to build settlement areas free of surface runoff into the sewage system, the systems are dimensioned for the local precipitation series of at least 30 years. For this purpose, the surface area of the systems must be large enough to demonstrably ensure the discharge and infiltration of the incoming rainwater, as this construction method does not require deep troughs or basins. The required system size, i.e. the volume of the substrate body, is dimensioned in such a way that the heavy rainfall events can be safely absorbed within the substrate. As a rule, contaminated surface water is discharged into the systems, which is why the manufacturer of such a substrate must be able to prove that the purification performance can be reliably maintained. Ultimately, water should also be able to seep away and the protection of groundwater is an essential prerequisite. There are now many examples of projects using the DrainGarden© system. The different variants are shown using a few examples.

Ober-Grafendorf

The center design in OberGrafendorf (Lower Austria), which was carried out in 2022 and 2023, can be cited. The asphalted roadway areas were reduced to the required minimum, additional green spaces were created with trees, bushes and shrub beds, and a pergola with climbing plants and shaded seating and information boards were erected. Walking areas were designed partly with a light-colored, water-bound surface and partly with a light-colored paving. Parking spaces were partially unsealed with grass liner stones and a suitable substrate and a bus shelter with an extensive roof was installed. Particular care was taken to ensure that the rainwater could run off as extensively as possible into the green areas and thus into the substrate planted with trees, shrubs and perennials.

Federal school center HAK/HTL in St. Pölten

The existing asphalted parking lot at this federal school center was hydraulically decoupled from the combined sewer by unsealing measures in 2019. Flooding in the basement has not occurred since then and the parking spaces are increasingly shaded. Trees and shrubs were planted in the strip-shaped green areas. The water supply for the facility is provided exclusively by rainwater from the surrounding parking areas. The combination with the conversion of the workshop roof from a sheet metal roof to an extensive green roof has proved particularly successful, which in a broader sense represents an effective unsealing measure.

Settlement in the market town of Prottes

A new residential area in the market town of Prottes (Lower Austria) was equipped with DrainGarden© substrate bodies in such a way that the entire public areas can do without rainwater drainage and the systems can also absorb heavy rainfall events without any problems. The water is fed into the green areas and is also used to supply the plants with water, whereby the filtered water is also reintroduced and can subsequently seep away.

Schulgasse in Herzogenburg

In the municipality of Herzogenburg, Schulgasse was equipped with green areas and substrate bodies in 2015, which were planted with trees and shrubs and ensure the drainage of public areas. Irrigation or gas exchange takes place directly via the surface of the substrate without watering the plants. The existing sewer system is effectively relieved.

Summary

In principle, it is therefore possible in almost all urban areas to design various types of facilities according to the sponge city principle and thus achieve considerable improvements in terms of climate resilience in public squares, residential streets and parking lots as well as commercial and private areas. However, the local boundary conditions such as existing buildings, fixtures, other land uses and soil conditions must be taken into account during the planning process. Regardless of which construction method is chosen and used, it is necessary to have the systems worked on by expert planners to ensure correct and economical design and technically correct implementation. When implementing projects, it always makes sense to involve local residents and users of the areas in the design process. Finally, it should be mentioned that there are also federal and state subsidies for many sponge city projects.

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Bibliography and list of standards

[1] ÖWAV Rule Sheet 45: Surface irrigation by infiltration into the subsoil, published by the Austrian Water and Waste Management Association, Vienna, 2015.

[2] ÖNORM B 2506 Parts 1 to 3: Rainwater infiltration systems for drains from roof areas and paved surfaces. Part 1: Application, hydraulic design, construction and operation, August 1, 2013. Part 2: Qualitative requirements for the rainwater to be infiltrated and requirements for the design, construction and operation of cleaning systems, November 15, 2012. Part 3: Filter materials – requirements and test methods, July 15, 2018.

[3] Federal Law Gazette II No. 98/2010 as amended: Qualitätszielverordnung Chemie Grundwasser – QZV Chemie GW, March 29, 2010.

[4] ÖWAW 78/2023: Sediment transport processes – From catchment to single stone. Zeiser, A.; Rath, S.; Grimm, K.; et al.: Ãœberlegungen zur Dimensionierung und Ausführung des Systems Schwammstadt für Bäume, 449, ed: Austrian Water and Waste Management Association, Vienna, 2023.

[5] Schwammstadt Graz, Referat Grünraum und Freiraumplanung, available at https://www. graz.at/cms/beitrag/10290985/7757216/Referat_Gruen_und_Freiraumplanung.html, checked on May 16, 2024.

[6] Streettree, project homepage, GrünStattGrau, available at https://gruenstattgrau.at/news/ projekt/streettree/, checked on May 16, 2024.

[7] DrainGarden planning aid, Zenebio GmbH, Vienna, 2024.