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More protection against heavy rain and heat, improved air quality and biodiversity
Overview
The German City of Hagen lies on the edge of the Sauerland low mountain range and has a hilly topography. Four rivers – the Ruhr, the Volme, the Lenne and the Ennepe – run through the city. As a result, large parts of the city are at risk of flooding, especially during heavy rainfall events. As part of a pilot project, specially developed innovative tree trenches (or tree drains) are now being planted. These are designed to make the city safer, more climate-resilient and thus more liveable.
Background
For several years now, the city of Hagen has been working to reduce the risk of flooding caused by heavy rainfall events. The urgency of this became clear in July 2021, when large parts of the city were flooded following torrential rainfall. This caused major damage to buildings, bridges and urban infrastructure. A further aim is to make the city more climate-resilient in general and to improve biodiversity.
Objectives
A tree trench plan was drawn up for the new development area Kuhlen Hardt. Complementing the traditional rainwater retention basins, this aims to allow all rainwater accumulating on the streets and pavements around the tree trenches to flow into them , thus also replacing the usual street drains in the sewer network. This is designed to
- reduce flooding during heavy rainfall
- increase groundwater recharge
- boost growth of the trees
- improve the urban climate in summer through shade and evaporative cooling
- increase air quality
- promote biodiversity
- by doing so, create liveable residential and commercial areas and
- reduce the need to water trees in summer.
Activities
The Hagen tree trench plan is the result of a hitherto unique interdisciplinary working group comprising staff from the following areas
- drainage planning
- road planning
- green spaces, sports and playgrounds
- urban green
- urban trees and road safety
- urban planning / urban land-use planning
Based on experiences and models from Stockholm and Bochum, know-how from the Klima.Werk future initiative, the BlueGreenStreets research project and other good practice examples from Leipzig and Mannheim, the working group developed its own model. This was adapted to Hagen's topography.
Effects
The Hagen model combines the Stockholm model with an infiltration trough up to 20 cm deep. An emergency overflow connected to the sewer system allows water that overflows from the trough to drain away, thus further ensuring road safety.
The planting pit, with a total depth of about two metres, consists of a 100-cm-deep substrate layer below the trough. This comprises a planting soil base of loess, lava and pumice, which is designed to ensure optimal root growth. Below this is an approximately 80-cm-deep 'coarse layer' – a coarser-grained, loose soil layer that retains water well. By slurrying the substrate layer into the coarse layer, the tree is also enabled to root well in this soil layer. The coarse layer is drained in order to prevent prolonged water logging. This is to prevent root rot in case of frequent rainfall and persistent sogginess in the winter months.
Tree varieties were selected that often grow in floodplain landscapes, tolerate temporary flooding well and can also cope with the salt infiltration from road salt in winter. Examples include alder, tree hazel, swamp oak, English oak, field maple, Norway maple and elm.
The development plan in the pilot project now envisages an initial nine roadside tree trenches. Additional ones might be built on the tree-lined central square to enhance its amenity. Implementation is expected to begin in early 2024. Based on the experience gained there, the plan is to extend the tree trench model to the entire city of Hagen over the coming decades. This means that wherever new trees are planted or old trees are replaced, this will be realised in the form of infiltration trenches, topography permitting.
Conclusions
Tree drains can become an important element of urban planning for making cities more climate-resilient, species-rich and thus more liveable. Alongside green roofs and façades, modern mobility masterplans, unsealing of surfaces, public and private gardens and low-carbon architecture, they are an important component of modern sustainable urban development.
The interdisciplinary working group not only strengthened person-to-person links across departments. The colleagues now also have a much better understanding of each other's working methods and concerns, and have thus also become more aware of green urban development as a whole. The experts from drainage planning no longer see themselves as 'drainage experts' but as 'water managers'.
further information
Research project BlueGreenStreet der HafenCity University Hamburg
Published: 06/06/2023
Contact
Lars Kiesewetter
Wirtschaftsbetrieb Hagen AöR
Drainage planning officer
Tel.: +49-2331/36 77-130
Fax.:+49-2331/36 77-59 99
E-Mail: lkiesewetter(at)wbh-hagen.de
Images
Categories:
Connective Cities
Integrated urban development
Public space
City and biodiversity
Cities and climate change
Urban renewal
Regions:
Europe
Germany
Hagen
