New tree bed on the market square to relieve the sewer system and store rainwater
28.06.2022 – To replace the felled plane trees on the redesigned market square, the city council has decided on an innovative approach to rainwater management and significantly larger tree beds. The Office for Planning and Building has proposed a sustainable option for retaining and storing rainwater on part of the market square.
"In future, when it rains, the water in a section will not be drained into the sewer system, but will be directed into a large tree pit, where it will be stored, retained and seeped away on site or absorbed by the trees," explains Paul-Gerhard Weiß, Head of Planning and Construction. The construction is based on the so-called Stockholm construction method, which is already being used in other municipalities across Europe. "Even if we don't notice any difference in the design on the surface of the market square once the construction project has been completed, the underground construction of the market square conversion contributes to climate protection in line with the sponge city principle," continued Weiß.
The tree bed for the two trees will have a volume of around 140 cubic meters. The bed will be constructed from layers of coarse gravel into which fine substrate will be slurried. The two-stage installation of gravel and substrate will create ideal growing conditions for the trees. This guarantees an uncompacted, rootable yet stable substrate in which the trees can develop a strong and deep root system. Above this layer is a so-called aeration layer, on which the standard structure for the surface of the market square is then built.
The new construction method was coordinated with the drainage and road maintenance departments of Stadtservice and the environmental agency. This system reduces the amount of irrigation required and eliminates wastewater charges for the retained rainwater. The trees to be planted are urban climate-resilient trees (cord tree, botanically Sophora japonicum 'Regent') with a trunk circumference of 35-40 centimetres and a height of five to seven meters, which have a rapid juvenile growth rate and are considered to be bee-nourishing trees.
The new tree bed is estimated to cost around 75,000 euros. This includes planning and implementation with infiltration as well as the necessary shafts. It will be created on the western side of the square, where the fountain will also be located.