Implementing the sponge city principle

It is time to act

The effects of climate change have been more noticeable than ever before in Europe and also in Germany over the last couple of years. Long periods of high temperatures in combination with low rainfall in the spring and summer of 2018 and 2019 put a strain on us humans and also had visibly harmful effects on the natural world.

The summers of 2018 and 2019 from a meteorological viewpoint

With a mean temperature of 10.5 °C, 2018 was the hottest experienced so far in Germany since regular records began in 1881. What is striking was the long-lasting drought from February to November. The combination of high temperatures and low precipitation makes 2018 a particularly exceptional year; The 30°C mark was often exceeded. By the end of November 2018, the total precipitation figures were also on the way to setting a record: the year 2018 takes 4th place for dryness after 1959, 1911 and 1921 in the records since 1881.

In 2019 a new national record was set for heat in Germany: from 24 to 26 July an exceptional heat wave prevailed in the West of the country over three consecutive days with maximum temperatures of over 40°C; the first time in this country since systematic weather records started to be taken. New record figures have been set at various measurement stations with a national all-time record of 42.6°C on 25 July 2019 at the Lingen weather station in Emsland. In other countries in western Europe too, e.g. in the Netherlands, Belgium and Luxembourg, numerous local and other regional records have been registered. Precipitation levels: also back on a deficiency record course.

The long-term effects of too little rainfall in 2018 and the lack of precipitation in 2019 will have long-lasting effects – not only for agriculture and forestry a disaster of the century.

The effect of drought on urban trees

Even more severely affected by drought were urban trees in parks and along roadsides. It was obvious that the trees were in bad shape, so that local authorities were calling on townspeople to water the trees in their street. Both municipal employees and residents irrigated the trees with drinking water. Using tankers and buckets, water was poured directly around the trunk so that trees, whose roots had not yet managed to leave the far too meanly dimensioned planting pit, could be safely supported. Older trees, for which it can be assumed that they too were planted in tree pits which were too small but whose roots have nevertheless managed to extend beyond the planting pit, definitely did not benefit from this irrigation water! In addition, the deeper lying layers of soil in towns and cities dried out, with this effect being reinforced by the high degree of sealing and reduced groundwater formation.

sponge city principle: use of the underground street space

An example of use of the underground street space.
(Picture source: RWE-Magazin June 2006, modified by K. Schröder)

The effect of construction methods for tree pits on urban tree vitality

At an early stage, a great pioneer of the sponge city idea, Klaus Schröder, already pointed out the consequences, and also the background, of tree pits which are designed to be too small. He did this in a speech at the Austrian tree forum in Vienna on March 2009:

Naturally, and particularly in times when the coffers are not exactly overflowing with money, the question is posed of whether the expense needed today for optimising the location of trees makes sense. But such planting, done for reasons of sustainability and durability, is only “expensive” at first glance. Because: who is not familiar with the sight of those pitiable specimens in “cheaply” prepared locations which waste away after just a few years, in the proverbial “too little to live, too much to die” situation, in order finally, a few years later, to need replacing. If one really thinks about the false investment in this type of tree planting, the return on the capital employed, the cost of years of pointless care and finally one takes account of the expenditure for replacement with new trees, one must become convinced that the outlay for providing the best growth conditions is, even under economic aspects, a good capital investment. Even disregarding the unfulfilled function for which the trees were originally planted.

Vital trees and reduced maintenance costs are achievable targets which make up for the additional initial expenditure for optimally planned planting. However, the necessary investments should be made at the time of planting and not some time afterwards in order to correct mistakes. When planting trees in towns and cities, the latest findings as regards planting technique / construction technology must be taken into account and put into practice!

Presentations like these have also contributed to the fact that planting techniques for trees have developed further. Tree pit methods have been developed for planting trees with substrates which ensure ideal soil aeration in the root space. The fact that rainwater can be used to irrigate the urban trees planted in this way is obvious. The so-called “Stockholm solution”, a synonym for a method of planting in tree pits in which highly compactable coarse ballast forms the support structure, combines the elements of root space and storage space for rainwater. As a result, the maintenance costs for urban trees are already reduced.

The sponge city principle

But this method goes much further: in a report by the Bundesinstitut für Bau-, Stadt- und Raumforschung (BBSR) on provisions for flooding and hot weather by means of urban development, these methods are described as the so-called sponge city principle. It is the aim of these modified applications to make the surfaces of towns and cities more suitable than ever for taking up and storing rainwater. By this type of nearly natural rainwater management, green spaces can become natural “refrigerators” for the town, in that they have a sufficient supply of water. This cooling capacity can be increased by the storage of rainwater, measures to improve the soil and a constant supply of water to the vegetation. The promotion of the sponge city principle and the development of sustainable storage and irrigation systems are therefore described as central tasks for the future for climate-adapted towns and cities.

With a view to the effects of long-lasting dry periods on urban trees described in the introduction it seems logical to enlarge the root spaces and their space for storing rainwater further. A seemingly unsolvable task in the urban underground space if one thinks of the intensive uses to which this is put. However, it does become solvable if this construction method can be used in the pipeline trenches of drains and sewers.

sponge city principle in the street space

The sponge city principle in the street space. By the use of large-grain, broken materials
with a large storage volume, the soil in the pipeline trench becomes a rainwater storage
space and provides extended root space.

Ductile iron pipe systems – solutions with a robust soil-pipe system

A pipe system which is able to be installed in this coarse bedding material is produced from ductile cast iron and protected with a cement mortar coating against corrosion and mechanical loads. The TYTON®, type push-in joints used are root-resistant and impervious to external water pressure.

Cement mortar coating can be used in broken bedding material with a maximum aggregate size of up to 63 mm and individual pieces of max. 100 mm diameter.

With the soil-pipe system, the pipeline trench with the ductile iron pipes beneath the road becomes a means of storing rainwater. Water from non-polluted surfaces, such as roof areas (with the exception of roofs with copper or zinc coverings), can be directly fed into this storage space. Polluted rainwater will first be treated and then introduced into the rainwater storage space. For example, DIBt-certified systems which are available on the market can be used for treating the water. The water is either used to irrigate the tree roots growing in the pipeline trench or it trickles away as in an infiltration ditch system.

The civil engineering for the planting of urban trees has been further developed and, with the soil-pipe system for ductile iron pipes as described, a possibility has been demonstrated for incorporating the so far unused large volume of pipeline trenches into the planning as well. We have to act now in order to make the urban infrastructure climate-proof too. With the inclusion of urban green spaces in the planning of underground infrastructures, there will be room to manoeuvre which must be put to good use.

Christoph Bennerscheidt, EADIPS FGR


The article was slightly shortened by the editors. You can find the complete article with various illustrations as a PDF in the download area under Downloads Annual Issues EADIPS FGR.