The sponge city principle – from pipe-soil-systems to soil-pipe-systems – solutions with ductile iron pipe systems

Figure 1:

Flooding after heavy local rainfall in
Dortmund- Marten in 2008
[Source: NRW Police]

 

1 Introduction

At least since the recent events of torrential summer rain it has become clear that municipal drainage systems can no longer cope with the volumes of water occurring. Increased surface run-off along uncontrolled flow paths can result in local flooding with devastating damage to infrastructures and buildings (Figure 1).

But it is not merely the volumes of water which are causing us problems. The increasing heat levels in predominantly urban areas with a high density of buildings and concrete or tarmac surfaces are also reducing the quality of life in our towns and cities. In order to counteract these effects of climate change, which are sometimes catastrophic in both cases, it is necessary for us to revise our drainage and climate control concepts for cities. In addition to the use of temporary flood-plain areas and the provision of defined flow paths for a safe run-off of rainwater above ground in case of heavy rainfall events – so-called urban flash floods – or increasing the number of green roofs to improve the evaporation rate, the use that the soil is put to in and around the streets of our towns offers as yet unused opportunities for action in order to implement measures against urban flash floods and overheated cities.

2 The sponge city principle

In a report by the Federal Institute for Research on Building, Urban Affairs and Spatial Development (BBSR) on flooding and heat control by urban development, these measures are referred to as the “sponge city principle”. The aim of these changes in use is to make the surfaces of towns and cities more capable of absorbing and storing volumes of rainwater than they currently are in our urban environment. With this type of near-natural rainwater management in towns, green spaces can become the town’s natural “coolers” in that they are sufficiently supplied with water. This cooling action can be enhanced by the storage of rainwater, soil-improvement measures and a continuous supply of water for 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 future climate-adapted cities [1].

The water mangament, too, has recognised the benefits of deliberate water evaporation for cooling cities in climate change as a new part of its remit. This is reflected in the currently available draft of DWA worksheet A 102 [2], for example. It describes measures for implementing the sponge city principle as a future drainage concept for “preserving the local water balance”. This includes vegetation with its contribution to evaporation. It also points out that conventional drainage processes, as combined or separate sewer systems, in their “pure form” where they drain away rainwater completely, clearly no longer meet the objective of a local water balance. In future planning projects, they should be gradually converted by the integration of – preferably – decentralised measures of stormwater management into modified systems, provided that there is room for manoeuvre.

The “de-coupling” of surfaces conducive to run-off from the existing sewer system looks like an effective approach for decreasing hydraulic loads on the water management system, improving flood protection and reducing the pollution and hydraulic stress which rainwater outflow causes to bodies of water [2].

Basically, the implementation of the sponge city principle – both in general terms and in terms of the DWA system of rules – means a process of decoupling in inner cities and town centres, where private land, the road space or other public surfaces can serve multifunctional uses for rainwater management, including the evaporation functions offered by vegetation.

3 The de-coupling potential

The de-coupling potential of road surfaces has already been determined on the basis of examples and is described in [3]. In residential streets in the districts of Erle and Resser Mark investigated in the city of Gelsenkirchen, the de-coupling potential achievable in the longterm should be up to 53 % and in the shortterm around 22 % (Table 1). As compared with results when determining the potential for de-coupling of roof surfaces, road surfaces offer approximately the same level of decoupling potential. Both for de-coupling measures on private land and for those on publicly used surfaces, what is needed is motivation in order for the measures to be implemented. On private surfaces this may be by reducing sewage charges. But for implementation in public spaces in the road space there must first and foremost be a readiness on the part of communities and public authorities to make the road space available for infiltration and/or retention systems where necessary [3].

Added to this is the fact that the technical solutions must be organised in such a way that they can be jointly sponsored by the various departments/administrative offices/independent operators in the city and that they meet systems of rules in terms of the materials and components used. Furthermore, preference must be given to solutions which do not result in any restrictions in the usability of the spaces used in this way. Solutions in the space beneath usable road surfaces are to be preferred. However, it must be borne in mind that these spaces are not exactly unused either. Sewers and drains, drinking water and gas pipelines, power cables and telecommunication lines are already installed underground and even now they are competing for space. Added to which are those unplanned underground supply networks, namely the root systems of trees, which apparently interact with sewers and utility pipelines in an uncontrolled manner [4]. Hence, in order to implement the sponge city principle, interdisciplinary cooperation, including urban greenspace departments, is necessary.

4 Structural engineering and planting requirements

For solving what, at first glance, looks like an insoluble task there are a few important linking elements. These are, on the one hand, the pore space in coarsely crushed bedding materials for sewers and pipelines which can be built over, can be used as storage for rainwater and at the same time offer trees sufficient space for root growth. On the other hand, there are piping systems which are sufficiently robust and are not damaged by sharp-grained bedding materials. In addition, they must be resistant against external water pressure and show evidence of being rootresistant.

Hence this soil-pipe-system differs from the usual pipe-soil-system where the soil is selected, or even modified, so that it simply ensures optimum bedding for the sewers and pipelines.

Figure 2 (top) shows an example of a coarsegrain crushed substrate, which ensures sufficiently large pore space for rainwater and tree roots, is capable of being highly compacted and, after completion, allows full use of the surface (Figure 2, bottom) [5].

When installed in the pavement area, the type of near-surface implementation of the “sponge city principle” shown interferes a very great deal with the pipeline space required by supply companies. This is also illustrated in Figure 3. By preference, drinking water and gas pipelines are installed under the pavement. Also, to be found here are the cables of power and communication network operators. Added to which is the fact that cellars are often located directly in the vicinity of the pavement and so, with the introduction of rainwater, planning and construction measures must be put in place to avoid water penetration in cellars. Against this background, it is advisable to implement a method for rainwater management in the pipe trenches of sewers.

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

A piping system which is to be installed in this coarse-grained bedding material is produced in ductile cast iron to EN 598 [6] and protected against corrosion and mechanical stress with a cement mortar coating to EN 15542 [7]. The TYTON^® push-in joints used are root-resistant and tight in terms of external water pressure. The cement mortar coating can be laid in crushed bedding material with a maximum grain size of up to 63 mm and individual grains up to max. 100 mm size [8].

Figure 4 (left) shows an example of the installation of such pipes in rocky bedding materials. Figure 4 (right) shows a ductile iron pipe with cement mortar coating and a TYTON^® push-in joint.

Figure 5 summarises the essential properties of the pipe bedding, the pipeline zone and the main backfill as well as the pipe characteristics during installation of the soil-pipe-system.

The integration of this soil-pipe-system in an urban environment is illustrated in Figure 6. The pipe trench with the ductile iron pipes becomes a storage space for rainwater beneath the road. Water from non-polluted surfaces such as roof spaces (apart from roofs covered with copper or zinc) can be directly discharged into this storage space. Polluted rainwater is first pre-treated and then discharged into the rainwater storage space. For pre-treatment, systems available on the market with DIBt certification can be used for example. The water is used either for irrigating the tree roots growing in the pipe trench or it trickles away as in an infiltration system. The decentralised storage, infiltration and evaporation of rainwater at the place where it falls has a number of positive effects:

• Improvement of the local water balance,
• Reduction of the number and level of combined sewer overflows,
• A decrease in the volume of contaminated rainwater discharged into bodies of water from the separate sewer system,
• Retention of rainwater during heavy rainfall events,
• Provision of root space for improved growth of trees in the city,
• Targeted irrigation of city trees with rainwater,
• Increase in the evaporation rate of trees with improved climate conditions in the ambient area,
• Improved protection of property by de-coupling rainwater downpipes from the building’s drainage system.

The positive effects on water and property protection and on the city climate are summarised in Figure 7.

6 Summary

When building roads and the structures of underground infrastructures, until now it has been the bedding of the pipes and the loadbearing capacity of the structure which are paramount. Highly compacted soils therefore tend to characterise the urban substrate. Wherever pore spaces occurred inadvertently in the ground, these were used by the root system of municipal and private trees, resulting in unintentional interactions with sewers and utility pipelines. Higher requirements for water quality as well as the effects on climate change mean that a change in thinking is necessary and that increased attention needs to be paid to the storage capacity of the soil so that rainwater can be managed where it falls into the soil. It has moreover been established that, with this type of near-natural rainwater management in our cities, green areas can become the city’s natural “coolers” because they are sufficiently supplied with water. This cooling effect can be heightened by the planned storage of rainwater, measures to improve the soil and a continuous supply of water for vegetation. The promotion of this so-called sponge city principle and the development of sustainable storage and irrigation systems are therefore a central task for the future of climate-adjusted cities.

To date, construction methods which take account of this principle have only been implemented in isolated cases. The soil-pipe system described here, consisting of crushed, large-grain bedding materials with a large pore space and robust ductile cast iron pipe systems, represent an important stage in the realisation of the sponge city principle. Also, the draft of DWA rule document A 102 [10] can be interpreted as an indication that in future the operators of drainage networks in particular should be involved in the planning, construction and operation of such decentralized rainwater management systems and thus make an essential contribution to dealing with the effects of climate change.

Bibliography

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