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Figure 1. 

1. Concrete block paving, with pervious 
joint material and bedding layer
2. Pervious concrete (as a load-bearing 
base layer)
3. Base and subbase aggreage layer
4. A layer for water detention
5. Subgrade
6. Geotextile
7. Draining of water, when needed

Two examples of permeable surfacing solutions. Rainwater penetrates the surface layer easily. Water runs into the highly porous sublayers, from which it eventually ­
1) is absorbed by the ground; 2) is channelled elsewhere or 3) is partly absorbed by the ground and partly channelled elsewhere.

Pervious pavements – new solutions to prevent urban flooding

Erika Holt, Hannele Kuosa, Juhani Korkealaakso, Terhi Kling, Kalle Loimula, Emma Niemeläinen, Harri Kivikoski | 4.6.2015

​Impermeable hard surfaces in cities overload stormwater networks and increase surface run-off. This increases the flow of impurities into natural waterbodies. New means of drainage and stormwater management are needed; demand for these solutions will rise as global warming brings increased rainfall volumes and intensities. 

Cities are growing and densifying. This means that more of their surface area is being covered by impermeable materials. The growth in areas covered by water-tight surfaces is having a variety of harmful impacts on buildings, the environment and people: stormwater networks may be overloaded, runoff water volumes grow and, in places, groundwater levels may fall. Harmful effects on the environment and waterways can be reduced through urban planning that takes into account hydrological factors. In addition, stricter environmental regulations require municipalities to find new ways of reducing the environmental load caused by impurities in runoff water.

In Finland, there is a greater need for new stormwater solutions due to growing rainfall volumes and harder autumn and winter rains as a result of climate change. Various scenarios put the likely rate of increase in annual Finnish rainfall at 12–24 percent this century. A rise in the number of rainy days and the frequency of torrential rain are affecting the volume of rainfall. It is estimated that, in Finland, the six-hour maximum rainfall volume – which occurs once every 50 years – will increase by an average of 30 percent by the end of this century. 

In the shift towards more sustainable stormwater management, each country must find the means that best suit local conditions, making use of the most natural and sustainable solutions tried and tested elsewhere. Many countries have drawn or are drawing up their own regulations on these issues. European and national directives and strategies have pointed to permeable surfacing as a means of reducing the harm caused by additional water-tight surfaces in built environments. 

Stormwater management is regulated by the EU’s Water Framework Directive (2000/60/EC) and the Flood Risk Management Directive (2007/60/EU). Various solutions for stormwater management in urban environments are presented in the EU’s Soil Sealing Guideline (2012). In Finland, stormwater management is regulated by a range of laws, such as the Land Use and Building Act, the Water Services Act, the Water Act, the Flood Risk Management Act, the Nature Conservation Act and the Act on the Maintenance, Cleaning and Clearing of Public Areas. In addition, the municipalities’ building ordinances and instructions include provisions on stormwater management. Finland drew up a National Strategy for Adaptation to Climate Change in 2005, which is currently being revised. Many municipalities have drawn up their own stormwater strategies and programmes which take account of adaptation to climate change. 

The Land Use and Building Act includes provisions on the new stormwater plans, which are comparable to technical plans and present stormwater management solutions and structures, while accounting for future increases in rainfall. Such plans must fulfil the applicable requirements for functionality, safety and comfort.


Development alongside other actors 

VTT’s Climate Adaptive Surfacesproject in 2012-14 involved the development of permeable pavement solutions suitable for Finnish conditions. Such surfacing is a structure whose surface layer comprises a monolithic pervious material (e.g porous asphalt pavement (PAP), pervious concrete pavement (PCP), or paving stones or tiles (concrete/natural stone)) which itself and/or whose seams, openings, filler and levelling-layer material enables the high filtration of water to the subsurface structures. 

The subsurface structures are dimensioned for slower stormwater release to the environment or water collection system. The pore space of this water-retaining, section of mineral layers is temporarily filled with a water quantity corresponding to the dimensioned volume of rainfall, before the water is absorbed by the underlying ground and/or exits the structure. Other, supplementary materials, products and structures, such as drainage and water collection systems, geotextiles and geobarriers, can be included in permeable surfacing structural solutions. Permeable pavement systems can be used in place of traditional water-tight asphalt, concrete and natural stone for low-volume roads.

The CLASS project was funded by Tekes – the Finnish Funding Agency for Innovation, VTT and 15 partners covering a wide value chain. The project involved major contributions in terms of work by these partners, and other kinds of collaboration with various industry-sector companies. In addition to several towns and cities, partners included material and goods suppliers, associations and designers. The project was implemented in cooperation with a Swedish parallel project, “Grå-gröna systemlösningar för hållbara städer”. Sweden had complimentary research on issues such as vegetation for stormwater management and creation of holistic good living environments. 

Thanks to the extensive cooperation involved, the know-how and competencies acquired were placed at the disposal of various actors and planners while the project was still underway. For example, the cities involved in the work (Espoo, Helsinki, Vantaa and Oulu) began implementation of their own pilot demonstration areas in 2014, before the CLASS project had ended. VTT has been engaged in the design of pilots for the application of know-how gathered during the initial research, including planning for the monitoring and long-term follow-up of pervious pavements’ in-situ performance. Theoretical knowledge was applicable to the latest real cases and Finnish design practices. 

At the conclusion of the research project, urban planners and stormwater designers also participated in the creation of practical guidelines aimed at actors in the sector. In addition, the active participation of materials suppliers and players representing sectors such as the construction and natural stone industry guaranteed the practical application of the recent research.

The research into permeable surfacing suitable for Finnish conditions drew considerable public attention abroad and in Finland. Such attention included extensive exposure in environmental, infrastructure and basic materials sector conferences, and in events and articles. Around 150 people attended the project’s closing seminar held in January 2015. The Finnish television channel MTV and YLE radio have also invited VTT to tell viewers about permeable surfacing solutions and the CLASS project. 


Suitability for Finnish conditions

Permeable surfacing solutions need to be able to handle sufficient stormwater volumes, while their load carrying capacity and other qualities must meet the requirements of the specific location. 

The CLASS project provided information on how such basic requirements can be met and the most suitable locations for permeable surfacing. Permeable surfacing works best in areas free of excess wear caused by regular, heavy traffic. Optimal applications included streets subject to less wear and tear, pedestrian and cycle paths, parking areas, various interconnecting areas, yards, market squares and fields, playing and sports fields and exercise areas. 

Permeable surfacing layers are already commonly used in other countries in areas with milder climates, in particular in Belgium, Germany, Japan and the United States. The winter conditions in Finland and the Nordic countries set special, local requirements for permeable surfacing layers with respect to durability. A key challenge in the CLASS project involved exploring the basis on which permeable surfacing layers would suit Nordic conditions in particular. 

In a cold climate, attention must be paid to the frost-resistance of individual materials and the overall behaviour of the surface and entire structure in winter conditions. Surfacing is subject to freezing and thawing cycles, frost upheave, de-icing chemical treatments and application of anti-slip grit and studded tyres. 

In sensative groundwater areas, it is important to consider the possible impacts of filtration of diluted de-icing salt in stormwater. In such cases, salt must not be allowed to reach groundwater via the surfacing. Attention must also be paid to risks associated with silty substances, such as grit, that may block porous, permeable materials but can be removed with proper spring maintanence coupled with proper design and construction of the pervious pavements. 

Alongside the development of materials suitable for winter conditions, attention was paid to determining the overall, actual permeability of the surfacing in cold climate conditions. In addition, recommendations were developed regarding frost durability and winter maintenance for Finnish conditions. For success, the pervious pavement should remain permeable at the planned filtration value and the surfacing should remain undamanged for its designed service life. 

The following factors are fundamental to guaranteeing a well-functioning surface throughout its life cycle: a design accounting for the environmental and wear conditions, material selections suitable for the conditions, and the appropriate construction practices and techniques. The correct maintenance procedures – particularly occasional cleaning to restore permeability – are also important. 

During the project, it was observed that more experience and data were needed, particularly regarding in-situe performance in actual Finnish conditions. Although the greatest possible use was made of international pervious pavement know-how, Finnish winter conditions and practices set special requirements for the pervious systems and their use. Further know-how and experience are continuously being gathered from the cities’ pilot projects and VTT is participating in the planning, follow-up and monitoring of these demonstrations during 2015 and beyond. 


Figure 2. VTT’s rig facility, which can be used to simulate the hydrological functioning of permeable surfacing (surface material and subsurface structures). Surface area 0.5 × 1.0 m2, height adjustable.


Development of surface materials

Pervious concrete pavement (PCP) is a fairly new material in Finland. The aim of the CLASS project was to form a clear idea of these materials’ basic characteristics such as its strength, porousness and permeability, and in particular to confirm its freeze and thaw resistance. Methods of demonstrating and ensuring the material’s high durability were also sought. The aim was to provide the concrete industry with a basis for continuing its own product development for the new material. 

Lemminkäinen Infra Oy, a participant in the project, engaged in its own product development on porous asphalt pavement (PAP). VTT had the role of investigating the new PAP products’ durability characteristics and hydrological performance as surface materials. 

In the case of concrete stone and natural stone surfaces, the seams or openings are made of permeable material, i.e. mainly cleaned fine aggregate lacking the smallest particle sizes (<1 mm) and with the smallest possible quantities of fines (<0.125 mm). The seams can also be made of specially bound and permeable material. 

During the project, a special filtration rig facility was prepared for laboratory research on surfacing and geotechnical structures. The entire surfacing structure and all of the layers, including the water-detaining subsurfaces, could be built with the rig. The rig has a surface area of 0.5 × 1.0 m2 and its height can be adjusted from 0.33 to 1.0 m depending on the thickness and section of the surfacing to be studied. It also includes a sprinkler device controlling rain intensity corresponding to climate change predictions, and an automatic measurement of the quantity of stormwater filtering through the structure as a function of time. The permeability (m/s) of the pervious pavement was also measured from above the rig, using relatively simple standardised methods which are applicable for field measurements at the pilot sites. 

Because all permeable surfacing layers are porous, they are naturally subject to partial blockage. The rig-based research included a method for assessing how clogging of the surface affected the hydrological functioning of the surfacing materiasl. VTT studied the permeability of different surfacing solutions when clean, clogged and cleaned using two different techniques. In addition, knowledge was gained about the cleaning techniques that can be used to restore permeability to a sufficient level to cope with torrential rain. 

The rig apparatus will be used in future research on new surfacing and subbase filtration solutions. Solutions will also be developed on the basis of new, innovative and ecological materials, which fulfil all of the  functional requirements. One key topic of VTT’s upcoming research is adding stormwater purification characteristics to permeable pavements and their subbase layers. 


Modelling tool

The CLASS project also included the development of modelling techniques which take into account the special characteristics and capabilities of new materials and permeable pavement systems. A modelling tool developed during the project supports the application of the results to the hydrological measurement and design of surfacing. 

The prototype tool is an Excel/VBA program. Within the calculation pages, this tool asks for the input data for hydraulic behaviour calculations related to permeable surfacing layers and creates an input text file compatible with the existing SWMM program. SWMM, the ´Storm Water Management Model´, has been developed and released by the US Environmental Protection Agency. It is mainly a stormwater runoff model for modelling both one-off and long-running rainfall in built environments. SWMM is an extremely flexible but complex simulation program enabling planners to analyse overall stormwater management. The modelling tool developed in CLASS supports the use of the SWMM as a calculation component.

The CLASS tool includes a range of databases based on the results of the CLASS project among others, as well as data gathered from CLASS project partners and literature. Such data can be of direct benefit in dimensioning and design processes related to permeable surfacing. The calculations performed using the tool give the runoff water quantities at different sites based on various development and management scenarios, using historical rainfall data, estimated rainfall taking account of climate change predictions, or shorter-term measured precipitation and heavy rainfall for the area in question. If necessary, the prototype can easily be developed into a software product for calculating the hydrological behaviour of surfacing structures. 


Planning, construction and maintenance guidelines

With the help of partners representing various organisations, the CLASS project ultimately led to the creation of guidelines on pervious pavement systems, which takes account Finnish conditions and planning practices. 

The main contents include the design principles, locations, dimensioning principles, instructions on the selection and quality control of materials and filler products related to pervious pavement layers, dimensioning guidelines in relation to structural, hydrological and frost and water-quality evaluation, as well as construction and maintenance directions.  

The full set of project reports and the publication Pervious Pavement Solutions – guidelines on design, construction and maintenance’ are available from the CLASS pages. 


Figure 3. Permeable, concrete stone surfacing layer. Parking area.


Pilots and further development

Even before the project, there was a clear need for pervious pavement solutions in Finland’s cities. There has long been a need for new, concrete means of stormwater management and the prevention of urban flooding. Planning of the C​LASS project pilot demonstrations in the cities involved (Espoo, Helsinki, Oulu and Vantaa) began in 2014 and the construction has started in spring/summer 2015. Pilots are required in order to gain practical experience of the planning, implementation and overall functioning of pervious pavements. Some technical factors also impact on the performance of permeable solutions in Finnish conditions. Useful information on these will be accumulated from the pilots and the related long-term performance monitoring. Such performance data will bring additional certainty for the basis of how permeable pavements can become a more widely used,  as a normal and successful method for stormwater management. The pilots will involve gathering experience and additional quantitative data through methodical monitoring  performance evaluations. 

VTT will have the role of contributing the specialist expertise developed and gathered during CLASS and its Swedish sister project to the pilots. The Chief Consultant of each project will be in charge of the design and dimensioning of each site. The pilots will involve the follow-up and measurement of freeze-thaw behaviour, the permeability and condition of various surfacings, surface conditions such as slipperiness, as well as runoff water quantities and the surface water level in water-detaining structures. Measurements will be performed on a more frequent basis during the first two years and then continuing occasionally during the long-term monitoring. A general aim is to verify the suitability of permeable surfacing for Finnish conditions. On the basis of the know-how gained during the pilots, it will be possible to verify and possibly revise the guidelines regarding the design, construction and maintenance of pervious pavement solutions in Finland.

The development of products and innovative solutions suitable for pervious pavements are providing Finnish industry with new business opportunities. The results obtained during the CLASS project provide a sound basis for this. VTT aims to be involved in further development such as work on quality assurance practices and the creation of new applications, material innovations and modeling for stormwater management. The latest plans involve issues such as the purification of urban stormwater using pervious pavements and ecological subbase materials, and the overall management of stormwater from heavy rain and of the related risks.

Actors from both Finland and abroad have shown extensive interest in participating in the follow-up projects. Further research is already being planned under the EU H2020 programme, the Nordic Built Environment programme, Tekes Smart Cities and within the framework of US-European cooperation.



Homesite, research reports, articles and presentations related to the CLASS project: VTT Technical Research Centre of Finland Ltd.

Pervious Pavement Solutions – guidelines on design, construction and maintenance. VTT TECHNOLOGY 201. 59 pp. + appendices



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