Information and Advisory Note Number 5                                                 Back to menu

Permeable surfaces - storm water control at source

1. Introduction

1.1 In most types of paving, the stability of the surface is maintained by excluding water from the underlying soil. The vast area of impermeable surfaces created by modem development interferes with water catchment systems, reducing the quantity of water reaching the aquifers and increasing water runoff and the risk of flash flooding. It may also increase problems of erosion, siltation and pollution downstream from urbanised areas.

1.2 The National Planning Policy Guideline (NPPG) on Planning and Flooding, states that the susceptibility of land to flooding is a material considerations deciding a planning application. An application for development can now be refused if it would result in a significant increase in surface water runoff relative to the receiving watercourse in flood prone areas. For new developments that increase the areas of hard surface, 100% runoff should be assumed for drainage design purposes unless permeable surfaces, soakaways or detention ponds are included as integral design features.

1.3 Storm water runoff can also have a significant effect on the water quality of the streams, rivers and waterbodies into which the urban drainage discharges. In a recent survey Forth River Purification Board (FRPB) found that runoff from urban areas was responsible for 14% of cases of poor river quality.

1.4 Permeable paving is a relatively recent development that combines surface stability and permeability. This technique appears to be widely used in the United States, where it is recognised that the increasing demand for car parking brings with it serious ecological problems.

1.5 While it is also used extensively in Japan and Europe, permeable paving has not yet been widely adopted in the UK.

2. Basic properties of permeable surfaces.

2 1 Like conventional paving, permeable surfaces consist of stone aggregate with some type of binder - either asphalt or cement, underlain by larger aggregate. The difference lies in the proportion of air space contained within the layers of the permeable surface, which is sufficient to allow water to flow through the material while the surface remains strong enough for pedestrian or light vehicular use.

3. Why use a permeable surface?

3.1 In the UK, stormwater has traditionally been controlled by building trunk sewers to concentrate the flow, and by treating the stormwater conveyed in combined systems at sewage treatment works. As the built area expanded, sewers were more frequently overloaded, causing flash flooding.

3.2 As the built area increases, the quantities of stormwater generated can overload these sewers and cause flooding. The earliest attempts to control the flooding involved increasing the capacity of the drainage system by laying larger pipes or duplicating sewers. This became increasingly costly and disruptive as the urban areas expanded, and tended simply to move the flooding problem further downstream.

3.3 In order to accommodate the increased flows, streams and rivers which received the stormwater had to be straightened and dredged to increase their capacities, reducing the nature conservation value and detracting from the recreational or amenity potential of the area. Watercourses can provide key natural heritage assets, particularly in or near urban areas.

3 4 There is a growing view that the Best Practicable Environmental Option as advocated by the Environmental Protection Act should include the control of water runoff at source. NPPG 7 also promotes the idea of integrated catchment planning, which includes environmentally acceptable strategies for controlling water runoff from urban areas.

3 5 In Sustainable Development and the Natural Heritage. SNH advocates a more sustainable approach to development in Scotland, one that does not put at risk the ability of the natural environment to meet the needs of future generations. There may be more sustainable ways of designing new developments as well as dealing with existing urban land to address the problem of flooding and pollution caused by water runoff.

3 6 The Nekar Project, for example, came about as a result of factory closures and the economic decline of an area near Stuttgart. The project involves collaboration between planners, designers and ecologists and aims to reduce the extent of impermeable surfaces and to encourage natural regeneration in the resulting open spaces.

3 7 Intercepting rainfall at source can provide significant benefits over the traditional "pave and pipe" approach, both in terms of flow attenuation and pollution control. Permeable surfaces offer an environmentally benign technique for controlling water runoff. They can help alleviate problems due to flooding, erosion and siltation and may also conserve groundwater resources.

3 8 Permeable surfaces can also moderate the ambient temperature of urban areas by evaporative cooling. More relevant to Scotland perhaps is the performance of permeable pavement in low or freezing temperatures. Tests in the UK suggest that for short spells of
cold weather, the air within the permeable surfaces acts as a sort of 'night storage heater', slowly releasing heat to the surface and melting frost. This could mean that winter salting of permeable surfaces might be avoided in all but the severest conditions.

3 9 The cost of materials and installation of a permeable surface may be in the order of 10% to 15% more than that for a conventional surface. This might be outweighed by the savings associated with the reduced need for infrastructure such as gullies, catchpits, sediment traps, oil separators and below ground storm water drainage systems.

4. Site specific criteria

4 1 The successful integration of a permeable surface within a development does require greater attention to design, construction and
maintenance than with conventional types of paving.


4 2 Systems can be designed to allow all rainwater to infiltrate to the underlying soil (the sub-grade) or, where there is a risk of soil or groundwater pollution, stormwater can be discharged from storage in the sub-base to a conventional piped sewerage system. Where balancing ponds are required because the storm water system is already at capacity, this type of underground reservoir can free the land that would otherwise be required for the pond, to be used for other purposes.

4 3 Where rainwater infiltrates to the ground, the soil percolation rate has to be taken into account and as a guide, the underlying soil should not contain more than 30% clay. The finished levels should lie at least 900mm above the water table and the slope of the bottom of construction has to be less than 3%, otherwise terracing of the sub-grade is needed. There is no fixed depth for the sub-base, this has to be assessed in each case, taking into account the slope of the site, percolation rate of the underlying soil and the design storm. Water should never be permitted to back up into the permeable surface, so the sub-base should be large enough to contain all the stormwater from a storm event with additional volume to allow for subsequent rainfall while the water is percolating.

A 50mm course of permeable macadam, 12-19mm diam aggregate
B 50mm course of 15-25mm diam crushed stone
C 300-500mm reservoir course of 25-200mm aggregate Depth vanes according to site conditions
D Compacted sub-grade
E Impermeable membrane such as "Visqueen" 1000 gauge plastic
F Flexible plastic permeable pipe
G Bedding layer, as C
H Concrete foundation
I Precast concrete kerb
J Flexible plastic permeable pipe
K Permeable membrane, such as "Terrain" 1000
L Uncompacted sub-grade


4 4 With all permeable surfaces, the construction should be kept free of silt at all times. In practice, it is important to keep the surface free of construction traffic and materials should not be stored on the surface. Ideally, the surface should be formed at the end of the construction process, once any earthmoving, topsoiling, grass seeding or planting is complete, to minimise the risk of soil clogging the surface. Site levels should be designed to minimise the likelihood of runoff from planted areas onto the permeable surface.

4 5 The effective operation of a permeable surface relies upon it being installed correctly. The greatest risk is from compaction of the base layers, and the clogging of pores by soil and other particulate matter.

5. Types of permeable surface

Permeable asphalt and macadam

51 Asphalt consists of a dense mix of mineral matter bound by bitumen. A mortar of fine aggregate, filler and high viscosity binder gives the material its strength. This is distinct from coated macadam, which consists of graded aggregate coated with tar or bitumen, which derives its strength from the interlocking of aggregate particles.

Permeable macadam

5.2 At first, macadam was produced using stone of one size, with only a small proportion of fine particles. These early surfaces were highly permeable, it is the requirement for surfaces to withstand ever-increasing traffic loads that has led to the development of denser, impermeable mixes. Permeable or open grade macadam may soften and break up in warm weather and the surfaces are also vulnerable to damage caused by vehicle turning movements, resulting in loss of porosity.

5.3 For these reasons, open grade macadam is not generally recommended for use in warm climates, or for roads or parking aisles where traffic is heavy. It does have the potential to be used in Scotland for residential or employee car parks. It may also be used in busier car parks, for parking bays or in strips beside the parking area to take the stormwater runoff from conventional paving.

Permeable asphalt

5.4 Over the last twenty years, great effort has been devoted to developing porous asphalt mixes for airport runways in an attempt to reduce the amount of spray generated as an aircraft lands. More recently this material has been used for resurfacing trunk roads, where the advantages include both spray reduction and reduced road noise levels. Porous asphalt has been used on a number of European roads, and tests are currently underway on trial surfaces in Broxburn, Forth village (Strathclyde), and on a section of road between the Barnton and Maybury
roundabouts, Edinburgh.

5 5 Porous asphalt is generally more expensive than non-porous, and requires very careful compaction if the permeability of the surface is not to suffer.

5 6 It is generally placed over a base of impermeable asphalt. The depth of porous asphalt has not yet been standardised, as its use is still experimental, but in Germany the layer is usually 40mm. The underlying impermeable asphalt is graded to shed water at the edge of the road into standard drainage systems.

5 7 Heavy pressure is applied to ensure firm compaction of the impermeable layer and the underlying sub-base. This cannot be done for a permeable surface as the procedure will reduce the permeability. A compromise solution should be possible, bearing in mind that in the USA, both Cahill Associates and Andropogon Associates have designed and built car parks that use a layer of permeable asphalt 65mm thick, often located over a sub-base that acts as a reservoir.

Porous concrete

5 8 Porous concrete consists of a uniformly graded aggregate bound with Portland cement. It is more suitable for use where the climate is warmer and has been used in Florida, for example, where heat could cause permeable asphalt to rut. The potential of porous concrete should be explored further, particularly as it is known to be able to withstand heavier loads than porous asphalt.

Concrete block paving

5 9 Permeable concrete block paving must contain 15-20% void and be capable of being laid on a free draining sub-base. Most commercially available blocks are therefore unsuitable, having only a small gap between blocks, and needing to be laid on a compacted sand bed, which becomes impermeable as silt from runoff accumulates in the sand layer.

5.10 Cee-py blocks were developed in the UK after observing the tendency of grass-concrete to become impermeable after the soil contained within the blocks is compacted by vehicle movements. Cee-py blocks are similar to the familiar concrete block paviours but have a raised disc on the surface, which prevents the loading of the car tyre being applied to a pattern of 50mm diameter percolation holes. These holes are filled with gravel, and the blocks are laid on a bed of gravel over a sub-base of free draining crushed stone.

5 11 At Shire Hall, Reading, the Department of Architecture, Royal County of Berkshire used Cee-py concrete blocks to surface an infiltration trench along one side of a 0 6 hectare car park. The car park, constructed in 1986, was surfaced with conventional block paving and graded to fall into the trench. The permeable surface lining the trench overlay a sub-base of free-draining aggregate, which allowed stormwater to infiltrate to groundwater. In 1992, field measurements on the percolation holes found an infiltration rate of 2,600mm/h on average, which was sufficient to ensure full interception of surface runoff under rainfall intensities of 60mm/h over the car park. This performance was very satisfactory as the trench surfacing had not been maintained during the six years of operation.

512 Although the Cee-py block is no longer available, other products are appearing on the market all the time, and it is likely that British manufacturers will start to offer a range of permeable concrete products as demand increases.

"CEE-PY" TYPE CONCRETE BLOCKS (Courtesy of Professor Chris Pratt)

6. Obstacles to the wider use of permeable surfaces

6 1 Wider acceptance by authorities in the UK is perhaps constrained by fears that the permeable surface may become clogged over time, subject to frost heave or the cause of ground water contamination.

Surface clogging and groundwater contamination

The rate at which a surface may become clogged is uncertain, but it appears unlikely that clogging of small areas will affect the overall performance of the surface. Clogging can be avoided to some extent by careful design and construction, by siting shrub beds in order to avoid soil laden runoff spilling onto the surface, for example.

In the USA, it is recommended that permeable car parks are vacuumed on a regular basis. This is likely to be an option for only the most high maintenance, prestigious schemes in this country. Pressure hoses can also be used to clean the surface, although there is a risk that fine particles can be driven further into the pavement. Small areas of clogged surface can be repaired by drilling the affected areas with a small diameter drill bit. If block pavers are used, it is a relatively simple and cheap task to lift individual blocks and replace the subsurface layers as required.

6 2 A small car park of 200m2 at Nottingham Trent University was surfaced with permeable macadam as an experiment over five years ago. Despite little maintenance, the surface still remains free-draining. Monitoring of this surface suggests that any pollutants present in the stormwater - from oil or petrol spillage, for example - tend to accumulate inside the permeable macadam, mainly on the geotextile at the base of the construction, and there is little evidence of any contamination of either the groundwater or local watercourses.

6 3 FRPB have reported removal of up to 80% of sediment, 60% of phosphorus and 80% nitrogen as well as high rates of organic matter and trace metals from stormwater by permeable surfaces.

6 4 Evidence from France, Japan, Sweden and the United States supports the theory that with shallow infiltration into surface soils, especially into the weathered zone, pollutants are often intercepted or dispersed, through becoming chemically or physically attached, or by degrading.

6 5 If a permeable surface has to be used where spillage of pollutants is likely, the use of a sub-base reservoir surrounded by an impermeable membrane is recommended. This should provide the necessary attenuation to control downstream flooding without modification to the existing sewer system.

6.6 The risk of groundwater contamination would be minimised since the stormwater would discharge via the sub-base reservoir to the sewer system. The permeable surface would act as a "sink" in this case, concentrating the pollution problem at a known location.

6 7 Any material removed during routine maintenance must be treated as special waste, as it may be enriched with absorbed or attached pollutants. This may appear to be a major disincentive to the use of source control where surface pollutants are present, but would change were realistic charges levied on the disposal of polluted stormwater.

Frost Heave

6.8 Tests of porous asphalt surfaces in the US indicated that the pore space allows sufficient space for freeze expansion and that frost heave is not likely to occur.

Divided responsibilities

6.9 One further constraint to the wider adoption of permeable surfacing may be the complex pattern of statutory responsibility here. For example, in the case of a retail development with parking, the design of the car park would be the responsibility of the developer and would be approved by the planning authority. If stormwater were to be routed to a local watercourse, the construction of the drains to do this would also be the responsibility of the developer. But if the stormwater were to be discharged to the sewerage system, this would then become the responsibility of the sewerage authority. The construction of a pavement is a regional responsibility, whereas the cleansing of it would probably be done by the district authority. The benefits of permeable surfacing would, therefore, have to be demonstrated to a number of organisations and any legal or fiscal implications fully identified. This will also be the case following local authority reorganisation in April 1996.

7. Lessons from elsewhere

7 1 Japan appears to be leading the field in the use of stormwater source control techniques. Here, surface water disposal is seen to be more of a local issue and source control techniques are regarded as a useful way to reduce the need for large, expensive public ventures such as new sewage treatment works.

7 2 By 1984 some 250 Hectares of the suburbs of Tokyo were connected to a local infiltration and storage drainage system, called the Experimental Sewer System. As a result, the peak flow to the Shakujii River, which previously received all the stormwater runoff from the catchment, had been reduced by a factor of 2.5. In 1989, Tokyo had 37 Hectares of permeable pavement and 716km of infiltration trenches. So far, no groundwater contamination has been observed, although this is something that will be closely monitored in the future.

7.3 It is in the United States, however, that advice about the design and maintenance of source control techniques is most readily available, mainly due to the history of using permeable pavements for car parks there. The technology was originally developed in the
United States to reduce surface ponding on airport runways, but permeable surfaces are now used extensively by American landscape architects for various types of paving schemes.

7 4 These techniques are also becoming more common throughout Europe. In Sweden permeable macadam car parks laid on a sub-base of free-draining, crushed stone aggregate is said to cost 25% less than conventional macadam surfacing when taking all construction and drainage costs into account. Ground conditions of rock or clay have meant that infiltration into the ground is limited and stormwater has to be discharged from the sub-base through drains at the edge of the construction.

8. Reservoir structures

8.1 These have been used extensively in France to take highway drainage. They usually consist of a free-draining, crushed stone sub-base, contained within a membrane into which water from a conventional paving surface is then fed.

8.2 Reservoir structures can reduce the rate of stormwater runoff by storing water within the sub-base. This is the case whether or not there is any reduction in water volume by infiltration into the soil. The storage capacity of the sub-base may be 100-200mm rainfall which could allow discharge from the reservoir at a more suitable time and so reduce the chance of flooding. Water could also be passed to treatment works overnight, for example, when industrial and household discharges are lower or might also be recycled in situ for garden irrigation or car washing.

8.3 Reservoir structures contained within an impermeable membrane can also be used in a wider range of ground conditions. The stormwater is removed from the sub-base via a drain, to discharge to a sewer or for use in a suitable re-cycling facility. Analysis of the drain effluent from these structures indicates that the construction provides treatment of the stormwater similar in standard to that provided by a sewage treatment works. Because the aggregate in the sub-base remains wet, the volume of stormwater ultimately discharged into the stormwater system is often reduced as a result of evaporation from the surface. This arrangement is recommended for use if there are concerns about the potential harm to any adjacent tree or shrub planting arising from contamination of the soil by runoff.

9. Swales

9.1 Swales are grassed depressions, which lead surface water overland from a drained surface to a storage or discharge system. They are usually dry with stormwater evident only after a storm. Sediment from the runoff is removed as the water passes over the surface of the swale. Although any oil residues and organic matter retained in the top layer of soil may be broken down by soil bacteria, swales are probably of most use where there is no risk of groundwater pollution.

9 2 The main advantage of swales is their low cost In the United States in 1987, for example, the cost of construction and drainage of a 4 hectare car park using swales and a conventionally paved surface was 1/6th of that using a permeable pavement. The principal cost saving was in the type of sub-base material used. Whereas the traditional road pavement materials were readily available, the free-draining, clean stone for the permeable pavement had to be specially transported to site.

9.3 Swales can be used as infiltration areas and as conveyance channels for stormwater, if discharge is available off site. They require regular grass cutting but it is fairly apparent when maintenance is required. Swales are very commonly used in the United States, often in addition to a permeable surface where additional stormwater storage is required.

A - 25-200mm diam free draining aggregate
B - 100mm diam perforated plastic pipe
C - Uncompacted sub-grade

9 4 Where soil permeability and water table permit, infiltration trenches (shallow excavated trenches backfilled with stone to create a subsurface reservoir), infiltration basins (surface impoundments where stormwater is stored until it gradually filters into the subsoil) and French drains may all provide control of runoff at source, allowing water to gradually fitter into the subsoil and eventually to the water table. Most work best in combination with filter strips (vegetated areas designed to accept overland sheet flow from an upstream development), gullies or silt-traps which can remove excessive solids. The choice of the most appropriate system will depend upon the site characteristics and the objectives of a specific application. Please see FRPB guidance for further details.

9.5 Shallow, permanently damp areas planted with marsh vegetation can reduce pollutants by a complex interaction of chemical physical and biological processes. Successful design of constructed wetlands is complicated and their performance and longevity is dependant on the care with which they are designed and installed. The flow attenuation and pollutant reduction provided by permeable surfaces is likely to enhance the performance of a wetland, and the use in combination is worth further investigation. Please see FRPB guidance for further details.

10. Potential in Scotland

10 1 Examples of these techniques are needed to demonstrate to developers that there are no great economic constraints to using source-control techniques and give local authority engineers the confidence that the techniques will work if they approve them.

10 2 Car parking and other paved areas in new industrial, commercial or residential developments located within catchments that are prone to flooding during short duration, high intensity storms probably offer the best scope for using permeable surfaces.

10 3 Providing these sites are not contaminated, or at risk from surface pollution, permeable macadam or concrete block paving may be used with a sub-base with a high storage capacity, to act as a reservoir releasing stormwater into the underlying soil at an appropriate rate. (Note that all surface water discharges completed since 1985,

except those to soakaways, require the formal consent of the appropriate river purification authority).

11. Acknowledgements

11 1 This advice note is based upon a study commissioned jointly by SNH and Scottish Enterprise National, which was carried out by Cobham Resource Consultants, Edinburgh, in association with Professor Chris Pratt of Coventry University. SNH intends to publish the full report of the findings of this study early in 1996 under the title Permeable Surfaces. This will be available from Publications Section, Scottish Natural Heritage, Battleby, Redgorton, Perth PH1 3EW.

12. Bibliography

The following are all held at RASD

Scottish Natural Heritage Sustainable Development and the Natural Heritage. Battleby October 1993.

Cobham Resource Consultants Permeable Surfaces. Unpublished report to Scottish Natural Heritage Edinburgh February 1994

Cobham Resource Consultants Permeable Surfaces II Unpublished report to Scottish Natural Heritage. Edinburgh. March 1995

Forth River Purification Board. Guide to Surface Water Best Management Practices. Edinburgh January 1995

Forth River Purification Board. Wetland Treatment Systems for Urban Drainage. A practical design guide Edinburgh. February 1995

Scottish Office, Planning and Flooding. National Planning Policy Guideline 7,1995

13. Contact for advice and information:

Nigel Buchan - Landscape Advisor, Landscape and Restoration Branch, 2 Anderson Place, Edinburgh EH6 5NP Telephone 0131 446 2429

Please note that the advice contained in this document is provided in good faith for general information only. It is beyond the scope of this note to provide site specific advice and it is therefore incumbent upon users to satisfy themselves that the designs and recommendations provided here are suitable in every respect before implementing them.

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