GB2404213A

GB2404213A - Water management system

Info

Publication number: GB2404213A
Application number: GB0415967A
Authority: GB
Inventors: Howard Lloyd Robinson; Colin Andrew Loveday; Nicholas Paul Toy; Benjamin John Sealey
Original assignee: Tarmac Ltd
Current assignee: Tarmac Ltd
Priority date: 2003-07-16
Filing date: 2004-07-16
Publication date: 2005-01-26
Anticipated expiration: 2024-07-16
Also published as: GB2404213B; GB0415967D0

Abstract

The water management system, primarily for managing storm water over a surfaced area and particularly such areas which support vehicular traffic and upon which there may be spillages, includes a first course 11 that comprises one or more areas of permeable material and one or more areas of impermeable material, a permeable understructure beneath the first course, the permeable understructure including a porous foundation layer 20 from which a drainage conduit 25 extends to a point beyond the boundary of the surfaced area. The permeable material may comprise porous asphalt that includes penetration grade bitumen, which may be modified with fibres, or polymer modified bitumen. Alternatively the permeable layer may comprise porous concrete. The impermeable layer may comprise impermeable asphalt or impermeable concrete. The one or more impermeable areas are preferably provided with a camber. The permeable understructure may include a supporting substrate 12 with upper 13 and lower 14 courses of asphalt, containing finer and coarser aggregate, respectively. The foundation layer may be laid on a natural or laid sub-grade 22 and may include a filter layer of particulate with interstitial cavities between the particles.

Description

24042 1 3 Title: Water Management System

Description of Invention

This invention relates to a water management system for managing storm water and/or spillage on a surfaced area.

As a result of an increasing proportion of the land surface being surfaced, particularly in urban regions, an increasing proportion of storm water is dealt with by draining immediately directly into man-made drains and into water courses and the like, thus placing a high burden on such drainage systems. In contrast, in unsurfaced areas, the water may pass into and dwell in the ground and seep into water courses and the like over a longer period.

Moreover storm water which runs off surfaced areas is not naturally cleansed by bacteria and the like action, as is water which passes into the ground. As a result, water courses and the like can become rich with contaminants which otherwise would have been cleansed from the water.

It has been proposed in GB patent 2294077 to provide a paved surface course through which water may pass into a substrate beneath, with there being a water impermeable membrane beneath the substrate, which membrane contains particles of the substrate in which water passing through the paved surface course may dwell, with there being drains from the substrate for the water.

However the use of an impermeable membrane restricts the capacity of the system in that in the event of a particularly heavy fall of water, the volume contained by the membrane may fill up, and thus water will need to be discharged via the drains, thus contributing to the burden imposed on local drainage systems by run-off water from other surfaced areas.

In accordance with the present invention we provide a water management system for managing storm water over a surfaced area, the system including a first course comprising one or more areas of permeable material and one or more areas of impermeable material, a permeable understructure beneath the first course, the permeable understructure including a porous foundation layer, there being a drainage conduit from the foundation layer to beyond the boundary of the surfaced area.

With the system of the invention, water may exit the system into the ground beneath the system as well as through the drainage conduit, and thus the system of the invention provides the advantages of the system disclosed in GB- A 2294077 whilst overcoming a major problem with this system.

The first course may comprise a surface course of the water management system. A top surface of the first course may be texturedlcoloured as desired suitable for its particular use, for example to provide for increased skid resistance.

The first course may have a thickness depending upon the use of the system. In one example, where the system is intended for medium to heavy traffic use and the understructure includes a supporting substrate between the first course and the foundation layer, the first course may have a thickness of between 10 mm to 50 mm, and preferably of about 25 mm. In another example, where the system is intended for only light, pedestrian traffic use and no supporting substrate is provided, the first course may have a thickness of between 40 mm and 80 mm.

The permeable material of the first course may comprise porous asphalt.

The porous asphalt may have a composition depending upon the use of the system. The porous asphalt may include penetration grade bitumen, such as 100/150 grade bitumen, of European Standard EN12591. The porous asphalt may include penetration grade bitumen modified with fibres and/or polymer modified bitumen. The latter is preferable when the system is to be used for vehicular traffic. The porous asphalt of the first course may include aggregate, ! the particles of which are all able to pass through a 14mm sieve, and preferably a 1 Omm sieve, and more preferably a 6mm sieve.

The permeable material of the first course may comprise porous concrete.

The impermeable material of the first course may comprise impermeable asphalt. The impermeable asphalt may have a composition depending upon the use of the system. The impermeable asphalt may have a composition which is at least substantially able to withstand stresses without fretting of the impermeable asphalt. The impermeable asphalt may have a composition which is at least substantially able to withstand stresses from vehicular traffic without fretting of the impermeable asphalt. The impermeable asphalt may have a composition which is able to at least substantially withstand turning stresses of vehicular traffic without fretting of the impermeable asphalt. The impermeable asphalt may have a composition which is able to withstand spillage of substances onto the asphalt.

The impermeable asphalt may have a composition in accordance with BS49871. The impermeable asphalt may have a denser composition than the composition of the permeable material of the first course when this comprises porous asphalt. The impermeable asphalt may include bitumen. The impermeable asphalt may include bitumen in a quantity which is determined by the type and density of aggregate of the impermeable asphalt. The bitumen may comprise any of, or a combination of any of, 40/60 grade bitumen, 50/70 grade bitumen, 70/100 grade bitumen, 100/150 grade bitumen, 160/220 grade bitumen, 250/330 grade bitumen. The impermeable asphalt may include aggregate. The aggregate may comprise coarse graded aggregate. The aggregate may comprise any of, or a combination of any of, rock, gravel, limestone, slag such as steel slag or blast furnace slag. The aggregate may have particles which are all able to pass through a 14mm sieve, and preferably a lOmm sieve, and 55% to 75% of which are able to pass through a 6.3mm sieve, and 19% to 33% of which are able to pass through a 2mm sieve, and 15% to 30% of which are able to pass through a lmm sieve, and 3% to 8% of which are able to pass through a 0.063mm sieve. The aggregate may have particles which are all able to pass through a lOmm sieve, and preferably a 6.3mm sieve, and 36% to 52% of which are able to pass through a 2.0mm sieve, and 20% to 50% of which are able to pass through a l.Omm sieve, and 7% to 23% of which are able to pass through a 0.25mm sieve, and 2% to 10% of which are able to pass through a 0.063mm sieve.

The impermeable material of the first course may comprise impermeable concrete. The impermeable concrete may have a composition depending upon the use of the system. The impermeable concrete may have a composition which is at least substantially able to withstand stresses without fretting of the impermeable concrete. The impermeable concrete may have a composition which is at least substantially able to withstand stresses from vehicular traffic without fretting of the impermeable concrete. The impermeable concrete may have a composition which is able to at least substantially withstand turning stresses of vehicular traffic without fretting of the impermeable concrete. The impermeable concrete may have a composition which is able to withstand spillage of substances onto the concrete.

The impermeable concrete may have a composition in accordance with BS8500. The impermeable concrete may have a strength class of C28/35. The impermeable concrete may have a consistence class of S2. The impermeable concrete may have a density in the range 2300 to 2400kg/m3. The impermeable concrete may comprise cement. The mean cement content may be 300kg per cubic metre of concrete. The cement may be Cem I or CII/B-V or CIII/A class cement. The impermeable concrete may comprise aggregate. The aggregate may be 4/20, coarse aggregate. The aggregate may be 0/4rnm fine aggregate.

The impermeable concrete may comprise water. The water may comprise approximately 7% by mass of the concrete. The water/cement ratio may be approximately 0.55. The impermeable concrete may comprise an air entraining agent. The air entraining agent may give 3.5% to 7.5% entrained air in the concrete. The impermeable concrete may comprise a water reducing agent. The impermeable concrete may comprise polypropylene fibres.

The system may include a first course comprising one or more areas of porous asphalt and one or more areas of impermeable asphalt. The system may include a first course comprising one or more areas of porous asphalt and one or more areas of impermeable concrete. The system may include a first course comprising one or more areas of porous concrete and one or more areas of impermeable asphalt. The system may include a first course comprising one or more areas of porous concrete and one or more areas of impermeable concrete.

The system may include a first course comprising one or more areas of porous asphalt, and one or more areas of impermeable asphalt and one or more areas of impermeable concrete. The system may include a first course comprising one or more areas of porous concrete, and one or more areas of impermeable asphalt and one or more areas of impermeable concrete. The system may include a first course comprising one or more areas of porous asphalt and one or more areas of porous concrete, and one or_more areas of impermeable asphalt. The system may include a first course comprising one or more areas of porous asphalt and one or more areas of porous concrete, and one or more areas of impermeable concrete. The system may include a first course comprising one or more areas of porous asphalt and one or more areas of porous concrete, and one or more areas of impermeable asphalt and one or more areas of impermeable concrete.

An area of impermeable material may be provided at each part of the system which is required to at least substantially be able to withstand stresses without fretting of the material. An area of impermeable material may be provided at each part of the system which is required to at least substantially be able to withstand stresses from vehicular traffic without fretting of the material.

An area of impermeable material may be provided at each part of the system which is required to at least substantially be able to withstand turning stresses from vehicular traffic without fretting of the material. Such turning stresses may be caused, for example, by vehicles having power steering turning into and out of parking bays in a car park. An area of impermeable material may be provided at each part of the system where there is a likelihood of substances being spilled onto the material. Such an area may comprise, for example, an area where substances are moved into and out of vehicles or buildings, such as a loading bay. Impermeable areas are useful in this circumstance, as if substances are spilled onto areas of a permeable material, these areas can be clogged by the substances, limiting their porosity.

The one or more areas of impermeable material may be at least partially surrounded by the one or more areas of permeable material of the first course.

Thus a water management system is provided which has permeable areas which allow drainage of water (but may have less ability to withstand stresses from e.g. vehicular traffic and spillage), and impermeable areas which are able to withstand stresses from e.g. vehicular traffic and spillage. The one or more areas of impermeable material may each be provided with a camber, to enable water to run off the area, for example onto one or more areas of permeable material and/or over the boundary of the system.

The understructure may include a permeable supporting substrate. The permeable supporting substrate may include permeable asphalt. The permeable supporting substrate may include an upper course of permeable asphalt and a lower course of permeable asphalt, in which aggregate of the asphalt of the lower course is coarser than aggregate of the asphalt of the upper course.

The upper course of the permeable supporting substrate may be a binder course. The upper course may include aggregate all the particles of which are able to pass through 20 mm sieve and preferably a 28 mm sieve. The upper course may include permeable asphalt which contains penetration grade bitumen, such as 100/150 grade bitumen. The upper course may include permeable asphalt which contains penetration grade bitumen bound by fibres and/or polymer modified bitumen. The upper course may have a thickness of between 30 mm and 80 mm and preferably of about 60 mm.

The lower course of the permeable supporting substrate may be a base layer. The lower course may include aggregate all the particles of which are able to pass through a 40 mm sieve. The lower course may include permeable asphalt which contains penetration grade bitumen, such as 160/220 grade. The lower course may have a thickness of between 30 mm and 80 mm and preferably of about 70 mm.

The system may include a semi-permeable membrane. The semi- permeable membrane may be provided beneath the supporting substrate of the understructure. The semi-permeable membrane may be provided beneath the foundation layer. The semi-permeable membrane may be a synthetic geotextile membrane, although any other suitable kind of membrane may be provided which permits the passage of water therethrough and which may substantially prevent the passage of contaminant particles into the foundation layer and ground beneath. Thus contaminants will dwell in the permeable supporting substrate and this will give time for bacteria and the like to cleanse the contaminants.

The porous foundation layer may include a filter layer of particulate material with interstitial cavities between the particles. The particle sizes may vary such that all of the particles are able to pass through a 300 mm sieve, but none of the particles are able to pass through a 6mm sieve. The porous foundation layer may have a thickness of at least 200 mm and preferably of about 295 mm, but may have a greater thickness as required. The porous foundation layer may be laid on a natural or laid sub-grade.

The drainage conduit may extend to a man-made or natural drain, or may be a simple soak away. In each case, preferably the drainage conduit extends into the foundation layer and has passages for water from the foundation layer through walls of the conduit.

An embodiment of the invention will now be described with reference to the accompanying drawing, which is a schematic section through a water management system in accordance with the present invention.

Referring to the drawing a water management system 10 for a surfaced area includes a first course 11, and an understructure including a permeable supporting substrate 12 (which includes an upper binder course 13 and a lower base layer 14), a semi-permeable membrane 16 beneath the supporting substrate 12, and a porous foundation layer 20, which is laid on a natural or man- made sub-grade 22. One or more drainage conduits 25 extend from the foundation layer 20 beyond a boundary of the surfaced area over which water is managed by the system 10 of the present invention.

The design and construction of each of the courses/layers is critical to proper performance of water management for the surfaced area and thus the make-up of each of the courses/layers will now be described.

In this embodiment, the first course l l comprises a surface course of the water management system. A top surface of the first course 11 can be textured/coloured as desired suitable for its particular use, for example to provide for increased skid resistance.

The first course 11 has a thickness depending upon the use of the system 10. In this embodiment, where the system is intended for medium to heavy traffic use and the understructure includes the supporting substrate 12 between the first course 11, the first course 11 has a thickness of between 10 mm to 50 mm, and preferably of about 25 mm.

The first course comprises one or more areas of permeable material and one or more areas of impermeable material.

The permeable material of the first course may comprise porous asphalt.

The porous asphalt may have a composition depending upon the use of the system. The porous asphalt may include penetration grade bitumen, such as 100/150 grade bitumen, of European Standard EN12591. The porous asphalt may include penetration grade bitumen modified with fibres and/or polymer modified bitumen. The latter is preferable when the system is to be used for vehicular traffic. The porous asphalt of the first course may include aggregate, the particles of which are all able to pass through a 14mm sieve, and preferably a 1 Omm sieve, and more preferably a 6mm sieve.

The permeable material of the first course may comprise porous concrete.

The impermeable asphalt may have a composition in accordance with BS49871. The impermeable asphalt may have a denser composition than the composition of the permeable material of the first course when this comprises porous asphalt. The impermeable asphalt may include bitumen. The impermeable asphalt may include bitumen in a quantity which is determined by the type and density of aggregate of the impermeable asphalt. The bitumen may comprise any of, or a combination of any of, 40/60 grade bitumen, 50/70 grade bitumen, 70/lOO grade bitumen, 100/150 grade bitumen, 160/220 grade bitumen, 250/330 grade bitumen. The impermeable asphalt may include aggregate. The aggregate may comprise coarse graded aggregate. The aggregate may comprise any of, or a combination of any of, rock, gravel, limestone, slag such as steel slag or blast furnace slag. The aggregate may have particles which are all able to pass through a 14mm sieve, and preferably a l Omm sieve, and 55% to 75% of which are able to pass through a 6.3mm sieve, and 19% to 33% of which are able to pass through a 2mm sieve, and 15% to 30% of which are able to pass through a lmm sieve, and 3% to 8% of which are able to pass through a 0.063mm sieve. The aggregate may have particles which are all able to pass through a lOmm sieve, and preferably a 6.3mm sieve, and 36% to 52% of which are able to pass through a 2.0mm sieve, and 20% to 50% of which are able to pass through a l.Omm sieve, and 7% to 23% of which are able to pass through a 0.25mm sieve, and 2% to 10% of which are able to pass through a 0.063mm sieve.

The impermeable concrete may have a composition in accordance with BS8500. The impermeable concrete may have a strength class of C28/35. The impermeable concrete may have a consistence class of S2. The impermeable concrete may have a density in the range 2300 to 2400kg/m3. The impermeable concrete may comprise cement. The mean cement content may be 300kg per cubic metro of concrete. The cement may be Cem I or CII/B-V or CIII/A class cement. The impermeable concrete may comprise aggregate. The aggregate may be 4/20, coarse aggregate. The aggregate may be 0/4mm fine aggregate.

The system may include a first course comprising one or more areas of porous asphalt, and one or more areas of impermeable asphalt and one or more areas of impermeable concrete. The system may include a first course comprising one or more areas of porous concrete, and one or more areas of impermeable asphalt and one or more areas of impermeable concrete. The system may include a first course comprising one or more areas of porous asphalt and one or more areas of porous concrete, and one or more areas of impermeable asphalt. The system may include a first course comprising one or more areas of porous asphalt and one or more areas of porous concrete, and one or more areas of impermeable concrete. The system may include a first course comprising one or more areas of porous asphalt and one or more areas of porous concrete, and one or more areas of impermeable asphalt and one or more areas of impermeable concrete.

The permeable material of the first course 11 permits storm or spillage water on the surfaced area to pass through the first course 11 to the permeable supporting substrate 12 beneath.

The permeable supporting substrate 12 includes an upper binder course 13 which includes permeable asphalt, aggregate of the permeable asphalt being of a size able to pass through a 28 mm sieve, but may be passable through a 20 mm sieve if desired. Thus the aggregate is in general coarser than the aggregate used for the first course 11.

Because the supporting substrate 12 is not subjected to direct wear, the permeable asphalt may include penetration grade bitumen, e.g. 100/150 bitumen, although the binder course 13 at least of the substrate 12 may if desired in particularly heavy traffic conditions, include penetration grade bitumen modified wit' fibres andlor polymer modified bitumen.

The lower base layer 14 of the substrate 12 is made of permeable asphalt, but with aggregate of a size which is able to pass through a 40 mm sieve. The base layer 14 may include a penetration grade bitumen, such as 160/220 bitumen.

The binder course 13 may have a thickness of between about 30mm and 80mm, and typically of about 60 mm, whilst the base layer 14 may have a thickness of between about 30 mm and 100 mm and typically of about 70 mm.

The semi-permeable membrane 16 may be made of any suitable material with passages for water, but the passages are of a sufficiently small size that the semi-permeable membrane 16 retains particulate contaminants which may pass from the top surface l la of the system 10, through the first course l l, and through the supporting substrate 12. One suitable material for the semi- permeable membrane 16is a woven geo-textile material, made of a synthetic material with passages for water which are not readily perceivable to the naked eye.

The porous foundation layer 20 beneath the semi-permeable membrane 16 is provided by particulate material which may be natural rock, such as limestone, and/or a recycled material such as an industrial by-product such as slag materials, or brick grog for examples only.

The particle sizes of the foundation layer 20 may vary such that all of the particles are able to pass through a 300 mm sieve but none of the particles are able to pass through a 6 mm sieve. The foundation layer 20 may have a thickness of at least 200 mm and preferably of about 295 mm, but may have a greater thickness as required.

In each case, the particles of the foundation layer 20 provide a filter for water passing through the foundation layer 20, the particles providing between them, interstitial cavities in which water may dwell, whilst bacteria for example, may naturally cleanse the water.

Slowly the water may drain through the filter foundation layer 20 to the sub-grade 22. However to provide for the escape of water from the system 10, within the foundation layer 20 there are provided drainage conduits 25 (only one is shown in the drawings) which in this example are of the kind having through walls thereof, passages (smaller than the minimum particle size of 6mm of the material of the foundation layer 22) or being porous to permit water to drain through the walls and into the interiors of the conduits 25.

The conduits 25 extend downwardly to and outwardly beyond a boundary of the surfaced area over which water is to be managed by the system of the invention, and in one embodiment, the conduits 25 connect with man- made drains or a natural water course, or the drainage conduits 25 may simply permit the drained water to soak away into the ground, beyond the surfaced area.

However if desired, the conduits 25 may include a drainage control valve to enable the rate of water drainage from the foundation layer 20 to be controlled, or the rate of drainage may be restricted by reducing the cross sectional dimension of the interior flow passages of the conduits 25. In each case preferably the rate of drainage is controlled so that water tends to dwell sufficiently long in the filtering foundation layer 20 for the water naturally to be cleansed, before the water is released tothe sub-grade 22 beneath or passes from the system 10 via the drainage conduits 25.

The asphalt for the system 10 of the present invention, or at least for one of the layers of the system 10, may be laid from a hot or cold asphalt mix as desired.

In another example, the semi-permeable membrane 16 need not be provided in which case the substrate will rest on the foundation layer, but by careful design and construction of the various layers, water may be retained in the system and released in a controlled manner into the sub grade 22 and into the drainage conduits 25.

It will be appreciated that the actual thickness of the first course 11 and the construction of the various layers, may be adjusted to suit specific site traffic requirements. For example, in the particular embodiment described, the system 10 is intended for medium to heavily trafficked sites, e.g. carrying a million standard axle loadings, such sites including for examples only, urban routes, retail parks, lightindustrial estates, feeder routes onto housing estates, etc. It is envisaged that a system 10 for only light pedestrian traffic use, may not require the understructure to include a supporting substrate 12. Rather the first course 11 may be laid directly onto the foundation layer 20, in which case the first course 11 might have a thickness of between 40 mm and 80 mm.

If desired, the first course 11 may include a plurality of layers rather than the single layer construction described with reference to the drawing.

In the embodiment described with reference to the drawing in which the first course 11 is partly provided by porous asphalt, it will be appreciated that the asphalt is laid in a hot state, such that the asphalt may be made to flow to provide the configuration of first course 11 shown, by rolling or the like, the asphalt setting as it cools to provide the surface course 11.

The first course 11 is provided by a gettable material which is laid onsite in a Plowable state, and sets on-site to provide the first course 11.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A water management system for managing storm water over a surfaced area, the system including a first course comprising one or more areas of permeable material and one or more areas of impermeable material, a permeable understructure beneath the first course, the permeable understructure including a porous foundation layer, there being a drainage conduit from the foundation layer to beyond the boundary of the surfaced area.

2. A system according to claim 1 wherein the first course has a thickness of about 10 mm to 50 mm, or a thickness of between 40 mm and 80 mm.

3. A system according to claim 1 or claim 2 wherein the permeable material comprises porous asphalt.

4. A system according to claim 3 wherein the porous asphalt includes penetration grade bitumen.

5. A system according to claim 3 or claim 4 wherein the porous asphalt includes penetration grade bitumen modified with fibres and/or polymer modified bitumen.

6. A system according to any of claims 3 to 5 wherein the porous asphalt includes aggregate the particles of which are all able to pass through a 14 mm sieve.

7. A system according to claim 6 wherein the porous asphalt includes aggregate the particles of which are all able to pass through a 10 mm sieve.

8. A system according to claim 7 wherein the porous asphalt includes aggregate the particles of which are all able to pass through a 6 mm sieve.

9. A system according to any preceding claim wherein the permeable material comprises porous concrete.

lO. A system according to any preceding claim wherein the impermeable material comprises impermeable asphalt.

11. A system according to claim 10 wherein the impermeable asphalt includes bitumen comprising any of, or a combination of any of, 40/60 grade bitumen, 50/70 grade bitumen, 70/100 grade bitumen, 100/150 grade bitumen, 160/220 grade bitumen, 250/330 grade bitumen.

12. A system according to claim 10 or claim 11 wherein the impermeable asphalt includes aggregate, having particles which are all able to pass through a 14mm sieve, and preferably a lOmm sieve, and 55% to 75% of which are able to pass through a 6.3mm_sieve, and 19% to 33% of which are able to pass through a 2mm sieve, and 15% to 30% of which are able to pass through a lmm sieve, and 3% to 8% of which are able to pass through a 0.063mm sieve.

13. A system according to claim 10 or claim 11 wherein the impermeable asphalt includes aggregate, having particles-which are all able to pass through a lOmm sieve, and preferably a 6.3mm sieve, and 36% to 52% of which are able to pass through a 2.0mm sieve, and 20% to 50% of which are able to pass through a l.Omm sieve, and 7% to 23% of which are able to pass through a 0.25mm sieve, and 2% to 10% of which are able to pass through a 0.063mm sieve.

14. A system according to any preceding claim wherein the impermeable material comprises impermeable concrete.

15. A system according to any preceding claim wherein the impermeable material has a composition which is at least substantially able to withstand stresses without fretting of the impermeable material.

16. A system according to claim 15 wherein the impermeable material has a composition which is able to at least substantially withstand stresses of vehicular traffic without fretting of the impermeable material.

17. A system according to any preceding claim wherein the impermeable material has a composition which is able to withstand spillage of substances onto the impermeable material.

18. A system according to any preceding claim wherein an area of impermeable material is provided at each part of the system which is required to at least substantially be able to withstand stresses without fretting of the impermeable material.

19. A system according to any preceding claim wherein an area of impermeable material is provided at each part of the system where there is a likelihood of substances being spilled onto the impermeable material.

20. A system according to any preceding claim wherein the one or more areas of impermeable material are at least partially surrounded by the one or more areas of permeable material of the first course.

21. A system according to any preceding claim wherein the one or more areas of impermeable material are each provided with a camber, to enable water to run off the impermeable area.

22. A system according to any preceding claim wherein the permeable understructure includes a supporting substrate.

23. A system according to claim 22 wherein the permeable supporting substrate includes an upper course of permeable asphalt and a lower course of permeable asphalt, in which aggregate of the asphalt of the lower course is coarser than aggregate of the asphalt of the upper course.

24. A system according to any preceding claim wherein the foundation layer includes a filter layer of particulate with interstitial cavities between the particles.

25. A system according to any preceding claim wherein the foundation layer is laid on a natural or laid sub-grade.

26. A system according to any preceding claim wherein the drainage conduit extends into the foundation layer and has passages for water from the foundation layer through walls of the conduit.

27. A water management system for a surfaced area substantially as hereinbefore described with reference to and/or as shown in the accompanying drawing.