Abstract:
A water detention system comprises a sub-base of crushed rock or stone overlying an impermeable layer which may be naturally-occurring, as in an impermeable sub-grade, or may be formed by an impermeable membrane laid over the sub-grade prior to the sub-base layer. Over the sub-base layer is an incompletely impermeable layer the impermeability of which is compromised by openings in the form of slits or by spacing between adjacent strips forming the layer. These openings allow water to percolate downwardly through the layer into the sub-base, but substantially inhibit the escape of moisture by evaporation thereby serving to retain the collected water. Above the incompletely impermeable layer may be a laying course of finer particulate material such as pea gravel over which may be laid a wear surface of slabs or blocks to form an area for traffic, such as a roadway or parking area.

Description:
PRIOR APPLICATION DATA 
     This application claims priority to and is a continuation-in-part of U.S. patent application Ser. No. 11/891,200 now abandoned, filed on Aug. 8, 2007 entitled A Water Detention System Incorporating a Composite Drainage Membrane, which is a continuation of PCT Application No. PCT/GB2006/000474, filed on Feb. 9, 2006, entitled A Water Detention System Incorporating A Composite Membrane, which claims priority to Great Britain patent application number 0502861.8 filed Feb. 11, 2005 and Great Britain patent application number 0516866.1 filed Aug. 17, 2005. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a composite drainage membrane, and to a water detention system incorporating such a membrane. 
     RELATED ART 
     the use of suds (sustainable urban drainage systems) is increasing with the increasing awareness of the economy of installation and value in decontaminating and managing the water collection and drainage systems leading to water courses for the disposal of water falling on pavement surfaces. Known drainage systems are built to cope with a maximum expected precipitation, which may be exceeded from time to time. Changing meteorological conditions, however, are leading to situations where the peak rainfall for which a drainage system may have been designed is being exceeded increasingly frequently. Upgrading of systems to cope with increased amounts of run-off is extremely costly. There is also the contaminating and polluting effect of motor traffic resulting in heavy metals, hydrocarbons, rubber dust, silt and other fine detritus becoming deposited on the surfaces of roadways and car parks and subsequently being washed into the water courses causing long term pollution. 
     Sustainable urban drainage systems utilising permeable pavements and underlying layers of crushed rock over an impermeable sub-grade, or provided with an impermeable lining membrane, may be used to collect and store water for other purposes such as irrigation. When used for this purpose, however, especially in regions of high temperature, evaporation of the stored water, even though located in subterranean voids, can result in effective loss of a large proportion of the water collected. 
     The present invention seeks, therefore, to provide means by which such systems can be improved to allow rapid infiltration of water into the voids in the sub-base, without there being an opportunity for equally rapid escape by evaporation. 
     SUMMARY 
     The present invention finds particular utility in connection with the provision of pavement surfaces, that is hard, load-bearing surfaces made from paving elements such as slabs or blocks, or continuous material such as concrete or asphalt. However, the present invention is not limited to application solely in this field, and may find utility in connection with a wide range of forms of water run-off management, storage, and precipitation re-utilisation systems, particularly those suitable for use with rainwater, as well as systems for decontamination of run-off water and for the use of subterranean water for heat exchange purposes. 
     According to a first aspect of the present invention, therefore, there is provided a water detention system characterised by comprising at least a sub-base of particulate material in a layer having a substantial number of voids, an overlying permeable layer of particulate material, and a composite membrane comprising a first, permeable layer, a second impermeable layer and spacer means between the first and second layer, the spacer means acting to maintain at least part of the first and second layers out of contact with one another and to allow the movement of liquid in the space between them, the composite membrane being so positioned that water collecting on its surface can infiltrate into the sub-base from the edges of the composite drainage membrane or through openings formed in the second layer. 
     When used as a separating layer over a sub-base of particulate material defining a plurality of voids, therefore, the composite membrane allows the infiltration of water passing through the permeable layer into the space between the two layers and then travelling laterally, towards the edges of the composite membrane, from which the water can escape into the sub-base. 
     The form of the composite membrane may vary depending on the particular exigencies of use. For example, in some circumstances it may be quite sufficient for the individual layers simply to be placed in juxtaposed relation one over the other loosely without a bonding between the layers. Because overlying layers will in practice be placed on top of the membrane, for example a laying course and a wearing course, there will be no effective lateral forces between the layers requiring them to be bonded together. For convenience in handling of the membrane, however, they may nevertheless be held together in fixed relation and in one embodiment the components of the membrane are held together by adhesive bonding. Alternatively, however, the component may be held together by fixing elements such as, for example, staples. 
     In a preferred embodiment of the invention the spacer means comprise a mesh or grid, and in particular a plastics mesh has been found to be particularly appropriate. Of course, since lateral transport of the water between the two layers spaced by the mesh is required a mesh structure which formed closed cells would be of little value and it is preferred, therefore, that the mesh is formed in such a way as to provide communicating or open cell structure when the mesh is placed between the two layers. This may be achieved, for example, by using a mesh formed of overlapping or “woven” filaments. 
     Another way in which lateral transport of water may be achieved lies in the use of a plurality of discrete elements as the spacer means. Such discrete elements may be irregularly spaced over the surface of the membrane between the two layers or, in order to minimise on the material used, may be regularly spaced over this surface, it being appreciated that regular spacing allows wider separation of the spacer elements. Indeed, it will be appreciated that although the spacer elements hold the two layers out of contact with one another in the region of the elements themselves, it is possible for the two layers to touch between the regions contacted by the spacer elements. In this case the two layers may be secured together between the discrete elements and this, of course, would assist in maintaining the discrete elements in determined positions spaced over the area of the membrane. 
     Although discrete elements in the form of studs, pebbles, beads or other granular material may be used, these could alternatively be elongate, possibly even spanning the entire width of the membrane, formed as rods, bars or tubes. 
     It is also within the ambit of the present invention for the second, impermeable layer to be formed with surface formations acting themselves as the spacers. Thus local inspissation, corrugation or embossment of the second layer may serve to hold other regions thereof in the required spaced relation with respect to the permeable layer. 
     Permeability of the first layer may be achieved by forming this as a woven or non-woven textile material, in which case the fibres or filaments may be heat bonded to make a strong resistant material suitable for use as a geotextile. 
     The present invention also comprehends a water detention system comprising at least a sub-base of particulate material in a layer having a substantial number of voids, and an overlying composite membrane formed by laying down successive layers in a substantially unbonded juxtaposition, and so positioned that water collecting on the surface can infiltrate into the sub-base at least from the edge of the membrane or through openings formed therein. The intermediate layer in such a structure may be made of stones or crushed rock laid to a depth of between a few cm to several tens of cm. 
     In a structure suitable for water detention the sub-base may overly an impermeable or at least substantially impermeable underlying layer, and this layer may be a geological formation such as a sub-grade or may be an introduced at least substantially impermeable, underlying layer in the form of a membrane. 
     The underlying layer need not necessarily be planar, and, indeed, there are circumstances which will be described in more detail below in which irregular further cavities or sumps, or at least one cavity or sump, may be of particular value. 
     Above the composite membrane of the water detention system there may be a further particulate layer and this may be a laying course for a wearing layer which may comprise a plurality of paving elements and which, in a preferred embodiment, may be blocks or slabs having means defining openings between them when laid in juxtaposed relation. 
     Alternatively, the wearing layer may comprise a substantially continuous layer of permeable material such as asphalt, porous concrete or the like. 
     A water detention system formed in locations other than under urban pavements may also be formed, and in such a case the particulate material overlying the composite membrane may itself constitute a wearing layer (for example, gravel laid to a path or drive, or a larger standing area). It could also be entirely unrelated to any traffic or parking system, in which case the further layer may be overlain by soil and/or vegetation. This is of particular value where the water detention system is provided primarily for collection and storage of water for purposes other than simply management of the water run-off. It may be stored, for example, for further use in irrigation, as wash water or even for use in other agricultural environments, such as drinking water for animals. 
     Infiltration of water resulting from precipitation is achieved particularly effectively if the membrane is laid in strips over the sub-base, and such strips may be lain in such a way that adjacent strips are spaced from one another (in which case water infiltration is maximised) although adequate water infiltration may equally be achieved if the strips of the composite membrane are laid abutting one another or overlapping one another. The strips may be laid on a perfectly horizontal surface of the underlying sub-base, or this may be shaped, for example domed or inclined, to receive the composite membrane. 
     The present invention also extends to the provision of a pavement structure having an underlying water detention system as defined hereinabove and/or using a composite membrane as defined herein. 
     Further, the invention may also be considered to lie in a method of forming a water detention system which may comprise the steps of laying a sub-base of rigid insoluble hard particulate material of a defined size range over an at least substantially impermeable sub-grade or a preliminarily positioned at least substantially impermeable membrane and overlying the sub-base with a substantially unidirectionally porous layer able to allow water to infiltrate from above into the sub-base, but which is such as substantially to resist loss of water from the sub-base by evaporation. This method also comprises overlaying the substantially unidirectionally porous layer with a further layer of particulate material. 
     The method of the invention may further comprises the steps of compacting the material of the sub-base prior to application of the substantially unidirectionally porous layer. 
     If the substantially unidirectionally porous layer is a composite membrane comprising at least an impermeable layer, a permeable layer and spacer means holding the two layers apart over at least a part of their area, as described hereinabove, these may be applied one at a time to the sub-base to build up the at least substantially unidirectionally porous layer. Indeed, the spacer means may itself comprise a layer of stones or crushed rock. 
     Alternatively, the substantially unidirectionally porous layer may be a composite membrane as herein defined preliminarily formed before application to the sub-base. 
     The present invention may also comprehend a heat exchange structure comprising a substantially enclosed volume bounded by a lower water-impermeable stratum or layer and containing a sub-base of rigid substantially incompressible particulate material, overlain by an at least partly permeable membrane which allows water to enter the substantially enclosed volume but resists evaporative escape therefrom. This system also comprises one or more heat exchange pipes for directing a heat exchange fluid therethrough and located so as to pass through water trapped in the substantially enclosed volume. 
     The substantially enclosed volume may include a channel through which the heat exchange pipe passes, and such channel may be formed by the membrane defining a lower boundary of the enclosed volume. In order to ensure that thermal contact is made with the water even in the most adverse circumstances the channel may be formed as a sump in the bottom of the enclosed volume and the pipe or pipes pass through this sump. 
     The rigid substantially incompressible particulate material may be crushed rock. 
     Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
       Various embodiments of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an enlarged cross sectional view of a membrane formed as an embodiment of the present invention; 
         FIG. 2  is an exploded view of a mesh layer forming part of the membrane of  FIG. 1 ; 
         FIG. 3  is a cross sectional view through a water detention system formed as an embodiment of the present invention and incorporating a membrane of the general type illustrated in  FIG. 1 ; 
         FIG. 4  is a schematic view of an alternative membrane having tubes, rods or bars as spacers; 
         FIG. 5  illustrates the use of beads as spacers; 
         FIG. 6  illustrates one laying configuration for the membrane of  FIG. 1  in a water detention system such as that of  FIG. 3 ; 
         FIG. 7  illustrates a further alternative laying configuration; 
         FIG. 8  is an enlarged cross sectional view of a membrane formed as an embodiment of the present invention; 
         FIG. 9  is a cross sectional view through a water detention system formed as an embodiment of the present invention and incorporating a membrane of the general type illustrated in  FIG. 1 ; 
         FIG. 10  is a cross sectional view of one laying configuration for the membrane of  FIG. 1  in a water detention system such as that of  FIG. 2 ; 
         FIG. 11  is a plan view of the configuration of  FIG. 3 ; 
         FIG. 12  is a plan view of an alternative configuration of  FIG. 3 ; 
         FIG. 13  is a plan view of an alternative laying configuration for the membrane of  FIG. 1 ; 
         FIG. 14  is a cross section view through a heat exchange structure formed as an embodiment of the present invention and incorporating a membrane of the general type illustrated in  FIG. 1 ; and 
         FIG. 15  is a perspective view of alternative composite membrane suitable for use in the water detention system of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , the membrane generally indicated  10  comprises a first layer  11  of non-woven geotextile fabric comprising a plurality of filaments bonded together and having the following properties. 
     Thermally-bonded non-woven geotextile meeting the following specifications: 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Mechanical Properties 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Wide Width Strip Tensile 
                 EN ISO 10319 
               
               
                   
                 Mean peak strength 
                 8.50 kN/m 
               
               
                   
                 Elongation at peak strength 
                 28% 
               
               
                   
                 CBR Puncture Resistance 
                 EN ISO 12236 
               
               
                   
                 Mean Peak Strength 
                 1575N 
               
               
                   
                 Trapezoidal Tear Resistance 
                 ASTM D4533 
               
               
                   
                 Mean Peak Strength 
                 325N 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Hydraulic Properties 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Pore Size 
                 EN ISO 12956 
               
               
                   
                 Mean AOS O 90   
                 0.145 mm 
               
               
                   
                 Water Flow 
                 EN ISO 11058 
               
               
                   
                 Mean Flow VI H50  10−3 m · s−1(1/m 2 s) 
                 80 
               
               
                   
                 Water Breakthrough 
                 BS 6906: Part 3 
               
               
                   
                 Mean Head 
                 50 mm 
               
               
                   
                 Air Permeability 
                 ISO 9237 
               
               
                   
                 Mean Flow 
                 2875 l/m 2  · s 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Typical Physical Properties 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Mass EN 965 
                 130 g/m 2   
               
               
                   
                 Roll width 
                 4.5 &amp; 1.5 m 
               
               
                   
                 Roll length 
                 100 m 
               
               
                   
                 Colour 
                 Green 
               
               
                   
                   
               
             
          
         
       
     
     The composite membrane  10  also includes a flexible second layer  12  of impermeable plastics material (such as polyethylene or similar) and sandwiched between the first and second layers  11 ,  12  is a geogrid or mesh layer (such as high density polyethylene or similar)  13  spacing the two first-mentioned layers apart and providing a plurality of drainage passageways for water to travel parallel to the plane of the backing layer  12 . 
       FIGS. 2   a  and  2   b  show two alternative forms of the geogrid  13 . This layer is intended to hold the geotextile layer  11  spaced from the impermeable backing layer  12  and to provide drainage channels or passages for water to travel parallel to the plane of the layer  12 . For this purpose the grid must provide spaces between itself and the layer  12  when placed in contact with it, and in the embodiment of  FIG. 2   a  this is achieved by forming the grid  13  of a plurality of “wovenwarp” filaments  14  interlaced with a plurality of “weft” filaments  15 . After weaving, the filaments  14 ,  15  are pressed together and heated to cause bonding in the overlap region such as that identified by the arrow  16  so that the geogrid is stable dimensionally. Passages for water flow are formed by the overlapping filaments as identified by the regions  17  identified in  FIG. 2   a.    
     A similar, but more economical geogrid is illustrated in  FIG. 2   b  where the warp filaments  14 ′ are first laid in parallel rows and/or overlaid by the “weft” filaments  15 ′ which are thereafter pressed and heated to bond the grid together at the intersections  16 ′. The heating causes partial interpenetration of the material of the warp and weft filaments, but as will be appreciated along the length of either row of filaments there are wide spaces through which water can travel even when the grid is placed in contact with an impermeable surface. 
       FIG. 3  illustrates in cross section a typical water detention system formed utilising the membrane illustrated in  FIGS. 1 and 2 . The water detention system illustrated in  FIG. 3  underlies a hard paved surface  18  defined by a plurality of individual blocks  19  laid closely spaced with no grouting between them so that channels (not shown) in the sides of the blocks can allow rainwater falling on the surface  18  to pass through into an underlying layer  20  formed as a bedding course for the blocks  19  and composed of relatively small particulate material such as gravel in the range of about 5 mm to about 20 mm. 
     Beneath this is a sub-base  21  of crushed rock of angular form and a size range of about 163 mm to about 10 mm between which are a significant number of voids providing storage space for water infiltrating through the permeable wearing surface  18 . Between the sub-base  21  and the laying course  20  is a composite membrane layer generally indicated  22 . This may have the same structure as described in relation to  FIG. 1  and, in this embodiment, the membrane  22  is laid in elongate strips  22   a ,  22   b ,  22   c  with spaces  23  between the edges of adjacent strips. Over the spaces  23  is laid a protective strip  24  of porous geotextile material, which may be the same material as that which constitutes the layer  11  of the membrane  10  of  FIG. 1 . A regulating layer  29  of smaller stones may be laid between the sub-base  21  and the composite membrane  22 . 
     The edges of the installation are defined by a kerb  25  in suitable haunching  26 , and escape of water is prevented by a strip  27  of impermeable material laid under the adjacent strip  22   c  of composite membrane and extending up the adjacent face of the kerb  25  between that and the layer of blocks  19 . The edging strip  27  thus forms a vertical limb  27   a  and a horizontal limb  27   b . An impermeable layer or membrane  28  defines the lower boundary of the sub-base  21 , lying between this and the sub-grade  29 . The membrane  28  likewise extends up the face of the kerb  25  adjacent the limb  27   a  of the edging strip  27  to define an enclosed space below the wearing surface constituted by the blocks  19 . 
     A sump  30  is formed by a channel membrane  36  beneath the sub-base  21  and extending downwardly into the sub-grade  29 . The sump  30  is filled with a granular material  32  which is smaller in size than the material of the sub-base  21 . 
     At the bottom of the sump  30  are laid pipes  33  for a heat exchange system. As described herein the water detention system may be used for multiple purposes and not every feature of this embodiment would necessarily be employed in a practical installation. Where the water detention system is provided to act as a heat sink, for example, it is convenient to maintain a significant body of water within the region defined by the sub-base  21  and the sump  30  so that heat yielded from the pipes  30  (through which, in use, a heat exchange liquid or fluid flows from the appliance or installation generating or using the heat which is lost to or drawn from the surrounding water). A further description of such a heat exchange system is to be found in British Patent Application No 0418391.9. 
     Alternative forms of composite membrane are illustrated in  FIGS. 4 and 5 , in which the same reference numerals have been used as those in  FIG. 1  to identify the same or corresponding component parts. Thus, the upper geotextile layer  11  is spaced in the embodiment of  FIG. 4  from the lower impermeable plastics membrane  12  by a regular array of rods or bars  40  spaced from one another along the length of the strip of membrane  12 . The bars  40  extend from side to side of the membrane and define elongate channels in the composite membrane encouraging water to flow in one of two opposite directions. The bars  40  may be secured to the membrane  12  by adhesive, friction welding or other technique, or, as shown in  FIG. 4   a , may be bonded in place by forming the membrane  12  around each rod  40  whilst in a mobile state so that, upon curing or hardening, the membrane  12  itself retains the rod  40  in position. 
     In  FIG. 5  the geotextile  11  is spaced from the membrane  12  by an irregular set of beads  41  spaced over the surface of the membrane  12  and either secured in place by adhesive or located by a direct connection of the geotextile  11  to the membrane  12  by way of fixing elements such as staples  42  over a defined region to form, in effect, pockets between which the beads  41  are trapped. 
       FIG. 6  shows a laying pattern for the composite membrane in a water detention system similar to that illustrated in  FIG. 3 . Again, the same reference numerals have been used to identify the same or corresponding components. Here, the composite membrane  22  is again laid in strips  22   a ,  22   b ,  22   c , but in this case they are laid overlapping one another over a regulating layer  29  and under a bedding course  20  overlain by blocks  19  which allow infiltration of water. This laying configuration still allows water to permeate through the permeable membrane  22  since water flowing onto, for example, the strip  22   a  can exit from each of the two opposite edges  22   a ′ and  22   a ″, and in this latter case the water flows onto the adjacent layer  22   b  from which it can escape through the edge  22   b ′. Water collecting in the sub-base layer  21 , however, has an effectively continuous impermeable membrane above it, and evaporation of the water contained in the sub-base  21  even when high temperatures exist above the wearing layer  18  is strongly resisted. 
       FIG. 7  illustrates another alternative laying configuration in which, however, the regulating layer  29  is formed into a cambered or domed configuration matching the dimensions of the strips  22   a ,  22   b ,  22   c  so that the infiltration of water through the membrane  22  into the sub-base  21  is encouraged by gravity. This laying configuration has the disadvantage, however, that the cambered regulating layer  29  must be formed with a shape which is reasonably accurate so as to receive the individual strips  22  of the composite membrane. 
     Turning now to  FIG. 8 , there is shown an assembled structure forming a composite membrane, generally indicated  10  for use in a water detention system of the type described above. The membrane comprises a first layer  11  of non-woven geotextile fabric composed of a plurality of filaments bounded together to form a porous web having properties as set out in relation to the web described with reference to  FIG. 1 . 
     The composite membrane  10  also includes a flexible second layer  12  of impermeable plastics material (such as polyethylene or similar), and sandwiched between the first and second layers  11 ,  12  is a layer  13   a  of crushed rock or stone spacing the two first-mentioned layers apart and providing a plurality of drainage passageways for water to travel parallel to the plane of the backing layer  12 . This layer of stone may have a thickness of about 75 mm and have been graded to include particles predominantly of a size 20 mm to 5 mm. 
     The composite membrane  10  may act as an evaporation control membrane as will be explained in more detail herein. 
       FIG. 9  illustrates in cross section a typical water detention system formed utilising the membrane illustrated in  FIG. 8 . The water detention system underlies a hard paved surface  18  defined by a plurality of individual blocks  19  laid closely spaced with no grouting between them so that channels (not shown) in the sides of the blocks can allow rainwater falling on the surface  18  to pass through into an underlying layer  20  formed as a bedding course for the blocks  19  and composed of relatively small size particulate material such as gravel in the range of about 5 mm to about 20 mm, more particularly 6 mm. 
     Beneath this is a sub-base  21  of crushed rock or stone of angular form and graded to have a size range of about 63 mm to about 10 mm between which are a significant number of voids providing storage space for water infiltrating through the permeable wearing surface  18 . Between the sub-base  21  and the laying course  20  is a composite membrane layer generally indicated  22 . This may have the same structure as described in relation to  FIG. 8 . 
     In this embodiment, between the sub-base layer  21  and the underside of the composite membrane  22 , a thin blinding layer of regulating stone  29  is provided having a size range of about 20 mm to about 5 mm and having a thickness of about 50 mm. This layer  29  helps to protect the second layer  12  of the composite membrane  22  from puncture by the larger and more angular rocks and stones of the sub-base layer  21 . 
     Further, the embodiment of  FIG. 9  has a stabilisation layer  50  shown. This may be a geotextile or a geo-grid such as manufactured by Tensar™. The purpose of this layer is to help stabilise the sub-base  21  and prevent it from being reduced in thickness, which in turn would reduce the volume of water which could be stored within it, due to traffic or natural weathering. 
     At the base of the structure of  FIG. 9  a substantially impermeable layer  28  is shown. This layer  28  may be a man-made impermeable layer such as sheets of substantially continuous plastics, a naturally occurring sub-grade such as a competent rock formation, or an imported naturally occurring material such as clay. This element  28  is not a pre-requisite but does enhance water retention. 
       FIG. 10  illustrates how the second layer  12  of the composite membrane  22  may be formed of overlapping separate sheets  12   a . The sheets are overlapped along an edge  12   b  and tapes  12   c  are adhered to the two adjacent sheets  12   a  at the overlap  12   b  to produce a larger continuous sheet. Holes  12   d  may then be punched through the sheets  12   a  in either a regular pattern as shown in  FIG. 4  or an irregular pattern (not shown). 
       FIG. 11  shows this regular pattern in plan view together with the taped section  12   c  and the overlap  12   b.    
       FIG. 12  shows alternative openings within the second layer  12 . Rather than holes  12   d  slices, slashes or cuts  12   e  are made within the second layer  12 . 
       FIG. 13  illustrates another alternative to the holes  12   d  of  FIGS. 3 and 4 . In this embodiment, the second layer  12  is made up of adjacent sheets  12   a  which are spaced apart with a gap  12   f  left therebetween. These gaps  12   f  act as the openings to allow water to flow through into the sub-base but to minimise evaporation from the sub-base by minimising the area of sub-base which is not covered by an impermeable layer. 
     In  FIG. 14  the water detention system of  FIG. 9  is adapted to become a heat exchange structure. This is achieved by having a sump  30  formed within the base of the system. The sump is lined with an impermeable layer  36  which could be an extension of the membrane  28 . At the bottom of the sump  30  are laid pipes  33  for a heat exchange system. Within the sump  30  a granular material  32  is placed which is smaller in size than the material of the sub-base  21  to protect the pipes from damage due to sharp edges and corners. 
     The impermeable layer  28  is also shown to continue up one side of the sub-base  21 , composite membrane  22 , bedding layer  20  and pavement  18 . If necessary this layer can be continued around all sides of the structure so as to make it waterproof and to retain as much water within it as possible. Water could then be regulated to flow out of the structure by means of a valve (not shown) placed through the impermeable layer  28  at a selected point. 
     As described herein the water detention system may be used for multiple purposes and not every feature of this embodiment would necessarily be employed in a practical installation. Where the water detention system is provided to act as a heat sink, for example, it is convenient to maintain a significant body of water within the region defined by the sub-base  21  and the sump  30  so that heat yielded from the pipes  30  (through which, in use, a heat exchange liquid or fluid flows from the appliance or installation generating or using the heat) is lost to the surrounding water. A further description of such a heat exchange system is to be found in British Patent Application No 0418391.9. 
       FIG. 15  shows an alternative at least substantially unidirectionally permeable membrane which may be used in place of the membrane  11  in water detention systems such as those shown in the illustrative drawings referred to hereinabove. In  FIG. 15  the unidirectionally porous membrane  40  comprises only two layers, namely an upper permeable non-woven layer  41  and a lower permeable woven layer  42 . The upper, permeable, non-woven layer may be composed of plastics filaments of a material commonly used for production of geotextiles for the building industry, which is sufficiently porous to allow water to pass therethrough when it accumulates above the membrane and develops a slight “head” resulting in a hydraulic pressure sufficient to cause the water to pass through the fabric. 
     The underlying woven plastics layer  42  is composed of closely woven flat plastics strips  43  (in the warp direction) and  44  (in the weft direction). The weave is sufficiently tight that the interstices  45  between adjacent interwoven filaments are extremely small and widely spaced in relation to the overall area covered by the interlocking filaments. Again, these are of a size such that, when water builds up above the composite membrane to provide an hydraulic pressure the liquid water will pass through the interstices  45 , albeit being slowed by the relatively small cross sectional area of these openings to allow water to build up in a sub-base underlying the membrane as described hereinabove in relation to the preceding Figures. When it is used in areas of low rainfall or when it is desired for any reason to retain captured water in the sub-base a rise in temperature in the air (and/or the ground) above the membrane which may cause evaporation at the surface of the water retained in the sub-base will not result in substantial loss of retained water since the water vapour cannot readily pass through the composite membrane in the reverse direction due to the small size of the interstices between the woven filaments  43 ,  44 . The close bonding of the two layers  41 ,  42  together also contributes to this effect. This results in a simple, economical and surprisingly effective unidirectionally porous membrane which resists evaporative loss from the sub-base.