One of the problems of retaining water to form a reservoir, even at relatively shallow depths of around 2 meters, is the quantity and strength of the materials required to build a structure of sufficient reliability. In remote rural communities of poorer countries the problem can be particularly acute as resources available to fund the building of such structures are often insufficient.

According to a first aspect of the invention there is provided a liquid retaining structure comprising an elongate generally horizontal member which in transverse cross section has the form of an arch extending, in use, above ground between a first position on a liquid retaining side of the horizontal member and a second position on the opposite side, the arch defining a liquid retaining face at least part of which lies outwards of a wall of a notional form of the arch which extends symmetrically between the two positions.

Preferably, the notional arch is a catenary arch.

The liquid retaining face may be of bulge-like form. Preferably, the bulge-like form comprises a section of the liquid retaining face which is arranged to extend upwardly with respect to the first position and outwardly from the interior of the arch and which then turns inwardly and upwardly back on itself.

In a preferred embodiment, the arch is in pure compression when the liquid retained has a given depth.

Preferably, the horizontal member is of hollow construction, the shape preferable comprising a wall of suitable thickness. The hollow construction is of particular advantage from the point of view of the amount of material required to produce the structure and again leads to a considerable cost saving compared to a conventional dam structure.

The cross-sectional shape of the horizontal member may be determined by providing a hanging catenary in substantially pure tension between first and second positions, and applying to one part of the catenary a force directed away from an area contained within the catenary which corresponds to the force which would be applied at that depth if the catenary were inverted to form a liquid retaining arch, the application of forces being repeated at depth intervals to deflect the catenary from its normal hanging

position. In that way, the part of the catenary to which the forces are applied may conveniently define the shape of the liquid retaining face of the arch. The hanging catenary can then be inverted so that the deflected catenary will provide an arch shape which is intended to be in at least substantially pure compression when liquid is being retained. That shape can then be translated into the shape an actual structure which forms the horizontal member. If desired, the hanging catenary may be computer generated and the forces applied theoretically to define the shape of the liquid retaining force.

The elongate horizontal member is preferably formed with alternate ridges and troughs over its length. In that respect the horizontal member may comprise a plurality of segments each of which may have a concave liquid retaining face. The segments are preferably integral with each other so as to form a unitary liquid retaining structure.

According to a second aspect of the invention there is provided a method of erecting a liquid retaining structure which comprises the step of locating an elongate horizontal member as described above in a liquid flow path.

A water retaining structure in accordance with the invention will now be described by way of example with reference to the accompanying

drawings in which Figure 1 is a view of a conventional hanging catenary in pure tension; Figure 2 is a view of the catenary of Figure 1 inverted so as to extend upwards to form an arch on pure compression; Figure 3 is a diagrammatic view similar to Figure 2 showing one side of the arch acted upon by water; Figure 4 is a view similar to Figure 1 showing a hanging catenary deflected by the application of outward forces; Figure 5 is a view of the deflected catenary inverted to form a liquid retaining structure; Figure 6 is a diagrammatic perspective view of a preferred form of liquid retaining structure in accordance with the invention; and Figure 7 is a vertical cross section taken longitudinally through part of the liquid retaining structure of Figure 6 and drawn to a larger scale.

Looking at Figure 1, a conventional catenary 10 hangs between two fixed positions 12,14 in pure tension in known manner. In Figure 2, the catenary 10 is inverted and the forces in the catenary 10 are in exactly the opposite direction from those in Figure 1 to provide an arch 16 in pure compression, again in known manner. If an arch as shown in Figure 2 were subjected to load due to pressure P of water 18 acting on one side 20 of the

arch 16 as in Figure 3, the arch would no longer be in pure compression and would experience a bending moment.

Looking at Figure 4, and in accordance with the present invention, a catenary can be modified in shape and in order to provided zero or substantially zero bending movement when it is used to retain water or other liquid. Taking a conventional hanging catenary 10 formed as a multiplicity of identical links 21, the catenary extends around an interior area 22. For a given theoretical depth H of water 18, the pressure P is applied as a negative pressure (suction) to the catenary 10. Beginning at a joint i between first and second links 21 a, 21 b adjacent the position 12, the pressure P1 (depth D1 1 multiplied by water density) is applied to the joint i thereby deflecting the catenary away from the interior 22 by a given amount.

Next going on to the joint ii between the second and third links 21 b, 21 c the pressure P2 at depth is again applied outwardly to the joint ii. The procedure is repeated for the full depth H of the water 18 until the catenary takes on the deflected shape 1 Oa shown in Figure 4. If desired, the catenary 10 of Figure 4 can be depicted on a VDU screen of a computer and the pressures applied theoretically to the depicted catenary. The pressures P1, P2 directed away from the interior 22 correspond to the pressures which would be applied

at depths D1, D2 if the catenary were inverted to form a liquid retaining arch.

The final shape 1 Oa is then inverted as shown in Figure 5 to form an arch 23 and it will be seen that the arch comprises a section 24, the major part of which lies outwards of a wall 25 of the symmetrical catenary 10 of Figure 4 inverted and superimposed notionally on the arch 23 in Figure 5. The section 24 in Figure 5 is in the form of a rounded bulge and initially extends over a portion 26 outwardly from the interior 22 and upwardly from the position 12 then turns back on itself over a portion 28 and further upwardly for the full depth H of the water 18. In that way, the section 24 provides a water retaining side for the arch 23. The section 24 may be less or more bulge-like than shown dependent upon the size of the arch 23 and the pressures involved.

Referring now to Figures 6 and 7, a water retaining structure in accordance with the invention comprises an elongate horizontal member 30 which has shape in vertical cross-section corresponding to that shown in Figure 5. The horizontal member comprises a skin 32 formed from a material such as concrete which may be cast on a shaped former and suitably reinforced so as to define the hollow interior 22. It will be noted that the horizontal member comprises ridges or cusps 34 between adjacent segments 35 which are either integral with each other, or suitably secured to each other as pre-formed parts,

so as to define a unitary water retaining structure. The cross-section through the structure as shown in Figure 7 indicates that there is a certain amount of sag between each ridge 34 which forms a trough 36 all round the segment 35.

The depth S of the sag may be of catenary form and helps to provide some strength to the structure in the axial (longitudinal) direction.

The skin 32 has a thickness t which may be substantially constant.

The horizontal member 30 is secured to the ground at positions 12, 14 and water is allowed to rise up the water retaining section 24 to the depth H, the final depth being controlled, say, by a suitable overflow. Levels which exceed the depth H will create bending moments in the arch 23 that may give rise to the need for steel reinforcement depending on the particular geometry of the structure. As the shape of the catenary 1 Oa was determined on the basis of depth H, the water pressure on the water retaining section 24 will create substantially pure compression in the skin 32 leading substantially to zero bending moment in the arch 23 and hence in the horizontal member 30.