EP0407432B1

EP0407432B1 - Method and device for dissipating wave energy and controlling erosion in coastlines

Info

Publication number: EP0407432B1
Application number: EP89903981A
Authority: EP
Inventors: Donald Edward Dorrell
Original assignee: DORRELL Donald Edward
Current assignee: DORRELL, DONALD EDWARD
Priority date: 1988-03-22
Filing date: 1989-03-22
Publication date: 1995-09-27
Anticipated expiration: 2009-03-22
Also published as: WO1989009308A1; AU3365189A; DE68924421D1; AU630605B2; DE68924421T2; EP0407432A4; NZ222653A; EP0407432A1

Abstract

A device (10) for dissipating the energy of waves impinging on a shore, comprising a massive body which can be placed in the water adjacent a shore to impede the waves, the body having paggages (20, 22, 26) therethrough which in use are disposed in the general direction in which the waves move towards the shore. Each passage comprises an inlet facing toward and an outlet facing away from waves moving towards the shore, a portion adjacent its inlet in which upper and lower faces of the passages (30-32, 36-38, 42-44) are parallel and a portion adjacent its outlet in which the upper and lower faces of the passages slope upwardly and are convergent to promote streamline flow and cause water passing through the passages towards the shore to issue from the outlet in an upward direction. A number of the devices may be placed side by side.

Description

This invention relates to a device for dissipating wave energy and controlling erosion in coastlines
The erosion of coastlines such as beaches is a problem which has been the subject of considerable concern in the past. While there is now extensive knowledge of how coastlines are eroded by the sea, practical attempts to solve the problem inexpensively appear to have met with limited success. One reason for this is probably that it appears to be inescapable that whatever means is employed to dissipate the energy of waves impinging on a coastline, the primary cause of coastal erosion, such means must of necessity be massive and therefore expensive. Any attempt to reduce the mass of an object placed in the sea to dissipate wave energy carries with it the increased risk that the object will be displaced by the sea in rough conditions. Furthermore most known such means cause erosion to adjacent shore lines.
In US Patents 4367978 and 4479740, both to Schaaf et al., mules are disclosed which are intended to be placed in the sea adjacent a coastline in order to dissipate wave energy. Each mule is of generally prismatic form and comprises passages disposed generally in the direction in which waves move towards the shore. The mule disclosed in 4367978 appears to be a development of that shown in 4479740, the passages being of refined design intended according to the disclosure to direct downwardly some of the energy of the waves impinging on the upper and lower surfaces of the passages, thereby to assist in keeping the module in place on the sea bed especially in rough weather and enabling the mule to be less massive.
US Patent 4711598 to Schaaf et al. discloses a device for dissipating the energy of waves in water according to the preamble of claim 1. The device comprises a massive body which can be placed in the water adjacent a shore to impede the waves, the body having a base from which arise opposed front and back faces and a plurality of passages which extend through the body and have respective inlets at the front face and respective outlets at the back face. The passages in the Schaaf device have upper and lower surfaces which are generally parallel throughout. However, the lower surfaces have, adjacent the passage outlets, upwardly inclined portions which, although they are noted as being intended to deflect water upwardly, are rather small and meet the lower surfaces at a sharp angle. They would thus be more likely, in the Applicant's experience, to promote turbulence rather than substantial upward deflection of water passing through the passages.
It is an object of the invention to provide a device for dissipating the energy of waves impinging on a shore of, for example, the sea or a lake.
According to the invention there is provided a device for dissipating the energy of waves in water, the device comprising a massive body which can be placed in the water adjacent a shore to impede the waves, the body having a base from which arise opposed front and back faces and at least one passage which extends through the body and has a median plane which is disposed in the general direction in which the waves move towards the shore, the passage having an inlet adjacent the front face and facing towards the waves moving towards the shore, an outlet adjacent the back face and facing away from the waves moving towards the shore, wherein the at least one passage comprises surfaces including, relative to the base, upper and lower surfaces which in a zone of the passage adjacent the inlet comprise mutually parallel portions and in a zone of the passage adjacent the outlet comprise curved portions which blend smoothly with the respective mutually parallel portions and are shaped to slope upwardly away from said base and converge so as to cause the water stream passing through the passage inlet towards the shore to issue from the outlet in an upward direction.
Further according to the invention there is provided apparatus for dissipating the energy of waves in water including a device as defined in the immediately preceding paragraph mounted on at least one leg which supports the device so that in calm conditions it is located above the water.
The device and apparatus of the invention operate according to principles substantially different from those of the modules of the aforementioned patents to Schaaf et al. These principles may be summarised as follows:

a) As explained more fully below, the lowest passage (as also the other passages) is designed to remain clear of sediments or other deposits. The shape of the passages promotes streamline flow of water therethrough. This has the effect of washing sediments more readily through the passages. Moreover the sediments emerge from the passage at greater speed which, because of their momentum, causes them to carry clear of the device before being deposited on the sea bed.
b) The upward slope of the passages at the outlet ends tends to increase this distance of carry. However, perhaps more importantly, the fact that the water issues from the outlets in an upward direction reduces the turbulence of the water close to the sea bed on the shore side of the device thus inhibiting backwash erosion. It also applies a reactive force to the device which tends to push the device down onto the sea bed so that displacement of the device is less likely. And still further, the energy of the water emerging from the passages is directed towards the surface thereby tending to dissipate energy in the portion of the wave closer to the surface. This reduces both the height of wave run-up onto the adjacent shore and also the speed of backwash from the shore. It is this latter characteristic which, as explained more fully below, makes the device suitable for placement inside an outlying reef in water which in ordinary weather conditions is placid so that no erosion of the adjacent shore takes place in normal conditions. The device functions to dissipate wave energy only in storm conditions.
c) The streamline flow of the water through the passages reduces the frontal resistance of the device to wave impact so that there is less likelihood of the device being shifted by the waves.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further discussed with reference to the accompanying drawings in which various embodiments thereof are illustrated by way of example and in which:

Figure 1 is a cross-sectional side elevation of a device for dissipating the energy of waves impinging on a coast;
Figure 2 is a plan view of the device;
Figure 3 is a view (in the direction of arrow A) on what will be described as the back of the device;
Figure 4 is a view (in the direction of arrow b) on what will be described as the front of the device;
Figure 5 is a cross-sectional side view, similar to Figure 1 of a modified device; and
Figure 6 is a is a three-quarter side view of the device shown in Figure 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In Figures 1 to 4 there is shown a device which comprises a module 10 of reinforced concrete construction. The module has a base 12 from which arise parallel side faces 14 which are perpendicular to the base, a front face 16 and a back face 18, both of the latter being inclined from the perpendicular towards one another. The module is approximately of prismatic shape.
Three passages are formed in the module passing from the front face to the back face and located one above the other. The lower passage 20 is separated from the middle passage 22 by a dividing element 24 and the middle passage 22 is similarly separated from the upper passage 26 by a dividing element 28. The lower passage has a lower face 30, an upper face 32 constituted by the bottom of the dividing element 24 and side faces 34. Similarly the middle passage has a lower face 36 constituted by the top of the dividing element 24, an upper face 38 constituted by the bottom of the dividing element 28 and side faces 40; and the upper passage has a lower face 42 constituted by the top of the dividing element 28, an upper face 44 and side faces 46. The passages each comprise a first zone (which will be called a parallel zone) adjacent the front face 16 in which the upper and lower faces are substantially parallel to the base; and a second zone (which will be called an inclined zone) adjacent the back face in which the upper and lower faces are inclined upwardly and blend smoothly into the parallel zone. By way of example the parallel and inclined portions of the face 30 are indicated at 30' and 30'' respectively.
The upper and lower faces of each passage are convergent in the inclined zones. Moreover, in the example shown in the drawings, these faces are inclined at equal and opposite angles to the (imaginary) longitudinal axis of the passage which itself intersects the back face 18 of the module at 90^o. The inclined zones therefore form in each case what is in effect a nozzle the effect of which is discussed hereafter.
The dividing elements function as a series of vanes which offer little resistance to the water while guiding it in the desired path through the module. The front end 48 of the dividing element 24 is set in a substantial distance from the front face 16 of the module to define what will be called a chamber 50 which is in effect bounded by the lower face of the lower passage, the side faces of the lower and middle passages and the upper face of the middle passage. It should however be noted that the dividing elements 24 and 28 are substantially mutually identical in cross-section so that the front end 49 of the dividing element 28 is also set in a small distance from the front face of the module.
The modules are placed side by side in line in the sea adjacent a coast with the front faces 16 facing the prevailing direction from which waves impinge on the shore. In Figure 3 a portion of a module is shown to the left of one module 16 and the dotted line to the right of the one module 16 indicates yet another module. In most cases it is likely that the modules will be placed directly on the sea bed so that they are normally at least partially immersed in the sea. The water to which motion is imparted by the energy in the waves moves into and through the passages. In the case of the lower and middle passages 2O, 22 however the water first enters the chamber 50 which tends to act as a trap, preventing any water from escaping back outwardly therefrom and thereby forcing it to pass into the passages. The parallel zones of the upper and lower surfaces of each passage (which are nominally horizontal) are intended to have a minimal resistance to the water passing through the passage. However as the water passes further along and out of the passage the upwardly inclined zones have the effect of imparting to the motion an upward component. Moreover the pressure changes occurring as the water passes through the nozzle portion have the effect of applying a reactive force to the module which, because of the said upward component of motion, tends to push the module down onto the sea bed, thereby tending to resist displacement of the module. The shape and disposition of the passages of the module 10 therefore generate forces for keeping the module in place which are entirely different to those operating on the Schaaf modules. Moreover, the passages are such as to streamline the flow of water therethrough as far as possible. As noted above this will tend to have the effect of causing sediment in the water to be swept through the passage with the water, minimizing the deposit of sediment in the passages or in front of the module. This is an important characteristic of the invention not found in earlier proposals for building up a shore line.
The module is provided with an upper face 90 which comprises a very short horizontal zone adjacent the front face 16 and an upwardly inclined zone adjacent the back face 18. The inclined zone is curved similarly to the upwardly inclined zones of the lower faces of the passages 20, 22 and 26. The flattened portion 91 of the face 90 between the inclined zone and the back face 18 is to avoid a face 90 terminating in a sharp edge which would be susceptible to damage and erosion. The purpose of the face 90 is to provide a further upward impetus to water flowing thereover.
In Figure 4 a module 10 is mounted on cast concrete legs 80 which are themselves cast into holes in the sea bed. The legs may be flared at their upper ends as at 82 to enable each pair of legs to support one side of each of two modules mounted alongside one another. The modules may be attached to the legs by reinforcing rods which are cast into the upper ends of the legs and pass into holes formed in the modules which are afterwards filled up with concrete or a suitable adhesive. The purpose of this arrangement is to provide protection for a shore which is protected by an outlying reef. In these circumstances in ordinary weather conditions the water in which the modules 10 are placed is relatively tranquil and the modules 10 are placed at a height which is, in the present example, well above the high water mark shown in dotted outline at 84. However in storm conditions waves form in the water and these waves are so high as to impinge on or even overtop the modules 10. It will be noted that in the present case there will be very little sediment in the water and the purpose of the modules is not so much to cause sediment to be deposited on the shore side of the modules 10 as to dissipate the energy of the waves in stormy conditions and thereby to prevent erosion of the beach or shore. In normal weather conditions the modules are located above the water so that natural coastal processes can take place with a minimum of disturbance caused by the legs 80.
The number of legs which are required to support each module may vary. In some circumstances a single such leg may be sufficient. The module has a channel shaped recess 92 cast into the base 12. The recess has an upper face 93 which is substantially parallel to the lower face 30 of the passage 20 so that the recess flares upwardly adjacent the back face of the module. The recess terminates short of the front face of the module. The recess serves two functions. When the module is mounted on the columns the recess helps to relieve upward pressure applied to the base of the module by a wave passing through and beneath the module. This pressure has a tendency to lift the module. Furthermore, when the module is placed on a sea bed comprising sand, the recess greatly reduces the non-permeable area of the base on the shore side of the module. This helps to prevent long shore scour in this area. The recess will quickly fill with sediment but will still allow hydraulic pressure applied to the lower face of the base to be relieved through the sediment.
Where conditions are expected to be exceptionally rough holes 94, 96 and 98 may optionally be cast into the sides, top or base respectively of the module 10. The purpose of the holes 94 is to impart a vertical component of energy to a wave which overtops the module. The holes 94 and 96 also help relieve hydraulic pressure applied by a passing wave. Both of these effects help to reduce the forces applied to the upper portion of the module which exert a considerable moment tending to overturn the module. The holes 98 relieve hydraulic pressure on the lower face of the module thus reducing any tendency for the module to be shifted under such pressure.
Whether laid on the sea bed or supported on columns as shown the modules may be anchored to one another by means of straps 86 (in the form of cables, chains or the like and indicated in dotted outline only) attached to U-bolts 88 cast into the back faces of the modules.
The dimensions of the module and especially the geometry of the passages may vary according to what is found most suitable for the locality in which they are to be used. The module illustrated in Figures 1 to 4 (which are to scale) is 2 metres long and 1.7 metres wide at the base. The slant height of the front face 16 is 1.488 metres and it is disposed at an angle of 52^o to the base. The slant height of the back face 18 is 1.414 metres and it is disposed at an angle of 65^o to the base. The width along the back face of each passage is 16 cm. The inclined zone of the upper and lower face of each passage and the upper face 90 intersects the back face 18 at an angle of 85^o. The radius of the circle joining the inclined zone with the parallel zone of each upper or lower surface is 60 cm. The remainder of the dimensions of the module 10 may be derived by scaling the drawings.
The modules are cast in moulds. For the purpose of extracting the modules from the moulds it is necessary for the parallel zones of the passages to be tapered at a very small angle. This angle is insufficient to affect the operation of the module and should be ignored in construing the term "parallel zones".
Referring now to Figures 5 and 6, the module 10' has passages 20', 22' and 26' which are substantially similar in configuration to those of the module 10. However the module 10' has a crown which is considerably more pointed than the top of the module 10.
Various other modifications which are illustrated in the module 10' may also be incorporated in the module 10. The modules 10' may be joined to one another by short cables or the like which are passed through holes 60 in the sides of the modules, a head being swaged in place at each end of the cable. One or more bars 62 may be placed across the entrance to the chamber 50'. These bars are useful to prevent divers, boulders or other large objects from being swept into the chamber or passages. The bars may also function as a ladder. Bars placed vertically across the seaward face with a spacing slightly less than the width of the outlets of the passage on the shoreward face would prevent blockage of the passages by boulders and other water-borne debris. The modules may further be anchored in place in exceptionally rough localities by means of a log or other suitable mooring device attached to the module by a steel cable 66.
The angle of inclination of the front and back faces of the module and the inclined surfaces may be varied to suit different conditions, as may be the dimensions of the modules. For example, under certain circumstances the length of the base and side walls may be increased without necessarily increasing the relative length of the passages. It is also envisaged that the inclined zones of the upper and lower faces of one or more of the passages may embody portions which vary in inclination. For example, central portions 36''' and 38''' of the upper and lower faces of the middle passage (shown in dotted outline in Figure 5) may be inclined at a steeper angle than portions of the faces to either side of the central portion.
In a further alternative the module may be constructed in two pieces, joined together along the dotted line indicated at 70 in Figure 5. The lower piece may be used as a module by itself or alternatively with the upper piece joined to it (by bolts or other suitable fastenings) either before or after the lower module is located in place in the sea. The upper piece might be detached later if it was found to be redundant.
Placement of a line of the modules adjacent a shore can help in the first place to reduce or prevent erosion of the shore. It can also help to build up a shore or beach by causing sand or other matter to be deposited on the sea bed between the line of the modules and the existing shoreline. be

Claims

A device for dissipating the energy of waves in water, the device comprising a massive body (10,10') which can be placed in the water adjacent a shore to impede the waves, the body having a base (12) from which arise opposed front and back faces (16,18) and at least one passage (20,22,24,20',22',24') which extends through the body and has a median plane which is disposed in the general direction in which the waves move towards the shore, the passage having an inlet adjacent the front face (16) and facing towards the waves moving towards the shore, an outlet adjacent the back face (18) and facing away from the waves moving towards the shore, characterised in that the at least one passage (20,22,24,20',22',24') comprises surfaces including, relative to the base, upper and lower surfaces (30,32, 36-38, 42-44) which in a zone of the passage adjacent the inlet comprise mutually parallel portions and in a zone of the passage adjacent the outlet comprise curved portions which blend smoothly with the respective mutually parallel portions and are shaped to slope upwardly away from said base and converge so as to cause the water stream passing through the passage inlet towards the shore to issue from the outlet in an upward direction.
A device as claimed in claim 1, characterised in that the mutually parallel portions are substantially horizontally disposed in use.
A device as claimed in either preceding claim, characterised in that adjacent the outlet the respective curved portions are disposed at equal angles on either side of said median plane of the passage.
A device as claimed in any one of the preceding claims, characterised in that the front face and the back face are inclined towards each other and the body has a plurality of said passages (20,22,26,20',22',26') therethrough located one above another and each having a said inlet adjacent the front face and a said outlet adjacent the back face.
A device as claimed in claim 4, characterised in that the lower surfaces (30,36,42) of the passages are mutually parallel and the upper surfaces (32,38,44) of the passages are also mutually parallel.
A device as claimed in claim 5, characterised in that the median plane of each passage is normal to the back face (18) at the outlets.
A device as claimed in any one of claims 4 to 6, characterised in that it comprises at least three said passages (20,22,26,20',22',26') which are separated from one another by two dividing elements (24,28,24',28') located one above the other, each dividing element having an upper face constituting the lower surface of the passage above it and a lower face constituting the upper surface of the passage below it, the two said dividing elements being substantially identical in cross-sectional shape.
A device as claimed in any one of claims 4 to 7, characterised in that it has an upper face (90) comprising a zone adjacent the front face and which curves to slope upwardly in a zone adjacent the back face.
Apparatus for dissipating the energy of waves in water characterised in that it comprises a device (10,10') as claimed in any one of the preceding claims mounted on at least one leg (80) which supports the device so that in calm weather conditions it is located above the water.