Abstract:
The present invention is a device and a method for protecting part of a coastline against erosion. The method uses the device as an elongated fin with a longitudinal axis lying generally in a horizontal plane. The fin is supported by a suspension arrangement so that it can swing in the horizontal plane. Adjusting the longitudinal axis of the fin, in the horizontal plane, causes the fin to deflect from the water flow a component that counteracts with a generally coast-parallel water flow component. This component originates from the water flow and/or from a wave that is not parallel with the coastline. The fin is carried by a suspension arrangement which enables the fin to swing in the horizontal plane. The suspension arrangement is constructed to increase the fin angle relative to the coastline when the direction of the waves relative to the coastline increases, and to decrease the fin angle relative to the coastline when the direction of the waves relative to the coastline decreases.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of protecting part of a coastline against erosion, by placing in a wave breaking zone, a device to dampen water flow generated by the breaking waves. 
     The invention also relates to an arrangement for protecting a part of a coastline against erosion. The arrangement includes a device being placed in a wave breaking zone and adapted to dampen water flow generated by the breaking waves. 
     In a zone where the water is so shallow that waves have an influence on the sea bed, sea-bed material will be suspended by waves and transported inwards and outwards perpendicular to the coastline, this material settling at roughly the same rate as it was suspended, thereby leaving the coastline more or less intact. However, the water flow will often have a flow component along the coastline. Such a flow component occurs when waves are angled to the coastline in the breaking zone. This results in sand and other erosion material/beach and coast material being moved along the coastline. Sand that is washed away along the coast from one place to another is not always replaced to the same extent from a part of the coast located upstream. Thus, sandy beach can be washed away or built-up and extended even in the case of relatively small changes in the direction of the incoming waves at the part of the coast in question. This phenomenon whereby a sand beach is washed away and re-built can be amplified by general water flows in the coast region. 
     Such coast erosion is a well known problem and proposed solutions include the use of beach facings, wave breakers, groynes, artificial reefs, pneumatic wave dampers, artificial “seaweed” (bottom-secured oil-filled hoses) among other things. 
     One problem with these known solutions, however, is that they are very expensive to construct and are either built to provide the intended function for a specific predominant wave direction or to dampen incoming waves. None of the solutions is aimed at actively influencing the coast-parallel net loss of sediment along a heavily eroded coast section. 
     In reality, however, wave direction varies in a manner that cannot be fully anticipated, and consequently fixed structures along the coastline can sometimes give a less desirable result. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide a method that can be carried out effectively at low cost, and to provide an effective arrangement that can be manufactured and mounted at a relatively low cost and which will function effectively in the case of differing wave directions, and which is able to adjust automatically to an optimal setting with respect to variations in wave direction, such as variations that have been observed over long periods of time. 
     One object of the invention is therewith also to provide a corresponding method. 
     This object is achieved by using a device in the form of an elongated fin with a longitudinal axis lying in a horizontal plane. The method includes the step of providing a support for the fin so that it can swing in the horizontal plane. This is accomplished by adjusting the longitudinal axis of the fin in the horizontal plane. The fin deflects, from the water flow, a component that counteracts with a generally coast-parallel water flow component that originates from the water flow and/or from a wave that is not parallel with the coastline. This occurs when the angle of the wave direction relative to the coastline increases. Alternatively, when the angle of the wave direction relative to the coastline decreases, the suspension arrangement decreases the fin angle relative to the coastline. 
     The method is also achieved with the arrangement according to a device which is an elongated fin. The longitudinal axis of the fin is orientated in a chosen first fin orientation such as to cause the fin to deflect from the water flow a first flow component. The first flow component counteracts a coast-parallel water flow component originating from a general water flow and/or from a wave that is not parallel with the coastline. The fin is carried by a suspension arrangement which enables the fin to swing in the horizontal plane. The suspension arrangement is constructed to increase the fin angle relative to the coastline when the angle of the waves relative to the coastline increases, and when the angle of the waves relative to the coastline decreases, the suspension arrangement decreases the fin angle. 
     Further embodiments of the invention will be evident from the dependent claims. 
     The inventive arrangement can be considered fundamentally to include an elongated fin having a longitudinal direction that generally lies in the horizontal plane. The fin also suitably has a generally vertical extension, such as to be able to capture and dampen water flows that derive from the breaking waves. The fin is conveniently constructed from a buoyant material, so that the upper edge of the fin will lie essentially at the surface of the water. The fin is supported so as to be able to swing in the horizontal plane. The longitudinal axis of the fin is set in the horizontal plane to a selected orientation in which the fin deflects from the water flow a flow component along the fin. This current component gives rise to a coast-parallel, water-flow component that serves to counteract an undesired coast-parallel water flow, for instance a water flow that transports sand away from the coast section concerned. This undesired coast-parallel water flow may consist of a general water flow and/or of a flow component generated by a wave that falls obliquely to the coastline. The orientation of the fin relative to the coastline is then changed in dependence on changes in wave direction relative to the coastline, so that the current component, or rather the component parallel with the coastline, deflected by the fin is given a desired value for different angles defined by the waves with the coastline. 
     The fin suspension can be controlled by sensing, or detecting, the direction of incoming waves, and using the sensed wave direction to control setting means/suspension arrangements coupled to the fin. In preferred embodiments of the invention, the suspension may be designed to enable incoming waves themselves to set the fin to those directions permitted by the suspension arrangement. According to one embodiment of the invention, the suspension arrangement may include a mechanism that has two anchoring points that are mutually spaced along the coastline, two coupling points spaced along the fin, and two links which are each connected between an anchoring point and a coupling point so as to intersect one another. The anchoring points may, for instance, comprise sea anchors, i.e. anchoring devices that are embedded in the sea bottom. 
     Alternatively, the suspension arrangement may include a fixed post, a fixed block or like element, and a centered U-shaped element on the rear side of the fin, wherein the post extends through the U-shaped element and wherein the U-shaped element is constructed to allow the fin to self-adjust in accordance with the abovementioned pattern for waves incoming in different directions. 
     A system for protecting a coastline or beach against erosion may include a plurality of protective arrangements mounted along the coastline. 
     When applying the inventive technique, a reduction in erosion can be expected by virtue of the fact that the invention retards the coast-parallel transportation of erosion material or sand, wherewith this erosion limiting effect can also result in a build-up of the coastline concerned, because the sedimentation possibilities with respect to coast-parallel material transportation are favoured. The invention can therefore also be applied to build-out or extend a coastline that would otherwise be kept constant or be eroded due to coast-parallel water flows and currents. 
     The fins will preferably be placed in the water in the coast zone where the incoming waves are broken, i.e. where the incoming waves generate water flows that cause erosion and the transportation of material. 
     The fin may beneficially be arranged to float in the water close to the surface thereof, wherewith the vertical extension of the fin, or its height, will preferably correspond to about half the depth of the water in the proximity of the fin. The fin will suitably have a height which corresponds at most to 0.9 times the depth of water. The fin may have a length of, for example, 3-5 m, although the length of the fin will depend primarily on the mechanical strength requirements in respect of the wave climate concerned. 
     The fin may be moored or tethered with a line that will prevent the fin from moving away from the beach to any great extent, for instance in the case of an off-shore wind. The mooring line, or some other corresponding device, may be arranged to restrict the angle of the fin relative to the incoming waves, where the wave direction causes the fin to adopt an angle that exceeds a given value from the coastline. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     The invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings, in which 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an inventive arrangement from above; 
     FIG. 2 shows the fin of said arrangement angled in relation to the coastline as a result of incoming waves defining an angle therewith; 
     FIG. 3 is a sectional view taken on the line III—III in FIG. 2; 
     FIG. 4 illustrates another embodiment of the inventive arrangement; 
     FIG. 5 illustrates the arrangement of FIG. 4 influenced by waves that define an angle with the coastline; 
     FIG. 6 illustrates an inventive arrangement from above, with geometric signs; and 
     FIG. 7 is a schematic version of FIG. 6 as a basis for the geometrical relationships. 
    
    
     DESCRIPTION OF THE INVENTION 
     Shown in FIG. 1 is a coastline  1  and essentially parallel waves  2  that break in a breaking zone whose outer limit is indicated by the broken line  3 . The sea and the coastline  1  will be affected by breaking of the waves  2 . This can result in erosion of the coastline  1  and the suspension of sea-bed material. 
     The inventive arrangement comprises a fin  10  that is placed in the wave breaking zone  4  between the lines  1  and  3  shown in the figure. Because the waves  2  are generally parallel with the coastline  1 , the fin  10  is mounted so as to extend generally parallel with the coastline  1 . The fin  10  is suspended from a suspension arrangement  20 , so as to enable the fin  10  to be angled relative to the coastline  1 , in dependence on the direction of the incoming waves  2 . As shown in FIG. 1, this suspension arrangement is comprised of two anchoring points  21 , achieved with sea anchors or the like embedded in the sea bed. The fin  10  has two connection points  22  that are mutually spaced along the fin  10  and that lie essentially equidistantly from the longitudinal centre of the fin  10 . (The distance between the anchoring points  21  is, as a rule, much shorter than the distance between the connection points  22 , although the ratio of said points will depend on the “lift-coefficient” of the fin). Connecting elements  23 , for instance links or wires, extend between the points  21  and  22  while intersecting one another. The fin  10 , which may have a length in the order of 3-5 m, is suitably arranged to float with its upper edge in the proximity of the surface of the water, wherewith the fin will have a height in the region of 0.3-0.9 d where d is the depth of water at the fin  10 . The perpendicular distance to the coastline  1  between the points  21  and  22  may be 5,6 m for instance. As evident from FIG. 3, links  23  may extend directly or with a division between  21  and  23  from respective anchoring points  21  to the upper edge and bottom edge of the fin  10 , so as to hold the fin in a chosen, generally vertical position. 
     The suspension arrangement  20  is constructed to set a larger angle between the longitudinal axis of the fin  10  and the coastline  1  when the waves  2  begin to define an angle with the coastline l, as in the case illustrated in FIG.  2 . 
     The flow of water in towards the coast resulting from the breaking wave will be dampened when the wave passes generally at right angles over the longitudinal axis of the fin  10 , although the angular difference between the fin  10  and the wave  2  will cause a part S 1  of the fluid flow of the wave to be deflected along the fin  10 . As indicated generally to the right of FIG. 2, the fluid flow S resulting from the wave  2  can be divided into a component S 2  which is perpendicular to the coastline  1 , and a component S 3  which is parallel with the coastline. Correspondingly, the fluid flow S 1  will be divided into a component that is perpendicular to the coastline  1  and a component S 10  which is parallel with the coastline  1 . It will be evident from FIG. 2 that the component S 10  moves in a direction opposite to the component S 3 . The component S 10  is thus able to slow down the coast-parallel fluid flow and the material transportation created by the component S 3 , thereby enabling the transportation of material, e.g. sand, from the coastal region inwardly of the fin to be counteracted. This effect is thus able to limit erosion of the coastline  1  or to promote an extension of the coastline, as a result of establishing more favourable sedimentation conditions for material suspended in the water in the region of the inventive arrangement. 
     As can be seen from FIG. 1, means are provided for limiting the angle to which the fin  10  is inclined relative to the coastline  1 . These means  40  are also adapted to keep the fin  10  in the region between the coastline  1  and the anchoring points  21  in the event of an off-shore wind or in the event of other conditions that strive to move the fin  10  outside the anchoring points  21 . 
     The means  40  comprise an anchoring point  25  on the sea bottom, for instance in the form of a sea anchor or like arrangement, a line  26  which is coupled between the anchoring point  25  and the fin  10  and the length of which determines the swinging area of the fin  10  and thus the angle positioning area relative to the anchoring arrangement formed by the anchoring points  21 . 
     FIG. 4 illustrates an alternative embodiment of the invention, comprising a vertical post/block  31  stationarily mounted on the sea bottom, and a generally U-shaped element  32  which is mounted centrally on the outwardly facing side of the fin  10 . The post/block  31  is received in the area defined by the U-shaped element  32  and the fin  10 . The U-shaped element  32  is symmetrical relative to a central vertical plane and has its deepest part in its symmetry plane, wherein the U-shaped element  32  is constructed in general so as to give the fin a larger angle of inclination relative to the coastline  1  than the angle between the incoming waves  2  and the coastline  1 , so that the U-shaped element  32  in co-act ion with the post  31  will cause the fin  10  to operate in generally the same way as in the embodiment according to FIG.  1 . 
     In order that respective corner regions  321  of the generally U-shaped element  32  shall be displaced into contact with the post  31 , such that the fin  10  will give rise to the desired, deflected flow component for the corresponding obliquely incoming wave, there is provided a setting arrangement  40 . 
     This setting arrangement  40  may include a line  33  that extends between the legs of the U-shaped element, wherein a running block  34  is arranged to run on the line  33 . The block  34  is, in turn, connected to a line  35  that extends through a running-eye  121  carried by an anchor anchored to the ground/sea bottom, wherein one end of the line  35  is connected to a buoy  36  whose position of boyancy is such as to generate a tensile force in the line  35 . Alternatively, the line  35  may be replaced with an elastic line or the like that is anchored to the ground in the indicated position, namely between the fin  10  and the coastline  1 . The angle at which the fin is inclined is determined by the balance between the force that acts perpendicularly on the fin, the shearing stress exerted by the fin on the deflected water flow, and the force in the line  35 . The line  35  may also be used to define the maximum angle of inclination of the fin when contact is made between the buoy  36  and the eye or loop  121 . 
     The post  31  can be replaced with a running block which accommodates the U-shaped element  32  and which, in turn, is supported from some fixed point. 
     In the embodiment illustrated in FIG. 3, the stabilizing line  23 ,  23 ′ extends fully from respective anchorage points  21  to the fin  10 , although it will be understood that the lines  23 ,  23 ′ may be mutually joined at a short distance behind the fin  10 , wherewith a single connecting line extends from the anchoring point  21  to the point at which the lines  23 ,  23 ′ are joined. 
     Littoral (coast-parallel) sediment transportation is described as a rule with the aid of different empirical expressions. A common feature of all these descriptions is the high significance of the wave&#39;s angle, i.e. the wave crest angle, to the coast, since the coast-parallel component of the wave is directly dependent on this angle. If it is assumed that the fin is able to “twist” a part of this wave crest such that it will approach the coast from an opposite direction (with a coast-parallel component in an opposite direction), it will enable the erosion inhibiting properties of the fin to be related to the natural transportation of sediment. 
     EXAMPLE 
     Ex: The CERC-formula for calculating coast-parallel sediment                  Q   =     K       (       p   s     -   p     )          g   ′        a                   P   ls             [   A   ]                                
     where Q=sediment transport, K=coefficient, p s =sand density, p=water density, g=gravitational constant, a′=sand porosity 
     The term p ls  describes the coast-parallel component of the wave energy flux.                P   ls     =       pg   16          H   ab   2          C   gb        sine                 2                   α   b               [   B   ]                                
     where H sb =significant wave height of the breaking wave, C gb =the group velocity of the breaking wave, a b =the angle of the breaking wave to the coast. 
     Assume that the fin changes direction of part of the wave to angle −a′. Sediment transportation will then be influenced in two ways, firstly that part of the wave which changed direction to −a′ will brake the natural transportation of sediment, and secondly the energy flux in the original direction a b  will be smaller. It is believed that the influence of the fin on sediment transportation can be described as                F   ls     =     A        [         pg   16          H   sb   2          C   gb        sine                 2                   α   b       +     η        pg   16          H   sb   2          C   gb        sine                 2                   α   ′         ]               [   C   ]                                
     and the influence of Q as:                  Q   =     K       (       p   s     -   p     )          g   ′        a                   (       P   ls     -     F   is       )             [   D   ]                                
     The term η describes a sort of fin efficiency (i.e. how much of the wave energy changes direction) and A describes how much of the wave energy flux is influenced. The terms will probably be dependent on the configuration of the fin (shape, height and length) and the angle of attack of the fin, and the distance between the fins with respect to a whole system. The angle a′ will be dependent on the angle of the incoming wave to the coast and also to the attack angle and configuration of the fin. 
     It is necessary to determine a desired design angle before the forces that act on the fin can be calculated (under normal conditions). Laboratory trials indicate that the optimal angle φ between the fin and the wave crest is about 40°. Coast-parallel lines in FIG.  6 . are designated  1 ′. 
     The design angle φ (FIG. 6) will be a function of the angles a, and a 2  which, in turn, depend on the “lifting capacity” of the fin and the length L, a and b (cf also FIG.  7 ). 
     Some geometrical relationships based on FIG. 7, which in turn is based on FIG. 6 are listed below.                sin                 φ     =       sin                 γ     =       2      sin                   α   1        sin                   α   2             2        sin   2                     α   1       +     2          sin                2          α   2       -       sin   2          (       α   1     +     α   2       )                       [A]                 sin                   α   2       =       sin                   α   1       -       b   L          sin        (     ϕ   +   β     )                   [B]                 cos                   α   2       =       a   L     +       b   L          cos        (     ϕ   +   β     )         -     cos                   α   1                 [C]               L   =         a                 sin                   α   2       +     b                   sin        (       α   2     +     (     ϕ   +   β     )       )             sin        (       α   1     +     α   2       )                 [D]               0   =       sin                   α   1       -     sin                   α   2       -       b                   sin        (       α   1     +     α   2       )            sin        (     ϕ   +   β     )             a                 sin                   α   2       +     b                   sin        (       α   2     +   ϕ   +   β     )                       [B+D]                                
     Force and moment equilibrium                             M1   :         F   2        sin                   α   2       -                    F   D     2        cos                 ϕ     -                    F   L     2                   sin                 ϕ         =                  0   ⇒                F   2       =                      F   L        sin                 ϕ     +       F   D        cos                 ϕ         2      sin                   α   2                     [   E   ]                 M2   :         F   1        sin                   α   1       -                    F   D     2                   cos                 ϕ                -                    F   L     2                   sin                 ϕ         =                  0   ⇒                F   1       =                              F   L        sin                 ϕ     +       F   D        cos                 ϕ         2      sin                   α   1                    [   E   ]     +     [   F   ]       ⇒       F   1        sin                   α   1         =       F   2        sin                   α   2                   [   F   ]                 M3   :                    m   a          F   D        sin                   (     γ   +   ϕ     )       -                  m   a                     F   L        sin                   (     φ   -   ϕ     )           =                  0              ⇒                             F   D       F   L         =                  sin                   (     φ   -   ϕ     )         sin                   (     φ   +   ϕ     )                   [   G   ]                 β   :                  F   L     -       F   1        cos                   (       α   1     -   ϕ     )       +       F   2        cos                   (       α   2     -   ϕ     )                      =              0           [   H   ]                 β   -     90        °   :                  F   D     -       F   1        sin                   (       α   1     -   ϕ     )       -       F   2        sin                   (       α   2     +   ϕ     )                          =              0           [   J   ]                   F   D       F   L       =         sin                   α   2        sin                   (       α   1     -   ϕ     )       +     sin                   α   1        sin                   (       α   2     +   ϕ     )             sin                   α   2        cos                   (       α   1     -   ϕ     )       -     sin                   α   1        cos                   (       α   2     +   ϕ     )                       [   J   ]       [   H   ]       =     [   K   ]                                  
     The ratio F D /F L  (C D /C L , tow/lift coefficient) and the length a will be specific for a given fin. The parameters L and b (and therewith also the angles a 1  and a 2 ) need to be adapted so that the desired design angle, φ, will be obtained for the largest possible span of β (the angle of the incoming wave). 
     EXAMPLE 
     Assume: F D /F L =1.5 (relatively high value) for φ=45° 
     Choose: a (distance between attachments 22 on the fin)=4 m 
     Step 1 
     The fins are first dimensioned so that they self-adjust to the desired design angle at the dominating wave direction. 
     EXAMPLE 
     Set φ=45°, β=20°. 
     L, b, a 1  and a 2  are taken from the equation [K] and [B+D]. Ex 
     a=4 m, φ=45°, β=20°, gives 
     F D /F L =1.5 (for one type of fin), L=6.48, b=0.85, a 1 =80° and a 2 =60°. 
     Step 2 
     It is of interest to obtain an understanding of to which angle the fin will self-adjust in other wave directions. 
     Assume a value for φ+β. Use equation [B] and [C] (with L and b from step 1) to find a 1  and a 2 . Use equation [K] or [G] to obtain the relationship between F D /F L  and φ (F D /F L  varies with φ, depending on the configuration of the fin). 
     (Assume for the sake of it that F D /F L =1.5 for all φ). 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 φ + b 
                 a 1   
                 a 2   
                 φ 
                 β 
               
               
                   
               
             
             
               
                 50° 
                 77.4° 
                 61.1° 
                 43.0° 
                 7° 
               
               
                 60° 
                 79.2° 
                 60.3° 
                 44.4° 
                 15.6° 
               
               
                 70° 
                 80.9° 
                 59.8° 
                 45.6° 
                 24.4° 
               
               
                 80° 
                 82.3° 
                 59.5° 
                 46.5° 
                 33.5° 
               
               
                   
               
             
          
         
       
     
     The anchoring lines will preferably be constructed so that they will not both break in the event of a breakdown. In the case of extreme loads (i.e. loads greater than the dimensioned load), one line will preferably be able to break before the other line, therewith reducing the load on the remaining line and enhancing its possibilities of retaining the fin until it can be repaired. 
     Because the fins float in the water, some form of warning mark should be fixed to the fin, for instance a flag or mark similar to those used to show the presence of fishing gear at sea. 
     In order to prevent the fin floating away from the coast, e.g. as the result of off-shore winds, some form of restricting line will preferably be used inwardly towards the coast. This line can also be used to give the fin a maximum angle to the coast. 
     The lines will preferably be dampened, so as to reduce wear and the risk of breakdown or displacement of the bottom anchorage. It is proposed in this respect that some form of spring is used, for instance rubber springs, and that the springs are connected parallel with a short section of the line, so as primarily to take-up jerks in the line by stretching elastically. Such springs are used for dampening jerks in the mooring lines of leasure craft and are available commercially. 
     The illustrated embodiments include a suspension means which, in co-action with the fin  10 , gives the fin the desired angular setting in relation to the incoming waves, so that the fin will generate therealong a current or flow that counteracts the coast-parallel flow component of the waves that are angled to the coast line. 
     In the embodiment shown in FIG. 1, the fin  10  is parallel with the coast line  1  and the waves  2  are also parallel with the coastline  1 . It will be understood, however, that the suspension means  20  can be constructed to hold the fin  10  in a non-parallel relationship with the line I and the waves  2 . For instance, if it is known that the coast suffers a net loss of material/sand to the “left” in FIG. 1, the lengths of the lines  23  can be adapted so that the fin will deflect a water-flow component to the “right” in FIG. 1, even when the incoming waves are parallel with the coastline. 
     The side of the fin that faces towards the waves has been shown to be concave in the horizontal plane, this curvature being sufficiently large to ensure that the part of the fin that faces towards the coastline will approach the coastline direction with a given wave direction (e.g. the dominant wave direction). However, the curvature of this surface should not be so large as to risk flow being deflected outwards from the coastline for other frequently occurring wave directions. Naturally, the deflecting side of the fin may be straight. 
     The purpose of a curvature is to guide the coast-parallel flow component generated at the fin in certain cases. The curvature can otherwise be said to function to improve the lifting coefficient of the fin. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be recognized by one skilled in the art are intended to be included within the scope of the following claims.