GB2442847A

GB2442847A - Sediment accretion device and method

Info

Publication number: GB2442847A
Application number: GB0719224A
Authority: GB
Inventors: Philip Edward Ball; Richard Sydney Silvester
Original assignee: SHOREFORM Ltd
Current assignee: SHOREFORM Ltd
Priority date: 2006-10-12
Filing date: 2007-10-02
Publication date: 2008-04-16
Anticipated expiration: 2027-10-02
Also published as: GB2442847B; GB0719224D0; GB0620253D0

Abstract

Apparatus for controlling movement of sediment in water adjacent a shore comprises a structure 12 erected on an underwater substrate. The structure has a substantially convex horizontal profile extending around its periphery to produce reflected wave crests 19 from incident wave crests 11. The structure has a horizontal width of no more than about four times the average incident wavelength and no less than about half the average incident wavelength. The structure has a height sufficient for incident waves to be at least partially reflected from it. A further structure may be erected on the underwater structure but spaced from the original structure.

Description

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TITLE: SEDIMENT ACCRETION DEVICE AND METHOD

DESCRIPTION

TECHNICAL FIELD

The present invention relates to a method and apparatus for controlling movement of waterbome sediment adjacent a shore, and particularly, but not exclusively, to the eventual formation of headlands and bays therebetween that are stable against erosion.

BACKGROUND ART

: *.* Defence of the shore against the ravages of the sea is a difficult challenge. Apart S...

from climate-induced sea-level rise we have the combination of tidal currents and s. 25 wave action to erode the beach margins and threaten nearby property. S...

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* The usual type of protection considered has been along the lines of groynes, sea walls, offshore breakwaters or beach material replenishment. s * .

Sediment will accumulate on the up-shore side of a groyne, but it is on the down-shore side of a groyne that erosion can occur during storms, where the concentrated rip current can take beach material further out to sea than would have been the case without the groynes. r

Sea walls are subject to increased littoral (longshore) drift and massive erosive forces at their foundations, due to the interaction between obliquely incident waves and their reflections.

Sediment will be deposited shoreward of a breakwater parallel to the original shore, yet severe storms may cause relatively large volumes of water to enter the region shoreward of a series of breakwaters, producing rip currents of such intensity that all the sediment collected behind the structures can be washed away in one event. In any case, a breakwater of any orientation remains subject to the same erosive forces as at the foot of a seawall.

Beach replenishment is an obviously undesirable means of shore protection due to the need for continual maintenance and dredging.

Conventional construction methods use heavy rock or cast concrete units for the outside face of breakwaters, with the inner core usually consisting of quarry-run rubble. Problems with this system include armour units becoming dislodged, broken or thrown about by freak waves, waves penetrating the core causing armour units to settle, and scour of sediment at the bed of the structure can undermine the whole structure.

: Berm breakwaters have also been considered, where quarry-run material, rather than S...

""S heavy armour units, is used throughout. Whilst storm waves can readily enough *** ** 25 move individual stones, the assembly as a whole is able to take a shape that is stable *.S.

against storms. It is possible for the entire structure to move steadily down-shore.

Headland control is the most promising line of defence against wave-induced erosion.

The idea is to place structures offshore along a shore at regular spacing so that stable bays can form between them. The new bays would be oriented such that the persistent swell no longer approaches the beach at an angle, but rather, is normal to the beach. The idea has been developed from observations of stable bays around the

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world, confirmed by a crucial model study and used with success both in Singapore and Western Australia.

Inerodible offshore structures, spaced along the shore, allow sandy bays to form between them that are stable from wave-induced erosion. As long as the structures are in deep enough water and spaced far enough apart, the detail of their design should not significantly affect the bays between them. The seabed contours inside the bays must join up with those around the headland-forming-structures between them, in order to create a continuous shoreline. If this transition can be made sufficiently smooth, the headlands can then be expected to be as stable as the bay.

Headlands do not form part of the stable bays themselves, but rather bridge the end of one stable bay to the beginning of the next. Natural headlands would themselves be founded on rock, but this would make a scheme of artificial headlands unfeasibly expensive. More favourable would be a continuation of the stable shelving beaches of the bays themselves.

The common element in nearly every erosive situation is that the waves arriving almost continuously at a shore (the most common waves, that is to say, the persistent/prevailing swell) approach the shore at an angle. Typically, the wavelength of the persistent swell will be in the region of around lOOm. Conventional construction of a headland requires that the average incident swell direction be estimated, for instance by looking for stable bays in the near vicinity. If there are S... none present, there is at present no clear way of designing and spacing a series of *...

headlands. * S

It is therefore desirable to form headlands, which may act as the ends of stable bays S.....

* that are stable (e.g. dynamically stable) against erosion such as that from wave- :.:::. induced vortices at sharp corners, or from short-crested waves induced by long *: straight sections.

It is also desirable to have a headland shape that, during a storm, initiates the formation of a sand bar close to the original shore that will be returned to the bay it came from. r

Furthermore, it is desirable to have a headland that may be constructed without prior knowledge of the prevailing swell direction.

DISCLOSURE OF THE INVENTION

In accordance with a first aspect of the present invention, there is provided a method of controlling movement of sediment (for instance, sand) in water adjacent a shore (of an ocean, sea, lake, etc.), comprising: determining at a predetermined location an average incident wavelength of waves travelling towards the shore; and erecting at the predetermined location a structure on an underwater substrate (for instance, the sea bed), the structure presenting a substantially convex horizontal profile to waves travelling toward the shore, the profile having a horizontal width of no more than about four times the average incident wavelength, and the structure having a height sufficient for incident waves to be at least partially reflected therefrom.

In this way, waves are dispersed on the substantially convex surface of the structure, and forced to deposit at least some of the sediment suspended in the water in the vicinity of the substantially convex horizontal profile of the structure, ultimately forming a beach around the structure. A relatively small simple core structure can be used to generate a large island or bank, or ultimately a headland. The resulting feature is protected from incoming waves by a sloping beach, and will be largely : naturally formed. The substantially convex horizontal profile of the structure may be uniformly curved (e.g. circular), smoothly curved or faceted (for instance it may * 25 consist of several vertical walls joined along their edges such that the angles between adjacent walls are each greater than 180 degrees, e.g. a hexagonal or octagonal section). Since the ratio of island diameter to wavelength is one determinant of its :, ::: accretion performance, once equilibrium has been reached the diameter essentially fixes the width of beach available as a buffer against storms. This dimension is parallel to the wave celerity vector, as opposed to the broadside dimension needed to accrete the beach when the structure is still an island.

At least a portion of the profile may substantially face the waves travelling towards the shore, in order for at least part of an incident wave to be reflected back along its incident path. Hence, some sediment is likely to be deposited in front of the portion of the profile substantially facing the waves. The profile may have a horizontal width of not less than about half the average incident wavelength. Below this threshold, the incident waves are unlikely to be substantially affected by the structure, yet the structure will be vulnerable to the full force of the incoming waves. The convex profile may have a maximum horizontal depth (i.e. in the direction of the average incident wave motion) approximately equal to half the maximum horizontal width, so that optimum dispersion of the reflected waves is achievable. It is preferable that the structure presents a substantially convex profile to waves incident on the structure, in particular, waves incident from any seaward direction, and preferably waves incident from any direction. It is also preferable that the structure does not present a concave profile to waves incident on the structure, in particular, waves incident from any seaward direction, and preferably waves incident from any direction.

The average wavelength may be a modal average wavelength. The structure may be located in water of an average depth of no more than a fifth of an average wavelength at the predetermined location, preferably the average depth is to be no more than a tenth of an average wavelength at the predetermined location, for instance, about 1 im. The method may further comprise depositing sediment (such as sand, shingle or gravel) adjacent the structure other than by incident wave action, in order to increase : the speed at which an island, bank or headland forms. * * * *..

* 25 The wavelength of incident waves may be between substantially 20m and * : substantially 200m. Accordingly, the profile may have a horizontal width of between * substantially lOm and substantially SOOm. For example, the profile may have a minimum horizontal width of lOm and a maximum horizontal width of SOOm. *

****** * S The predetermined location may be chosen such that sediment deposited on the shore side of the structure leads to formation of a land bridge. The predetermined location may be within eight average wavelengths of a shore.

The method may further comprise erecting a further structure on the underwater substrate, spaced from the predetermined location, the further structure presenting a substantially convex horizontal profile to waves travelling toward the shore, the profile of the further structure having a maximum horizontal dimension of no more than about four times the average incident wavelength, and the further structure having a height sufficient for incident waves to be at least partially reflected therefrom.

For substantially similar reasons to those described above, the profile, positioning and use of the further structure may include similar features as for the first mentioned structure. The spacing of the further structure from the predetermined location may be chosen such that sediment deposited on the shore side of the further structure leads to formation of a land bridge, and the location of the further structure may be within eight average wavelengths of a shore.

The further structure may be closer to the shore than the predetermined location is from the shore.

The further structure may be spaced from the predetermined location between three and eight times the distance of one of the structures from the original shore, and preferably about five times the distance of one of the structures from the original shore. This should provide an adequate spacing for the formation of a stable bay between the two structures. Alternatively, a stable bay may form between a single structure and an existing headland, be it natural or artificial.

The stable bay will comprise a beach that will join the beach at an end of the stable bay, formed around a structure, producing a smooth transition between stable bay beach and headland beach. If a structure that is used to form the headland of an up-shore stable bay is also used to form the headland of a down-shore stable bay, then a single beach will be produced connecting the two stable bays.

if littoral drifi is prevented at one location along the shore, regions down-shore will be starved of sediment and may therefore suffer net erosion. Starting construction of a headland just up-shore of an area where sediment is not needed, or is undesirable, like a dredged channel or harbour mouth, would stop littoral drift into such a place and would reduce the need for maintenance dredging as well as protecting the up-shore beaches. Headlands could then be added progressively up-shore until an entire section of shore is stabilised. Hence the predetermined location may be immediately up-shore of a region into which sediment deposition is undesirable.

In accordance with a second aspect of the present invention, there is provided an apparatus for controlling movement of sediment in water adjacent a shore, comprising: a structure erected on an underwater substrate, the structure presenting a substantially convex horizontal profile to waves travelling toward the structure, the profile having a maximum horizontal dimension of no more than about four times an average wavelength of waves travelling toward the structure, and the structure having a height sufficient for the waves to be at least partially reflected.

As explained above, at least a portion of the profile may substantially face the waves travelling towards the shore, the profile may have a maximum horizontal width of no less than about half the average incident wavelength, the profile may have a maximum horizontal depth of about half the maximum horizontal width, the average wavelength may be a modal average wavelength, and the structure may be located in water of an average depth of no more than a tenth of an average wavelength at the predetermined location. It is preferable that the structure presents a substantially : convex profile to waves incident on the structure, in particular, waves incident from any seaward direction, and preferably waves incident from any direction. It is also preferable that the structure does not present a concave profile to waves incident on I...

the structure, in particular, waves incident from any seaward direction, and preferably waves incident from any direction. * ** * . * ****

The structure may have a ring-like cross-section/planform, formed by a peripheral wall. In this way, the interior of the ring-structure may be filled with ballast to add further reinforcing mass. The region inside the structure may be isolated from the waves, allowing the space to be used for material of any grade or quality. This form of structure is immensely resilient to movement by waves. It is naturally strong and wide at the base without the need to reinforce the base of the wall, as is necessary in conventional structure construction, especially in deep water.

The outside of the wall may be surrounded by material coarser than the sediment S normally in circulation, in order to protect the wall from storm attack.

The structure may be constructed from a plurality of ring-like objects, stacked one atop another. The diameter of each ring-like object may be different to another, such that smaller ring-like objects may be placed atop larger ring-like objects. Each ring-like object may be a bag filled with mortar, where the mortar may be set in situ, in order to aid construction of the structure.

The use of a circular shape for the structures, especially when combined with a gently sloping skirt of suitably coarse material, either naturally accreted or placed there manually, ensures that a portion of the profile of the structure is normal to incident waves from any direction, thereby naturally accreting sediment adjacent to the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention shall now be described with reference to the figures in which; S. * S*S * *** Figure 1 shows schematically a first embodiment of the invention comprising a s.. 25 vertical cylindrical structure; S.'.

Figure 2 shows schematically the expected development of a beach around the structure of figure 1; and Figure 3 shows schematically a second embodiment of the invention comprising a * hexagonal prismatic structure.

DESCRIPTION OF EMBODIMENTS OF THE iNVENTION

Figure 1 shows schematically a first embodiment of the present invention illustrating the behaviour of waves (11) incident on a vertical cylindrical structure (shown in heavy solid lines) (12) that extends above the surface of the water, although it should be apparent that such a structure need not extend substantially above the surface. The structure (12) is located on an underwater substrate that is assumed to be substantially horizontal, so that refraction of the incident waves (II) can be neglected. Incident and diffracted wave crests (11) are shown as solid lines while reflected wave crests are shown as broken lines (19). The cylindrical structure (12) presents a convex profile to the incoming waves that has a horizontal diameter of about four times the wavelength of the incident waves. A portion of the profile that substantially faces the waves is defined herein as the front (13).

Reflected wave energy is spread due to the dispersive effect of a convex reflector.

Hence, close to the reflecting profile, the potential mass transport of the reflected waves will counter the mass transport towards the structure of the incident waves, yet further from the reflecting profile, the mass transport towards the structure of the incident waves is not significantly diminished. Therefore, some sediment is likely to be deposited in the region of the front (13).

Within the regions adjacent the perimeter of the structure, located 45 either side of the front (10), interference between incident and reflected waves is likely to cause sediment to be moved from the front around to the back (14). This would be added to : that deposited by incident waves diffracting into the region behind the structure. The *S..

S..... whole of the sediment load extending across an incident crest length of about two s..., 25 structure diameters should be deposited somewhere around the structure each time a *5*S * crest passes. There is unlikely to be any erosion, unlike when using a substantially straight wall. * *s * I S S...

The typical swell has a period of 12 seconds. Storm waves have a shorter period (5-8 seconds) and higher amplitude. The periphery of the structure should be at least a mean storm wavelength in diameter, so that the structure is not subjected to the full force of a storm wave, in the case of removal of all the material surrounding the structure. If the radius is too large, its boundaries begin to look like straight shore, r with those parts of it oblique to the waves in danger of erosion. Between one and four average wavelengths in diameter would seem to be ideal.

A beach may be allowed to accrete naturally around the periphery of the structure.

Alternatively, the structure may be surrounded with a skirt of coarse material. If this alternative construction method is pursued, it is not necessary to begin with a vertical reflecting wall, though making the wall circular does offer the easiest way of adding a sloping skirt of coarse material. A skirt of course material would find its natural angle of repose and extend the effective diameter of the structure considerably. Granular material (2-4 mm particle size), for instance, makes a natural angle of about 110, which would extend the toe of the headland another 30 m for 6 m depth of water, or m for 10 m depth, if a 50m diameter structure is considered.

In one alternative embodiment, shown schematically in figure la, the structure (12) may comprise a peripheral wall (1). The peripheral wall (1) may be composed of geo-textile bags (2) filled with a mortar mix (3). Each bag (2) may be formed into a ring and then concentrically stacked. Preferably, the diameter of each ring is different to another ring, such that smaller rings may be placed atop larger rings. Each ring may be of approximately 50m diameter, 1 meter in height and 2.5 meters thick. The space inside the resultant ringed wall of mortar filled bags may be filled with any grade of material to add yet further reinforcing mass.

in a further alternative embodiment, shown schematically in figure Ib, a core of * .** mortar (3) filled bags (2) is surrounded with quarry run material, rubble, shingle or *...., 25 any other mixture (4) substantially coarser than sand (5), yet finer than armour units or boulders. Shingle etc. (4) moves more slowly than sand (5) and is only exposed to storm waves after the beach itself has been removed. It thus serves as a next line of :.:::. defence of the core structure (2), especially against bed scour. The core units (2) prevent any overall down-shore movement of the shingle etc. (4), which is for the most part surrounded by sand (5). The shingle etc. (4) will only be exposed if a storm persists, and the core (2) only in a 1 in 100 year event.

Figure 2 shows schematically the expected contour shapes (20) of the island resulting from sediment accretion around the structure of figure 1, and the land-bridge (26) formed between the island and shore (27) subject to 45 persistent swell (11).

Although the rate of sediment deposition initially will be greatest directly in front (13) of the wall, it will be moved around to the back (14). A beach (25) will develop around the structure. Incident waves will both break at the beach and be refracted by it in towards the structure. The accumulation of sediment behind the structure would eventually build up a land bridge (for example, a tombolo) (26) between it and the original shore (27), forming a headland (28). As soon as a land bridge (26) has formed between a structure and the original shore (27), beach growth will proceed rapidly in the direction of the incoming waves (11) carrying sediment. As the size of the beach (25) increases further, the energy intensity of the reflected waves will be less affected by the curvature and eventually a state of dynamic stability will prevail where the accretion will be balanced by erosion.

Incorporating a plurality of such structures along a shore (27) encourages sandy bays to form that are stable against erosion.

A round structure (12), especially one with a rubble skirt sloping gently out to sea, will gently disperse a rip current more successfully than conventional methods can achieve. A sand bar created by a rip current during a storm will thus begin to form closer to the headland than the prior art allows, with the bar material ready to return to : *.* the beach (25) that it came from once the storm passes, or even back to the headland * , S (28) should the storm persist from the same direction. * S S...

In one particular embodiment where a 45 persistent swell (11) approach angle is assumed, a suitable size for a cylindrical structure (12) would be 50m diameter. For a chosen spacing of 200m between structures, the minimum beach (25) width is likely to be 50m, and the bay indentation may reach a maximum of 80m; however, this is more likely to be 75 m from the centre of the structure. In a preferred embodiment, the headland (28) spacing is of the order of five times the offshore distance of the headland (28) tip from the original shore (27), for a deepwater 12 second period swell approach angle of 45 . Refraction towards the shore will reduce this to a value nearer for 5 metres depth of water at the structure. The water depth between each structure (12) and the shore (27) should preferably be between 1 and 2 meters to allow some passage of sediment before the gap is closed completely.

The biggest contrast between the cylindrical structure (12) and the prior art is that sediment is accreted right up to the base of the structure (12). The prevailing sediment movement is thereby around the structure (12) where we can expect the land bridge (26) to form. The value of having some kind of beach (25) seaward of the headland is that the straight segment of stable beach is more robust at depth, with mild beach (25) slopes extending all the way to the headland (28). Storm attack on this beach (25) segment should thus yield an offshore bar significantly more parallel to the stable bay beach (25) than for any situation where we cannot expect mild beach (25) slopes near the headland (28). Being normal to the prevailing swell (11) makes it much more likely that the entire offshore bar should return to the beach (25) it came from.

Figure 3 shows schematically a further alternative embodiment of the present invention that comprises a structure (42) with a hexagonal cross section that extends above the surface of the water, shown in heavy solid lines. Again, incident and diffracted wave crests (11) are shown as solid lines while reflected wave crests (49) are shown as broken lines. The structure (42) is located on an underwater substrate that is assumed to be substantially horizontal, so that refraction of the incident waves can be neglected. The structure shown has three segments of wall facing the waves (42a, 42b, 42c), presenting a convex profile to the incoming waves that has a horizontal width of about three times the wavelength of the incident waves and a : :* horizontal depth of about equal to the wavelength of the incident waves. One wall of the structure (42b) substantially faces the waves and is defined herein as the front ::: wall. * .e

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::* 30 As can be seen, the reflected wave energy is dispersed due to the convex profile.

* Hence, some sediment is likely to be deposited in the region of the front (43). Within I..

the regions adjacent the walls of the structure oblique to the incident waves (42a, 42c), interference between incident and reflected waves is likely to cause sediment to sediment to be moved from the front (43) around to the back (44). This would be added to that deposited by incident waves (11) diffracting into the region behind the structure (42). The whole of the sediment load extending across an incident crest length of about two structure diameters should be deposited somewhere around the structure each time a crest passes.

The average incident swell direction need not be known for a structure having a profile substantially convex from all directions. A single headland would develop stable beaches either side of it and thereby provide a guide for extending protection in either direction.

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Claims

1. A method of controlling movement of sediment in water adjacent a shore, comprising: determining at a predetermined location an average incident wavelength of waves travelling towards the shore; and erecting at the predetermined location a structure on an underwater substrate, the structure presenting a substantially convex horizontal profile extending substantially around its periphery, the profile having a horizontal width of no more than about four times the average incident wavelength and no less than about half the average incident wavelength, and the structure having a height sufficient for incident waves to be at least partially reflected therefrom.

2. A method of controlling movement of sediment in water adjacent a shore according to claim 1, in which at least a portion of the profile substantially faces the waves travelling towards the shore.

3. A method of controlling movement of sediment in water adjacent a shore according to any preceding claim, in which the profile has a horizontal width of no less than about half the average incident wavelength.

4. A method of controlling movement of sediment in water adjacent a shore according to any preceding claim, in which the average wavelength is a modal average wavelength. I. ) ..*

5. A method of controlling movement of sediment in water adjacent a shore S...

according to any preceding claim, in which the structure is located in water of an average depth of no more than a tenth of an average wavelength at the predetermined *e S. S. * * location.

* .. 30 * * S

6. A method of controlling movement of sediment in water adjacent a shore S.....

* according to any preceding claim, further comprising depositing sediment adjacent the structure other than by incident wave action. r

7. A method of controlling movement of sediment in water adjacent a shore according to any preceding claim, in which the predetermined location is chosen such that sediment deposited on the shore side of the structure leads to formation of a land bridge.

8. A method of controlling movement of sediment in water adjacent a shore according to claim 7, in which the predetermined location is within eight average wavelengths of a shore.

9. A method of controlling movement of sediment in water adjacent a shore according to any preceding claim, further comprising erecting a further structure on the underwater substrate, spaced from the predetermined location, the further structure presenting a substantially convex horizontal profile to waves travelling toward the shore, the profile of the further structure having a horizontal dimension of no more than about four times the average incident wavelength, and the further structure having a height sufficient for incident waves to be at least partially reflected therefrom.

10. A method of controlling movement of sediment in water adjacent a shore according to claim 9, in which at least a portion of the profile of the further structure substantially faces the waves travelling towards the shore.

11. A method of controlling movement of sediment in water adjacent a shore :. 25 according to any one of claims 9 or 10, in which the profile of the further structure has *::: a horizontal width of no less than about half the average incident wavelength.

12. A method of controlling movement of sediment in water adjacent a shore according to any one of claims 9 to 11, in which the further structure is located in : *. 30 water of an average depth of no more than a tenth of an average wavelength at the * further structure.

*1SS** r

13. A method of controlling movement of sediment in water adjacent a shore according to any one of claims 9 to 12, further comprising depositing sediment adjacent the further structure other than by incident wave action.

14. A method of controlling movement of sediment in water adjacent a shore according to any one of claims 9 to 13, in which the spacing of the further structure from the predetermined location is chosen such that sediment deposited on the shore side of the further structure leads to formation of a land bridge.

15. A method of controlling movement of sediment in water adjacent a shore according to any one of claims 9 to 14, in which the location of the further structure is within eight average wavelengths of a shore.

16. A method of controlling movement of sediment in water adjacent a shore according to any one of claims 9 to 15, in which the further structure is spaced from the predetermined location between three and eight times the distance of one of the structures from the original shore.

17. A method of controlling movement of sediment in water adjacent a shore according to any one of claims 9 to 16, in which the predetermined location is immediately up-shore of a region into which sediment deposition is undesirable.

18. Apparatus for controlling movement of sediment in water adjacent a shore, comprising: a structure erected on an underwater substrate, the structure presenting a substantially convex horizontal profile extending substantially around its periphery, S...

the profile having a horizontal dimension of no more than about four times an average S...

wavelength and no less than about half the average incident wavelength, and the structure having a height sufficient for the waves to be at least partially reflected * *. 30 therefrom. * . . S... **... * S e

19. Apparatus for controlling movement of sediment in water adjacent a shore according to claim 18, in which at least a portion of the profile substantially faces the waves travelling towards the shore.

20. Apparatus for controlling movement of sediment in water adjacent a shore according to any one of claims 18 or 19, in which the profile has a horizontal width of no less than about half the average incident wavelength.

21. Apparatus for controlling movement of sediment in water adjacent a shore according to any one of claims 18 to 20, in which the average wavelength is a modal average wavelength.

22. Apparatus for controlling movement of sediment in water adjacent a shore according to any one of claims 18 to 21, in which the structure is located in water of an average depth of no more than a tenth of an average wavelength at the predetermined location.

23. Apparatus for controlling movement of sediment in water adjacent a shore according to any one of claims 18 to 22, in which the structure has a ring-like cross-section, formed by a peripheral wall.

24. Apparatus for controlling movement of sediment in water adjacent a shore according to any one of claims 18 to 23, in which the structure is constructed from a plurality of ring-like objects. *I * S.,

**** **

25. Apparatus for controlling movement of sediment in water adjacent a shore S.,.

according to claim 24, in which each ring-like object is a bag filed with mortar. *... * * *.

S * S * *5 * S 5 S...

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