GB2246382A - Breakwaters - Google Patents

Abstract

A structure for defence from the sea comprises a breakwater or harbour wall 14 which is exposed to wave action at least at high tide on a beach 20 subject to alongshore sediment transport A, said breakwater 20 having an extent in the direction of said sediment transport. The structure further comprises at least one groyne 30 extending seawards of the breakwater with the breakwater extending upstream of the groyne relative to direction of sediment transport. The beach level seawards of the breakwater and upstream of the groyne is thereby maintainable at a higher level than would otherwise be the case, thereby reducing incident wave height at the breakwater and accordingly reducing the structural requirements of said breakwater. Each groyne may be submerged. Breakwater 14 and each groyne may be of any known construction, e.g. a stone-stabilised rubble mound (Fig. 2). A groyne may be placed first to build sediment on which the breakwater is built. A groyne may extend for similar purposes from a sea wall (Fig. 3). <IMAGE>

Description

BREAKWATERS This invention relates to structures and methods of construction applicable to harbour and sea walls.

On many coasts, whether around the United Kingdom and Europe or elsewhere, the action known as alongshore sediment transport occurs. It is caused by prevailing winds for which their predominant direction causes generation of waves in the sea that travel towards and approach the shoreline at an oblique angle, i.e. their crests are not parallel to the shoreline. In shallow water the waves break and their energy and momentum is re-diverted into water flow generally parallel to the shoreline. Regions of such diverted water flow are known as swash zones and water flow can be highly turbulent, indeed sufficient for sediment and shingle to be carried along the coast in substantial quantities over a period of time. The results, if uncontrolled, are generally undesirable by way of beach erosion, even shoreline recession.

Control to reduce those generally undesirable effects is well known by way of using groynes, which are generally vertical barriers extending outwards, i.e. to sea, normally substantially perpendicularly to the shoreline. Beach material builds up on the more windward, or upstream relative to alongshore sediment transport, sides of the groynes and significant erosion takes place on their leeward or downstream sides, but generally with an overall increase in the beach level over a period of time. Groynes are well known structures, usually built using timber.

Coastlines subject to alongshore sediment drift are generally considered to present problems for harbour and other sea defence construction, particularly as it tends to be in their nature that there are no obvious natural features from which advantage can be taken.

Harbour structures, in particular, for such coastlines based on traditional design criteria would be very expensive, particularly in relation to their major elements, specifically breakwaters.

Considering a breakwater in general terms, say of a normal rock-stabilised rubble-mound construction, its cost is highly dependent on size of rocks required as costs and handling problems increase as the required rock size increases. The size of rocks to be used has often been determined by the well established Hudson's formula, from which required individual rock mass is proportional to the cube of wave height. To design for extreme situations or worst cases the so-called significant wave height associated with a storm with an expected return period of 50 years is often used. In practice, largest wave height at an inshore coastal site is however further controlled by water depth. It is generally accepted that the maximum wave height cannot exceed about 80% of the water depth. Devising a stable structure is not a problem using such criteria, but costs would tend to be high.

It is an object of this invention to provide a structure and construction system for sea defence structures, particularly harbours, that is more economical in terms of materials and costs, which will self-evidently be achieved if required individual rock masses and overall quantities can be reduced.

According to one aspect of this invention a harbour structure comprises an outer wall or breakwater behind or inside which the harbour area is afforded, and a groyne or series of groynes extending out to sea from the outer side of the breakwater.

On an exposed beach of the type generally subject to alongshore sediment transport, and having a breakwater substantially parallel to the direction of alongshore sediment transport, it is preferred that at least one groyne extend seawards from the downstream end of the breakwater. A typical harbour outer wall will have two limbs, one extending generally outwards from the beach, say between high and low water marks, and the other as said breakwater extending generally in the direction of the beach and of water flow in the swash zone, usually parallel to the low water mark, often along it, to a toe marking the seaward side of entrance to the harbour, or harbour mouth.The outwardly extending limb of the harbour outer wall is often most efficiently substantially perpendicular to the beach and the swash zone water flow, with the other limb of the outer wall substantially perpendicular to the outwardly extending limb. However, application of this invention is to be seen as limited only by the harbour outer wall having some extent along the swash zone in the direction of swash zone water flow due to prevailing winds, i.e.

not to any particular shape, such as sharply angled; and to a further groyne structure going seawards of said other extent from a position leeward or downstream of a harbour wall extent to be made of more economical construction, i.e. only preferably and advantageously from the end or toe of that extent. Indeed, a curving or part polygonal outer wall might well have a groyne going from its maximum seaward excursion.

The intended effect and purpose of the groyne, or series of groynes if the breakwater is particularly long, is to utilise alongshore sediment transport action in raising the level of the beach on the same side of the groyne or groynes as at least part of the harbour outer wall. At that side of the groyne or groynes there will then obviously be a reduction of water depth and thus of wave height to be taken into account in specifying the construction of the harbour outer wall.

Raising of beach level can be, and preferably is where practical, further contributed to (or at least time required for natural build up reduced), by excavation within the intended harbour area, i.e. inside the position of the harbour outer wall, and deposition of removed material within and outside the intended width of the harbour outer wall, say from centrally of its highest part and outwards.

Resulting harbour structures will, of course, be entirely stable in that their seaward extending groyne (or groynes) will continue its beach height determining effects, as may seaward extent of the harbour outer wall also. At least most of the extent of the groyne (or groynes) will be beyond the low water mark, and thus normally be submerged. The extent of the groyne (or groynes) should be sufficient to produce an outer beach slope that is sufficiently gentle to avoid undue reflection of water from breaking waves back out to sea, thus avoiding counter-productive seawards sediment transport effects. Moreover, scouring action at the other side of the groyne extending outwards from the breakwater toe can have useful effects in maintaining depth of an entry channel to the harbour mouth.

According to another aspect of this invention a method of constructing a harbour involves raising beach level before building at least outward part of width of the harbour's outer wall upon that raised beach level, and utilising a groyne or series of groynes extending seawards of the outer wall at least to assist in obtaining and/or maintaining a desired raised beach level.

Advantageously, such a groyne or groynes can be built first and serve to accomplish at least part of desired raising of beach level due to natural action of alongshore sediment transport. Alternatively or additionally, and as already indicated herein, desired raising of beach level is at least contributed to by deposition of material excavated from the intended harbour area.

Another view of the invention could be the construction of a breakwater with end groynes, whether or not a harbour predominately (at least other than its junction with the groyne) using smaller building components, such as rocks, than would be required from traditional calculations/criteria.

Preferred specific implementation of this invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a harbour layout; and Figure 2 is a section through the harbour outer wall; and Figure 3 is a plan view of a seawall layout.

In the drawing, a harbour area 10 is indicated within an L-shaped outer wall or breakwater. A first limb 12 of the outer wall is shown extending seawards substantially perpendicular to the coastline 20 from high water mark H to low water mark L. A second limb 14 of the outer wall is shown extending at right angles to the first limb 12 as a breakwater substantially parallel with the coastline 20. The extent and direction of the second limb 14 of the harbour wall from its corner to the first limb 12 is shown as generally parallel to the direction of alongshore sediment transport indicated by arrow A, arrow W indicating prevailing wind and wave directions oblique to the coastline 20. Entrance to the harbour 10 is shown at harbour mouth 16 between end or toe 18 of the second limb 14 of the main harbour wall and a secondary harbour wall 22 extending outwards from the high water mark H.Application of the teaching of this invention is mainly in relation to the main harbour wall 12,14 and involves the provision of a groyne 30 associated therewith and specifically shown extending directly out to sea from the position of the toe 18.

The harbour wall 12,14 is upstream of the groyne 30 relative to the direction of alongshore sediment transport (arrow A), and transported beach material will naturally build up against the upstream side of the groyne 30, thereby reducing water depth and wave height incident the harbour outer wall 12,14. Further groynes upstream of groyne 30, in relation to alongshore sediment transport, may be required if limb 14 is of long extent. Rock size requirements for stability of the outer wall or breakwater construction can thus also be very considerably reduced from what would otherwise need to be specified, often by as much as half or more.

Moreover, the overall quantity of rock needed for the outer wall 12,14 will also be considerably reduced.

Figure 2 shows the original beach level at OB, the increased beach level at IB resulting from beach material 26 built up due to or using the groyne 30, also the spring tide high water level at SH, all for the typical harbour construction of Figure 1 with the outer wall 14 typically on the spring tide low water mark SL, also where effective water depth relative to the spring tide high water level is approximately halved using a groyne of appropriate height, see dashed at 30H, which could represent a timber groyne, and/or with rubble mounds 30A,B indicated to each side, but which would most usually represent a simple rubble mound construction, i.e. without any central timber. As a section through the outer wall limb 14, the height of the increased beach level IB and its slope seawards also roughly represents the top of the groyne 30 extending seawards of the harbour mouth 18.That slope then also -represents the effective outer beach and should be sufficiently gentle, typically 1 -in- 10 or less, to avoid reflection of incident waves in a manner leading to beach material transport out to sea, thus effectively determining the required length of the groyne 30. In turn, that affects practical compromise of achievable increase of beach level and height and constructional specification for the main harbour wall or breakwater 14. Figure 2 shows that construction as being of stonestabilised rubble-mound type with an outer slope 14A shortened by beach level increase. Its longer inner slope 14B is shown above and steeper than the fall-off sloping 26F of the built-up beach material 26, which is thereby further stabilised, and extends down to the bottom of any excavation of level (EL) for the harbour itself, i.e. inside the harbour wall.

It will be appreciated that as a section through the seaward extending harbour wall limb 12, the original beach level OB will vary by rising in the coastward direction. The slope of the increased beach level IB will normally be small along the outer breakwater 14, certainly such that sediment transport effects should not produce depletion, and there will not normally be a problem for any height 30H of groyne against which alongshore sediment transport action at the location concerned, or dumping of other material, achieves its intended increase of beach height before breakwater building begins.

Two parts of the illustrated harbour layout are subject to any conventional construction approach, i.e.

without beach height manipulation, namely the toe 18 of the breakwater 14 and any secondary harbour wall 22, i.e. relative to likely wave action conditions resulting from other than prevailing winds. Even so, for the toe 18, material scouring action resulting from alongshore sediment transport at the downstream side of the groyne 30 could produce relatively deep water conditions at the toe 18 and wave action from less severe non-prevailing winds may not even be depth limited. To such extent that larger rock sizes are needed for the toe 18, that requirement is for a small part only of the harbour wall 12,14 and will, in any event, be less than would be required (but for application of this invention) for the remainder of the harbour wall 12,14 upstream of the groyne 30 as most severe winds are normally only associated with prevailing wind direction.

Construction of a harbour layout such as illustrated conveniently begins with construction of the groyne 30, which can be of any suitable type, whether conventional rubble-mound or timber or whatever.

Further work can then be delayed until alongshore sediment transport action has built up beach material at the upstream side of the groyne 30 to the extent intended, conveniently at least sufficiently for excavation and dumping to complete requirements. It might be useful for the initial groyne construction to extend through the intended position of the main harbour wall 12,14, perhaps especially as a retaining wall where the harbour area 10 is to be excavated and excavated material deposited outside and along the groyne thereby speeding up establishment of the desired raised beach level. However, it could be satisfactory and simplest to build first both of the groyne and the landward extent of the harbour wall, especially if principal reliance is required on natural build up of beach height and time allowed for that to take place.

Thereafter, the main harbour breakwater 14 is built according to specifications relying on increased beach level, thus in a relatively economical manner, and with further enhanced efficiency by use of any excavated sand and shingle to supplement natural build-up.

Including whatever may be required for the relatively small volume at the toe 18, rock size and overall quantity required for a harbour as illustrated, or of any other shape so long as at least partially upstream of a seaward extending groyne or groynes and at least for that part, is very much reduced. No limitation is, of course, intended to the use of natural rock or specific construction in the manner illustrated in Figure 2. Indeed, any other suitable materials and constructions can be used, for example using quarry stone, precast concrete dolos, concrete blocks even cast concrete itself, or some combination such as upper wall parts of concrete as is popular and convenient in use.

It will be further appreciated that, for a layout as illustrated, the groyne 30 in extending from the spring tide low water mark will almost always be submerged. It may need to be marked by visible upstands or buoys anchored thereto, but that is actually more advantageous than not as the natural scouring action for beach material at the downstream side conveniently aids producing a deeper water entry channel to the harbour mouth, and marking same is useful. Excavation of the harbour area 10 enables provision of floating berths even at low tide.

It should be appreciated that not only can shape and constructional details be varied compared with what is illustrated, but that location relative to high and low tide water marks could be different, for example with the main harbour wall limb 14 or equivalent seawards or coastwards of the spring low tide mark.

For one specific site being considered, an original beach slope of 1-in-30 could be reduced to about 1-in-lO by a 50 metre groyne going from 5 metre height to zero and protect a harbour outer wall (14) of 100 to 200 metres length. The resulting maximum water depth at that outer wall would then be reduced from 8 metres to 3.5 metres and required rock sizes would be reduced by over 50%, indeed as much as 90%.

Finally, reduction of water depth by the use of groynes on coasts subject to alongshore sediment transport can be used to produce more economical designs of other coastal constructions such as sea walls or armoured slopes for coastal defence purposes, thereby reducing wave height and water depth at the construction site. Again one groyne would be built on the downstream side (in relation to the alongshore sediment transport) as shown in Figure 3. This illustrates a groyne 40 extending from a seawall 44 downstream of prevailing winds W and alongshore sediment transport (arrow A), seaward of from a high water mark H beyond low water mark L. Further groynes upstream may be required for a construction of long extent. As well as being used for new constructions, such measures can be deployed for existing constructions, for example where becoming exposed to increasing water depths and hence wave heights, say as a result of global warming and other changing climatic conditions.

A general aspect of this invention thus comprises a combination of a coastal construction having an extent along the direction of alongshore sediment transport and one groyne extending seawards from the construction downstream (in the sediment drift direction) of its structure to be made more economic or maintained adequate for its purpose.

Claims (11)

1. A structure for defence from the sea comprising a breakwater which is exposed to wave action at least at high tide on a beach subject to alongshore sediment transport, said breakwater having an extent in the direction of said sediment transport, which structure further comprises at least one groyne extending seawards of the breakwater with the breakwater extending upstream of the groyne relative to direction of sediment transport, such that beach level seawards of the breakwater and upstream of the groyne is maintainable at a higher level than would otherwise be the case, thereby reducing incident wave height at the breakwater and accordingly reducing the structural requirements of said breakwater.

2. A structure according to claim 1 in which the breakwater upstream of the groyne serves as a harbour wall by affording a protected harbour area behind or shorewards of it.

3. A structure according to claim 2 in which the breakwater is L-shaped with a secondary limb extending shorewards in bounding said harbour area which is excavated.

4. A structure according to claim 2 or 3, in which the breakwater is of rock faced rubble mound construction and extends downstream to a toe at the harbour mouth, the toe being constructed of generally larger rocks than the remainder of the breakwater.

5. A structure according to claim 4, in which the groyne extends seawards from the toe.

6. A structure according to claim 3, 4 or 5, in which the harbour area is also bounded by a secondary harbour wall landwards and spaced from the toe.

7. A structure according to any preceding claim in which further groynes extend seawards from the breakwater at spaced intervals from the first mentioned groyne at its downstream end.

8. A structure according to any preceding claim, in which the breakwater extends substantially wholly parallel to the direction of alongshore sediment transport.

9. A structure according to any preceding claim, in which the groynes extend substantially perpendicular to the breakwater.

10. A structure arranged and adapted to operate substantially as hereinbefore specifically described with reference to and as shown in any one of Figs. 1 to 3.

11. A method of constructing a breakwater comprising raising beach level before building at least outward part of width of breakwater's outer wall upon that raised beach level and utilising a groyne or series of groynes extending seawards of the outer wall at least to assist in obtaining or maintaining a desired raised beach level.