MODES FOR CARRYING OUT THE INVENTION

[0028] The present invention relates to a method of protecting a shore from erosion. The method involves placing a reef on the water floor, the reef located, oriented and dimensioned to modify the direction of travel of waves destined for the shoreline. By modifying the wave direction, the water to shore interaction may be modified to reduce erosion.

[0029] Referring to the accompanying drawings, FIG. 1 shows an orthographic view of a portion of a reef 1 located on a floor of a body of water. The floor may be a sea bed 2. The reef 1 includes an elongate member oriented at a pre-selected angle to the direction of travel of the waves approaching the reef 1. The direction of travel of the waves is referenced in FIG. 1 by arrow W. The reef 1 is located adjacent to the shore at a distance therefrom depending on the particular requirements for the shore. In practice, reefs may be typically placed anywhere from 20 to 1000 metres from the shore.

[0030] As a wave approaches the reef 1, the right hand side of the wave will contact the front end 3 of the reef 1 first. As the presence of the reef 1 acts to shallow the depth of water, the speed of the wave travelling over the end 3 begins to slow. The portion of the wave-front opposite end 4 continues travelling at the same speed until it reaches reef 1. The relative change in speed of travel of the portions of the wave-front causes the wave-front to be refracted towards the right as it travels towards the shore.

[0031] This change in direction of travel of the waves, properly selected, can result in less erosion of the shoreline. For example, if the waves approach the shoreline at an angle closer to the perpendicular, currents moving along the shoreline may be reduced.

[0032] Three important factors that influence the design of the reef 1 are the offshore wave climate, water depth and coastal orientation. Because the wave refraction effect will vary with water depth and reef elevation above the seabed, it will be necessary to align the reef in each environment to achieve the desired outcome of aligning the waves more closely with the coastal orientation. Wave angle relative to the shoreline at the point where the wave breaks is a factor that determines the strength of the currents inside the surf zone. It may be necessary to use computer modelling to align the reef so that wave angles at the breakpoint are modified to achieve the desired aim of reducing or negating the longshore currents which cause erosion. It will be necessary to ensure that the problem of variability in the wave climate and variations in open coast wave heights and angles are accounted for by aligning the reef so that the average of all the longshore currents resulting from each natural condition is reduced or negated. The reef 1 would normally be placed beyond the natural breakpoint so that the angles can be modified prior to reaching this zone. However, some reefs may be placed inside the breakpoint when the waves have widely varying heights.

[0033] The present invention can accommodate for varying shoreline angles relative to incoming wave direction by providing a plurality of reefs 1 located adjacent to each other and designing each reef to modify the direction of travel of the waves depending on the orientation of the shoreline relative to the direction of travel of the incoming waves. The plural reefs may be interconnected, but act as different reefs as the orientation and size varies along the shore.

[0034] When selecting the orientation of the reef 1, consideration should be given to any “end effects” which may result. For example, if the wave angle is modified so as to substantially eliminate erosion along a first portion of the shore, another portion of the shoreline may degrade due to no longer having a feed of material from the first portion. In this case, a balance may be struck, by reducing the erosion along one portion while still providing sufficient material for the second portion.

[0035] The reef 1 may be typically located in water depths of 2 meters to 1 5 meters and project upwards by at least 1 meter from the sea bed 2. The extent of projection upwards is selected according to the requirements for the shore and may project up to the surface of the water. The reef 1 should not normally extend above the water line, as this will create a block to the waves rather than modifying the direction of travel and optionally the shape of the waves as they pass over the reef.

[0036] The reef 1 may be constructed from any material that is stable on the sea bed 2. In a preferred form, the reef 1 may be formed from geotextile bags that are pumped full of sand or other particulate material and laid to form the required shape. Alternatively, the reef 1 may be constructed from rocks, concrete, or any other durable material.

[0037] Referring to FIGS. 2a-c, a schematic representation of three embodiments 1a, 1b and 1c of a reef are shown. Each reef 1 is oriented at a predetermined angle relative to the direction of travel of the wave W and act to change the direction of travel of the wave to a new direction W′. FIG. 2a shows a convex reef 1a relative to direction W, FIG. 2b shows a concave reef 1b and FIG. 2c shows a linear reef 1c. The shape of the reef 1 may be selected according to whether the waves need to be diffracted, converged, or maintained at their current energy. It will be appreciated by those skilled in the art, that the profile of the elongate member 1 may be varied widely depending on the particular requirements of the shore to which the reef 1 is to protect.

[0038] FIG. 3 shows a preferred range of orientations which the reef 1 may be oriented relative to the direction of travel of the wave W. The actual angle selected depends on the required direction of the wave after it passes over the reef 1, but as shown in FIG. 3 will typically be selected from angles between 5° to 80° relative to direction of travel of the wave W. Generally, the more the reef 1 is angled towards the orthogonal to the direction of travel along the wave front, the less the direction of travel of the wave will be changed. Variations in the height of the reef 1 along its length may be used in combination with the angle of the reef to influence the direction of travel of the waves. In one embodiment, the longitudinal axis of the reef 1 may be oriented substantially orthogonal to the direction of travel of the waves so as to rely solely on the height variation (or depth change) to refract the waves. However, less options for wave shape modification may be available if such a reef were used.

[0039] Multiple reefs 1 may be placed in series (i.e. along the direction of travel of the wave W) to fine-tune the wave direction or wave shape or to provide a cumulative effect if larger changes are required. Also, multiple reefs 1 may be provided in parallel along the shore until the required portion of the shore has a reef substantially adjacent to it. This may be useful for protecting extended portions of shore.

[0040] FIGS. 4A-C show three examples of possible cross-sections of the reef 1. The cross-section of reef 1 may be varied depending on the required wave shape. For example, the reef 1 may have a cross-section, which is selected to encourage the formation of waves suitable for surfing. The height of the reef 1 may also be varied along its length to influence the direction of travel of the wave and modify the properties of the wave as required. The reef shown in FIG. 4A will cause the wave breaking to be most intense on the steepest segment at the deeper part of the reef, and this will act primarily on larger waves. The reef shown in FIG. 4B will cause the wave to shoal slowly, but break abruptly on the steeply rising segment, thereby creating an intense breaking wave during smaller wave conditions. The reef shown in FIG. 4C will result in the breaking intensity to not vary due to seabed slope factors, but the wave height and period will separately determine wave breaking intensity.

[0041] To protect a shoreline from erosion, an appropriate reef 1 needs to be selected and its orientation also selected. This may be achieved by performing computer modelling of the shoreline and wave patterns to find the required orientation, profile and cross-section of the reef 1. As wave direction is variable, a statistical analysis may be performed to establish the optimum orientation. In one embodiment, the method of determining the orientation of the reef 1 may be dependent on the average or mode direction of travel of incoming waves.

[0042] By way of example, a shoreline may be experiencing erosion adjacent to a natural headland. The currents generated by the waves may sweep down the headland and along the beach, carrying sand away and causing erosion. A solution would be a reef that rotates the waves at the downstream end of the beach. This rotation would negate the currents and cause at least a partial block to the sediment movement. The sediment would then be expected to collect in the zone of low flows and to create a beach salient (like a bulge in the width of the beach). Further build up of sand would then collect on the upstream side of the salient. The input variables may include the wave climate describing the long-term distributions of wave angles in deep water offshore and the corresponding wave height. Usually computer models would be used to account for refraction and height transformation over the natural seabed contours to the offshore tip of the reef. The same models would then be used to assess the rotation and height modification over the reef, and the reef would be adjusted (e.g. reef angle, height above the seabed, volume, width, length, offshore positioning and depth of reef crest) to achieve the desired outcome of negating the currents in the surf zone along the shoreline.

[0043] Thus, there is provided a method of shore protection which modifies the direction of travel of waves and/or wave properties using a reef and which may be located below the water line.

[0044] Where in the foregoing description reference has been made to specific components or integers of the invention having known equivalents then such equivalents are herein incorporated as if individually set forth.

[0045] Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the scope of the appended claims.