NL1039528C2 - Breakwaters against tsunami and storm waves. - Google Patents

Description

Breakwaters against tsunami and storm waves

The invention is on breakwaters for coastal protection against tsunami and storm waves. In one configuration the breakwater of the invention returns the wave impulse moments and kinetic 5 energies; in another configuration it affects them to destruct among themselves.

Breakwaters for coastal protection against tsunami and storm waves are known for centuries. The devastating effects of tsunamis (e.g. in Sumatra on December 26 2004 and in Japan on March 11 2011) and heavy storm waves (e.g. hurricane Katrina 200S and Irene 2011 in the USA) have shown that the existing breakwaters, which are of the impulsive (impact) type, are 10 inadequate against tsunami and heavy storm waves. Onshore and offshore breakwaters are proposed with perforation in the wave facing parts to disperse part of the incoming waves, e.g. patent documents [1,2]; in patents documents [3-5] a curved breakwater is proposed to splash back part of the incoming wave, to damp the incoming waves. As the magnitudes of the impacting parts are unknown, it remains unsure whether such breakwaters will have the 15 structural strength to withstand future tsunamis. A breakwater of the impulsive type, which is strong enough for heavy storms, need not be good enough for the long tsunami wave, taking also into consideration that the tsunami generating earthquake, which is of magnitude 7 or larger, may already have a destructing effect on the breakwater and its connection to the foundation.

The characteristics of tsunami and storm waves are known from hydrodynamics. Here some 20 aspects are mentioned as information. Like other travelling water waves, a tsunami wave is characterized by a crest (top of the wave), a trough (bottom of the wave), the wave length L (distance from top to next top), the amplitude A = (top - bottom)/2, the velocity v and the period T = L/v. A tsunami can be generated by underwater earthquake, landslide, volcanic eruption and cosmic body impact that perturbed the equilibrium state of the water. In case of underwater 25 earthquake, usually at the boundary of two approaching tectonic plates, the overriding plate suddenly shifts upward (order of meter) and displaces the overlaying incompressible water upward, over a considerable surface (hundreds of square km), causing a tsunami wave with long wave length L, of 200 - 800 km. The water tends to restore equilibrium under influence of gravity and a travelling wave starts from the centre of the perturbed water. As the ocean depth D 30 is of the order of 5 km, which is smaller than L/20, one has the shallow water situation: the (horizontal) velocity v is given by v = V (g D) where g = 9.8 m/s2. With D = 5000 m the result is v = 221 m/s = 800 km/h, the speed of a jetliner. For L = 200 km, T = 15 min., for L = 800 km, T = 1 hour. In the open ocean the amplitude A is rather small, about 1 m and the tsunami can go unnoticed. The energy flow of a water wave per unit width, of one wave length L and velocity v 1039528 2 is given by: P = g pA2 L v / 4, where p is the density of water in kg/m3. For comparison, a wind generated wave, even heavy storm wave, has a wavelength of the order 20 - 200 m. For D larger than L/2 on has the deep water condition, v = 1.25 Vl. For L = 100 m, v = 12.5 m/s (45 km/h), T = 8 s. Even for the same velocity v and amplitude A, a tsunami wave with L = 200 km has 5 much more energy flow during the crest period than a storm wave of20*200 m. When a tsunami wave approaches the shore, the sea depth is smaller and so the velocity in the front of the wave becomes smaller than in the back, cf. the formula for v. The wavelength becomes smaller, but the period remains constant. Therefore the amplitude becomes greater. This is called shoaling. It explains why the tsunami is so deadly when it reaches the coast: the run-up on shore can become 10 10 m or more, sustained by the long length of the incoming wave, causing flooding of the coastal zone. The crest of the wave is followed or preceded by the trough. If the latter case happens in a tsunami, the sea retreats several km from the shore (this is called drawdown), a sign (warning) that the crest of the tsunami will be coming after that 15 The novel breakwater of the invention will now be described with the help of Fig. 1. The breakwater consists of a half cylinder (1) of length 1, smooth inner surface with the semi-circle radius R, and a smaller concentric hollow inner cylinder (2) with circle radius r, i.e. the inner cylinder is a tube of thickness dr, of length 1 and smooth outer surface, closed at both circular ends of the tube. The inner side of the half cylinder (1), the front side, is facing the incoming 20 waves. The back side of the half cylinder can be cast as integral part of the supporting structure (3) or the half cylinder can be of certain thickness fixed to a supporting structure. In Fig. 1 a cross section perpendicular to the common cylinder axis is shown. It can be used as a return-type breakwater, with the cylinder axis horizontal and perpendicular to the incoming wave, returning the impulse moment (also called momentum), which is proportional to v, and kinetic energy, 25 which is proportional to v2, of the incoming wave. Alternatively it can be used as an annihilation-type breakwater, with the cylinder axis vertical, so that the inner cylinder acts as a bi-directional roundabout, dividing the tsunami or storm waves in two halves moving toward each other for a clash among themselves. Mutual collisions of the two halves annihilate their impulse moments as well as their kinetic energies. Fixing provisions (4) are just examples of 30 many alternatives, depending on configuration, placed fixed onshore or floating offshore and further practical situations. Frictional effects between the breakwater and the water waves are considered negligible, compared to the wave momentum and energy involved.

The breakwater of the invention of the return-type will now be described in more detail, in an offshore floating configuration, Fig. 2a where (1) - (4) are the same as in Fig.1. The inner 3 cylinder (2) divides the breakwater in a lower and an upper opening. The lower opening is the inlet for the incoming water wave. The inlet capacity is enclosed between the lowest side of the half cylinder and the lowest side of the small cylinder as the maximum waterline MWL. The waterline will be the still waterline, when there is no tsunami wave, as well as the wave waterline 5 of the long tsunami wave as the breakwater will float up and down with the tsunami wave. Therefore the hydrostatic pressure on the floating breakwater does not change with or without a tsunami or storm wave. Another advantage of die floating breakwater is that it is not sensitive to the tsunami generating earthquake. The upper opening is above the wave waterline, such that ho incoming water wave enters into the upper part. If that would happen, its impulse moment and 10 kinetic energy will be destructed by a counter part from below, as will be discussed next in the annihilation type configuration. Due to the speed of the incoming water wave, water enters into the lower opening and makes an upward turn, using some of its kinetic energy, and exits the upper opening in the opposite direction as the incoming wave, creating a counter stream over the incoming tsunami wave. From energy and momentum conservations it follows that in the IS stationary state the breakwater does not undergo net energy and momentum change. The kinetic energy and momentum of the tsunami are returned back by the breakwater. At the end of one period the returned wave compensates the incoming trough of the tsunami wave. The same happens with the next, usually few, tsunami waves. In the absence of tsunami or storm, if the water waves do not have enough kinetic energy to complete a turn, the kinetic energy will be 20 converted into potential energy, which may be used to damp the incoming waves, or the incoming waves can be utilized for power generation.

A symmetric return-type breakwater, Fig. 2b, is of particular interest for the protection of a volcanic island, on the one hand against incoming tsunami and storm waves, on the other hand as protection of the external world against a tsunami that might result from volcanic eruptions and 25 rock fall into the sea that can generate a giant tsunami, like the 524 m wave on July 9 1958 Lituya Bay Alaska USA [6]. Volcanic islands with such potential dangers are in the Canary Islands and in Hawaii. This potential catastrophe is a matter of great concern and debates since several years. Concerted arrays of the symmetric return-type breakwaters around such an island would help to prevent a giant tsunami to hit the surrounding world.

30 In case of water waves generated by storm winds, wind momentum enters the upper part, while water wave momentum enters the lower part of the breakwater. The difference of them is returned. It does not matter which one is bigger. The storm surge will not reach the coast and coastal flooding by sea surge is thus prevented by the breakwater. In addition the storm wind up till the top of die breakwater is also blocked by the breakwater.

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Numerical example. The tsunami, that hit Japan on March 11 2011, had waves of 6 m high at 20 km off the coast, whereas the run-up on the coast was 10 -12 m. Such waves can be blocked by the return-type floating breakwater, placed 20 km off the coast, for example with R = 8 m and r = 2 m, so inlet opening is 6 m. A fin (5) at rear end of the breakwater helps to align the 5 breakwater perpendicular to the incoming wave. The breakwater is fixed on the sea bed with anchor (6) and chain or cable (7), Fig. 2, with at least 6 m over length than the still water depth to allow for upward motion of the breakwater due to the tsunami wave. The length 1 of the breakwater can be chosen, for example 50 -100 m. The width, of the order of 20 m, is determined by the use of the materials and the requirements for the stability of a floating body, 10 known from ship dynamics. Two parallel arrays of such breakwaters can be used for coastal defence: a front array with each breakwater separated from the neighbours by a distance equal to the length of a breakwater, to prevent clashing, and a second array, with the breakwaters in the openings of the front array. The distance between the two arrays is of the order of the length of a breakwater, with provisions of freeway for ships, see Fig. 4 for both types of breakwater. Further 15 advantages of offshore floating breakwaters are: no flooding of the coastline, shorter breakwater array length than the irregular coastline, the gain can be a factor of 5, no debris coming from the ocean (8) that would hit the breakwaters, not hindering the sea sight, creation of still water between breakwater arrays and the coast (9), which offers opportunity for coastal socioeconomic development.

20 Onshore placement of the return-type breakwater is also possible. For an inlet capacity of say 10 m run-up the radius R = 12 m, r = 2 m and the lowest point of the inlet set at the level of the still water line, at high tide. The height of the breakwater is 2R = 24 m. To solve the sea sight problem the upper part of the half cylinder can be folded dawn in idle and folded up when there is a tsunami coming. Failing the mechanism for fold-up, the returned tsunami wave should 25 automatically fold up that part. Another solution is to make part of the half cylinder transparent, with commercially available pvb foil reinforced glass plates fixed on a structure.

The annihilation-type breakwater will now be described in more detail. Fig. 3 is a horizontal cross section, where (1)- (4) are the same as in Fig.1, (6) is anchor and (7) is chain or cable as in Fig. 2. In this case the common axis of the cylinders is vertical, of height h, with the front side of 30 the breakwater facing the incoming waves. As can be seen from Fig. 3a, the inner cylinder (2) serves as a bi-directional roundabout, leading the left and right parts of the incoming wave to clash against each other in the cul-de-sac formed by the semi cylinder (1) and the inner cylinder (2), thus destroying their impulse moments and kinetic energies. A possible remnant e.g. by asymmetry is returned in the opposite direction as the incoming wave.

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For a tsunami wave of 6 m at 20 km offshore, the floating annihilation type breakwater can be of dimensions, for example: h = 10 m, R = 8 m and r = 2 m, submerged for 6 m under the waterline and 4 m above the waterline. The same advantages of the return-type hold for the annihilation-type floating breakwater. In this case the kinetic energy of the tsunami is converted 5 into potential energy. The incoming water mass comes to stand still at the cul-de-sac and is pushed downward and upward by the incoming wave, dispersing the potential energy, creating secondary waves which are not further considered here. The breakwaters can be assembled in a row of 3 to 6 units, Fig. 3b, and arranged in arrays off the coast (9) in the same manner as described for the return-type breakwater, Fig. 4. In case of storm waves, in addition of blocking 10 the water waves for the mentioned capacity, the wind wave impulse momentum and kinetic energy above the water waves till the top of the breakwater destruct each other. So if the breakwater is used against storm waves it is preferable to make the height h greater to block more of the storm wind. A symmetric annihilation-type is obtained, with the horizontal cross section given as Fig. 2b but with the cylinder axis vertical and can serve the same purpose as the 15 symmetric return-type breakwater. In the absence of tsunami and storm, the incoming waves can be utilized for wave power generation.

On shore placement of the annihilation-type breakwater is not suitable as breakwater because still water accumulates at the breakwater location and will cause flooding of the surroundings. However, in combination with an offshore breakwater, of the return or annihilation type, to block 20 the sea surge, such breakwater incorporated as sea side balcony, with 1 = 3 m, R = 3 m, r = 0.5 m, of a multi story apartment building, can serve as a wind-breaker to block the storm wind for the height of the building. Curved and plane reinforced glass plates are commercially available to integrate them in the design of such a storm wind annihilation-type balcony, with sea sight.

Finally, the half inner cylinder facing the incoming water waves can be modified in many 25 ways, for example to improve the floating capability and stability of the breakwater, without changing the essential aspect of the invention. Such modifications are enclosed in the claims.

References [1] KR 20110069409 (A), date 23-06-2011.

30 [2] KR 20110069408 (A), date 23-06-2011.

[3] US 3595026, date 27-07-1971.

[4] WO 2009/049464 Al, date 23-04-2009, EP 2206835 Al, date 14-07-2010.

[5] WO 2011/123870 Al, date 06-10-2011.

[6] G. Pararas-Carayannis, Science of Tsunami Hazards 17 (1999) 193-206.

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Addition to the text and redressed claims

This addition and redressed claims take the search report and written opinion into consideration. Apart from this addition, no any change is made in the original text and figures.

The references [D1-D4] of the search report deal with annihilation type [Dl] and return type 5 [D2-D4] breakwaters, aspects in common with the present invention. However, the invention stressed that impact type breakwaters are inadequate for tsunami and heavy storm waves, p. 1 lines 9-18. Therefore in the invention the wave momentum does not impact on the breakwater and that kinetic energy is not absorbed by the breakwater, p. 2 lines 22-25, p. 3 lines 1-3, 14-17 and Fig. 2a for the return type, p. 2. lines 25-29 for the annihilation type.

10 It is mentioned that the annihilation type is not suitable on shore, p.5 lines 17-18, because of flooding of the surroundings, exactly what happens in [Dl], In addition, it is known that on shore breakwater structures suffer from enhanced hydrostatic pressure of flooding and are vulnerable to the tsunami generating earthquake, while the floating types of the invention maintain the same hydrostatic pressure with or without tsunami and are insensitive to earthquakes p. 3 lines 6-8, 15 which is of great advantage in breakwater design, preferred by the invention.

It is noted that in D2 Fig. 1 the pontoon 1 is partly immersed such that the incoming wave impacts on the pontoon, aspect that is avoided in the invention, see also Fig. 2a, where the maximum waterline MWL is just below the inner cylinder to avoid water momentum impact.

D3 Fig. 4 describes a breakwater containing a semi-circular tsunami reversal device 1, 20 tsunami leading board 2 for tsunami absorption port 4, a tsunami blow-up leading board 3 for tsunami blow-up port 5 and in front of that another breakwater, D3 Fig. 2, which supposedly affects the tsunami wave to become a planar flow, D3 [0024]. Both the front and rear breakwater are of the impact type and therefore do not satisfy the requirement of the present invention.

D4 uses cylindrical wave absorbing bodies 1, figure and abstract: wave engulfed in the 25 cylinder in slant upward direction breaks incoming waves. D4 is obviously of the impact and absorbing type, which does not satisfy the requirement of the invention.

Therefore, despite some common aspects with the invention, it is concluded that D1-D4 are of the impact types and do not satisfy the requirement of the invention. They do not affect the inventive aspect of the invention. The claims are redressed accordingly.

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Dl = DE2242949 A, 1974-03-14 D2 = SU1719529 A, 1992-03-15 D3 = JP2001059212 A, 2001-03-06 D4 = JP2003239247 A, 2003-08-27 1 039 528

Claims (5)

1. Breakwater for coastal protection against tsunami and storm surge waves, arranged fixed on the coast or floating at sea on cable or chain anchored on the seabed near the coast, 5 merals characteristic that a bend of 180 degrees is formed by the inner surface of a half cylinder and the outer surface of a smaller cylinder, the axis of which coincides with the axis of the half cylinder, which device is arranged as a return-type breakwater, with the cylinder axis horizontal and perpendicular to the water waves forming the lower gap between the two flow into cylinder surfaces, then up the bend and return above the opening in the opposite direction as the inflow, without exchange of impulse and kinetic energy between breakwater and water waves. Breakwater as claimed in claim 1 in a floating arrangement, which device is arranged as an annihilation type breakwater, with the cylinder axis vertical, wherein one half of the water waves flows into one opening and the other half the other opening, with the bend 15 bumping towards each other halfway through the bend and therefore both the impulses and the kinetic energies of the two halves annihilate each other, without exchange of impulse and kinetic energy between the breakwater and water waves. 3. Breakwater as claimed in claim 1, wherein for the view of the sea the upper part of the half cylinder can be folded down towards the small cylinder, is folded down and is mechanically folded up in the event of a tsunami or storm, in the absence of the mechanism due to the force of the water is raised. Breakwater as claimed in claim 1 or 2, in a floating arrangement, with a fixation to the shore by means of a floating or non-floating pier or pontoon for the horizontal resistance to the waves, with a sliding in the vertical and hinged in the vertical connection 25 to the breakwater, which connection, just like the breakwater, goes with the water level. Breakwater according to claim 1 or 2, which device is arranged on a building, whether or not as an integrated part thereof, as a windbreaker against storm winds. 1039528