Difference between revisions of "Floating breakwaters"
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| − | == | + | {{Review |
| + | |name=Job Dronkers|AuthorID=120| | ||
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| − | Floating breakwaters | + | |
| + | ==Use of floating breakwaters== | ||
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| + | [[Image:FBimage006.JPG|350px|thumb|right|Figure 1 Example of floating box-breakwater (Fezzano,SP-Italy; courtesy of INGEMAR srl)]] | ||
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| + | Floating breakwaters provide a relatively cheap solution to protect an area from wave attack, compared to conventional fixed breakwaters. It can be effective in coastal areas with mild wave environment conditions. Therefore, they are used for protecting small craft harbours or marinas or, less frequently, the shoreline, aiming at erosion control. Some of the conditions that favour floating breakwaters are<ref name=MC>McCartney, B. 1985. Floating breakwater design, J. of Waterway, Port, Coastal and Ocean Engineering 111: 304-318</ref>: | ||
#'' Poor foundation:'' Floating breakwaters might be a proper solution where poor foundations possibilities prohibit the application of bottom supported breakwaters. | #'' Poor foundation:'' Floating breakwaters might be a proper solution where poor foundations possibilities prohibit the application of bottom supported breakwaters. | ||
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#'' Visual impact:'' Floating breakwaters have a low profile and present a minimum intrusion on the horizon, particularly for areas with high tide ranges. | #'' Visual impact:'' Floating breakwaters have a low profile and present a minimum intrusion on the horizon, particularly for areas with high tide ranges. | ||
#'' Breakwater layout:'' Floating breakwaters can usually be rearranged into a new layout with minimum effort. | #'' Breakwater layout:'' Floating breakwaters can usually be rearranged into a new layout with minimum effort. | ||
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==Types of floating breakwaters== | ==Types of floating breakwaters== | ||
| − | Floating breakwaters are commonly divided into four general categories: | + | Floating breakwaters are commonly divided into four general categories<ref name=MC/>: |
# Box | # Box | ||
# Pontoon | # Pontoon | ||
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# Tethered float. | # Tethered float. | ||
| − | For each category, some types of floating breakwaters are shown in Figures 1 - 5. The first three types have been | + | For each category, some types of floating breakwaters are shown in Figures 1 - 5. The first three types have been more widely investigated by means of physical models and prototype experience than the last one. |
| − | + | Floating breakwaters often consist of several interconnected modules. Connections are either flexible, allowing preferably only the roll along the breakwater axis, or pre- or post-tensioned, to make them act as a single unit. In the latter case the efficiency is higher, but the forces between modules are higher. The modular assemblage and the mooring system are primary points of concern for this kind of structures. | |
| − | Connections are either flexible, allowing preferably only the roll along the breakwater axis, or pre or post tensioned, to make them act as a single unit. In the latter case the efficiency is higher, but the forces between modules are | ||
| − | Large breakwaters are frequently built with used barges, ballasted to the desired draft with sand or rock.Floating breakwaters are | + | The performance of a floating breakwater depends on the strongly non-linear interaction of the incident wave (that may partially overtop the module and is in general short-crested and oblique) with the structure dynamics. The forces induced by the mooring system and the connections between the modules complicate the interactions. Accurate design is necessarily based on the combination of numerical and physical models<ref>Martinelli, L. and Ruol, P. 2006. 2D Model of Floating Breakwater Dynamics under Linear and Nonlinear Waves, 2nd Comsol User Conference, 14 Nov., Milano</ref>. |
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| + | Large breakwaters are frequently built with used barges, ballasted to the desired draft with sand or rock. | ||
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| + | Floating breakwaters are most effective for wave damping when their width <math>W</math> is of order of half the [[wavelength]] <math>L</math>. | ||
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| + | The natural period of oscillation of a floating breakwater is of the order of <math>\; T \sim 2 \, \pi \, \sqrt{\large\frac{M}{\rho g A}\normalsize} \;</math>, where <math>\rho</math> is the seawater density, <math>g</math> the gravitational acceleration, <math>A</math> the horizontal section of the breakwater, and <math>M</math> the mass. This period should be much longer than the wave period to avoid resonance. These requirements imply that floating breakwaters are not suited in areas with long-period high waves. | ||
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===Box breakwaters=== | ===Box breakwaters=== | ||
| − | Box type breakwaters are used most frequently (see | + | Box type breakwaters are used most frequently (see figures 1 and 2). Most box-type breakwaters have been constructed of reinforced concrete modules. Reinforced concrete modules are either empty inside or, more frequently, have a [[core]] of light material (e.g. polystyrene). In the former case the risk of sinking of the structure is not negligible. The width and depth (draft) are usually limited to a few meters. |
| − | + | [[Image:FBimage002.JPG|450px|thumb|center|Figure 2 Box breakwaters<ref name=MC/>.]] | |
| − | [[Image:FBimage002.JPG|450px|thumb|center|Figure 2 Box breakwaters<ref name= | ||
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===Pontoon breakwaters=== | ===Pontoon breakwaters=== | ||
Pontoon types are effective since the overall width can be of the order of half the [[wavelength]]. In this case the expected attenuation of the wave height is significant. See also Figure 3. | Pontoon types are effective since the overall width can be of the order of half the [[wavelength]]. In this case the expected attenuation of the wave height is significant. See also Figure 3. | ||
| − | [[Image:FBimage003.JPG|450px|thumb|centre|Figure 3 Pontoon breakwaters<ref name= | + | [[Image:FBimage003.JPG|450px|thumb|centre|Figure 3 Pontoon breakwaters<ref name=MC/>]] |
===Mat breakwaters=== | ===Mat breakwaters=== | ||
| − | Within the mat category, the most used are made with tires. | + | Within the mat category, the most used are made with tires. They have a low cost, they can be removed more easily, they can be constructed with unskilled labour and minimal equipment, they are subjected to lower anchor loads, they reflect less and they dissipate relatively more wave energy. However, they are less robust and suitable only in mild wave climates. |
| − | [[Image:FBimage004.JPG|450px|thumb|centre|Figure 4 Mat breakwaters<ref name= | + | [[Image:FBimage004.JPG|450px|thumb|centre|Figure 4 Mat breakwaters<ref name=MC/>]] |
===Tethered float breakwaters=== | ===Tethered float breakwaters=== | ||
| − | Tethered float types are | + | Tethered float types are not much used. Two schemes are shown in figure 5. |
| − | [[Image:FBimage005.JPG|450px|thumb|centre|Figure 5 Tethered float breakwaters<ref name= | + | [[Image:FBimage005.JPG|450px|thumb|centre|Figure 5 Tethered float breakwaters<ref name=MC/>]] |
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| + | ==Wave transmission== | ||
| + | Many laboratory experiments have been performed to establish empirical formulas for the wave transmission of floating breakwaters, especially for the box-type breakwaters<ref>Koutandos, E. and Prinos, P. 2005. Design formulae for wave transmission behind floating breakwaters. XXXI IAHR congress, paper 4081</ref><ref>Alizadeh, M.J., Kolahdoozan, M., Tahershamsi, A. and Abdolali, A. 2014. Experimental Study of the Performance of Floating Breakwaters with Heave Motion. Civil Engineering Infrastructures Journal 47(1): 59 – 70</ref><ref>Moghim, N. and Botshekan, M. 2017. Analysis of the performance of pontoon-type floating breakwaters. Hong Kong Institution of Engineers Transactions 24: 9–16</ref><ref>Elsheikh, A.K., Mostafa, Y.E. and Mohamed, M.M. 2022. A comparative study between some different types of permeable breakwaters according to wave energy dissipation. Ain Shams Engineering Journal 13, 101646</ref>. The parameters considered in these experiments are the width <math>W</math> (along the wave propagation direction), the draft | ||
| + | <math>D</math>, the incident wave height <math>H_i</math> (most experiments considered regular waves), the wavelength <math>L</math> and the depth <math>h</math>. The dependence of the wave transmission coefficient <math>C_t=H_t/H_i</math> (where <math>H_t</math> is the transmitted wave height) on these various parameters shows a fairly large spread between the different experiments. However, in a qualitative sense, the dependency is similar: the transmission coefficient decreases for increasing values of <math>W</math>, <math>D</math> and <math>H_i</math> and increases for increasing values of <math>L</math> and <math>h</math>. The transmission coefficient is most sensitive to the ratio <math>W/L</math>. | ||
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| + | ==Application== | ||
| + | Floating breakwaters can provide suitable protection measures for small boat harbor at some locations. However, they must be properly designed for the site conditions with an understanding of their limitations<ref name=MC/>. | ||
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==See also== | ==See also== | ||
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* [[Shoreline management]] | * [[Shoreline management]] | ||
| − | == | + | |
| + | ==References== | ||
| + | <references/> | ||
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| + | ==Other sources== | ||
*Allyn N., E. Watchorn, W. Jamieson and Y. Gang, 2001. Port of Brownsville Floating Breakwater, Proc. Ports Conference. | *Allyn N., E. Watchorn, W. Jamieson and Y. Gang, 2001. Port of Brownsville Floating Breakwater, Proc. Ports Conference. | ||
*Briggs M, Y. Ye, Z. Demirbilek and J. Zhang, Field and numerical comparisons of the RIBS floating breakwater, J. of Hydraulic Research, 40(3), 289-301. | *Briggs M, Y. Ye, Z. Demirbilek and J. Zhang, Field and numerical comparisons of the RIBS floating breakwater, J. of Hydraulic Research, 40(3), 289-301. | ||
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*Isaacson M. (1993): Wave effects of floating breakwaters, Proc. of the 1993 Canadian Coastal Conference, May 4-7, Vancouver, British Columbia, 53-66. | *Isaacson M. (1993): Wave effects of floating breakwaters, Proc. of the 1993 Canadian Coastal Conference, May 4-7, Vancouver, British Columbia, 53-66. | ||
*Isaacson M. and S. Sinha, 1986. Directional wave effects on large offshore structures, J. of Waterway, Port, Coastal and Ocean Engineering, 112(4), 482-497. | *Isaacson M. and S. Sinha, 1986. Directional wave effects on large offshore structures, J. of Waterway, Port, Coastal and Ocean Engineering, 112(4), 482-497. | ||
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*Martinelli L., Zanuttigh B., Ruol P., 2007. Effect of layout on floating breakwater performance: results of wave basin experiments . Proc. Coastal Structures '07, Venice. | *Martinelli L., Zanuttigh B., Ruol P., 2007. Effect of layout on floating breakwater performance: results of wave basin experiments . Proc. Coastal Structures '07, Venice. | ||
| − | * | + | *PIANC. Floating breakwaters - a practical guide for design and construction PTC2 report of WG 13 – 1994 |
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*Richey E.P. (1982): Floating Breakwater Field experience, West Coast. Report MR 82-5, U.S. Army, Corps of Engineers, Coastal Engineering Research Center; Springfield, Va, 64 pp. | *Richey E.P. (1982): Floating Breakwater Field experience, West Coast. Report MR 82-5, U.S. Army, Corps of Engineers, Coastal Engineering Research Center; Springfield, Va, 64 pp. | ||
*Ruol P. and Martinelli L., 2007. Wave flume investigation on different mooring systems for floating breakwaters. Proc. Coastal Structure '07, Venice. | *Ruol P. and Martinelli L., 2007. Wave flume investigation on different mooring systems for floating breakwaters. Proc. Coastal Structure '07, Venice. | ||
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{{author | {{author | ||
Revision as of 23:02, 2 May 2023
Contents
Use of floating breakwaters
Floating breakwaters provide a relatively cheap solution to protect an area from wave attack, compared to conventional fixed breakwaters. It can be effective in coastal areas with mild wave environment conditions. Therefore, they are used for protecting small craft harbours or marinas or, less frequently, the shoreline, aiming at erosion control. Some of the conditions that favour floating breakwaters are[1]:
- Poor foundation: Floating breakwaters might be a proper solution where poor foundations possibilities prohibit the application of bottom supported breakwaters.
- Deep water: In water depths in excess of 6 m, bottom connected breakwaters are often more expensive than floating breakwaters.
- Water quality: Floating breakwaters present a minimum interference with water circulation and fish migration.
- Ice problems: Floating breakwaters can be removed and towed to protected areas if ice formation is a problem. They may be suitable for areas where summer anchorage or moorage is required.
- Visual impact: Floating breakwaters have a low profile and present a minimum intrusion on the horizon, particularly for areas with high tide ranges.
- Breakwater layout: Floating breakwaters can usually be rearranged into a new layout with minimum effort.
Types of floating breakwaters
Floating breakwaters are commonly divided into four general categories[1]:
- Box
- Pontoon
- Mat
- Tethered float.
For each category, some types of floating breakwaters are shown in Figures 1 - 5. The first three types have been more widely investigated by means of physical models and prototype experience than the last one.
Floating breakwaters often consist of several interconnected modules. Connections are either flexible, allowing preferably only the roll along the breakwater axis, or pre- or post-tensioned, to make them act as a single unit. In the latter case the efficiency is higher, but the forces between modules are higher. The modular assemblage and the mooring system are primary points of concern for this kind of structures.
The performance of a floating breakwater depends on the strongly non-linear interaction of the incident wave (that may partially overtop the module and is in general short-crested and oblique) with the structure dynamics. The forces induced by the mooring system and the connections between the modules complicate the interactions. Accurate design is necessarily based on the combination of numerical and physical models[2].
Large breakwaters are frequently built with used barges, ballasted to the desired draft with sand or rock.
Floating breakwaters are most effective for wave damping when their width [math]W[/math] is of order of half the wavelength [math]L[/math].
The natural period of oscillation of a floating breakwater is of the order of [math]\; T \sim 2 \, \pi \, \sqrt{\large\frac{M}{\rho g A}\normalsize} \;[/math], where [math]\rho[/math] is the seawater density, [math]g[/math] the gravitational acceleration, [math]A[/math] the horizontal section of the breakwater, and [math]M[/math] the mass. This period should be much longer than the wave period to avoid resonance. These requirements imply that floating breakwaters are not suited in areas with long-period high waves.
Effectiveness
Box breakwaters
Box type breakwaters are used most frequently (see figures 1 and 2). Most box-type breakwaters have been constructed of reinforced concrete modules. Reinforced concrete modules are either empty inside or, more frequently, have a core of light material (e.g. polystyrene). In the former case the risk of sinking of the structure is not negligible. The width and depth (draft) are usually limited to a few meters.
Figure 2 Box breakwaters[1].
Pontoon breakwaters
Pontoon types are effective since the overall width can be of the order of half the wavelength. In this case the expected attenuation of the wave height is significant. See also Figure 3.
Figure 3 Pontoon breakwaters[1]
Mat breakwaters
Within the mat category, the most used are made with tires. They have a low cost, they can be removed more easily, they can be constructed with unskilled labour and minimal equipment, they are subjected to lower anchor loads, they reflect less and they dissipate relatively more wave energy. However, they are less robust and suitable only in mild wave climates.
Figure 4 Mat breakwaters[1]
Tethered float breakwaters
Tethered float types are not much used. Two schemes are shown in figure 5.
Figure 5 Tethered float breakwaters[1]
Wave transmission
Many laboratory experiments have been performed to establish empirical formulas for the wave transmission of floating breakwaters, especially for the box-type breakwaters[3][4][5][6]. The parameters considered in these experiments are the width [math]W[/math] (along the wave propagation direction), the draft [math]D[/math], the incident wave height [math]H_i[/math] (most experiments considered regular waves), the wavelength [math]L[/math] and the depth [math]h[/math]. The dependence of the wave transmission coefficient [math]C_t=H_t/H_i[/math] (where [math]H_t[/math] is the transmitted wave height) on these various parameters shows a fairly large spread between the different experiments. However, in a qualitative sense, the dependency is similar: the transmission coefficient decreases for increasing values of [math]W[/math], [math]D[/math] and [math]H_i[/math] and increases for increasing values of [math]L[/math] and [math]h[/math]. The transmission coefficient is most sensitive to the ratio [math]W/L[/math].
Application
Floating breakwaters can provide suitable protection measures for small boat harbor at some locations. However, they must be properly designed for the site conditions with an understanding of their limitations[1].
See also
Articles about breakwaters in general:
Articles about detached breakwaters:
Articles about shore or coastal protection:
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 McCartney, B. 1985. Floating breakwater design, J. of Waterway, Port, Coastal and Ocean Engineering 111: 304-318
- ↑ Martinelli, L. and Ruol, P. 2006. 2D Model of Floating Breakwater Dynamics under Linear and Nonlinear Waves, 2nd Comsol User Conference, 14 Nov., Milano
- ↑ Koutandos, E. and Prinos, P. 2005. Design formulae for wave transmission behind floating breakwaters. XXXI IAHR congress, paper 4081
- ↑ Alizadeh, M.J., Kolahdoozan, M., Tahershamsi, A. and Abdolali, A. 2014. Experimental Study of the Performance of Floating Breakwaters with Heave Motion. Civil Engineering Infrastructures Journal 47(1): 59 – 70
- ↑ Moghim, N. and Botshekan, M. 2017. Analysis of the performance of pontoon-type floating breakwaters. Hong Kong Institution of Engineers Transactions 24: 9–16
- ↑ Elsheikh, A.K., Mostafa, Y.E. and Mohamed, M.M. 2022. A comparative study between some different types of permeable breakwaters according to wave energy dissipation. Ain Shams Engineering Journal 13, 101646
Other sources
- Allyn N., E. Watchorn, W. Jamieson and Y. Gang, 2001. Port of Brownsville Floating Breakwater, Proc. Ports Conference.
- Briggs M, Y. Ye, Z. Demirbilek and J. Zhang, Field and numerical comparisons of the RIBS floating breakwater, J. of Hydraulic Research, 40(3), 289-301.
- Gesrah M.R. 2006. Analysis of 5 shaped floating breakwater in oblique waves: I. Impervious rigid wave boards. Applied Ocean Research, 28(5) 327–338.
- Isaacson M and O.U. Nwogu, 1987. Wave loads and motions of long structures in directional seas, J Offshore Mech Arct Eng 109, 126–132.
- Isaacson M. (1993): Wave effects of floating breakwaters, Proc. of the 1993 Canadian Coastal Conference, May 4-7, Vancouver, British Columbia, 53-66.
- Isaacson M. and S. Sinha, 1986. Directional wave effects on large offshore structures, J. of Waterway, Port, Coastal and Ocean Engineering, 112(4), 482-497.
- Martinelli L., Zanuttigh B., Ruol P., 2007. Effect of layout on floating breakwater performance: results of wave basin experiments . Proc. Coastal Structures '07, Venice.
- PIANC. Floating breakwaters - a practical guide for design and construction PTC2 report of WG 13 – 1994
- Richey E.P. (1982): Floating Breakwater Field experience, West Coast. Report MR 82-5, U.S. Army, Corps of Engineers, Coastal Engineering Research Center; Springfield, Va, 64 pp.
- Ruol P. and Martinelli L., 2007. Wave flume investigation on different mooring systems for floating breakwaters. Proc. Coastal Structure '07, Venice.
- Sannasiraj S.A., V. Sundar and R. Sundaravadivelu, 1998. Mooring forces and motion responses of pontoon-type floating breakwaters, Ocean Eng., 25 (1), 27-48.
- Silander, J. 1999 Floating Breakwater and Environment.
- Tsinker G., 1994. Marine structure engineering: specialized application, Chapman & Hall, International Thomson Publishing Inc.
- Van der Meer J.W., R. Briganti, B. Zanuttigh and B. Wang, 2005. Wave transmission and reflection at low-crested structures: Design formulae, oblique wave attack and spectral change, Coastal Eng., 52(10-11), 915-929.
- Yamamoto T., 1981. Moored floating breakwater response to regular and irregular waves. Applied Ocean Research 3, 27–36.
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