Floating Breakwater Design

FLOATING BREAKWATER DESIGN By Bruce L.fMcCartney,1 M. ASCE

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'(f) ,,_ ABSTRACT: {Floating breakwaters are inventoried. The various types are separated into 4 general categories, wMehSale Box, Pontoon, Mat, and Tethered Float. The Tethered Float was identified as a special category but lacked sufficient prototype experience for detailed analysis. Advantages and disadvantages of the Box, Pontoon and Mat are presented. Hydraulic model test results and prototype experience for these 3 types are presented. Alternative mooring systems and anchorage methods are summarized. The engineering studies ttstt* aBy^needed for a suitable design are outlined. Costs and design data for selected prototype installations are tabulated.-]

INTRODUCTION

Floating breakwaters are used increasingly as a means of protecting small boat marinas in the United States. This paper presents an inventory of typical floating breakwater types,, model test information, prototype installations, a n d design considerations." ANALYSIS

Floating breakwaters have m a n y potential applications for boat basin protection, boat r a m p protection, a n d shoreline erosion control. Some of the conditions that favor floating breakwaters are: 1. Poor Foundation.—Floating breakwaters m a y be the only solution where poor foundations will not support bottom-connected breakwaters. 2. Deep Water.—In water depths in excess of 20 ft (6.1 m), bottomconnected breakwaters are often more expensive t h a n floating breakwaters. 3. Water Quality.—Floating breakwaters present a m i n i m u m of interference with water circulation and fish migration. 4. Ice Problems.—Floating breakwaters can be removed and towed to protected areas if ice formation is a problem. They may be suitable for areas w h e r e s u m m e r anchorage or moorage is required. 5. Aesthetics.—Floating breakwaters have a low profile and present a minimum intrusion on the horizon, particularly for areas with high tide ranges. 6. Breakwater Layout.—Floating breakwaters can usually be rearranged into n e w layout with m i n i m u m effort. a Hydr. Engr., Hydr. and Hydrology Div./ Office Chief of Engrs., U.S. Army Corps of Engrs., Washington, D.C. 20314. Note.—Discussion open until August 1, 1985. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on August 30, 1982. This paper is part of the Journal of Waterway, Port, Coastal and Ocean Engineering, Vol. I l l , No. 2, March, 1985. ©ASCE, ISSN 0733-950X/ 85/0002-0304/$01.00. Paper No. 19610.

304

J. Waterway, Port, Coastal, Ocean Eng. 1985.111:304-318.

BOX

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SOLID R E C T A N G L E

R E I N F O R C E D CONCRETE UNITS A R E T H E MOST COMMON T Y P E .

S T A N D A R D B A R G E SIZES ON I N L A N D WATERWAYS A R E 195' X 35' X 12' A N D 1 7 5 ' X 26' X 1 1 ' I N C L I N E D BARGES (ONE E N D SU BMERGED1HAVE BEEN T E S T E D . PONTOON

TWIN P O N T O O N

FLO TAT/ON BALLAST

C A T A M A R A N SHAPE

ALSO C A L L E D ALASKA

OPEN COMPARTMENT

TYPE

•*- WOOD SHEET SECTION

nr.

^M=

•*

DECK IS OPEN WOOD FRAME.

MAT S C R A P T I R E S S T R U N G ON P O L E F R A M E W O R K OR B O U N D T O G E T H E R WITH C H A I N OR B E L T I N G . FOAM F L O T A T I O N IS U S U A L L Y N E E D E D .

T I R E MAT

L O G R A F T C H A I N E D OR CABLEDTOGETHER.

TETHERED

FLOAT

FLOAT TETHER ANCHOR - / '

F L O A T S P L A C E D IN ROWS. V

PLATFORM ^

SECTION

=•

ARRANGEMENT SIMILAR TO SPHERES. S T E E L DRUMS WITH B A L L A S T S C A N B E USEO IN L I E U O F T I R E S .

FIG. 1.—Various Types of Breakwaters 305

J. Waterway, Port, Coastal, Ocean Eng. 1985.111:304-318.

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FLOATING BREAKWATER TYPES

In recent times, many types, of floating breakwaters have been modeltested and some have been constructed. Ref. 8 provides a comprehensive survey of floating breakwater types. These breakwaters can be subdivided into four general categories: box, pontoon, mat, and tethered float. Some floating breakwaters in each category are shown in Fig, 1. The box, pontoon, and mat types have considerable model and prototype experience, which is summarized in this paper. The tethered float is only identified as a unique type worthy of a separate category. However, since the known published information (summarized in Ref. 8) is experimental, the tethered float breakwater is not examined further in this paper. PROTOTYPE PERFORMANCE

Performance of prototype installations was recently investigated for the east and west coasts of the U.S. These surveys (2,14) showed that used tire mats were used primarily on the east coast and concrete box or pontoon types, on the west coast. The tire mats had a high failure rate or exhibited poor performance due mainly to placement at sites where wave forces exceeded the mooring system capacity, flotation material was lost, or flexure failure of connection materials occurred. Better analysis of site conditions and a proper engineering design of the tying materials and mooring system could decrease the failure rate. Although modular connection and mooring hardware are their weaker points, the concrete box or pontoon breakwaters have generally performed satisfactorily; however, their first cost is considerably higher than tire breakwaters. Life cycle costs between various types of floating breakwaters have not been developed. TIRE MAT BREAKWATER

Three types of tire mats have been model-tested and constructed: Wave Maze, Goodyear, and Wave-Guard (also called Pole-tire and pipe-tire) as shown in Figs. 2-4. Advantages of the tire mat'breakwater are: (1) Low cost; (2) easily removed and beached for maintenance or to prevent ice damage; (3) can be constructed with unskilled labor and minimal equipment; (4) lower anchor loads than box type; and (5) much lower reflected waves than box type. Disadvantages are: 1. Lack of buoyancy.—Loss of trapped air in tire crown, marine growth, and silt accumulation in tire bottom can sink the breakwater. In order to ensure flotation, foam is usually needed for extra buoyancy and regular maintenance is needed to control marine growth. Holes in tire bottom can be used to prevent or reduce silt accumulation. 2. Design life.—Design life of the tire breakwater is still unknown. A properly designed breakwater and mooring system with adequate maintenance may have a 15-20-yr life. 3. Limited application.—Present use has shown that tire mat breakwaters are only effective in mild wave climates (less than 3-ft high, 3308

J. Waterway, Port, Coastal, Ocean Eng. 1985.111:304-318.

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FIG. a.-r-Wave Maze (Ref. 11) , Cornet tires are .rotated 100°

Note: Each individual module is 1.98 by 2.13 by 0.76 m.

'

FIG. 3.—Goodyear (After Ref. 7)

B-

-

-H

fl flf1)c=H^H=H=H=i-

-Q

=

-CZMZ3C3{[]J

FIG. 4.—Wave-Guard or Pipe Tire (After Ref. 10)

307

J. Waterway, Port, Coastal, Ocean Eng. 1985.111:304-318.

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