Engineering of the Submerged Breakwater at Maiden Island

This page contains numerous links to scientific papers to help proved scientific and engineering perspectives to using Reef Balls as submerged breakwaters.  It is not, however, intended to provide site specific engineering advise for specific locations.  There are a large number of variables that impact the design of submerged breakwaters including tidal range, bathometry, long shore transports of sand, wind and wave climates, proximity to other structures, sand grain size, beach slope, and others.  Consult with the Reef Ball Foundation Inc. Services Division before using Reef Balls in any type of submerged breakwater application. 

History of Reef Balls as Submerged Breakwaters

Dr. Lee Harris, a professor at the Florida Institute of Technology and a consulting coastal engineer has been working with the Reef Ball Foundation Inc. and Reef Ball Foundation Inc. Services Division since 1996 to pioneer the use of Reef Balls as a submerged breakwater to control, and in some cases even reverse, erosion on beaches.  

 

Beaches typically form behind natural submerged barrier reefs because these reefs reduce wave energy, so the concept was simply to mimic such systems in both the ability to reduce wave energy and also to provide diversity of habitat to support a healthy reef ecosystem.  From an engineering standpoint, there were many steps taken over several years in order to master the specifics needed to provide solutions for a variety of site specific issues.  The first step was to complete complex wave tank studies to determine just exactly how stable Reef Balls are and whether anchoring systems would be required in specific areas.  Any submerged breakwater needs to be engineered so that it cannot be moved by storms and waves.  As part of stability tests, Reef Balls were also placed in a wind tunnel to determine drag coefficients.   As a result of the modeling, it was then possible to do wave force calculations at proposed project locations. 

Dr. Harris' first Reef Ball submerged breakwater project was completed in 1998 at the Gran Dominicus Hotel in the Dominican Republic.  The results were so successful that the next project was the hotel right next to the Gran Dominicus, the Iberostar.  Reef Ball Foundation Inc. Services Division then began developing specific Reef Ball shapes and sizes (mostly larger) such as the Super and Goliath sized Reef Balls to fit a wider range of beach slopes.  More beach restoration projects followed in the Cayman Islands, Cancun, Puerto Progresso, and construction is currently underway for the Canola Hilton in Dominican Republic.   There were many additional spin-offs of the technology including protection of seawalls and even historic Yalkubul lighthouse in Yucatan, Mexico.  

In 2003, the U.S. Army Corps of Engineers, after extensively reviewing existing breakwater designs selected Reef Ball Breakwaters as a new option worthy of serious trials and commenced a large demonstration project at Miami Beach under a program called 227.  This project included major wave reduction modeling in scales from 1 on 10 to 1 on 20 at the U.S. Army, Corps of Engineers, Engineer Research and Development Center in Vicksburg, Mississippi, the largest wave tank facilities in the world. 

As a result of the physical modeling of Reef Ball configurations in wave tanks and wind tunnels, along with data from existing Reef Ball breakwaters, breakwaters can now be designed with confidence to meet a client’s specific application. 

History of Reef Ball as Artificial Reefs

Reef Ball artificial reefs represent the world's leading technology in designed reefs.  Since 1993, Reef Ball has built and deployed over 3,500 reefs in 46 countries using over 500,000 Reef Ball modules.  These reefs were built for a variety of biological purposes creating fishing reefs, diving reefs, snorkeling reefs, and a wide range of habitat restoration projects.  Biologically, Reef Balls have a wide range of special features to create high quality habitat for marine creatures, resulting in reefs that mimic natural reefs in form and function.  

By 1999, the Reef Ball Foundation felt there was a valid application for artificial reefs in tropical coral reef areas suffering degradation from bleaching, destructive fishing practices, and coastal development. However, slow recruitment and growth of hard corals discouraged the use of artificial reefs as tools for coral reef restoration because humans expect results faster than artificial reefs could deliver.  Fortunately, scientists were advancing their understanding of coral reproduction, and an increasing number of  methods for asexually reproducing corals by propagation or fragmentation were developed and perfected by aquarists.  However, what was lacking was an inexpensive and rapid system to extend this exciting work from aquariums to the natural marine environment.  The Reef Ball Foundation worked with experts from around the world to develop techniques to collect, multiply and attach corals to Reef Balls with minimal mortality, and over several projects, developed the coral attachment adapter system.  The Reef Ball Foundation has propagated corals in American Samoa, Malaysia, Maldives Islands, Bahamas, Curacao, USA, Kuwait, Mystique, Dominica, and now Antigua. This system has proven so simple and effective,  that volunteers with minimal training can successfully carry it out. The Reef Ball Foundation has now became the leading group in this technique of coral reef restoration.  

Maiden Island

The Stanford Development Company, Ltd. was working with Greg Morris and Associates and the Caribbean Oceanography Group (Dr. Alfredo Torruella) to develop a traditional breakwater to protect the beaches of Maiden Island.  In trying to find better environmental approaches, they contacted the Reef Ball group for a proposal.  After visiting the site, Todd Barber, Chairman of the Reef Ball Foundation realized that virtually all the technologies developed by Reef Ball over the last ten years could be applied to this single site to create the worlds largest, and most complete restoration project ever attempted. What was needed was a new type of Ocean Engineering with conservation and restoration philosophies incorporated in every engineering design detail.  The Reef Ball Foundation brought Dr. Harris together with Greg Morris and Dr. Alfredo Torruella, and added conservation and restoration experts to the mix. Using this multi-disciplinarian approach yielded a project able to achieve the new level of Ocean Engineering.

Environment

The Maiden Island windward Reef Ball breakwater is located on the eastern shore of Maiden Island, about 1 km off the northeast coast of Antigua.  Maiden Island is approximately 26 acres in size and experiences significant trade winds which generate during most of the year waves in the 2-4 foot range. Antigua has a minimal tidal range of 16 inches.  Sub tidal habitat from 0.5m to 7m is dominated by a dense meadow of Turtle Grass (Thalassia testudinum) with occasional small rocky outcrops supporting a diverse assemblage of hard and soft corals and numerous algae.

Breakwater Objectives

The breakwater was designed to achieve the following main objectives 

  1. Improve the recreational value of the windward beach by reducing the wave energy sufficiently to allow the creation of a stable new beach using natural sand, replacing the existing coarse dredge spoil beach.
  2. Maintain sufficient environmental flow and water exchange behind the breakwater to ensure the survival of existing seagrass and transplanted corals and algae.
  3. Provide wave attenuation to reduce the average turbidity of the water to enhance water quality for hard corals.
  4. Minimize visual impact by remaining below the waterline.
  5. Provide guided snorkeling and diving trails with maximum diversity and abundance of marine life.
  6. Minimize the loss of existing seagrass within the footprint of the breakwater.
  7. Mimic as closely as possible a natural fringing coral reef system.

 

SECTIONS BEYOND THIS SIGN ARE WORKS IN PROGRESS
...AND NOT TO BE PUBLISHED YET but feel free to browse.

The Work

Oceanography and Bathymetry

<<to be written by Dr. Alfredo Torruella>>

Site Survey and Auto CAD Drawings

 

<<DRAFT SECTION...to be written by Dr. Lee Harris & Dr. Alfredo Torruella>>

The first tasks were to establish baseline survey data of depth contours which could then be used for CAD assisted design drawings, and install temporary survey markers to guide construction and determine the elevation of the Biological Tide Line.  Dr. Lee Harris and Todd Barber used scuba gear, short lengths of rebar and high visibility cord to mark the 4 foot and 7.5 foot depth contour lines..   Rebar stakes were placed at 30 foot intervals, and the force required to drive in each rebar was recorded to help approximate bottom firmness.  The team also noted the width between 4.5 foot, 5 foot and 6 foot contour lines which corresponded to standard Reef Ball sizes.  Squares that formed between the two contour lines every 30 feet were labeled alphabetically for additional investigations. 

 

Stanford Development Group, Ltd.'s survey crews used 'Total Station surveying methods' to determine coordinates and elevations for each of the rebar stakes.   From a known survey point, the exact altitude of the Biological Tide Line was computed and used as the baseline elevation for the CAD drawing created from the survey work.  .  The temporary rebar construction markers where then adjusted to the planned breakwater location based on the survey

 

A pass was made over the site using bottom penetrating sonar, however, the resolution was not sufficient for characterizing sediments sufficiently to design the required anchoring systems.  Therefore Dr. Harris and Dr. Torruella, working with assistants, used a jet probe and rebar/hammer method to obtain depth of sand over hard bottom at surveyed marks established by the survey team.  This provided the necessary data to calculate the ability of the sediments to support the modules, degree of settling, and type of anchoring required.

Engineering Design

<<DRAFT SECTION...to be written by Dr. Lee Harris, Todd Barber, Dave Lennon, Greg Morris & Dr. Alfredo Torruella>>

Several issues complicated the design.  The bottom varied greatly from soft sand over 8 feet to hard bottom in the south, to areas of very firm shell less than one foot thick over hard bottom in the north..  This required four separate and new anchoring methods to be developed to prevent the modules from horizontal and vertical movement, and to be able to withstand hurricane forces. 

 

An additional complication was that thirty-two coral colonies were found within the intended footprint of the breakwater so the breakwater had to be designed around them.  This was achieved by incorporating the colonies into the snorkeling passages and diving trails that needed to penetrate the breakwater for swimming access.  The snorkeling and diving section of the breakwater has a spacious mix of Reef Balls, therefore the gaps between the modules were able to accommodate the existing corals.

 

In some areas, there was very little shelf room to place the five rows of Reef Balls that the Wave Tank data indicated would provide the desired level of wave attenuation.  Because one of the goals was to maximize swimming area inside the breakwater, normally the largest sized Reef Balls were used, but in areas of narrow shelf, some smaller units were used to maintain the desired design height.  

 

Design and Testing of Anchoring Systems

<<DRAFT SECTION...to be written by Dr. Lee Harris, Todd Barber, and Greg Morris>>

Anchoring Methods

There were a variety of anchoring methods used, in many cases, combinations of the methods shown below were used.  Each methods was tested in the field for anchoring ability and engineering calculations determined which anchoring combination would be required for stability. 

Anchoring Cones

For 'Normal' bottom (rebar could be driven to two feet by a sledge hammer with normal effort and there was a healthy biological seagrass root system for resistance to settlement) Anchoring cones were used.   The four anchoring cones on the above Goliath Sized Reef Ball were cast monolithically when the Reef Ball was made and are designed to slowly settle over a period of months  into the sea grass bed root system.  After settlement, the cones will prevent lateral movement of the Reef Ball during storms.   Although somewhat difficult to see in this photo but clearly visible in the photo below, between each cone has a 1.5 inch pad that is designed to keep the module above the root systems to maintain the designed height since the roots are slightly below the seabed.

Anchoring Spikes

In areas where the bottom was 'hard packed' (so hard that it was difficult for rebar had to be driven in by a sledge hammer) pre-cast spikes with #5 fiberglass re-enforcement were cast into the Reef Ball bottoms.  Engineering pressure tests indicated the size of the spikes needed to be self penetrating by the weight of the units when installed.  These spikes provide resistance to lateral movement.  The outside and inside rows of Reef Balls had solid bottoms (shown above) and were 25-50% heavier than the hollow bottom units in the center rows.  This was done to maximize stability.

 

Above is a picture of the base Reef Balls are cast upon showing several of the adaptations made for the Maiden Island project.

 

Battered Anchoring Pilings

In areas where the sand was 'soft' (so soft that rebar could be put to a two foot depth using hand pressure only) battered pilings were used.  Three piles in a 'tripod' pattern were jetted in until they reached the hard bottom below the soft sand.  During installation, the crews first jet to the bottom with a 4 inch pipe to determine the exact depth.  Next they would insert a pile of the appropriate length and use the smaller 1/2 inch PVC pipe precast into the pile to jet it to the bottom.  Three pieces of #3 fiberglass rebar were added to each precast piling for strength. Pilings were made in ranging from 8 feet to 3 feet in one foot increments.

 

Anchoring With Battered Fiberglass Rebar

In areas where there was hard bottom, or for smaller sized Reef Balls located inside of the breakwater in the wave protected area, 3-4 pieces of battered #5 fiberglass rebar where drilled or driven into the seabed. Holes for both the fiberglass rebar and the piling anchoring system were cast into every Reef Ball made to provide multiple anchoring options.

Anchoring With Rock 'Cones'

'Layer Cake' Reef Balls were cast upside down, so instead of using anchoring cones, we selected natural rocks about the size and shape of the anchoring cones.  The concept is similar, the units settle by themselves over time or they are assisted by jet pump which provides additional stability from lateral movement and overturning.    

Anchoring With Articulating Concrete Slabs

The Miami reefball submerged breakwater project included articulating concrete slabs and below are links to the AutoCAD drawings of the design.  

However, use of the Battered Anchoring Pilings was selected for Maiden Island over the Articulating Concrete Slabs as being more cost effective since diving contractors are less expensive in Antigua than in Miami.

Engineering Monitoring

<<to be written by Dr. Alfredo Torruella>>

 

 

<<=BACK TO THE MAIN PRESS RELEASE

 


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