Performed by: Dr. Lee E. Harris, Ph.D., P.E.
Associate Professor of Ocean Engineering & Oceanography
Division of Marine & Environmental Systems
Florida Institute of Technology
Melbourne, FL USA
Performed for: Ing. Miguel Angel Cabral Espinal
EMPACA Ecotourism & Environmental Projects
Santo Domingo, Dominican Republic
This report presents the existing conditions and recommendations for the Reef Ball™ submerged breakwater constructed offshore of the Gran Dominicus Resort, located on the south Caribbean coast of the Dominican Republic near Bayahibe (east of Santo Domingo and LaRomano). Figure 1 shows an aerial photograph of the project area taken on April 28, 2001. This breakwater was installed during the summer 1998, and this report includes background information on the design and performance of this breakwater over the past 2-1/2 years.
The latest site visit was made on April 27-30, 2001. Initial site visits and surveys were performed in the winter and spring of 1998. Post construction surveys of the Reef Ball artificial reef submerged breakwater were performed during November 1998 (following the direct hit by Hurricane Georges in September and the large swell waves from Hurricane Mitch in October) and in February 1999.
Figure 1. April 2001 Aerial Photograph of Gran Dominicus
Three submerged breakwater segments using three rows of Reef Ball artificial reef units.
Approximately 450 Reef Ball artificial reef units were installed offshore of the Gran Dominicus Resort during the summer 1998 to form a submerged breakwater for shoreline stabilization, environmental enhancement and eco-tourism. Shortly following the installation, the area was affected by Hurricanes Georges (direct hit by a category 3 storm) and Mitch (large swell waves from category 5 storm) during the fall 1998.
The individual units used for the breakwater were 1.2m high Reef Ball units and 1.4m high Ultra Ball units, placed in three 3-row segments in water depths of 1.6m to 2m. The units were fabricated with the maximum volume of concrete for the molds. To increase the strength and workability, and decrease the pH of the concrete, microsilica and ADVA additives were used. Number five fiberglass rebar were used to pin the units to the bottom, and the central cavities of the units were filled with rocks to provide additional habitat and weight, with each individual Reef Ball unit weighing approximately 6,000 pounds.
The photograph in Figure 2 shows the original installation configuration of the submerged breakwater using 3 rows of Reef Ball units. The photograph in Figure 3 shows an underwater view of the breakwater during a survey. Biological activity includes substantial marine growth on the units, with numerous benthic and pelagic species inhabiting the Reef Balls. The rocks inside the Reef Ball units provide small interstices, which provide habitat and hiding areas for numerous small benthic and pelagic organisms.
Figure 2. Original submerged breakwater using 3 rows of Reef Ball units
Photograph taken in November 1998.
Figure 3. Underwater photograph of the seaward row of Reef Balls.
Photograph taken during survey in February 1999 (after Hurricane Georges and Mitch).
The direct hit on the project area by Hurricane Georges, a category three hurricane, followed by large swell waves from Hurricane Mitch, a category five hurricane that passed south of the Dominican Republic, greatly tested the stability of the Reef Ball artificial reef units. The project area experienced large waves, elevated water levels and strong currents associated with the storm surge and hurricane conditions accompanying Hurricane Georges in September. In October, large swell waves generated by Hurricane Mitch directly impacted the project area. However, the Reef Ball units remained stable, and the originally installed configuration of the Gran Dominicus project remained intact following these two severe storm events. Underwater inspection of the Reef Balls on 20 November 1998 indicated that none of the Reef Ball units moved from their placed positions.
Comparison of photographs taken before and after the installation of the Reef Ball artificial reef submerged breakwater show that sand had accreted and the beach was building prior to Hurricane Georges. The storm surge and wave conditions accompanying this category three hurricane greatly exceeded the levels for which the submerged breakwater can provide wave attenuation and shoreline protection, so that significant erosion of the beach and dune occurred, as shown in Figure 4.
Figure 4. Eroded beach following Hurricane Georges and Mitch.
Photograph taken in November 1998 at center of Gran Dominicus, and the offshore breakwater with the western gap and PVC marker pile can be seen offshore.
This submerged breakwater was not designed to be effective during the storm surge conditions accompanying hurricanes. Hurricanes Georges and Mitch produced meteorological and oceanographic conditions that created elevated water levels due to the storm surge and waves. On top of this storm surge, the large waves and strong currents accompanying the hurricanes produced severe erosion of the shoreline, with large volumes of beach sand washed away, leaving a one to two meter high escarpment which undermined several palm trees in the project area and adjacent beaches to the east. The shoreline that had been sand prior to the hurricanes consisted of an exposed rock ledge. Much of the sand in the project area was transported down the cost to the west, and was trapped between the larger headlands of the Dominicus public beach, so that this beach is very wide with a large volume of sand, even after the effects of the hurricanes.
The Gran Dominicus Beach Resort was scheduled to open in December 1998, so that the sudden loss of beach was catastrophic. A site visit in November 1998 was performed, with a report and recommendations submitted. The report recommended the addition of sand to the beach as the most direct way of establishing the beach, with the natural post-storm beach recovery, aided by the Reef Ball breakwater, expected over the longer term. The addition of large rocks to widen and increase the wave attenuation of the Reef Ball breakwater was also discussed.
Gran Dominicus added some sand to the beach, and placed a considerable amount of small rocks seaward, landward, and on top of the submerged Reef Ball breakwater, as shown in Figure 5. This photograph shows that small rocks were used, instead of the larger rocks that would be stable under wave attack. In addition to widening and raising the elevation of the breakwater above the water level, rocks were added to connect the eastern end of the breakwater to the shore, as shown in Figure 6.
Due to the use of the smaller rocks that were added to the Reef Ball breakwater, wave action easily displaced the rocks, scattering them landward of the Reef Ball. Figure 7 shows two aerial photographs that deliniate the original Reef Ball breakwater from the scattered smaller rocks.
Figure 5. Placement of smaller rocks on top of Reef Ball breakwater.
Photograph taken in February 1999 during construction.
Figure 6. Rock placement on top of Reef Ball breakwater and connected to shore.
Photograph taken in February 1999 during construction.
Figure 7. Deliniation between the orignal Reef Ball breakwater and smaller rocks.
Aerial photographs taken April 28, 2001.
The photographs in Figures 8 through 10 show that the Reef Ball breakwater has been very effective in assisting with the stabilization of the shoreline, with a significant increase in beach width and elevation along the Gran Dominicus shoreline. In fact, more sand is accreting than is required for the recreational beach use, so that Gran Dominicus has removed sand from the beach area and stockpiled further landward. This sand can be used to improve the beach for Phase 2 of Gran Dominicus, and for repairing the beach landward of the breakwater if necessary following a severe storm event.
As shown in these photographs, the removal of the smaller rocks landward of the Reef Ball breakwater would greatly increase the available swimming area between the shoreline and the breakwater. The gap widths have greatly been reduced due to the smaller rocks (western gap has been reduced from 20m to 4m and eastern gap reduced from 12.5m to 5m), and removal of the rocks in the gaps between the breakwaters would restore the gaps to their original size. Therefore the smaller rocks landward of the Reef Balls and in the gaps between the Reef Ball breakwater segments should be removed as soon as possible. This may reduce the effectiveness of the breakwater slightly, but this may help with slowing the accretion of sand and increased beach width beyond what is required for Gran Dominicus.
More monitoring of the project and more maintenance of the beach are highly recommended measures to ensure the continued beach use and swimming areas for the Gran Dominicus Beach Resort. It is recommended that the existing Reef Ball breakwater be restored to its original design and installed configuration, by removing the smaller rocks that were added after Hurricanes Georges and Mitch. Beach maintenance should be performed to remove sand from the beach area if the beach width continues to increase and reduce the swimming area.
The construction of a larger breakwater system further seaward of the existing Reef Ball breakwater is not recommended due to the high cost and satisfactory performance of the existing system. An additional one or two rows of Reef Ball units can be added immediately seaward of the existing 3 rows of Reef Balls, if added width and height of the breakwater is found necessary after the removal of the smaller rocks.
Figure 8. Increased beach with at Gran Dominicus looking west.
November 1998 (top photo) compared with April 2001 (lower photo)
Figure 9. Increased beach with at Gran Dominicus looking east.
Summer 1998 (top photo) compared with April 2001 (lower photo)
Figure 10. Increased beach with at Gran Dominicus.
February 1999 after small rocks added on top of breakwater and sand fill (top photo), compared with April 2001 showing large natural acretion (lower photo).