POST-HURRICANE SITE INSPECTION
OF THE
REEF BALL ARTIFICIAL REEF SUBMERGED BREAKWATER
AT
GRAN DOMINICUS BEACH RESORT
NEAR LA ROMANA, DOMINICAN REPUBLIC23 November 1998
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
Introduction
Approximately 450 Reef Ball artificial reef units were installed offshore of the Gran Dominicus Resort during the summer 1998. This report presents the observations of the Reef Ball artificial reef submerged breakwater during site inspections performed on 20 and 21 November 1998, following the direct hit by Hurricane Georges in September and the large swell waves from Hurricane Mitch in October, including recommendations for post storm restoration of the beach area.
Reef Ball Unit Stability
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.
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.
Underwater inspection of the Reef Balls on 20 November 1998 indicated that none of the Reef Ball units moved from their placed positions. 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.
Shoreline Conditions
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.
Hurricanes Georges produced meteorological and oceanographic conditions that created elevated water levels due to the storm surge. On top of this storm surge, the large waves and strong currents accompanying the hurricane 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 covered with sand prior to Hurricane Georges now consists 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.
Slow natural recovery of the beach is occurring, with some sand returning to parts of the shoreline, and sand has accumulated up to 0.3m thick in the offshore region between the shoreline and the breakwater. However, this post-hurricane recovery is expected to be slow, especially due to the large volume of sand that was lost in the Gran Dominicus project area and beaches to the east, and the resulting steep beach slope and dune escarpment.
Summary
Prior to the two major hurricanes, Georges and Mitch, the Gran Dominicus project area had a substantial sandy beach, but this was severely eroded due to these two major storm events. A large volume of sand was eroded from the beach in the project area and the beaches to the east. The Reef Ball artificial reef submerged breakwater survived the two major hurricane events with no displacement of the units. The breakwater does provide wave attenuation during normal sea conditions, as evidenced by the build up of the beach following its installation. However, with the severity of the erosion that occurred due to the two major hurricanes, the beach is not expected to recover sufficiently prior to the opening of the Gran Dominicus Beach Resort in December 1998.
Respectively submitted: ___________________________________________
Date: 23 November 1998 Lee E. Harris, Ph.D., P.E., consulting coastal engineer
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