Submerged Reef Structures for Habitat Enhancement

and Shoreline Erosion Abatement

by Lee E. Harris¹

ABSTRACT: This paper presents the use of artificial reef structures as submerged breakwaters, providing both wave attenuation for shoreline erosion abatement, as well as artificial reef structures for habitat enhancement. An example of this technology is presented for a project constructed using Reef Ball^TM artificial reef units (shown in Figure 1) along the southern shore of the Dominican Republic near Bayahibe (east of Santo Domingo and LaRomano, as shown in Figure 2) during the summer 1998.

Approximately 450 Reef Ball^TM 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. The individual units used for the breakwater were 1.2m high Reef Ball^TM units and 1.3m high Ultra Ball units, with base diameters of 1.5 and 1.6 meters, respectively, and mass of 1600 to 2000 kilograms. Figure 1 shows an individual Reef Ball^TM unit, which is fabricated to provide void spaces and surface areas for habitat and biological growth. These artificial reef units were deployed by using large buoys inside and strapped to the Reef Ball^TM units to float the individual units offshore and set them in place by divers.

Figure 1. Individual Reef Ball^TM Unit. Figure 2. Three-Row Submerged Breakwater.

The design of the submerged breakwater system consisted of three segmented breakwater sections, using three rows of Reef Ball^TM units for each segment (Figure 2). The breakwater was installed in water depths of 1.6m to 2.0m, so that the units were 0.3m to 0.8m below the mean water level (the tide range in the project area is approximately 0.4m). In the fall of 1998 shortly after the installation of the breakwater system, a direct hit by Hurricane Georges (Category 3) and large waves from Hurricane Mitch (Category 5) impacted the project area, but not a single Reef Ball^TM unit was displaced or damaged. As shown in Figure 3, the beach and shoreline in the lee of the submerged breakwater system has been stabilized and has accreted sand, with no adverse impacts on adjacent beaches.

Figure 3. Increased Beach Width 1998 to 2001 at Center of Project - looking west.

1. Lee E. Harris, PhD, P.E., Assoc. Professor, Florida Institute of Technology, 150 W. University Blvd., Melbourne, FL 32901 USA; Phone: 321-674-7273; FAX: 321-674-7212; E-Mail: LHarris@fit.edu

Figure 4 shows the location of three profile lines surveyed to document the performance of the submerged breakwater system. Shoreline and sand volume calculations are shown in Table 1.The beach profile shown in Figure 5 shows that the Reef Ball^TM breakwater has been very effective in stabilizing and increasing the beach, with a significant increase in beach width and elevation along the project shoreline.

Table 1. Changes in Shoreline and Sand Volume Calculations 1998 to 2001

Profile Line	Shoreline Change (meters)	Sand Volume Change (m^3/m)
West	+10 m	+25.65 m^3/m
East	+13 m	+44.25 m^3/m
Control	0 m	+2.0 m^3/m

Figure 4. April 2001 Aerial Photograph.

Figure 5. Beach Profile across Breakwater near East Gap

The submerged breakwater project presented in this paper demonstrates the technology available to provide shoreline stabilization due to wave attenuation at sites with low tidal range and low to moderate wave climate (except during tropical storms and hurricanes). In addition, the use of artificial reef units for the breakwater provides habitat enhancement for the marine life, which can be enjoyed by divers and snorkelers. Application of this technology to other sites must consider the particular site specific conditions.

Acknowledgements: Support for this paper was provided by the National Shoreline Erosion Control Development and Demonstration Program. For Program information see: http://limpet.wes.army.mil/sec227/