Biogenic Reefs – A Living Defense For Tangier Island
Biogenic oyster reefs offer something conventional shoreline protection cannot: they are living structures capable of growing and adapting over time. Oyster larvae attach to existing shells and other hard surfaces. As generations of oysters grow on top of one another, the reef becomes larger, higher, rougher, and more complex. This three-dimensional structure disrupts incoming waves, reduces wave energy, encourages sediment to settle behind the reef, and can help protect marshes and shorelines from erosion. NOAA recognizes oyster reefs as a nature-based tool that, when properly located, can reduce wave energy, prevent erosion, and strengthen wetlands as protective barriers.
Perhaps most important for Tangier Island is the reef’s ability to grow vertically as sea level rises. In a landmark 2014 study published in Nature Climate Change, University of North Carolina researchers measured intertidal oyster-reef growth and found rates as high as approximately 11 centimeters—or more than four inches—per year under favorable conditions. The researchers concluded that oyster reefs have the potential to accrete rapidly enough to outpace projected rates of sea-level rise while submerged, eventually adjusting their growth as they reach their preferred intertidal elevation.
That makes an oyster reef fundamentally different from a seawall or static breakwater. Instead of simply deteriorating with time, a successful biogenic reef can continually add new oysters and shell, potentially increasing its protective capacity.
This concept is already being applied on a major scale through New York’s Billion Oyster Project, which is working to restore one billion oysters to New York Harbor. The organization specifically identifies shoreline protection among the benefits of restored reefs, including softening large waves, reducing flooding, and preventing erosion.
For Tangier Island, Marine scientists Russ Burke, Dave Schulte, and Romuald Lipcius became heavily engaged in large-scale native oyster restoration in the Chesapeake Bay in the early 2000s, and were soon joined by Rochelle Seitz, with her focus on living shorelines and expertise on secondary production. Collectively, they have evaluated nearly every type of shell (oyster, clam, whelk, etc.) and alternative substrate (granite, limestone, recycled and/or crushed concrete, and pre-fabricated concrete structures with or without an oyster shell veneer) oyster reef in Virginia waters, including on the Bayside and Seaside Eastern Shore, for oyster and mussel production, as well as for secondary production. In particular, Russ Burke has been investigating the viability of shell-embossed concrete structures for oyster and fish reefs, as well as for wave attenuators – we call these reefs “Biogenic Breakwaters.”
Three types of Biogenic Breakwater Reefs will be deployed along the shore to Protect Tangier Island.
X-Reefs
X-Reefs are innovative, nature-based structures designed to reduce wave energy, protect vulnerable shorelines, and create productive marine habitat. Their open, three-dimensional form allows water to move through and around the reef while creating turbulence that helps dissipate the force of incoming waves before they reach nearby beaches and marshes.
A key feature of the X-Reef is what happens as oysters begin to colonize it. The exposed legs and outer surfaces provide a hard substrate where oyster larvae can attach. As successive generations of oysters grow on the structure, their shells build outward and begin covering more of the reef’s exterior. This creates a rougher, thicker, and increasingly complex surface that can further disrupt wave flow, create additional turbulence, and provide more places for new oysters to attach.
Over time, the original manufactured reef can become the framework for a much larger living oyster reef. Layers of oyster shell can increase the effective size, surface area, and biological complexity of the structure while creating habitat for fish, crabs, and other marine life.
For Tangier Island, X-Reefs are intended to work with restored marsh, dredged sediment, granite sills, and other reef structures to create multiple layers of shoreline protection. As oyster growth expands over the reefs, the system has the potential to become stronger, more biologically productive, and more effective at reducing wave energy over time.
Atlantic Reefmaker
Atlantic Reefmaker (ARM) is a nature-based wave attenuation system allows water to flow through it while providing similar wave protection characterized by a traditional rock breakwater system (TRBS). The system is pile-based system that consists of concrete disks, referred to as ‘ecodisks’, that can be perched above the substrate by a mechanical support system (Figs. D12-D15). Square and octagonal disks have been created and installed.
The legs, or paddles, on the ‘water front’ side, or open water/river side of the structure, of the ecodisk direct water to the fiberglass piling. The wave energy through the structure can be controlled by how close or far the paddle is to the pile. When wave energy comes toward the structure with a typical 20% porous ecodisk, the paddles direct water energy to the pile where becomes concentrated and is forced to travel around the piling. When the water reaches the ‘landward’ side of the structure, the paddles direct the water outward, and water exits the structure. The wave energy dissipates as travels through the ecodisk because the flow is no longer concentrated. If the waterbody has a high sediment load, then, with a 20% porous structure, sediment in the water column will drop out, accreting in the area between the ARM structures and the shoreline.
The ARM structure has several advantages over other wave attenuation products and a TRBS. These advantages include: 1) working in horizontally-limited areas; 2) dissipating destructive wave energy;
3) working in high-energy environments; 4) providing habitat for marine fauna, both sessile and motile;
5) having the base unit setting above the substrate, thereby minimizing scour and sand/sediment re- distribution; 6) minimizing its ‘footprint’ to the substrate as impacts are limited to the fiberglass pile itself (12” diameter fiberglass pile equates to 0.785 ft2 of substrate impact per piling);
7) allowing for ‘flushing’ along the entire shoreline and between the open water and shoreline; 8) allowing for modular construction, which enables easy adjustments to accommodate for sea level rise by adding one or more ecodisks to the ARM structure without the need for additional environmental permitting or compensatory mitigation; 9) adjusting the structure location as it can be easily moved and relocated or removed; 10) modifying the porosity of the structure per design criteria, site conditions and/or along the piling; and 11) providing the ability to adjust the structure layout to irregular shorelines.
The modular construction of the structure provides other design options for a coastal designer. These options include varying the porosity of the structure along the pile itself to accommodate the designer’s preferred wave energy environment. An example includes having the 0% porous structure at the substrate and 20% porous structure at the top of the pile in order to manage the wave energy that flows through the ecodisks.
The other option, referred to the ‘lollipop’ version, involves setting the ecodisks into deeper water. With this example, the ecodisks are only set at the designed elevation in the upper water column, via the mechanical support system, where the wave energy exists. This ‘lollipop’ version of the structure means that ecodisks are not located along the pile from the substrate to top of the crest, such in 10’ deep of water, but the ecodisks would be focused only in the upper 4’ of the fiberglass pile, not the entire 10’ of pile above the substrate.
Additionally, the product consists of fiberglass rebar instead of steel rebar, as fiberglass rebar maintains a more constant cost than steel rebar. The fiberglass rebar also does not contract and expand with cooling and heating processes as steel rebar does. This feature will enable long-term structural durability. Sustaining Productive Fisheries and Strengthening Ecosystem Resilience
The ARM product was used to wave attenuation for seagrass coalescing and also essential fish habitat for the Bonner Bridge Seagrass project in Pamlico Sound, NC (Fig. D15). There has been tremendous seagrass coalescence at this project, and the initial mitigation requirement seagrass coverage has been exceeded as the five-year monitoring plan has been completed.
Resident reef fishes find refuge within the interstitial areas of the ecodisks, and the surface area of the ecodisk maximizes the opportunity for oyster community settlement and recruitment (Fig. D14). The perched nature of the ARM provides refugia for predator fish. ARM is the only structure in the Pamlico Sound and attracts not only the noted marine species, but also fishermen.
ARM was used as an alternative to TRBS at the Brunswick Town/ Fort Anderson in Brunswick County, NC (Figs. D12-D15). The structure dissipates vessel-generated wakes (VGW), and this protection of the coastal marsh has yielded many ecological benefits. This includes the natural recruitment and expansion of marsh grass along the protect shoreline whereas it was being washed away. The blue crab population at the site has grown tremendously, to the point that 46 crab pots were counted just offshore of the BTFA shoreline in July 2020. There are also two species of oysters growing on the ecodisk, along with barnacles, this site is the furthest inland colony of oysters within the Cape Fear River basin. As with Bonner Bridge, there has been in increase in fishery species utilizing the interstitial area of the ecodisks, and the site is a local recreational fishing hotspot.
The coastal marsh at BTFA (Figs. D12-D14) has been re-building in two ways. As water leaves the ecodisk, the wave energy is no longer concentrated and, therefore, any entrained sediment in the water column will
settle out of the water column and accrete on the shoreline. With 2013 as the baseline and through June 2021, the shoreline has extended river ward behind the Phase 1 and 2 areas. These changes include 2 feet of accretion behind the Phase 1 structure and 1.5 feet of accretion behind the Phase 2 structure.
There has also been a natural thin layer placement within the existing marshes at BTFA. After flood waters flow through the system to shore, it has to exit the flooded marsh to the river the same way that it entered – through ARM. The sediment in the flood waters settle out of the water column, and they are depositing to re-build the marsh.
Enhancing Community Resilience to Climate Hazards and Providing Other Co-benefits
ARM has the capability to provide storm surge protection to coastal environments and communities. The structure has been designed to sustain wave energy associated with large transoceanic vessels loaded with containers. The structure has withstood sustained high tide storm surges associated with tropical storms, particularly Hurricane Florence in 2018, at BTFA without any damage to the structure or required maintenance. At the Bonner Bridge project, the structure has sustained impacts associated with nor’easters and tropical storms, as well as 3-5 feet inundation associated with Hurricane Dorian in 2019, without any damage to the structure or required maintenance.
The successful wave attenuating capabilities of ARM have been documented via lab studies and field studies (Fig. D18). Lab studies were performed by the US Army Engineer Research & Development Center in Vicksburg, MS. The square and octagonal ecodisks were studied in a three settings: 1) like a typical TRBS, 2) submerged and 3) lollipop. Data from these studies will be used to refine project design. The study also modeled the 20% porous structure and a rotated ecodisk to mimic the 0% porous structure. Data from field studies conducted at BTFA used data from stationary buoys to collect the wave energy transmission between the river and shore side of the structure. This study delineated VGW and fetch energy, and collected data for the square ecodisks. For outgoing VGW, the ecodisks, on average, reduced energy by 68%. For all of the fetch events, there was 78% wave energy reduction and, for large storm events, wave energy dissipation averaged 68%.
Atlantic Reefmaker, LLC’s reef design attenuates wave energy and promotes oyster community development; note the Brunswick Town/Fort Anderson project at high tide (Photo Credits: Atlantic Reefmaker, LLC).
Sill Reefs
Our Reef Sill living shorelines combine the aesthetics and ecological benefits of an oyster shell bag shoreline with the stability of heavier materials and eliminates the use of plastic.
In an oyster-rich environment, we’ve created an oyster reef using the same chemical compound as an oyster shell, and our arrangement gives oyster spat an advantage to mature. Protected from shorebirds and other predators, The Reef Sill interstitial spacing provides the ideal nursery for them to flourish.