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LARGE-SCALE SEAGRASS MITIGATION FOR MIAMI HARBOR DREDGING

The Value of Navigation Dredged Material to Ecosystem Restoration & Coastal Resiliency

By: T. JordanSellers, Senior Biologist, U.S. Army Corps of Engineers, Jacksonville District, Environmental Branch

E.P. Summa, Chief, Planning Division, U.S. Army Corps of Engineers, Jacksonville District

B. Hope, Environmental Manager, PortMiami

C.J. Kruempel, Director of Coastal Services, Tetra Tech, Inc.

D. Nelson, Project Engineer, Great Lakes Dredge and Dock

A. McCarthy, Senior Scientist / Project Manager, CSA Ocean Sciences Inc.

M.S. Fonseca, Ph.D., Vice President, Science, CSA Ocean Sciences Inc.

S.R. Conger, Engineering Technical Lead, U.S. Army Corps of Engineers, Jacksonville District

C. Pomfret, Senior Project Manager, Great Lakes Dredge and Dock, LLC.

 

In 2015, over 118,000 seagrass plants were transplanted into a newly filled 17.1acre dredge hole in Miami, Florida. The Project  involved filling the historical dredge hole with navigationdredged material and seagrass planting as part of environmental mitigation  requirements for the deepening and widening of Miami Harbor, Phase III. It is on course to be one of the largest and most successful  actively planted seagrass mitigation projects to date.

PortMiami is the second largest economic driver in Miami-Dade County and is currently estimated to contribute $28 billion annually to south Florida’s economy. The Port has long been recognized as a vital portal for trade and commerce in the U.S. In 1990, Congress recommended specific improvements to PortMiami to enhance operational efficiency and safety of deep draft commercial vessels by providing a deeper channel with widening in certain areas. This recommendation was done in response to a request by PortMiami and the Miami-Dade County Seaport Department, working through the House Committee on Transportation and Infrastructure, to study the feasibility of improving navigation in Miami Harbor. The Port and other stakeholders believed that the navigation project could also reduce vessel operation costs, resulting in transportation cost savings. On October 29, 1997, through a resolution, the House Committee, in turn, authorized the U.S. Army Corps of Engineers to conduct the study and provided funding to begin the study in 1999.

Figure 1. Miami Harbor Deepening Project .

Figure 1. Miami Harbor Deepening Project .

FEASIBILITY STUDY AND SITE SELECTION

As part of the Feasibility Study and Environmental Impact Statement (EIS) process under the National Environmental Policy Act (NEPA), the project area (Figure 1) was evaluated to identify potential impacts and develop proposed avoidance and minimization measures. The process determined unavoidable direct impacts to approximately 0.2 acres of seagrass habitat and 7.7 acres of indirect impacts to seagrass habitats in Biscayne Bay that border the Port’s channels (Figure 2). These unavoidable impact predictions resulted in a recommendation that compensatory mitigation be included to replace the lost functions associated with those seagrasses.

Figure 2. Estimated Seagrass Impacts on the South Side of Fisherman’s Channel

Figure 2. Estimated Seagrass Impacts on the South Side of Fisherman’s Channel

Photo 1. Manatee grass (Syringodium filiforme) in Biscayne Bay.

Photo 1. Manatee grass (Syringodium filiforme) in Biscayne Bay.

Seagrasses (Photo 1) play a very important role in South Florida bay ecosystems by stabilizing sediments, filtering the water column, producing vast amounts of organic detritus, and sequestering large amounts of carbon. Additionally, these seagrasses serve as nursery habitat for commercial fish and invertebrate species like spiny lobster (Panulirus argus) as well as foraging and resting habitat for threatened and endangered species like the green sea turtle (Chelonia mydas) and the Florida manatee (Trichechus manatus latirostris). Biscayne Bay is designated critical habitat for the manatee (Photo 2) and the threatened Johnson’s seagrass (Halophila johnsonii). The bay also contains a large state Aquatic Preserve, and is designated as “Outstanding Florida Waters.” These classifications and associated regulatory protections were critical to the seagrass mitigation area selection process. Through an extensive series of meetings with local, state, and federal resource agency staff, filling of a previously dredged borrow area (circa 1958) in northern Biscayne Bay was identified as the best alternative for the required mitigation.

Photo 2. Florida manatee (Trichechus manatus latirostris) in Biscayne Bay.

Photo 2. Florida manatee (Trichechus manatus latirostris) in Biscayne Bay.

To replace lost seagrass functions and values, active restoration by filling approximately 24 acres of a 100+ acre borrow area, located north of the Julia Tuttle Causeway in northern Biscayne Bay (Figure 3) was selected as the preferred approach. The “Julia Tuttle” hole was dredged 1958-59 as fill material for the Julia Tuttle Causeway connecting Miami and Miami Beach, essentially creating a barrier to water movement and altering flow and subsequently water quality. Due to the depth of the hole in concert with low currents, flushing rates, and high nutrient content, the hole had not naturally filled with sediment over the intervening halfcentury and, thus, not re-colonized from the extensive remaining seagrasses to the north and south of the hole to recolonize the area.

Figure 3. Julia Tuttle Dredge hole prior to filling.

Figure 3. Julia Tuttle Dredge hole prior to filling.

Bathymetric surveys (Figure 4) and other evaluations of the dredge hole, which had once been conterminous seagrass habitat, were conducted to determine the most appropriate area for filling based on depth, current usage, and other planned projects. The site was historically referred to as “Cadillac hole,” as supposedly it had been used as a dumping area for vehicles, and a portion of the area is currently an active 55-acre artificial reef site utilized by Miami-Dade County’s Division of Environmental Resources Management for placement of bridge rubble and pre-cast concrete modules.

Figure 4. Bathymetry of the Julia Tuttle Dredge hole.

Figure 4. Bathymetry of the Julia Tuttle Dredge hole.

PRECONSTRUCTION, ENGINEERING AND DESIGN PHASE

After completion of the NEPA process in 2004 and execution of the federal Record of Decision (ROD) in 2006, the Project moved into the “Preconstruction, Engineering and Design” phase during which plans and specifications are developed and Florida Department of Environmental Protection (FDEP) authorization, including federal and state consistency determinations, is obtained.

As part of the permitting process, the FDEP and USACE conducted a “Uniform Mitigation Assessment Method” (UMAM), or functional analysis of the impacted seagrass beds, to determine the minimum amount of mitigation necessary to compensate for the loss of natural communities. The final seagrass mitigation requirement area was reduced from filling of 24 acres of the dredge hole without planting to filling at least 16.6 acres of the dredge hole, with planting 7.15 acres of the site with locally sourced manatee grass (Syringodium filiforme).

Concerns were raised over turbidity generated during filling operations at the seagrass mitigation site and potentially affecting seagrasses adjacent to the restoration site. In response, the FDEP permit special condition limited the use of dredged material to cap the dredge hole to .15% silts and clays, or the use of quarried material from upland sources with .5% silt and clays. To further minimize turbidity potentially affecting the surrounding habitat, the installation of turbidity curtains and monitoring of turbidity were required to determine compliance with the established threshold of 9 Nephelometric Turbidity Units (NTUs) above background levels. Any documentation of exceedances of the compliance limits would require modification of the fill placement process.

CONTRACT PLANS AND SPECIFICATIONS

Concurrent with the development of the FDEP permit and consultation under the Endangered Species Act, the USACE developed performance-based specifications, including incorporation of permit special conditions and environmental commitments cited in the EIS. Consultation under the ESA resulted in a pre-construction survey (Figure 5) requirement for the presence of the threatened Johnson's seagrass (Halophila johnsonii) within the Julia Tuttle Mitigation Site (JTMS). If Johnson's seagrass was not located, the final design for the fill site location could move closer to the edge of the adjacent seagrass beds, thereby potentially increasing the size of the mitigation acreage and the rate of natural colonization.

Figure 5. Pre-construction seagrass survey to document presence of Johnson’ seagrass (Halophila johnsonii).

Figure 5. Pre-construction seagrass survey to document presence of Johnson’ seagrass (Halophila johnsonii).

The permit and contract plans required the construction of a 16.6-acre mitigation site with a base fill elevation of -6 ft MLLW capped with an average of 2 ft of “select fill” (±0.5 ft) and a final target elevation of -4 ft MLLW maximum (Figure 6). The contract further required the use of weighted turbidity curtains to contain dredged material, minimize turbidity, and protect mobile protected species such as manatees and sea turtles from potential construction impacts and entanglement.

Figure 6. Typical cross-section of the Julia Tuttle Mitigation Site.

Figure 6. Typical cross-section of the Julia Tuttle Mitigation Site.

SITE DESIGN

The performance-based contract specifications intentionally provided the contractor with flexibility in siting and construction techniques to allow efficient and effective site restoration. Great Lakes Dredge and Dock, LLC (GLDD) was awarded the contract and began final site design soon after the USACE issued the Notice to Proceed on July 2, 2013. The requirements for the base fill excluded material with >15% fine silts and clays but put no upper bound on the particle size. The broken limestone rock from the harbor deepening was chosen for base fill based on its lower turbidity potential in the fill site, abundance and stability. The final design was centered in the widest section of the permitted area to minimize the perimeter of the site and meet -4 ft MLLW elevation target.

SITE CONSTRUCTION

Curtains and Pilings

During the mitigation design phase, GLDD engineers determined that construction of the JTMS would require more turbidity curtain (in linear feet) than they had ever deployed at one time on a project and that the original design to use anchors in the seabed around the site was not feasible. Instead, they opted for vertical piling stabilization of the curtains. GLDD installed 74 steel pilings approximately 97 ft apart to construct a steel pile perimeter around the site to which were affixed 100-ft long contiguous sections of turbidity curtains (Photo 3). Unanticipated rigorous and ongoing maintenance and replacement of the curtains throughout the 21-month construction period was required due to the much shorter (3-month) design life of the curtains.

Photo 3. Aerial photograph of turbidity curtains and support pilings surrounding the Julia Tuttle Mitigation Site.

Photo 3. Aerial photograph of turbidity curtains and support pilings surrounding the Julia Tuttle Mitigation Site.

Endangered manatees (T. Manatus latirostris) were commonly seen entering and exiting the curtained area before, during, and after placement operations and during seagrass planting. The curtains did not appear to hinder manatee access to and from the site and they were seen feeding on algae that had grown on the curtains (Photo 4).

Photo 4. Florida manatee feeding on algae growing on the turbidity curtains.

Photo 4. Florida manatee feeding on algae growing on the turbidity curtains.

Base Fill

Because Johnson’s seagrass was not documented, the Project fill area shifted north, closer to the existing S. filiforme bed margin, resulting in more restored acreage per unit of placed material—totaling 17.1 acres, or an additional 0.5 acre over the 16.6 required acreage. The majority (approximately 85%) of the required volume was derived from dredged rock from within the dredging Project limits (termed “base fill”). Approximately 560,000 cubic yards (CY) of excavated dredged material were transported in split-hull scows using push tugs to navigate to the JTMS hole. In total, 1,192 scow loads of base fill, limited to <1,000 CY/scow due to depth limitations, were placed in the hole between January 8 to October 19, 2014. The final grade for the base fill to a uniform depth of -6 ft MLLW.

Photo 5. Placement of select fill material into the Julia Tuttle Mitigation Site.

Photo 5. Placement of select fill material into the Julia Tuttle Mitigation Site.

Select Fill

The GLDD team determined that the sediment characteristics of the materials to be dredged from the Project limits were not of a consistent quality to ensure compliance with the technical specifications for the select fill (<5% fine silt and clay), requiring them to obtain the fill from a local quarry. A total of 85,000 CY (114,425 tons) of select fill material was transported on 285 barge loads from January 8 through July 7, 2015 and placed within with JTMS (Photo 5). A final grade of -4 ft MLLW was confirmed by a USACE bathymetric survey after smoothing the area with a drag bar.

Turbidity Monitoring

Turbidity exceeded the 9 NTU above background threshold five times during the 19-month fill site construction period. In compliance with the FDEP permit, when an exceedance was documented, fill operations ceased until water quality measurements confirmed that turbidity had returned to levels below background conditions. The turbidity sampling was done multiple times per day while placement was ongoing, and the contractor adaptively managed material placement based on the monitoring information to avoid violating the 9 NTU threshold limit.

Photo 6. Seagrass harvesting device—the “cookie cutter.”

Photo 6. Seagrass harvesting device—the “cookie cutter.”

SEAGRASS RESTORATION

Seagrass Harvest and Planting

Approximately 30 days following completion approval of the fill operations, over 118,000 apical-bearing S. filiforme (manatee grass) runners (rhizomes) were harvested, bundled into 29,000 planting units (PUs), and planted within the JTMS using the “bare-root staple” seagrass restoration method. The source of donor material for the transplants was the adjacent, relatively quiescent S. filiforme habitat north of the JTMS. Divers used a 0. 25-m2 device (“cookie cutter”) (Photo 6) to harvest S. filiforme rhizomes from discrete harvest sites located along pre-defined transects with a minimum of 2 m separation between each site. Divers manually lifted plants from the sediment within the cutter by hand, shook out sediment from the root mat back into the excavation and placed the plants in a mesh collection bag (Photo 7) that remained submersed until it was transported to the fabrication team for PU assembly.

Photo 7. Scientists unloading harvested seagrass material from mesh collection bag.

Photo 7. Scientists unloading harvested seagrass material from mesh collection bag.

Donor material was sorted in an onboard flowthrough system that kept the plants continually shaded and wetted with seawater to prevent desiccation. The fabrication team selected vegetative propagules that had either rhizome apicals that had emerged from the plants above the sediment surface (termed “aerial runners”) or sediment rhizome apicals. Rhizome apicals are needed for S. filiforme to spread across the seafloor. Much of the harvested material contained other non-target seagrass species or S. filiforme plants that did not support rhizome apicals and was discarded on the planting site. The apical-bearing rhizomes with several short shoots each (average 4.1 apicalbearing rhizomes PU-1) were bundled into Pus using biodegradable twist ties and a landscaping staple (Photo 8). Completed Pus were placed in a shallow tray until they were transported to scientific divers for planting within the JTMS.

Photo 8. Fabricated seagrass planting unit.

Photo 8. Fabricated seagrass planting unit.

The JTMS planting area was designed with a checkerboard pattern into 290, 10-m × 10-m planting plots, or 29,000 sq. m (7.15 acres) (Figure 7). Each plot was planted by divers on 1- m centers with 100 S. filiforme planting units (PU) using a movable PVC grid. Pus were inserted into the sediment after loosening it with a dive knife just to the depth to bury the rhizomes. Planting and the installation of 1,160 bird roosting stakes as a means to encourage natural fertilization from bird feces was completed within 45 days, with a small interruption due to the threat of a tropical storm.

Figure 7. Checkerboard planting grid.

Figure 7. Checkerboard planting grid.

Monitoring

Post-transplantation monitoring of the site, 30-days on average following final planting, demonstrated that there were no documented detrimental effects to the Pus from the use of quarried select fill material. Despite locally intensive grazing by manatees, PU survival was 97.6%, substantially greater than the FDEP-mandated requirement of 70% survival. There was visible extension of PU rhizomes with the observation of new shoot formation within 2 weeks of planting (Photo 9). The growth trend continued through October 2015, when the posttransplantation survivability survey was conducted.

Photo 9. Growing seagrass within two weeks after transplantation.

Photo 9. Growing seagrass within two weeks after transplantation.

Volunteer recruitment of S. filiforme, Halodule wrightii, Halophila engelmannii, H. decipiens and Thalassia testudinum was documented within the JTMS (Photo 10) with some areas densely colonized. Some volunteer recruitment, particularly the observation of robust T. testudinum plants, may have been facilitated by the disposal of non-target plant material during PU assembly by the fabrication team. Colonization of the site by various species of macroalgae (Halimeda, Batophora, and Caulerpa) and Cassiopeia (upside down jellyfish) was rapid and extensive.

Photo 10. Volunteer seagrass within the Julia Tuttle Mitigation Site.

Photo 10. Volunteer seagrass within the Julia Tuttle Mitigation Site.

CONCLUSIONS

Direct impacts to 0.2 acre of seagrass were predicted to occur as a result of the Miami Harbor dredging with longer-term secondary impacts associated with post-construction slope equilibration to be assessed in October 2016. The JTMS seagrass mitigation Project effectively replaces the seagrass ecosystem functions lost by the Miami Harbor Expansion Project. Barring any unforeseen disturbances, the Project is on course to be one of the largest and most successful actively planted seagrass mitigation projects to date and serves as a demonstration of beneficial reuse of dredged material in a large-scale seagrass restoration effort.

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