Dredging & Marine Construction

La Quinta Aquatic Habitat Flourishing in Corpus Christi Bay

In 2016 and 2017, the Port of Corpus Christi Authority (PCCA) and engineering company Mott MacDonald created 35 acres of shore and marsh grass habitat in a portion of Beneficial Use Site 6 (BUS-6) in Corpus Christi Bay. Named the La Quinta Aquatic Habitat Mitigation Project, it furthered the 200-acre site’s purpose as prime marsh and seagrass habitat.

The team submitted the project to the Western Dredging Association (WEDA) Environmental Commission as a candidate in its 2018 Mitigation or Adaptation to Climate Change category.

Marsh excavators building the habitat berms for planting Spartina alterniflora grass.

BUS-6 is a beneficial use island adjacent to the shipping channel. It was built to receive the new work material from the La Quinta Channel Extension Project that extended the La Quinta shipping channel by 1.4 miles, matching the 47-foot depth, with a width of 400 feet, including creating a turning basin fronting the La Quinta Gateway Multi-Purpose Terminal, which is now under construction.

More than 7.7 million cubic yards of material was dredged in the channel extension and deepening projects, and most was used to build BUS-6 and its levees, which include an earthen levee approximately nine feet high and 9,200 feet long. The creation of habitat conducive to the growth of seagrass and fringe estuarine marsh habitat is ongoing and will provide a location for future mitigation projects.

The La Quinta Aquatic Habitat Mitigation Project had as its goal to exceed regulatory requirements by creating more than 6.6 acres of marsh habitat and 19.2 acres of shoal grass habitat. This goal was exceeded as of March 2017 when 12.6 acres of smooth cordgrass and 25.3 acres of shoal grass were planted onsite.

During the 2014/2015 channel deepening, new work material was strategically placed in BUS-6 to be available for the creation of the marsh habitat. During Phase I of the mitigation project, PCCA and Mott MacDonald re-used this material to create habitat berms that were constructed and planted at elevations conducive to survival and propagation of smooth cordgrass.

In 2017 during Phase II, material from an upland dredged material placement area (DMPA) was beneficially re-used by hydraulically pumping it across the adjacent navigation channel to construct additional habitat berms and a geotextile-covered earthen protection berm to shelter the new marsh habitat from erosive waves and currents.
Mott MacDonald was contracted by PCCA to design and manage the project.

SMOOTH CORDGRASS
The smooth cordgrass Spartina alterniflora is a marsh grass that is common to the Corpus Christi Bay area. The team prepared planting sites, or berms, at elevations conducive to the survival of this grass in BUS-6, anticipating sea level rise in the next 20 years, as well as wave action within the site.

The team also evaluated coastal conditions, including storm waves and overtopping for the design of a protection structure, to ensure that conditions around the mitigation berms would remain below the threshold for Spartina growth. The platforms on which the grass was planted were designed to be partially inundated throughout each tidal cycle, and each included a crest to remain above water at high tide, to maintain growth as the sea level rises. The berms were surrounded by channels to promote water circulation throughout the site.

Three marsh excavators were brought in by Apollo Environmental Strategies Inc. to construct the mitigation berms, using the material that had been placed in BUS-6 for this purpose.

In 2016, Apollo team members transplanted Spartina alterniflora from the adjacent shoreline at the site of the La Quinta Multi-Purpose Terminal, onto the mitigation berms.

In Phase II of the project, a protection berm was incorporated around the mitigation area, covered with a geotextile fabric apron and ultraviolet light protection shroud to increase the durability of the structure. Phase II also included building an additional two acres of mitigation berm.

PROTECTION BERM

Operations on upper placement area PA-14 for material to create a protective berm around the mitigation area

To build the protection berm, contractor Shoreline Foundation Inc. (SFI) chose to pump coarse sands from upland placement area PA-14 through 1.5 miles of polyethylene pipeline.

SFI began pumping from PA-14 in October 2016, deploying excavators to redistribute the material as necessary to the protection and mitigation berm templates, after surrounding the work area with silt curtains to protect adjacent habitats.

Once the pumping operations were complete, installation of the geotextile scour apron began.

The geotextile scour apron was delivered in rolls of 100-foot sections, which were placed atop the berm as the underlying material was graded. Adjacent sections of geotextile were overlapped to ensure even coverage. The anchor tubes were filled by dredging excess material from the berm template into the tubes using a small slurry pump with a four-inch intake and outfall hose. Each anchor tube included several fill ports to enable uniform filling of the tube. These were then buried as necessary to construct the habitat bench.

Meanwhile, a bulldozer graded the berms to their target elevations, and an excavator created the channels. The last section of scour apron was installed in March 2017.

Spartina planting began in March 2017 on the aquatic habitat mitigation berms, and the transplants were inspected after installation and 60 days after planting. The survival survey showed that the units exceeded 50 percent survival and were uniformly distributed throughout the site.

HURRICANE HARVEY
Hurricane Harvey made landfall on August 26, 2017, approximately 25 miles east of BUS-6. Winds exceeded 100 miles per hour, bringing storm surges over five feet high, which overtopped the BUS-6 protection berm. But the berm dissipated the storm waves and the aquatic habitat mitigation berms and plants were not damaged.

Some sections of the scour apron were partially displaced, but the geotextile remained intact. The Spartina was unaffected and continued to flourish onsite largely due to the success of the protection berm design.

The seagrass Halodule wrightii was planted as shoal grass on 23.2 acres within BUS-6. This project proceeded independently of the earthwork activities and began prior to October 2016. The planting contractor selected the area for planting, avoiding several locations that had been naturally colonized with seagrass. Another two acres of shoal grass were planted around the aquatic habitat mitigation berms.

The initial 18 acres of shoal grass was completed by late fall 2016. In March 2017, more than 50 percent of the transplanted units had survived the winter and were flourishing. The two acres planted around the mitigation berms were also flourishing. Other species of seagrass, including Halodule wrightii, had appeared naturally in other location of the site by Spring 2017.

CLIMATE CHANGE ADAPTATION
The La Quinta Aquatic Habitat Mitigation project mitigates and adapts to climate change in several ways, said Luis M. Maristany, Mott MacDonald team leader.

The creation of several acres of seagrass and marsh habitats help sustain several other species and act as carbon sinks, he said. Smooth cordgrass often grows as a narrow strip of vegetation (fringe) extending from the higher marsh to the water but can grow in large fields near the heads of tidal creeks, which is common in the Corpus Christi area.

“Through expertise in coastal engineering and marsh creation we were able to optimize the berm design to create large fields of smooth cordgrass through careful planning and analysis of conditions onsite. The project has produced a large area where this vegetation is growing in volumes and densities not typically seen in the area,” Maristany said.

“Then there is the structural component of BUS-6, which protects the adjacent shoreline from waves and storm impacts,” he said. “Reduced wave action reduces sediment transport, which reduces maintenance dredging with its accompanying greenhouse gas emissions,” he explained.
BUS-6 is complete and will remain as is in the future, though there may be additional plantings. No additional dredged material will be placed in BUS-6, as it is now functioning as prime marsh and seagrass habitat, Maristany added.

Mott MacDonald team members on this project were: Luis M. Maristany, P.E., Coastal Engineer; Aaron G. Horine, P.E., Principal Coastal Engineer, and Paul D. Carangelo, REM, CESM, PWS, Coastal Environmental Planner.

Maristany presented the paper “La Quinta Terminal Mitigation Project: Large Scale Dredged Material Beneficial Re-use Facility for Estuarine Habitat Creation in Corpus Christi Bay, Texas” at the organization’s conference and expo in Norfolk, Virginia, in June.

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