The U.S. Army Corps of Engineers Mobile District has been at the forefront of implementing Thin Layer Placement of sediment as an innovative and sustainable method for managing dredged material in Mobile Bay, Alabama. This approach has become a cornerstone of the District’s strategy for maintaining navigable waterways while safeguarding the environment.
“Thin Layer Placement is a strategic method used to distribute dredged material in open water at a depth of 6 to 12 inches. This approach is part of a broader sediment management strategy and is designed to support ecological recovery,” said Jenny Jacobson, USACE Mobile District Environmental & Resources Branch chief. “Placing material in such thin layers creates an environment that allows for the rapid recolonization of species, enhancing the resilience and health of the ecosystem.”
The District’s commitment to maintaining the integrity of the Mobile Harbor is evident in its continuous monitoring of the channel’s depth. This proactive approach enables the District to optimize maintenance dredging events, ensuring that sediment is effectively managed and the harbor remains accessible for maritime traffic.
“The District actively engages with local communities and environmental organizations in Thin Layer Placement projects,” added Jacobson. “As part of the National Environmental Policy Act, we provide environmental documentation, including Environmental Impact Statements or Assessments, which are shared with the public.”
Approximately 6 million cubic yards of material are annually dredged from Mobile Harbor, with 4 million cubic yards explicitly coming from the Bay Channel. This substantial volume underscores the scale of operations required to keep one of the nation’s key harbors operational.
“Cutter head suction dredging is used in the channel to pump material through a pipeline at 1,500 to 3,000 cubic yards per hour. At the end of the pipeline, a diffuser plate is attached to spread the material evenly. The diffuser, often maneuvered back and forth using winches, ensures that the dredged material is placed in a thin layer, typically between 6 to 12 inches thick,” said Barry Dailey, USACE Mobile District Operations Special Projects manager. “GPS tracking is utilized to monitor and control the placement precisely, ensuring accuracy in the placement process. This technique is essential for controlled sediment distribution, supporting ecological recovery, and enhancing the area’s stability.”
In 2010, the Interagency Working Group was established to explore environmentally acceptable alternatives for in-bay sediment placement. This group, comprised of federal and state resource agencies, scientific experts, and local environmental non-governmental organizations, focused on evaluating sediment placement practices that would maintain sediment within the natural system while being environmentally sustainable.
A pivotal moment for TLP in Mobile Bay came in 2012 when an emergency event necessitated the rapid placement of dredged material within the bay to restore access to navigable channels. In response, USACE, in collaboration with the IWG, swiftly developed guidelines for this emergency placement, leading to the adoption of Thin Layer Placement. The TLP involves placing dredged material in layers no thicker than one foot, which minimizes environmental impact while keeping sediment within the bay’s natural system.
The emergency event provided a valuable opportunity for the USACE to study the effects of TLP in a real-world scenario. Extensive data was collected before, during, and after the placement activities to understand how the dredged material moved within Mobile Bay. This data was critical in developing a predictive model to forecast the material’s behavior under typical seasonal conditions and during extreme weather events, such as hurricanes.
The results of this year-long study, published in the 2014 report titled “Modeling of Thin Layer Placement of Dredged Material in Mobile Bay,” indicated that material placed using the thin layer method is not transported along the bottom as a large sediment mass. Instead, waves and currents remobilize the material into the water column, allowing it to reintegrate into the bay’s natural sediment transport system.
“In 2014, a long-term study demonstrated that TLP significantly benefited the bay by reducing erosion rates,” said Dailey. “The material, dredged using a cutter head dredge, is partially returned to the channel, and some disperse into the bay, but a substantial portion remains in place, helping to elevate the bay’s surface.”
The study found that approximately 65% of the material was widely dispersed by natural waves, winds, and currents, while 35% of the placed material returned to the channel. Moreover, the modeling indicated that dredged material placed in thin layers is approximately 45% less erodible than native sediment.
Building on these findings, the IWG and U.S. Army Corps of Engineers approved a Statement of Findings in 2014, which provided a framework for the long-term use of TLP in Mobile Bay. The plan included specific guidelines to ensure the practice remained environmentally sustainable:
Distance from Channel: Dredged material must be placed at least 2,500 feet away from the channel to avoid interference with navigation.
Layer Thickness: The material should be placed in thin layers, each less than one foot thick, to minimize environmental disruption.
Site Rotation: Placement sites should be rotated every 4–6 years, allowing natural processes to restore and maintain the bay’s health.
This strategic approach to sediment management allows for more effective channel management and maximizes the beneficial use of dredged material, ultimately contributing to Mobile Bay’s environmental resilience.
Thin Layer Placement has gained widespread acceptance as a best practice within both environmental and dredging industries, mainly due to its ability to balance operational needs with ecological responsibility. TLP is recognized by environmental experts and industry leaders as an innovative approach that aligns with sustainability goals, making it a preferred method for managing dredged material in sensitive coastal environments like Mobile Bay.
As one of the largest managers of dredged material in the United States, USACE has prioritized responsible and beneficial use of this valuable resource. In January 2023, Lieutenant General Scott Spellmon, the 55th Chief of Engineers, issued a “Beneficial Use of Dredged Material Command Philosophy Notice,” setting a goal for USACE to use at least 70% of its dredged material beneficially by 2030. The District is committed to achieving this ambitious goal through continued innovation, dedication, and collaboration with a broad range of partners and stakeholders.
The District routinely partners with local governments, federal and state agencies, and environmental organizations to restore the environment, reduce flood risk, and maintain navigable waterways. Current collaborations include projects with the Alabama State Port Authority, Alabama Department of Conservation and Natural Resources, Mobile County, the Town of Dauphin Island, USFWS Bon Secour National Wildlife Refuge, the Mobile Bay National Estuary Program, The Nature Conservancy, and other federal and state agencies. These partnerships are vital to enhancing coastal Alabama's environmental and economic resilience.
Through its ongoing commitment to innovation and collaboration, the District continues to play a crucial role in advancing sustainable dredged material management practices, ensuring that Mobile Bay remains a vibrant and navigable waterway for years to come.
“Studies are occurring nationwide to improve Thin Layer Placement,” said Jacobson. “We’re committed to staying at the forefront of these advancements to further enhance the environmental and operational benefits of Thin Layer Placement.”
| Date Taken: | 08.27.2024 |
| Date Posted: | 08.27.2024 12:14 |
| Story ID: | 479531 |
| Location: | MOBILE, ALABAMA, US |
| Web Views: | 46 |
| Downloads: | 0 |
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