An innovative approach to the beneficial re-use of navigation dredged material at Horseshoe Bend on the Atchafalaya River in the state of Louisiana, USA, relies on natural processes transporting and depositing the sediment to create islands of significant wildlife value. As this island has evolved, multiple other benefits have been realized: the newly established vegetation promotes carbon sequestration; and the island has formed a natural ‘training wall’, facilitating self-scour in the channel and thus significantly reducing dredging requirements, with significant resultant carbon savings.
The photo of the island behind the dredger California, is the river island at Horseshoe Bend on the lower Atchafalaya River, Louisiana, USA. This island is being self-designed by dredged sediment strategically placed upriver, allowing for the river’s energy to disperse the sediment. The dispersed sediment contributes to the island’s growth, thus creating environmental and other benefits.
During the 1990s, placement of shoal material dredged from Horseshoe Bend occurred at eight wetland development sites located along the river’s banklines adjacent to the channel. Capacity of these placement sites was nearly exhausted by 1999. Thus, to meet the anticipated disposal requirements for future channel maintenance, mounding of material at mid-river open water placement sites immediately adjacent to the navigation channel and upriver of a small naturally forming sandbar was selected on a demonstration basis to investigate the impacts of mid-river placement on shoaling trends downriver of the site. Beginning in 2002, strategic placement of the sediment dredged from Horseshoe Bend occurred at the mid-river open water placement area. Placement of between 0.5 to 1.8 million cubic yards of sediment was conducted every 1 to 3 years which influenced and contributed to the development of an approximately 35 ha island mid-river. The practice of strategically placing dredged sediments upriver of a naturally-occurring sandbar was conducted to aid the sandbar’s growth to produce greater environmental and economic benefits than otherwise would be present using more conventional dredged material placement practices.
Before sediment was placed in mounds upriver of the sandbar, the project team consulted with state (State of Louisiana, USA) and Federal (U.S. Fish & Wildlife Service) environmental agencies to obtain their feedback on the proposed innovative sediment placement approach as a more sustainable alternative compared to filling in wetlands or dumping the sediment downstream in Atchafalaya Bay.
No substantive difficulties were encountered during the planning and implementation of the demonstration project, though intra-agency uncertainty existed over downstream impacts which thus far have gone unrealized. Furthermore, existing legislation did not affect the application of the Engineering With Nature initiative for this project. No legal requirements were in place that hindered the application of the concept, potentially fostering application of the concept at other sites in the future.
A diverse assemblage of native plant and animal life has colonized the island. In the left photo is an extensive stand of the native American lotus (Nelumbo lutea). A juvenile tricolored heron (upper right) and a juvenile snowy egret (lower right) were observed in nests on the island during nesting season (July 2014).
The project used natural processes to maximum benefits, thereby reducing demands on limited resources, minimizing the environmental footprint of the project, and enhancing the quality of project benefits. As the island has enlarged it reduced the overall cross sectional area of the river, increasing flow through the navigation channel to velocities that were sufficient to reduce shoaling and maintenance dredging requirements. This led the U.S. Coast Guard to realign the navigation channel, reducing its length and eliminating sharp bends, thereby enhancing navigation safety, reducing fuel and carbon requirements and air pollution. The island is currently 35 ha in size and supports over 80 plant and over 20 faunal species within four distinct habitat types, including broadleaf forested, broadleaf scrub-shrub, persistent emergent, and aquatic bed. The self-designed Horseshoe Bend Island resulted in landscape and landform characteristics (e.g., distance from shore, flooding regime) that support a large, successful wading bird rookery. Signs of human activity were also noted on the island, as the presence of shotgun shells signified that the island was being used for hunting. Dark, organic-rich surface soils have developed on the island demonstrating that chemical reduction regularly occurs within island soils, resulting in carbon sequestration, nutrient cycling, and denitrification.
These investigations further quantifying the multiple benefits of using dredged material to create such riverine islands will provide a more complete understanding of the formation of the island so this concept can be integrated into other dredging projects, thereby providing substantial environmental, social, economic and climate benefits as part of maintenance dredging activities.
The Atchafalaya River island project exemplifies what can be achieved through the application of Engineering With Nature concepts and practices. Documenting such innovation exemplifying the approach and communicating across the technical community and with project partners and stakeholders is imperative to apply this innovation at other dredging projects and at larger scales.
Information regarding ecosystem classification and mapping, floral and faunal composition of the island, and background data supporting future research efforts are also needed to document and communicate environmental and other benefits being realized. This practice could be replicated in other rivers facing similar challenges, if the context is also comparable. The environmental benefits shown to date also suggest high value.
A U.S. Army Corps of Engineers Technical Note on this project can be found at: http://el.erdc.usace.army.mil/elpubs/pdf/ewn14-4.pdf
A link to the fact sheet for this project can be found at the USACE Engineering With Nature website: http://el.erdc.usace.army.mil/ewn/Action_Projects.html
An article about this project was published in 2014 in World Dredging Mining and Construction, 48(9-10): 14-16.
Morgan City, Louisiana, USA, North America
North America, Adaptation, Freight, Shipping, Technology, awareness
U.S. Army Corps of Engineers New Orleans District & Engineer Research and Development Center, Vicksburg, MS, USA
Burton Suedel US Army Corps of Engineers Engineer Research and Development Center, Vicksburg, MS USA email@example.com
“During the early stages of the project, everyone's attention was focused on engineering uncertainties. This preoccupation made initial surveying of the island’s rich floral and faunal communities that much more amazing. But greater benefits were hidden in and around the island. Soil horizons expressed biogeochemical signatures atypical of traditional dredge-and-fill sites, and the physical presence of the island allowed for development of a stable channel. Thus, it became apparent that obvious macro-benefits were outweighed by the island’s complementary roles in sequestering carbon in its soils and reducing dredging requirements and emissions.”
-Jeff Corbino, Project Manager, U.S. Army Corps of Engineers, New Orleans District, New Orleans, Louisiana, USA