4 minute read
Sand Island
Mobile County, Alabama, United States
Working with nature to build ephemeral islands in an ebb-tidal delta. Mobile Pass is one of the largest ebb-tidal delta systems in the United States. It provides safe and efficient navigation to harbors for the distribution of goods and services, serves as a natural source and sink of sediments for adjacent shoals and ephemeral islands driven by a prevailing wave approach and tidal-flow dynamics, and provides a natural wave break, dissipating wave energy approaching lee-side island beaches and dunes along Dauphin Island, Alabama. Since 1999, sandy material dredged as part of routine maintenance of the Bar Channel has been placed within the Sand Island Beneficial Use Area (SIBUA) by means of bypassing, but the limited placement area restricted dredge equipment and site capacity. In 2019, the SIBUA Northwest Extension was approved to continue the balancing of safe and efficient dredged material placement practices while also using natural transport mechanisms to move sediment westward along the shoal system. As part of the channel improvements and the National Fish and Wildlife Foundation’s Alabama Barrier Island Restoration Assessment, the U.S. Army Corps of Engineers (USACE)–Mobile District worked with partners, stakeholders, and industry incorporating lessons learned and design strategies for the placement of over one million cubic meters of dredged material, improving dredge efficiencies and providing migratory bird habitats.
Article Cover: Aerial view of the 2022 SIBUA dredged material placement of ephemeral islands in the foreground looking east toward Sand Island Lighthouse, Alabama. (Photo by Sam St. John, FlyTheCoast)
Producing Efficiencies
Revisions to dredged material placement were determined through analysis of long-term sediment transport processes, historical dredge records, and modeling of sediment transport and morphological changes. Historical topographic and bathymetric surveys and dredge records were compared to quantify past and present-day changes in the sand flux to develop sediment budgets that helped to inform modeling tools and placement strategies. This aided in identifying potential operational efficiencies through use of bimodal sand transport pathways for safe and efficient placement practices. To evaluate effectiveness and better understand future placement site capacity, SIBUA is proactively monitored and managed through hydrographic surveys and change analysis.
Using Natural Processes
The project worked with nature to build the ephemeral islands. Regional Sediment Management and beneficial use strategies in conjunction with Engineering With Nature were used to place dredged material along nearshore shoal systems, providing an estimated 401,391 cubic meters of beneficial use sediment placement per year to the downdrift ebb-tidal delta system. A prevailing southeast wave approach and tidal-flow dynamics coupled with ephemeral-island placement strategies maximized efficient dredged material placement and provided natural wave breaks and dynamic transit migratory bird habitats. The islands are the result of sand moving west along the perimeter of the delta shield.
Broadening Benefits
Placement of sediment to mimic ephemeral islands improved dredged material placement efficiencies while providing dynamic transit habitat features for migratory birds, including piping plover (Charadrius melodus) critical habitat. The placement provides sediment to the tidal delta shield, which acts as a natural wave break, dissipating wave energy approaching the lee-side island beaches and dunes that serve as second lines of defense for both natural (maritime forests and freshwater lakes) and built infrastructure (cultural resource sites, public educational facilities, campgrounds, and private developments) occurring landward. In addition, the natural wave break of the shoal system allows for recreational boating, kayaking, fishing, and bird-watching.
Promoting Collaboration
This effort was built upon decades of data, science, research, and knowledge. Prior work included sediment budget analysis and modeling by Applied Coastal Research and Engineering; coastal data collection and analysis from the University of South Alabama, Geological Survey of Alabama, and South Coast Engineers; sea floor and shoreline change studies by the U.S. Geological Survey (USGS); and updated bathymetric data sets by NOAA, USGS, and USACE. USACE Mobile District worked with partners and stakeholders to gain a fuller understanding of the system and to better define problems, objectives, and opportunities, thereby improving collaboration and leading to continued partnerships and innovative nature-based solutions.
