Swan Island Restoration

Sea level rise, coastal flooding, increased storm frequency, and other hazards pose increasing risks to our nation’s communities and natural resources. Nature-based solutions to infrastructure are those that incorporate natural features (e.g. dunes, reefs, marshes, and islands) for the purpose of flood and coastal storm risk reduction. Compared to conventional hardened infrastructure, nature-based solutions are more capable of adapting to a changing environment and can be self-sustaining. They also provide socio-economic and environmental benefits.

The National Centers for Coastal Ocean Science and The US Army Corps of Engineers, Engineering With Nature® (EWN®) Initiative work collaboratively at places like Swan Island, Maryland, to conduct research on the use of nature-based solutions to reduce risks from coastal hazards. Together, NCCOS and USACE are providing the, science, engineering and guidance to help coastal communities know how, where, and when to best construct nature-based solutions.

The Swan Island restoration is an example EWN 'principles in action' because it emphasizes the use of dredged sediment, natural processes, and ecosystems to achieve a broader suite of benefits for people and the environment.

To counter island loss in the Chesapeake Bay, in 2019 the USACE Baltimore District placed 60,000 cubic yards of sediments, dredged from a nearby navigation channel, to restore the footprint of Swan Island. The restoration plan included creation of dunes and high and low marsh, and the installation of 200,000 plants. The island was transformed from a low and highly fragmented marsh to one with a wider range of habitats that sit higher in the tidal frame.

The Swan Island restoration provides the research team with an opportunity to quantify and document island performance and benefits to inform the future planning, design and adaptive management of islands and natural features. In collaboration with USACE’s Engineering With Nature® Program, staff members from USACE Engineer Research and Development Center (ERDC), USACE Baltimore District, NOAA’s National Centers for Coastal Ocean Science (NCCOS), Maryland Department of Natural Resources (MDDNR) and the U.S Fish and Wildlife Service (USFWS) came together to conduct the pre- and post-placement restoration monitoring required to quantify project success.

The restoration of Swan Island is expected to increase the long-term resilience of the Island to sea level rise and erosion, shelter nearby communities and shorelines from waves, and serve as habitat for a range of wildlife. Currently, the lack of quantitative data on the resilience and ecological benefits of natural features like islands are often cited as a barrier to their widespread use for coastal protection.

During a series of USACE ERDC facilitated workshops, the project team identified objectives, defined monitoring parameters, and developed a conceptual model to guide the research team in monitoring and the adaptive management efforts. The conceptual model reflects the primary objectives which are to evaluate the capacity of Swan Island to reduce waves and erosion to nearby shorelines (i.e. engineering performance) and the resilience of the island to storms, sea level rise and other environmental stressors. For example the project team will address questions such as:

1. How have the restoration actions influenced the capacity of Swan Island to provide protection from wave energy to the town of Ewell? and

2. How will the protective capacity and ecosystem services provided by Swan Island be influenced by sea level rise?

The partners have monitored Swan Island annually for four years (2018-2021) to collect the hydrodynamic, topographic, ecological and sediment parameters necessary to evaluate project performance and benefits and inform adaptive management actions. Specific details of the monitoring and adaptive management are described in the Swan Island Monitoring and Adaptive Management Plan (in review, see below). These data will also inform the development and evaluation of integrated hydrodynamic and ecological simulation models.

The roots and rhizomes of vegetation have the capacity to stabilize sediments while their blades have a baffling effect on waves and currents which help to trap sediment and reduce erosion at the sediment-water interface. Below, feldspar clay is placed on the sediment surface (left). Two years later, a core collected in that same location (middle) reveals the amount of new sediment that has been deposited on top of the feldspar layer (right), providing a measure of sediment accretion over time.

Drone-based imagery georeferenced with RTK-GPS is collected annually to document change over time. Sturcture from Motion image processing is used to generate whole-island Digital Elevation Models (DEMs), Habitat Classification Maps, and to measure shoreline change. These data are critical to detecting change over time on the island and foundational to development and evaluation of predictive simulation models (see below).

The monitoring data are being used to develop predictive simulation models such as the Marsh Transect Model and the ADCIRC+STWAVE models to test hypotheses and answer questions about island performance, resilience and ecological services under a variety of sea level rise and storm conditions.

The marsh transect model simulates the processes of vegetative growth (above and below ground biomass), sediment accretion and erosion under various sea level rise and tide scenarios to predict island evolution. The inputs, outputs, and preliminary results are shown below.

ADCIRC (Advanced CIRCulation) was coupled with the STWAVE (Steady-state Wave) Model to simulate water elevations and wave heights during storm events. Preliminary results from a Hurricane Sandy simulation model run are shown below. The results from this modeling effort will allow the project team to quantify the capacity of Swan Island to reduce storm risk to nearby shorelines.

The project team developed a Monitoring and Adaptive Management Plan (MAMP) to track progress and serve as a blueprint for the monitoring (e.g. What, How, and How often data is collected) and the adaptive management approach. This includes reporting, data management, roles and responsibilities, performance metrics and decision thresholds for adaptive management.

The results of this research effort will address current information gaps and uncertainties surrounding the protective benefits and impacts of island restoration activities on surrounding ecosystems, and will be used to inform adaptive management of the Island as it matures over time. The approach developed here will be applicable to other island systems.

Multi-disciplinary partnerships are critical to leverage both the expertise and the resources required to monitor, evaluate, and manage island or other natural systems.