Sediment: a lifeline to vulnerable marshes
September 6, 2024
Coastal marshes are some of the most important habitats for improving water quality and protecting infrastructure along the shoreline, but rising sea levels pose a growing threat. With increased sea level rise, marshes are vulnerable and restoration efforts needed to provide a lifeline are often costly and difficult to arrange.
UNC Institute for the Environment Associate Director Susan Cohen and a team of co-investigators from UNC-Chapel Hill, the NOAA National Centers for Coastal Ocean Science, the United States Army Corps of Engineers and EA Engineering, Science and Technology are investigating ways to increase the resiliency of coastal marshes by leveraging science to improve coastal management.
The four-year project was funded through the NOAA National Centers for Coastal Ocean Science (NCCOS) Effects of Sea Level Rise (ESLR) Program and resulted in four conceptual designs for marsh restoration projects in North Carolina and Florida that take into account salt marsh geomorphology, wave energy, ecology and sea level rise.
According to Cohen, it is important to consider factors that affect marsh elevation when planning a restoration project, such as the rate of relative sea level rise, the rate of shoreline erosion and the amount of wave energy impacting the marsh. Restoration projects are also tailored to site characteristics, including the presence of navigation channels, fish habitats, aquatic vegetation and shoreline type.
Although project specifics may vary, methodologies focus on a common theme.
“Every restoration project that we propose revolves around keeping sediment in the local system because the currency of marshes is sediment,” Cohen said.
The dominant way marshes maintain their elevation and remain above sea level rise is through sediment accretion, according to Cohen. Marshes accumulate sediments by trapping particles carried in tidal waters, yet it is often not enough to keep up with sea level changes. Low sediment accumulation rates and the dredging of navigation channels pose specific problems for marsh elevation. This is especially true when dredged material is relocated to distant sites and no longer available to build up coastal marshes.
“Oftentimes dredged sediment is taken offshore or hauled to a site inland,” Cohen shared. “We shouldn’t take this sediment out of the system. We need to keep the sediment in the system, and we do that by matching up a restoration project to a nearby dredging activity.”
NCCOS has been supporting projects that inform effective sediment placement across the country. According to NCCOS, dredged sediment can be used to create new habitats or to raise the elevation of coastal habitats like marshes via thin-layer replacement, a technique designed to imitate natural sediment deposition.
Trevor Meckley, program manager for the ESLR Program that supports this work, shared that “some of the most innovative research aims to advance our understanding of coastal processes and our ability to model them so that dredgers can place sediment at a site strategically so that it both feeds habitats or nourishes the region without having to displace the natural habitat, leveraging the natural sediment deposition processes rather than fighting them.”
While several agencies and organizations are advancing the science to understand how best to apply dredged material in restoration, all approaches face challenges when it comes to coordination on the ground. Meckley shared that “despite the reality of these challenges, we are elevation deficient and can’t afford to dump elevation equity rather than utilize it. This is a value that the USACE has shared and is reflected in their 2023 memorandum that set a goal for beneficially using at least 70% of its dredged material by the year 2030.”
With the unified goal of retaining dredged sediments, teams like the one led by Cohen aim to collect data to better inform the process for repurposing sediment for marsh restoration.
“It’s not hard to find a marsh that is not keeping up with sea level rise over time,” Cohen said.
Despite the prevalence of vulnerable marshes, the team focused their conceptual designs on two sites in Carteret County, NC, and two sites in Jacksonville, FL, chosen for their proximity to important infrastructure, such as an airport, military base and national park.
Based on elevation sampling, old imagery and evaluations of site characteristics, the team used scientific data to develop specialized restoration plans for each marsh site in an attempt to improve how marsh elevation challenges are addressed. For example, the team proposes building out the marsh to the location of the 1959 shoreline using sediment and planting vegetation to create a hybrid shoreline due to the high-energy space at the Michael J Smith Air Field near Beaufort, N.C. The image below shows how close the marsh edge is to the runways, eroding approximately one meter/year and not accreting enough sediment.
According to Cohen, the next step is to secure funding for permits to implement the designs.
Story by Natalie Peoples
Natalie Peoples is a senior from Kensington, Maryland, pursuing a double major in journalism and environmental science. In addition to her work as a communications intern for the Institute for the Environment, she has experience in photojournalism and environmental research. Peoples plans to pursue a career in environmental journalism with a special interest in marine science.