Thin Layer Placement

Developing a Framework for Assessing the Benefits and the Feasibility of Thin Layer Placement in South Carolina

To address increasing coastal hazards along the South Carolina coast, natural resource managers and coastal decision-makers must consider proactive approaches to enhance coastal resilience while maintaining ecosystem function and services.

In this project, the S.C. Sea Grant Consortium, S.C. Department of Natural Resources, Louisiana State University, and the College of Charleston will develop a multi-sector decision framework for thin layer placement (TLP) in South Carolina marshes, which would serve as a demonstration and guide for other southeastern marshes in the United States.

Thin Layer Placement Graphic

“Thin-layer placement” also known as “thin-layer sediment addition” and “marsh nourishment”, is a process where sediment removed … during dredging is transported to a marsh by pipeline or barge and applied to the surface of the marsh by spraying a slurry of water, sand, and silt. (VIMS, 2014)

Thin layer placement, as it is defined by the U.S. Army Corps of Engineers, refers to the purposeful placement of thin layers of sediment to achieve a target elevation or thickness. (Berkowitz et al. 2019)

Project Team
Katie Finegan, P.E.
S.C. Sea Grant Consortium and Coastal Carolina University
Matt Gorstein
S.C. Sea Grant Consortium
Amanda Guthrie, Ph.D.
S.C. Sea Grant Consortium
Ke’Ziyah Williamson
S.C. Sea Grant Consortium
Andrew Tweel, Ph.D.
S.C. Department of Natural Resources
Giulio Mariotti, Ph.D.
Louisiana State University
Ellie Lovellette, Ph.D.
College of Charleston
Lexi Watson
S.C. Sea Grant Consortium Student
Amber Tymul
Louisiana State University Student
Timeline

Start date: January 1, 2023

Year 1 (2023)

  • Engage a diverse group of stakeholders to act as a project advisory council.
  • Begin data collection for economic, regulatory, ecological, and socio-cultural analyses.
  • Reconfigure MarshMorpho2D to simulate evolution of SC salt marshes.

Year 2 (2024)

  • Expand upon economic, regulatory, ecological, and socio-cultural analyses.
  • Simulate TLP in SC salt marshes using the calibrated Marsh Morpho 2D.

Year 3 (2025)

  • Conduct a crosswalk of all the analyses and combine with modeling.
  • Create an online interactive tool to report marsh evolution predictions under TLP.

Year 4 (2026)

  • Develop outreach products
  • Finalize web tool and framework

End date: December 31, 2026

Project Advisory Council
Heidi Anderson, P.E. Engineer, ATM
Liz Fly, Ph.D. Director of Resilience and Ocean Conservation, The Nature Conservancy
Alana Jenkins Co-Founder, Sea Island Heritage Academy
Jeremy Johnson, P.E. Navigation Branch Chief, USACE-Charleston
Lindsey LaRocque, Ph.D., P.E. H&H Engineer, USACE-Charleston
Katie Luciano, P.G. Coastal Geologist, SCDNR
Chris Mack, P.E.  Resilience, Adaptation, and Coastal Practice Lead, JMT
Pelor Richards Gullah Geechee Chamber Foundation and RichRiverTours
Denise Sanger, Ph.D. ACE Basin NERR Research Coordinator, Senior Marine Scientist, SCNDR
Erik Smith, Ph.D. Manager, NIWB NERR
Blair Williams Critical Area Permitting Section Manager, SCDES-BCM

Project Methodology and Anticipated Results

The project team will engage with a diverse group of stakeholders to define TLP success and discuss expected benefits of TLP.

To determine the economic feasibility of TLP, the team will engage with the United States Army Corps of Engineers to determine the estimated cost of TLP in comparison to current dredge disposal methods.

To determine the regulatory feasibility of TLP, the team will seek the opinion from the S.C. Department of Health and Environmental Control’s Office of Ocean and Coastal Resource Management. The regulatory analysis will provide a perspective of the TLP regulatory pathway in South Carolina, taking into account precedents set in other states where TLP has already been applied.

Ecological considerations of TLP will be accounted for in consultation with natural resource managers and scientists and ecological specialists.

The socio-cultural aspects of marshes will be captured through community survey and mapping events to understand where areas of importance are, areas that are experiencing loss, and what values the marshes hold to South Carolina communities.

To predict the functional success of TLP, the coastal evolution model MarshMorpho2D will be applied to South Carolina marshes. Different marsh types found in South Carolina will be modeled in MarshMorpho2D over a period of time, determined in consultation with stakeholders, under different sea-level rise scenarios, to show if a marsh will maintain, gain, or lose area when TLP is applied.

Developing the Framework

Potential TLP locations will be assessed from an economic, regulatory, ecological, and socio-cultural lens, and then representative sites will be implemented in Co-PI Mariotti’s geomorphological model (MarshMorpho2D) to evaluate the potential for success.

Thin Layer Placement Pac Diagram

Economics

Cost Analysis

This part of the project aims to evaluate “the estimated costs of thin layer placement as they compare to traditional (upland or offshore) disposal methods—particularly focusing on the costs associated with activities conducted after dredging the material”. This includes expenses related to piping or pumping material to a given location (e.g., fuel, labor, equipment costs) and depends on the volume of material, distance to a disposal site, and size of the disposal area. The project team is collecting data from past project reports, published bid abstracts from the US Army Corps of Engineers, and through consultations with dredging contractors.

Ecosystem Service Benefits

The practice of thin layer placement can potentially enhance coastal habitats’ resilience to sea level rise, thereby preventing habitats from being lost (e.g., converted to open water) or degraded. Healthy coastal habitats provide a variety of ecosystem service benefits to humans.

Ecosystem Services refer to the benefits that humans receive—directly or indirectly—from healthy functioning ecosystems. Coastal wetlands and salt marshes in South Carolina provide a variety of ecosystem services benefits to human communities—including nursery habitat for commercially and nutritionally important fish and shellfish species, coastal protection from storms through wave energy mitigation, recreation space, and cultural value, among others. If sea level rise were to destroy habitats, coastal communities would receive fewer ecosystem services compared to having an intact, healthy habitat. Following this logic, coastal restoration activities like thin layer placement can potentially avoid losses in ecosystem services, providing more benefits than if the habitat were left on its own.

To assess the change in the number of ecosystem services produced and the number of user groups benefiting from them when coastal wetlands convert to open water, the project team is using the EPA National Ecosystem Services Classification System (NESCS) tool. In order to assign value estimates to these services, the project team is using the Bluevalue literature database and estimates found in the report Assessing South Carolina’s Ocean Economy.

Output From the EPA NESCS Tool

Output from EPA NESCS tool.
Output from EPA NESCS tool.

Regulatory

The project team is partnering with the National Sea Grant Law Center​ to review relevant state and federal policies and Acts that relate to TLP permitting, such as the Clean Water Act (including the Sackett v. EPA ruling) and Coastal Zone Management Act. The team is additionally focusing on the permit process for Georgia and Maryland as these states have permitted and installed TLP projects. Results of this research are expected in Fall 2024​.

Ecological

To assist in model calibration efforts in September 2023, an Acoustic Doppler Velocimetry (ADV) instrument and five turbidity sensors were deployed by the S.C. Department of Natural Resources (SCDNR) along the tidal creek and marshes of Hutchinson Island and its main drainage, Twin Sisters Creek. The study site is depicted in the image below. The ADV was deployed in the main channel, while turbidity sensors were deployed at five sites, including three along the channel and two in the marsh (one soft marsh and one firm marsh).

Project Map

The Hutchinson Island study area is outlined with a red box.

A PVC pipe tube with a small red flag on top is affixed in the mud of a salt marsh.

The turbidity sensor is shown with piping and flag as placed in the soft marsh.

These instruments were retrieved in November 2023, and heavy biofouling was observed on all except the two marsh deployments. Elevation data was collected using RTK GPS at all sites, where practicable. Additionally, surficial sediment samples were also collected at three sites, including one creek bank, one from  the soft marsh, and one from  the firm marsh. These samples will be analyzed for sand/silt/clay percentages using the pipette method without the removal of organics.

To gather more information about the soft marsh site, four turbidity sensors were redeployed in December 2023, spaced approximately 10 m apart along a transect from the bank to an interior site. The ADV was also redeployed at a new site farther up the main channel. An additional sediment sample was collected in a depositional environment along the main channel, near the mouth. All instruments were removed in January 2024.

Social-Cultural

In the summer of 2024, three participatory mapping workshops were held in the Beaufort area, Georgetown area, and Charleston area of South Carolina to learn from community members about their thoughts related to coastal wetland erosion, coastal wetland restoration, and their experiences with environmental conditions that have changed or remain the same over time. The workshops identify coastal locations that have high social, economic, or cultural value.

The data collected will be provided to coastal management agencies to help them understand key on-the-ground anecdotes and experiences with marsh erosion and where it has occurred. The data will also be used in a decision support tool to inform coastal managers of potential human use overlaps and areas of high social value to be aware of if South Carolina considers a TLP project. This could involve  prioritizing areas to protect and/or areas to leave undisturbed  all of which would still need to go through the proper permitting and regulatory pathways in South Carolina.

Community members were compensated with a small stipend for participating. The Human Research Protection Program at the College of Charleston has reviewed the survey and mapping exercise, ensuring it meets all relevant requirements of the EU General Data Protection Regulations.

Two participants lean over a large map taking up the whole table, marking it with a red marker.

In June of 2024, residents from the area surrounding Beaufort, S.C. inspect maps to help identify the areas that are of socio-cultural importance to them and where they have observed habitat changes.

Modeling

MarshMorpho2D is a numerically efficient marsh evolution model, which can simulate marsh trajectories over decades to centuries.

The model calculates a tidal flow map by redistributing the tidal prism according to water depth and bed drag coefficient. Suspended sediment transport is simulated as a dispersion process, i.e., by considering the net effect of a tidal cycle.

Marsh vegetation is formulated as a function of elevation, which serves as a proxy for the hydroperiod. Vegetation affects the drag coefficient, sediment erodibility, effective settling velocity, bank slumping, and the in-situ production of organic sediments.

Pond dynamics include pond formation, pond expansion, and pond merging. Wind waves are calculated using a smoothed fetch model, whereas wind-induced edge erosion is implemented through a probabilistic algorithm.

Calibrated Model Run Video

Simulation of an idealized mesotidal marsh (3000 years total run), with constant Relative Sea Level Rise rate and seawars sediment supply.

Modeling Publications

Thin Layer Placement Resources

Engineering with Nature Podcast

In Season 6, Episode 9, hosts Sarah Thorne and Jeff King, Lead of the Engineering With Nature® Program at USACE, welcome Candice Piercy, Research Environmental Engineer at the Engineer Research and Development Center (ERDC), USACE; Ram Mohan, Senior Principal at Anchor QEA and Adjunct Professor at Texas A&M University; and Monica Chasten, Coastal Engineer and Project Manager in the Philadelphia District, USACE. Candice and Ram co authored the recently published Guidelines for How to Approach Thin-Layer Placement Projects with their colleague, Tim Welp, a renowned dredging expert who passed away in 2021. Tim inspired  the guidelines, and this episode is dedicated to him. Monica, a guest in Season 1, Episode 8, is an innovator and leading light in the dredging community responsible  for keeping the coastal navigation channels open in New Jersey and Delaware. She was an early adopter of Engineering With Nature and one of the first movers of beneficial use of dredge materials.

Reports

Ecosystem Service Basics

Tools

Webinars

In Spring 2019, the U.S. Army Corps of Engineers Philadelphia District partnered with the State of New Jersey, The Wetlands Institute, and the USACE Engineer Research and Development Center to launch the Seven Mile Island Innovation Laboratory (SMIIL). The initiative is designed to advance and improve dredging and marsh restoration techniques in coastal New Jersey through innovative research, collaboration, knowledge sharing and practical application.

Funding

Funding awarded from the U.S. Coastal Research Program and the NOAA National Sea Grant College Program.

This project is funded, in part, by the US Coastal Research Program (USCRP) as administered by the US Army Corps of Engineers® (USACE), Department of Defense. The content of the information provided in this publication does not necessarily reflect the position or the policy of the government, and no official endorsement should be inferred. The authors acknowledge the USACE and USCRP’s support of their effort to strengthen coastal academic programs and address coastal community needs in the United States.

Uscrp Logo

Contact for More Information

Katie Finegan, P.E.
S.C. Sea Grant Consortium and Coastal Carolina University

References

VIMS (Virginia Institute of Marine Science). 2014. Thin-layer sediment addition of dredge material for enhancing marsh resilience. College of William and Mary, Gloucester Point, Virginia.

Berkowitz, J. F., C. Piercy, T. Welp, and C. VanZomeren. 2019. Thin layer placement: Technical definition for U.S. Army Corps of Engineers Application. ERDC/EL Technical Notes Collection (ERDC/EL TN-19-1), Vicksburg, MS: U.S. Army Engineer Research and Development Center.

Top photo: High pressure spray application of dredged material by Bob Blama, CENAB, USACE.