Marsh Sill Evaluation

Why is estuarine shoreline stabilization important to our coast?

Estuarine shoreline erosion and coastal population growth have combined to make estuarine shoreline stabilization an important coastal management issue in North Carolina. Different types of stabilization structures have potentially varied impacts on the state's estuarine shoreline and the ecological benefits they provide.

Natural shorelines capture nutrients and sediment from stormwater before it enters our estuarine systems. They also provide feeding and nursery habitat for a multitude of species and dampen wave energy along the shoreline. Shoreline stabilization often leads to a change in these ecosystem services, especially when scaled to reflect growing coastal development. Bulkheads are currently the most commonly used method of shoreline stabilization in North Carolina. As understanding of ecosystem function has increased, new alternatives to bulkheads have emerged. These alternatives are designed to provide similar levels of shoreline stabilization while minimizing the reduction in ecosystem services compared to a bulkhead. The rock sill with marsh plantings, commonly referred to as a marsh sill, is the focus of this N.C. Division of Coastal Management assessment project.

What is a Marsh Sill?

Marsh sills are shore-parallel structures made up of two critical elements:

  1. An offshore low relief mound made of rock or oyster shell called a sill
  2. An intertidal area between the offshore sill and the upland containing emergent marsh vegetation

While marsh sills are considered "living shorelines" that maintain as many natural habitat elements as appropriate for the construction site, there is also concern that marsh sill construction can alter habitat types and may create new uplands that convert public land into private land. 

Evaluation Project

Given the potential opportunities and challenges with using marsh sills for estuarine shoreline stabilization, as well as the 50+ expected lifetime of most marsh sill projects, the N.C. Division of Coastal Management (DCM) was tasked with evaluating how current marsh sill projects are performing.

Researchers from DCM and the N.C. Coastal Reserve undertook a qualitative technical assessment of 27 existing marsh sills located throughout the state. Sills were evaluated on two criteria:

  1. Are the marsh sills performing their function as expected?
  2. What are the landowner and adjacent property owners’ (where marsh sills
    are located) perceptions of the marsh sill shoreline stabilization option?

The Division's efforts, together with simultaneous living shoreline research being conducted in the region, help provide qualitative and quantitative information regarding how marsh sills are performing in North Carolina and how they relate to larger estuarine systems and associated ecosystem services.

Final Report: N.C. Division of Coastal Management Assessment of 27 Marsh Sills in North Carolina