A new NCCOS study shows that spreading a thin layer of sediment on low-lying marshes promotes vegetation growth that builds elevation. The technique may be an effective way to restore coastal marshes and improve their resilience to sea level rise.

Six experimental plots were used for thin layer placement testing and data collection at Freeman Creek marsh, North Carolina. Credit: NOAA.

The rate of global sea level rise is increasing, threatening to drown coastal wetlands like marshes. At the same time, dredging, shoreline hardening, river damming, and diversions reduce the amount of sediment available for marshes to build elevation. In places where sea level rise outpaces marsh elevation gain, marshes will convert to unvegetated mudflats and eventually, to open water if no corrective action is taken.

Thin layer placement involves depositing sediment on a marsh using either a high pressure hose to spray sediments, known as "rainbowing," or as low pressure slurry delivered through a pipe suspended above the sediment surface. These techniques are designed to emulate natural sediment deposition processes. This project used the latter method to conduct a controlled, replicated, demonstration project.

This method, when combined with sediment sourced from routine navigation channel maintenance, can help retain sediment in coastal systems that would otherwise be placed in upland containment facilities or in offshore disposal areas, and benefit marshes that are not increasing in elevation fast enough to keep pace with sea level rise. As an added bonus, thin layer application can result in reduced dredge material disposal costs.

"We found a significant increase in marsh vegetation at the end of the first growing season that has lasted for four years and now demonstrates the marsh has an increased capacity to build further elevation," said Dr. Jenny Davis, NCCOS Research Ecologist and lead author of the study. "These data demonstrate that sediment addition can result in immediate benefits in terms of elevation gain, and longer term benefits by increasing the ability of marshes to trap additional sediments and continue to build elevation on their own. In other words, sediment application can result in marshes that are more resilient to sea level rise."

"The ‘low and slow’ application approach used here resulted in relatively consistent application depths without any noticeable impacts to the existing vegetation, but this approach is challenging to replicate on a large scale," notes Davis. Advances in placement technology that allow for deposition of relatively uniform amounts of sediment across a large area will be required for thin layer sediment placement to become operational on a large scale.

The research is part of NCCOS’s efforts to conduct science that supports coastal resilience through investigations on the performance of nature-based solutions in real-world conditions.

Citation: Davis, J., Currin, C., and Mushegian, N. 2022. Effective use of thin layer sediment application in Spartina alterniflora marshes is guided by elevation-biomass relationship. Ecological Engineering, 177; 106566. https://doi.org/10.1016/j.ecoleng.2022.106566