“Climate change adaptation” is a term that is thrown around a lot in research and management circles. Although it is a relatively new, and sometimes daunting, term it can often refer to the thoughtful repurposing of management strategies. A prime example of this is the use of thin layer sediment augmentation in tidal marshes. Thin layer sediment augmentation (hereafter sediment augmentation) refers to the practice of spraying a slurry of dredge spoils and water from a barge or a system of pipes onto a marsh surface. As discussed in a previous post (link), marsh habitats are dependent on a balance between processes that increase and decrease marsh elevation relative to sea-level. Marsh elevation increases as a function of deposition of sediment from external sources (i.e. rivers), and plant growth. Elevation decreases due to decomposition of organic matter (plants), erosion, compaction, and sea-level rise. Local tectonics can also cause an increase or a decrease in elevation. Thus, thin layer sediment augmentation artificially increases the amount of sediment deposited on the marsh surface. In a typical marsh, annual sediment inputs are measured at a scale of millimeters a year. Sediment augmentation artificially increases the amount of sediment deposited on a marsh, by up to 60 cm. Excitement is growing in the natural resource management communities about the potential of using sediment augmentation as a climate change adaption strategy.
Sediment augmentation was initially used in coastal marshes in Louisiana in the 1980’s to increase the elevation of marshes that were experiencing localized subsidence. Sediment augmentation has been used in the San Francisco Bay to restore tidal marshes and is currently used by the San Francisco Bay Joint Venture, San Francisco Bay Conservation and Development Commission, and San Francisco Estuary Institute who are collaborating on an online database designed to connect the wetland habitat restoration community and the dredging/ sediment supply community (called SediMatch link). These studies have pioneered the methodologies for sediment augmentation they have shown that marsh vegetation is extremely resilient to sediment augmentation. Sediment augmentation appears to be a promising strategy to increase the elevation of marshes which makes it a prime candidate for use as a climate change adaptation strategy. However, despite the decades of use there are still a lot of important questions about its efficacy. One of the most important unknowns is how much sediment is needed and for how long augmentation will increase the elevation of a marsh. There are concerns that too much sediment could cause a compaction before vegetation is established and sediment will be washed away. There is also uncertainty about the effects of increased sediment on nearshore ecosystems such as eelgrass beds, which are known to be sensitive to sediment. In response to these unknowns, a five year pilot study of sediment augmentation will be implemented at Seal Beach National Wildlife Refuge in Seal Beach, California. Thirty centimeters of sediment will be applied over a 10 acre region of the marsh and will be intensively monitored. Pre application monitoring of elevation, vegetation, suspended sediment concentrations, vegetation, and eelgrass have already been completed. The hope is that if the application of this sediment successfully increases the elevation of the marsh, repeated applications could help the marsh keep pace with sea level rise.
Khalil, S. M., Finkl, C. W., Roberts, H. H., & Raynie, R. C. (2010). New Approaches to Sediment Management on the Inner Continental Shelf Offshore Coastal Louisiana. Journal of Coastal Research, 264(1992), 591–604.
Ray, G. (2007). Thin layer placement of dredged material on coastal wetlands: A review of the technical and scientific literature, TN-07-1(December), 1–8.
Thin-layer Salt Marsh Sediment Augmentation Pilot Project - Seal Beach - U.S. Fish and Wildlife Service. (n.d.). Retrieved September 28, 2015
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