Effects of Bulkheads on Salt Marsh Loss: A Multi-Decadal Assessment Using Remote Sensing

Abstract

Coastal salt marshes and the ecosystem services they provide are on the decline, disappearing more rapidly than any other type of wetland in the United States. Salt marshes provide numerous ecosystem services, including storm protection, improved water quality, carbon sequestration, and critical habitat and nursery areas for commercially and recreationally important fish and shellfish species. Coastal development has risen considerably in the last several decades and has often led to shoreline hardening, whereby shoreline stabilization structures like bulkheads are used to protect against property erosion. Despite the widespread use of bulkheads and a growing body of evidence of their potential negative impacts, little is known about the effects of bulkheads on loss of salt marsh ecosystems.

To inform estuarine shoreline management, this study investigated the long-term effects of bulkheads on salt marsh loss using historic aerial imagery of Bogue, Back, and Core Sounds (Carteret County, North Carolina, USA) from 1981, 1992, 2006, and 2013. In addition to the effect of bulkhead structures, I investigated the role of wave energy on marsh loss in this system. Rates of marsh loss at landward bulkheads (i.e. bulkheads with adjacent salt marsh) were compared to ‘background’ rates of loss at natural marshes (i.e. non-stabilized controls). A combined wave energy index was developed to assess overall wave energy at a given site, including wind wave energy data from a previous simulation of the National Oceanic and Atmospheric Administration’s Wave Exposure Model (WEMo) and distances to commercial and recreational boat channels as proxies for boat wave energy. A two-way analysis of variance was used to determine the impact of shoreline type (bulkhead vs. natural marsh) and wave energy regime (low, medium, and high) on rates of marsh loss from 1981 to 2013. Additionally, a linear mixed effects analysis was used to determine the effect of shoreline type (bulkhead vs. natural marsh), wave energy regime (low, medium, and high), date (1981 to 1992, 1992 to 2006, 2006 to 2013), and their interaction on rates of marsh loss.

The results of this work suggest that rates of marsh loss are higher at bulkheads, as these structures appear to increase outer edge erosion, and they prevent marsh gain through upland migration. Many natural marsh sites experienced upland migration but gains in marsh through this landward expansion were still insufficient to offset marsh loss from erosion of the waterward edge. Additionally, rates of marsh loss from 1981 to 2013 were not significantly different among wave energy regimes. However, the highest rate of marsh loss occurred at landward bulkheads in high energy regimes. While not statistically significant, this observation supports the idea that the effect of wave energy on marsh loss at bulkheads may be amplified as wave energy increases because of wave reflection. My results also suggest that horizontal erosion rates of salt marsh correlate with rates of sea level rise (SLR), as the lowest marsh loss occurred during the period with the lowest rates of SLR (1992-2006), and the highest marsh loss was observed during the period with the most rapid rate of SLR (2006-2013).

The results of this study are intended to inform estuarine shoreline management. Since the assumption that bulkheads do not negatively affect public trust resources (e.g. salt marshes) is negated by this work, I provide several policy recommendations to begin leveling the playing field for bulkheads and living shorelines, including: 1) develop estuarine setbacks based on long-term erosion rates (as quantified by this study), 2) increase the price of bulkhead permits to incentivize the use of living shorelines, 3) incorporate the Living Shorelines Suitability Tool into the permitting process to help identify a site’s suitability for different stabilization techniques, and 4) implement and expand educational programs to inform property owners and the coastal engineer and contractor communities about living shorelines.

This study was the first to investigate multi-decadal effects of bulkhead structures on marsh loss in the Albemarle-Pamlico Estuary and provides useful information for better understanding the effects of shoreline hardening on salt marsh ecosystems. Ultimately, guarding against property erosion should not compromise the integrity of salt marsh ecosystems and the ecosystem services they provide to coastal communities throughout North Carolina.