Abstract
Sea level rise, increased storminess, and human population growth amplify coastal erosion problems, pressuring landowners to implement shoreline protection measures. Growing concern over the negative impacts of traditional shoreline protection methods has increased interest in nature-based solutions, called “living shorelines.” The goal of this study was to develop a better understanding of the characteristics that contribute to the vulnerability of salt marshes fringing Virginia’s coastal bays and to recommend appropriate shoreline stabilization techniques.
Using GIS and remote sensing data, a Marsh Vulnerability Index (MVI) was developed to relate and map disparate physical, biological, and climatological factors that contribute to salt marsh erosion and inundation. The MVI, which indicates the level of vulnerability for individual shoreline segments, was calculated for the marsh shorelines fringing the bays of the Virginia Coast Reserve (VCR). The majority of VCR shoreline (86%, 1,007 km) was designated as having very low to low vulnerability; 14% (165 km) was resolved as moderate to very high vulnerability. Vulnerability was generally higher along mainland marshes than marsh islands or backbarrier marshes. Moderate to high vulnerability was primarily associated with high wave exposure, low marsh buffer width, and proximity to boat ramps. At one comparison site with relatively high wave exposure, there was a significant positive relationship between MVI and historical shoreline change rates. This was not true at two other sites with less wave exposure, which indicate that other factors such as existing shoreline stabilization structures or low overall vulnerability can alter the relationship between the MVI and shoreline change rates.
A field study was carried out to investigate the effects of constructed oyster reefs and marsh vegetation – materials commonly used in living shoreline design – on dampening waves, the main driver of shoreline erosion. Constructed oyster reefs were effective at dampening waves up to 46% when water levels were low to moderate (< mean water depth); and, marsh vegetation was found to dampen waves by an average of 78% over a 20-meter marsh transect when water depths were high enough to flood the marsh. These results suggest that combining constructed oyster reefs with marsh vegetation may offer effective and sustainable long-term reduction in marsh vulnerability.
A spatial model, The Living Shoreline Explorer Model (LSEM), was developed to identify shorelines where conditions were suitable for the use of living shoreline stabilization methods. The LSEM designated 85% (237 km) of mainland shoreline along Virginia’s coastal bays as suitable for nature-based shoreline stabilization projects, with the remaining 15% recommended for more traditional hard stabilization. The LSEM can be used as a screening tool for coastal stakeholders in examining living shoreline stabilization options, with the understanding that successful living shoreline design and placement requires a site visit for a final determination.
The MVI and LSEM developed for this study offer coastal stakeholders context for understanding their shoreline and a guidepost for pursuing an appropriate shoreline stabilization plan. Data from both spatial models were incorporated into The Nature Conservancy’s Coastal Resilience web mapping decision-support tool, where they can be analyzed with other spatial data to visualize vulnerability and identify nature-based solutions to coastal erosion problems. Future iterations of the MVI and LSEM will benefit from access to this comprehensive data repository for marsh shorelines in the VCR.
