EVALUATING SEDIMENT, VEGETATION, AND SHORELINE DYNAMICS AT ROCK SILL LIVING SHORELINES IN CHESAPEAKE BAY

Abstract

Living shorelines are a natural and nature-based feature that is primarily composed of a restored or created narrow fringing marsh. Some designs incorporate elements of gray or hardened infrastructure such as low-lying rock sills. In many coastal areas including Chesapeake Bay, living shorelines with rock sills of various configurations have been implemented to replace aging gray infrastructure, address pervasive shoreline erosion, and adapt to changing conditions such as sea-level rise. Little is known about their long-term (> 5 years) efficacy to reduce or mitigate shoreline erosion, their impacts to the near-shore shallow waters and submersed aquatic vegetation, and if design-specific differences exist. In Chapter 1 of this study, we evaluated 100 living shorelines of three designs (continuous sill, headland breakwater, and segmented sill) aged 5-20 years for changes in shoreline position, post-installation change in marsh area, and submersed aquatic vegetation density patterns in the adjacent subtidal zone. All designs equally reduced lateral shoreline erosion since installation and facilitated rapid growth in marsh vegetation area that stabilized 5-10 years post-installation. Shifts in submersed aquatic vegetation density at the site-scale were explained by regional trends in submersed aquatic vegetation area. In Chapter 2, we evaluated the spatial patterns of sediment and vegetation characteristics in the intertidal and subtidal zones in segmented sill living shorelines and made comparisons to unaltered reference shorelines and to the continuous sill design. In segmented sill living shorelines, vegetation %cover, mud%, organic matter content, mass deposition rate, particulate carbon, nitrogen, phosphorus and their burial rates were consistently and often significantly lower within the gaps and/or behind the gaps than marsh behind the sill in the living shoreline and reference marshes. Behind the sill, sediment and vegetation characteristics were statistically similar to continuous sill and reference marshes. In the subtidal zone, no definitive spatial patterns emerged in sediment characteristics or in the distribution of submersed aquatic vegetation. This work provides the first quantification of spatial variability in segmented sill living shorelines, assesses impacts to near-shore submersed aquatic vegetation, and compares shoreline patterns among the most commonly implemented living shoreline designs in Chesapeake Bay.