Detecting hydrodynamic changes after living shoreline restoration and through an extreme event using a Before-After-Control-Impact experiment

Abstract

Stabilization of eroding estuarine shorelines using living shoreline techniques, including native vegetation and nearshore structural components, has the potential to combat erosion while increasing shoreline ecotone function. However, there are few detailed field studies and little quantitative data available to assess hydrodynamic changes that occur immediately following living shoreline implementation. To address this gap, detailed hydrodynamic observations were made along eroding and stable reference shorelines over a 16-month period spanning living shoreline stabilization, which included the landfall of a major hurricane (Irma) 9 weeks after stabilization. In the months following stabilization, planted vegetation was sparse and shoreline hydrodynamics were governed by water level relative to breakwater structures. When water levels were at or below breakwater crest elevation, current velocities were initially reduced by 62% and wave heights by up to 83%; however, at higher water levels, shoreline velocities at the stabilized site vastly exceeded those observed at a nearby bare control site. Sixteen months after stabilization, flow-vegetation interactions had become a dominant control over shoreline hydrodynamics, and current attenuation was similar to that observed in nearby mature mangrove vegetation. Additionally, turbulence dissipation rates at the stabilized site (2.2∙10−5 m2/s3) and vegetated reference site (1.1∙10−5 m2/s3) were an order of magnitude greater during boat wake events compared to the bare shoreline site (1.6∙10−6 m2/s3,p < 0.001). This first experimental assessment of hydrodynamic effects related to living shoreline stabilization indicates that more than one year may be required before planted vegetation meaningfully influences shoreline hydrodynamics.