Roberts Briggs, Tiffany

As sea level rises, saltwater migration can threaten coastal ecosystems and beach-dune environments, which negatively impacts coastal flora. This study uses ground penetrating radar (GPR) to evaluate the spatiotemporal variability of saltwater migration in the near shore at high lateral resolution (i.e. cm) by using daily micro tidal cycles as analogs to infer saltwater migration. Time-lapse GPR profiles were collected at low and high tide capturing phase lags of the tidal flux through different substrates. GPR measurements were collected at two sites in Miami with contrasting lithologies: a) Crandon Park, composed of unconsolidated sand; and b) the Barnacle Historic State Park, composed of the Miami Limestone Formation. Laboratory-scale GPR measurements were collected over samples mimicking field conditions. The results may be helpful to identify regions vulnerable to saltwater migration in the near shore based on lithological variability, and to mitigate negative impacts for flora in beach-dune habitats during sea level rise.

Beach morphology changes naturally with seasonal and event-driven variability in the wave climate, as well as due to anthropogenic activities such as erosion mitigation efforts. In 2017, category four Hurricane Irma caused beach erosion and dune overwash in Boca Raton, FL. Immediate post-storm perigean spring tides coupled with typical winter high-wind conditions imposed a regime of spatially and temporally extended meteorologic and morphologic variability. This study evaluates the morphologic evolution and post-storm recovery in the first year following Hurricane Irma. Time-series topographic surveys and surface sediment samples were collected. Patterns of accretion and erosion were evaluated with regionally measured water and wind levels. Recovery morphology was generally through berm-building, but lacked shoreline stability. Storm impact regime, mitigation structures, and sediment transport patterns drove the recovery. Total volume lost above the 0 m contour due to the storm was not fully recovered within the year, with a large volume measured in the south.

For the last several decades, beach nourishment has been a widely implemented erosion mitigation strategy along many developed coastlines. Measuring subsequent patterns of erosion and accretion can help elucidate local sediment transport trends, improve time scale predictions of profile equilibration, decrease renourishment intervals, and adjust future engineering design of nourishments. This study evaluates the morphologic evolution of two beach nourishment projects (e.g., characterized as a full and partial nourishment) at the same location in Boca Raton, Florida using time series beach profiles, surface sediment samples, and wave data. More than 85% of sediment volume was retained within the full nourishment six months after project completion, compared to 50% retained eight months after completion of the partial nourishment. Wave energy largely influenced immediate post nourishment change. Profile equilibration was controlled by high-energy events (i.e., hurricanes) for both nourishments.