Cigdem Akan

Coastal and riverine communities with anthropogenic congestion and natural and economic resources are vulnerable to climate change impacts, including rising sea levels and increasing severity and frequency of storms. Coastal habitats are increasingly being recognized as natural infrastructure that provides resilience against these stressors. However, few studies have analyzed coastal vulnerability at the landscape scale with finely resolved spatial data that account for habitats and demographics. This study aims to assess the coastal vulnerability of the St. Johns River and the adjacent Northeastern Florida Atlantic shoreline within the St. Johns River Water Management District. Unique to this study is that natural habitats, different sea level rise scenarios, and human demographics were considered. Specifically, the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST)® coastal vulnerability model with seven metrics (geomorphology, relief, natural habitats, sea level change, wave exposure, wind exposure, and surge potential) was used to create a coastal exposure index for shore points. Results from three sea level rise scenarios (current, 2050 Intermediate-High, and 2100 Intermediate-High, (Sweet et al. 2017) showed spatial correlations between coastal exposure indexes and habitat role values. Rising sea levels increased coastal exposure and habitat protective roles, while vulnerability in population density and poverty grew without habitats. Conversely, low vulnerability areas had high concentrations of mangroves. The results can help prioritize beneficial habitat types and areas where protection and/or restoration are needed to protect shorelines and disadvantaged communities. This type of coastal vulnerability study could aid resiliency planning efforts in Northeastern Florida and could be expanded upon for other socioeconomic, infrastructure, or ecosystem queries.

The St. Johns River, located in northeast Florida, USA, is a large watershed characterized by relatively flat topography, porous soils, and increasing urbanization. The city of Jacksonville, Florida is located near the downstream terminus of the river near the Atlantic Ocean. The lower portion of the watershed located downstream of Lake George is subjected to tidal exchange and storm surge from tropical storms and hurricanes as well as extra-tropical winter storms. Extreme flood events in the Lower St. Johns River can be caused by rain-driven runoff, high tide, storm surge or any combination of the three. This study examines the range of potential extreme flood discharges caused by rain-driven runoff within six larger sub-basins located in the Lower St. Johns River with special emphasis on the Pablo Creek sub-basin. The study uses multiple methods including published flood insurance data, two statistical hydrology methods, and model simulations to estimate an array of flood discharges at varying return frequencies. The study also examines the potential effects on flood discharges from future land use changes and the temporal distribution of rainfall. The rain-driven flood discharge estimates are then fit to a normal distribution to convey the overall risk and uncertainty associated with the flood estimates. Overall, the study revealed that a wide range of reasonable rainfall-driven flood estimates are possible using the same data sets. The wide range of estimates will help inform future resiliency projects planned in the study area by providing a more realistic set of bounds with which planning can proceed. The estimates derived herein for the Pablo Creek sub-basin can be combined with the independent or dependent effects of tide and storm surge in order to characterize the total flood resiliency risk of the region.

Large-eddy simulation (LES) of a wind- and wave-forced water column based on the Craik–Leibovich (C–L) vortex force is used to understand the structure of small-scale Langmuir circulation (LC) and associated Langmuir turbulence. The LES also serves to understand the role of the turbulence in determining molecular diffusive scalar flux from a scalar-saturated air side to the water side and the turbulent vertical scalar flux in the water side. Previous laboratory experiments have revealed that small-scale LC beneath an initially quiescent air–water interface appears shortly after the initiation of wind-driven gravity–capillary waves and provides the laminar–turbulent transition in wind speeds between 3 and wind speed but with different wave parameters (wavelength and amplitude) in the C–L vortex force. It is observed that longer wavelengths lead to more coherent, larger centimetre-scale LC providing greater contribution to the turbulent vertical scalar flux away from the surface. In all cases, the molecular diffusive scalar flux at the water surface relaxes to the same statistically steady value after transition to Langmuir turbulence occurs, despite the different wave parameters in the C–L vortex force across the simulations. This implies that the small-scale turbulence intensity and the molecular diffusive scalar flux at the surface scale with the wind shear and not with the wave parameters. Furthermore, it is seen that the Langmuir (wave) forcing (provided by the C–L vortex force) is necessary to trigger the turbulence that induces elevated molecular diffusive scalar flux at the water surface relative to wind-driven flow without wave forcing.

The St. Johns River watershed, located in Florida, USA, is the focus of planning efforts to improve the resiliency and management of water resources infrastructure from current and future flood threats. These threats are driven by intense urbanization of the basin combined with increased frequency and intensity of coastal storms, made worse by sea level rise. Research efforts have begun to develop a comprehensive system of coupled numerical simulation models of the entire watershed in order to assess the current and long-term risks from flood inundation. This study provides a discussion of the system-wide model design and preliminary development. The study first outlines the project area and various numerical models to be coupled together. Then, the study discusses preliminary model development efforts and challenges. This initial study revealed the overall complexity and size of the undertaking. Overall, the study’s primary conclusion is that while an integrated system of coupled models is feasible and potentially very useful for long-term planning, the computational challenges for such an undertaking are extensive.

Coherent vortices with horizontal swirl arise spontaneously in the wave-driven nearshore surf zone. Here, a demonstration is made of the much greater robustness of coherent barotropic dipole vortices on a sloping beach in a 2D shallow-water model compared with fully 3D models either without or with stable density stratification. The explanation is that active vortex tilting and stretching or instability in 3D disrupt an initially barotropic dipole vortex. Without stratification in 3D, the vorticity retains a dipole envelope structure but is internally fragmented. With stratification in 3D, the disrupted vortex reforms as a coherent but weaker surface-intensified baroclinic dipole vortex. An implication is that barotropic or depth-integrated dynamical models of the wave-driven surf zone misrepresent an important aspect of surf-eddy behaviour.

The ROMS‐WEC model [Uchiyama et al., 2010] based on an Eulerian wave‐averaged vortex‐force asymptotic theory of McWilliams et al. (2004) is applied to analyze 3‐D transient wave‐driven rip currents and associated intrinsic very low‐frequency (VLF) variability in the surf zone on a surveyed bathymetry under spatiotemporally uniform offshore incident waves. The 3‐D rip currents are substantially depth‐dependent due to the vertical recirculation, composed of pairs of counter‐rotating longitudinal overturning roll cells that promote surface convergence. The vortex force plays an important role in vorticity budget, preconditioning overall vorticity reduction. These rip currents are intrinsically unstable and contribute about 70% to kinetic energy (KE) as eddy kinetic energy (EKE), consistent with observations. The dominant fluctuation period fits the VLF band, at about 18 min. The current effect on waves (CEW) alters not only the mean rip structure, but also the associated turbulence as the modified cross‐shore EKE profile with considerable accentuation in the inner surf zone. Increased alongshore bathymetric variability proportionally intensifies KE and intrinsic EKE, whereas it reduces the VLF period. With a guide of a pseudo 2D model, we reveal that vortex tilting effect due to the horizontal vorticity inherent in the 3‐D rip currents promotes collapse of the 3‐D eddies through an enhanced forward kinetic energy cascade, leading to short‐lived, laterally‐stretched 3‐D eddies resulting in elongated filaments that decay more quickly than coherent, long‐lived, circular 2‐D eddies.