Ilgar Safak

Barrier island breach evolution: Alongshore transport and bay-ocean pressure gradient interactions

Physical processes controlling repeated openings and closures of a barrier island breach between a bay and the open ocean are studied using aerial photographs and atmospheric and hydrodynamic observations. The breach site is located on Pea Island along the Outer Banks, separating Pamlico Sound from the Atlantic Ocean. Wind direction was a major control on the pressure gradients between the bay and the ocean to drive flows that initiate or maintain the breach opening. Alongshore sediment flux was found to be a major contributor to breach closure. During the analysis period from 2011 to 2016, three hurricanes had major impacts on the breach. First, Hurricane Irene opened the breach with wind-driven flow from bay to ocean in August 2011. Hurricane Sandy in October 2012 quadrupled the channel width from pressure gradient flows due to water levels that were first higher on the ocean side and then higher on the bay side. The breach closed sometime in Spring 2013, most likely due to an event associated with strong alongshore sediment flux but minimal ocean-bay pressure gradients. Then, in July 2014, Hurricane Arthur briefly opened the breach again from the bay side, in a similar fashion to Irene. In summary, opening and closure of breaches are shown to follow a dynamic and episodic balance between along-channel pressure gradient driven flows and alongshore sediment fluxes.

Persistent Shoreline Shape Induced From Offshore Geologic Framework: Effects of Shoreface Connected Ridges

Mechanisms relating offshore geologic framework to shoreline evolution are determined through geologic investigations, oceanographic deployments and numerical modeling. Analysis of shoreline positions from the past 50 years along Fire Island, New York, a 50 km long barrier island, demonstrates a persistent undulating shape along the western half of the island. The shelf offshore of these persistent undulations is characterized with shoreface-connected sand ridges (SFCR) of a similar alongshore length scale, leading to a hypothesis that the ridges control the shoreline shape through the modification of flow. To evaluate this, a hydrodynamic model was configured to start with the US East Coast and scale down to resolve the Fire Island nearshore. The model was validated using observations along western Fire Island and buoy data, and used to compute waves, currents and sediment fluxes. To isolate the influence of the SFCR on the generation of the persistent shoreline shape, simulations were performed with a linearized nearshore bathymetry to remove alongshore transport gradients associated with shoreline shape. The model accurately predicts the scale and variation of the alongshore transport that would generate the persistent shoreline undulations. In one location, however, the ridge crest connects to the nearshore and leads to an offshore-directed transport that produces a difference in the shoreline shape. This qualitatively supports the hypothesized effect of cross-shore fluxes on coastal evolution. Alongshore flows in the nearshore during a representative storm are driven by wave breaking, vortex force, advection and pressure gradient, all of which are affected by the SFCR.

Nonlinear and directional effects on wave predictions over muddy bottoms: central chenier plain coast, Western Louisiana Shelf, USA

The sensitivity of wave-mud interaction on directionality and nonlinearity is investigated. A phase-resolving nonlinear wave model which accounts for directional wave propagation and mud damping is used to simulate wave propagation over a muddy shelf. Field data from an experiment conducted at the central chenier plain coast, western Louisiana, USA are used to validate the model. Recently, verification of a one-dimensional wave model with the field data showed that this model was able to replicate the evolution of wave spectra over muddy bottoms. In this study, unidirectional wave spectra were also run through the parabolic model, but with various initial angles. Linear wave model runs were also performed in order to gauge the effect of nonlinear evolution on the results. Significant wave height and total energy contained in three different spectral bands from the model are compared to the data over the shelf, and correlation metrics calculated. While the model generally performs well no matter the initial angle, at no point does a zero initial angle compare best to the data, indicating that a unidirectional model may be missing some of the dynamical features of wave propagation over a muddy shelf. Furthermore, despite similar correlation scores between linear and nonlinear model comparisons of bulk statistics, it is seen the linear model does not replicate some aspects of the spectral evolution (such as low-frequency generation and amplification) shown in the data and captured by the nonlinear model. Despite the relatively short propagation distance, the effects of both directionality and nonlinearity play a noticeable role in wave evolution over a muddy seabed.