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

Abstract

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.

Shoreface mesoscale morphodynamics: A review

“…However, advances in the study of the role of the shoreface in the coastal system and sediment budget suggest that shoreface sediment (even beyond the depth of closure) might in fact play a key role in nearshore morphodynamics. Indeed, several studies (Billy et al, 2013;Brunel et al, 2014;Lazarus and Murray, 2011;Safak et al, 2017;Schupp et al, 2006;Thomas et al, 2011;Valvo et al, 2006) have linked shoreline shape and mesoscale variation patterns to adjacent shoreface setting. Frihy et al (1991) examined the link between shoreline evolution and nearshore bathymetric change in the Nile Delta, Egypt, and found no significant relationship between shoreline change and nearshore (i.e.…”

“…Those studies show that the shoreface acts as a source and/or a conduit for onshore sediment flux and has a role in foreshore long-term (>10 1 years) morphodynamics; there is an apparent link between the volume of shoreface sediment and foreshore behaviour. Safak et al (2017) subsequently used a three-dimensional, hydrodynamic-based model to examine the effects of shoreface connected sand ridges on shoreline variability at Fire Island. Their results show that hydrodynamic processes and sediment flux are controlled by the effects of offshore geologic framework and that cross-shore flows between the shelf and the coast (i.e.…”

Correlation Between Shoreline Change and Planform Curvature on Wave‐Dominated, Sandy Coasts

“…Offshore bathymetric features, such as shoreface‐attached sand ridges, can influence both shoreline shape and shoreline change. Fire Island (NY 4) provides a clear example, displaying a roughening signal on scales greater than 1 km in our analysis (Figure , Table ), likely caused by the presence of shoreface‐connected sand ridges offshore (Safak et al, ). These features apparently act as a cross‐shore source of sediment, resulting in accretion and subtle convex bumps along the shoreline.…”

“…One of the shoreline sections, NY 4 (Fire Island, Figure 5), displays a roughening signal on both shortterm and long-term timescales despite not undergoing nourishment. This signal can be attributed to the presence of shoreface-attached sand ridges acting as an offshore sediment source (Safak et al, 2017;Section 4.3). The Virginia Barrier Islands are characterized by short, uninhabited islands with a strong tidal influence.…”

Geological constraints on mesoscale coastal barrier behaviour

“…Working at decadal timescales, Wijnberg (2002) showed shoreline behaviour along the Holland coast was strongly influenced by adjacent shoreface morphology, particularly the occurrence of shoreface-connected ridges (SCRs). These quasi-stationary features have also been linked to the development of long-term coastal concavities, for example on Fire Island, NY (Safak et al, 2017).…”