Inner-shelf circulation and sediment dynamics on a series of shoreface-connected ridges offshore of Fire Island, NY

Abstract

Locations along the inner-continental shelf offshore of Fire Island, NY, are characterized by a series of shorefaceconnected ridges (SFCRs). These sand ridges have approximate dimensions of 10 km in length, 3 km spacing, and up to ∼8 m ridge to trough relief and are oriented obliquely at approximately 30°clockwise from the coastline. Stability analysis from previous studies explains how sand ridges such as these could be formed and maintained by storm-driven flows directed alongshore with a key maintenance mechanism of offshore deflected flows over ridge crests and onshore in the troughs. We examine these processes both with a limited set of idealized numerical simulations and analysis of observational data. Model results confirm that alongshore flows over the SFCRs exhibit offshore veering of currents over the ridge crests and onshore-directed flows in the troughs, and demonstrate the opposite circulation pattern for a reverse wind. To further investigate these maintenance processes, oceanographic instruments were deployed at seven sites on the SFCRs offshore of Fire Island to measure water levels, ocean currents, waves, suspended sediment concentrations, and bottom stresses from January to April 2012. Data analysis reveals that during storms with winds from the northeast, the processes of offshore deflection of currents over ridge crests and onshore in the troughs were observed, and during storm events with winds from the southwest, a reverse flow pattern over the ridges occurred. Computations of suspended sediment fluxes identify periods that are consistent with SFCR maintenance mechanisms. Alongshore winds from the northeast drove fluxes offshore on the ridge crest and onshore in the trough that would tend to promote ridge maintenance. However, alongshore winds from the southwest drove opposite circulations. The wind fields are related to different storm types that occur in the region (low-pressure systems, cold fronts, and warm fronts). From the limited data set, we identify that low-pressure systems drive sediment fluxes that tend to promote stability and maintain the SFCRs while cold front type storms appear to drive circulations that are in the opposite sense and may not be a supporting mechanism for ridge maintenance.

Alongshore Momentum Balance Over Shoreface-Connected Ridges, Fire Island, Ny

“…The significance of observed acceleration due to shallowing agrees with modelled SFCR-related dynamics . Our alongshore observational agreement with Warner et al (2014) suggests, but does not verify, that the recognition of cross-shore pressure gradient-induced current veering is accurate.…”

“…Vis-Star et al (2007) described wave orbital velocity convergence and divergence which cause ridge maintenance, presenting the possibility of ridge growth without a transverse slope. Warner et al (2014) studied in-situ current measurements of Fire Island SFCRs at water depths near 20m.…”

Inner-shelf ocean dynamics and seafloor morphologic changes during Hurricane Sandy

“…The similarity in characteristics suggests a common set of processes is responsible for the origin and maintenance of these sedimentary structures (Trowbridge, 1995;Falques et al, 1998;Calvete et al, 2001;Nnafie et al, 2014;Warner et al, 2014). These SFCRs offshore of Fire Island vary in size and configuration but are, in general, on the order of 10 km in length, have a crest to trough relief on the order of 6 m, spaced about 3 km apart, and oriented obliquely from the coastline on average about 30 degrees clockwise (Fig.…”

Characterizing storm response and recovery using the beach change envelope: Fire Island, New York

“…This information has been used to link observations with what is known about localized sediment supply and geology of a system (e.g. Aagaard et al, 2004;Houser et al, 2008;Lentz et al, 2013;Schwab et al, 2013;Warner et al, 2014;Hapke et al, 2016), providing better understanding of the signature these controls may exert on an area.…”