Alongshore momentum balance over shoreface-connected ridges, Fire Island, NY

Abstract

Hydrodynamic and hydrographic data collected on the inner shelf of Fire Island, NY, over a region of shoreface-connected ridges (SFCRs), are used to describe wind-driven circulation over uneven topographies along relatively straight coastlines. The data reveal a predominantly alongshore flow, under westward wind forcing, with localized offshore current veering over the SFCR crests and an onshore veering over the adjacent troughs. Alongshore momentum balance analysis of the observations reveals that total acceleration (i.e., local and advective) and bottom stress are balanced by wind stress and pressure gradient. A numerical model is used to verify the experimentally derived results and to analyze the cross-shore momentum balance which is not resolved by the experimental data. Analysis of the numerical and field data reveal that the total alongshore pressure gradient force can be considered as the sum of a local and a regional component. The former is correlated with the alongshore advective acceleration that develops over the crest of the ridges resembling a Bernoulli-like pressure-flow relationship. The regional alongshore pressure gradient is related to wind stress with which it maintains a strong, negative relationship. Over the ridge crests the cross-shore veering of the flow is due to mass conservation, associated with a balance between cross-shore advective acceleration and local cross-shore pressure gradient. Under variable wind conditions the regional pressure gradients are more important, as compared to quasi steady-state conditions which increase the influence of the local scale pressure gradient. A time scale of 6 h is found as the temporal scale required for the local pressure gradients to make a significant contribution to the momentum balance.