Alongshore momentum balance analysis on a cuspate foreland

Abstract

[1] Nearshore measurements of waves and currents off Cape Hatteras, North Carolina, U.S.A, are used to investigate depth-averaged subtidal circulation and alongshore momentum balances in the surf and inner shelf region around a cuspate foreland. Data were collected on both sides of the cape representing shorefaces with contrasting shoreline orientation (north-south vs. northwest-southeast) subjected to the same wind forcing. In the nearshore, the subtidal flow is aligned with the local coastline orientation while at the cape point the flow is along the existing submerged shoal, suggesting that cape associated shoals may act as an extension of the coastline. Alongshore momentum balance analysis incorporating wave-current interaction by including vortex and Stokes-Coriolis forces reveals that in deep waters surface and bottom stress are almost in balance. In shallower waters, the balance is complex as nonlinear advection and vortex force become important. Furthermore, linearized momentum balance analysis suggests that the vortex force can be of the same order as wind and wave forcing. Farther southwest of Cape Hatteras point, wind and wave forcing alone fail to fully explain subtidal flow variability and it is shown that alongshore pressure gradient as a response to the wind forcing can close the momentum balance. Adjacent tide gauge data suggest that the magnitude of pressure gradient depends on the relative orientation of local coastline to the wind vector, and in a depth-averaged sense the pressure gradient generation due to change in coastline orientation even at km length scale is analogous to the effect of alongshore variable winds on a straight coastline.

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

“…Large scale tendencies for currents to follow bathymetric contour lines has been observed in other cases of inner shelf current analysis, along coastline orientations due to continuity effects (Kumar et al 2013) and along small scale ripple orientations due to bottom roughness (Madsen et al 2010). …”

“…terms, and the right hand side of the equation is grouped as a force term, defined as current-independent terms (Kumar et al 2013).…”

“…The dynamics in this region are dominated by the forcing of alongshore wind stress and alongshore pressure gradients, balanced by bottom stress response (Pettigrew 1980, Lentz and Winant 1986, Lentz 1994, Liu and Weisberg 2005, Gutierrez et al 2006, Fewings and Lentz 2010, Lentz and Fewings 2012. Coastal and bathymetric variations modify inner shelf circulation and the momentum balance (Dragos and Aubrey 1990, Yankovsky and Garvine 1998, van de Meene and van Rijn 2000a, Kirincich and Barth 2009, Kumar et al 2013). …”

Observations and 3D hydrodynamics-based modeling of decadal-scale shoreline change along the Outer Banks, North Carolina

“…Tidal forcing was imposed using constituents derived from the US Atlantic Coast ADCIRC database [45]. In addition to these hindcasts of waves, winds, and tides that were used as model forcings, field observations of waves from nearby buoys available (1996)(1997)(1998)(1999)(2000)(2001)(2002)) and a nearshore experiment in 2010 within the section of interest [40,31] were used for validation of the model.…”

Alongshore momentum transfer to the nearshore zone from energetic ocean waves generated by passing hurricanes

“…Previous studies have considered different drag coefficients inside and outside the surf zone [ Lentz et al ., ; Feddersen and Guza , ] and Kumar et al . [], for example, obtained a cross‐shore variable drag coefficient using regression analysis to account for wave‐enhanced roughness in shallower water. In the present study, we apply a cross‐shore constant drag coefficient of c d = 0.0023 that results in high correlation ( R = 0.80 for Bill; R = 0.93 for Earl) between the bottom stress and radiation stress gradient terms inside the surf zone.…”