Tidally influenced alongshore circulation at an inlet-adjacent shoreline

Continental Shelf Research
By: , and 

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Abstract

The contribution of tidal forcing to alongshore circulation inside the surfzone is investigated at a 7 km long sandy beach adjacent to a large tidal inlet. Ocean Beach in San Francisco, CA (USA) is onshore of a ∼150 km2 ebb-tidal delta and directly south of the Golden Gate, the sole entrance to San Francisco Bay. Using a coupled flow-wave numerical model, we find that the tides modulate, and in some cases can reverse the direction of, surfzone alongshore flows through two separate mechanisms. First, tidal flow through the inlet results in a barotropic tidal pressure gradient that, when integrated across the surfzone, represents an important contribution to the surfzone alongshore force balance. Even during energetic wave conditions, the tidal pressure gradient can account for more than 30% of the total alongshore pressure gradient (wave and tidal components) and up to 55% during small waves. The wave driven component of the alongshore pressure gradient results from alongshore wave height and corresponding setup gradients induced by refraction over the ebb-tidal delta. Second, wave refraction patterns over the inner shelf are tidally modulated as a result of both tidal water depth changes and strong tidal flows (∼1 m/s), with the effect from currents being larger. These tidally induced changes in wave refraction result in corresponding variability of the alongshore radiation stress and pressure gradients within the surfzone. Our results indicate that tidal contributions to the surfzone force balance can be significant and important in determining the direction and magnitude of alongshore flow.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Tidally influenced alongshore circulation at an inlet-adjacent shoreline
Series title Continental Shelf Research
DOI 10.1016/j.csr.2013.01.017
Volume 56
Year Published 2013
Language English
Publisher Elsevier
Contributing office(s) Pacific Coastal and Marine Science Center
Description 13 p.
First page 26
Last page 38
Country United States
State California
County San Francisco
Other Geospatial Ocean Beach
Online Only (Y/N) N
Additional Online Files (Y/N) N