Ebb-tidal delta and inlet dynamics as a control on adjacent beach morphology

Many of the world's largest coastal cities are located on estuaries or bays that provide protection for ships and facilitate trade. Therefore, the physical processes occurring at the adjacent open coast beaches are of societal interest given the large local population and urban infrastructure, yet are often complex due to the presence of a tidal inlet, ebb-tidal delta, or river mouth. In this dissertation, such a complex coastal site is examined. Hydrodynamics and shoreline change are investigated at Ocean Beach in San Francisco, CA (USA), a 7-km long sandy beach that is directly south of a large tidal inlet and onshore of an ebb-tidal delta. More than five years of monthly topographic surveys have been conducted at Ocean Beach between 2004 and 2009 and reveal that seasonal shoreline changes are the dominant signal and are linked to the seasonal variability in offshore wave height. The second largest signal is the counterclockwise rotation of the shoreline that results from multi-decadal contraction of the ebb-tidal delta.;Based on the observed shoreline response, a numerical model was used to determine the dominant forces driving alongshore circulation, which ultimately drives long-term shoreline change. Fourteen years of wave buoy data and a 24.8 hour representative tidal cycle were used to generate an ensemble alongshore momentum balance. The ensemble momentum balance reveals that the alongshore circulation at Ocean Beach is driven by the radiation stress gradients (caused by oblique wave approach), a wave-driven pressure gradient resulting from wave focusing by the ebb-tidal delta, and a tide-driven pressure gradient caused by tidal flux through the inlet.;A field experiment was carried out to provide direct measurements of the alongshore pressure and radiation stress gradients. The field measurements show that during the week long deployment the alongshore wave-driven pressure gradient was the largest term of the momentum balance and resulted from wave height differences outside the surf zone along the ∼11 m isobath of ∼10% over a 1 km alongshore extent. Radiation stress gradients were often five times less than the pressure gradient and mostly acted in the opposing direction. Wave height gradients of the magnitude observed outside the surf zone can be introduced by many common bathymetric features, suggesting that the alongshore pressure gradient, which is typically neglected, may be an important driver of flows at many other sites worldwide.