Drifter observations of the effects of shoals and tidal-currents on wave evolution in San Francisco Bight

Abstract

Detailed observations of wave evolution and wave–current interaction in tidal inlets and river mouths are practically non-existent. This is in part due to the practical difficulty of installing and maintaining fixed instruments in this harsh environment with large waves, strong currents, dynamic seabed morphology, and often busy ship traffic, but also due to the fact that it is difficult to resolve the spatial variability and evolution of the wave and current field from an array of point measurements. This work explores the use of newly developed small, free-drifting buoys to collect wave and current measurements in a coastal inlet. The instruments, referred to as wave-resolving drifters (or WRD), are small and lightweight enough so that they can be deployed and retrieved from small vessels, and relatively inexpensive so that large numbers can be used. The surface-following drifters resolve the three-dimensional wave orbital motion and surface current field by combining GPS and accelerometer measurements. We validate the WRD platform and its sensor package in open ocean conditions in Monterey Bay by comparing the WRD observations to observations made by a collocated 40cm-diameter Datawell Waverider buoy. To study wave evolution in the San Francisco Bight, 30 WRDs are deployed near the San Francisco Bay entrance (Golden Gate) during peak ebb tide so that the drifters flow out of the bay, and into the incident wave field. Wave statistics are estimated through local ensemble averaging of drifter observations, and ensemble-averaged wave spectra are used to capture the wave evolution through the inlet area. Comparisons with numerical simulations of the simulating waves near shore (SWAN) model help identify the various processes acting on different frequency ranges of the wave field, and ray diagrams show the distinct effects of refraction by variable depth on the lower-frequency swells and refraction by currents on the higher-frequency wind waves. This combined analysis demonstrates the potential of using relatively inexpensive surface-following drifters to investigate surface dynamics in a complicated and energetic coastal inlet.