The influence of oceanic swell on flows over an estuarine intertidal mudflat in San Francisco Bay

Abstract

In this study, we examine the role that remotely forced ocean waves play in the hydrodynamics of an intertidal, estuarine mudflat. The observations indicate that long-period (10–20 s) ocean waves are a potentially important source of near-bed energy and shear stress in this environment. Over a two-week period in February 2001, we deployed an autonomous SonTek Hydra system on a mudflat in Central San Francisco Bay, and measured velocity and sediment concentration approximately 10 cm from the bed using an acoustic Doppler velocimeter (ADV) and an optical backscatter sensor (OBS). The experiment continued through wet (high tide) and dry (low tide) periods over an entire spring–neap cycle, and thus included the variation of near-bed velocity over a range of timescales. Results show that during large ebb tides, tidally forced flows dominate the near-bed dynamics during calm conditions. Wind waves dominate whenever the wind direction exposes the mudflat to wind coming off the bay (from the south and southwest), as occurs during winter storms. During periods when tidal forcing is limited and wind waves are small, remotely forced ocean swells become an important energy source. These motions appear in the burst samples at frequencies between 0.1 and 0.04 Hz and their energy correlates well ( ρ>0.8) with ocean swell measured from a buoy offshore of San Francisco. Spectral analysis of data shows that the average energy of ocean waves per tide varied between 2 and 15% of total energy load. Moreover, extreme values in the distribution of ocean waves bring episodic bursts of greater energy onto the estuarine mudflat, which may influence local suspension of sediments.