Near bottom velocity measurements in San Francisco Bay, California

The ability to accurately measure long-term time-series of tidal currents in bays and estuaries is critical in estuarine hydrodynamic studies. Accurate measurements of tidal currents near the air-water interface and in the bottom boundary layer remain difficult in spite of the significant advances in technology for measuring tidal currents which have been achieved in recent years. One of the objectives of this study is to demonstrate that turbulent mean velocity distribution within the bottom boundary layer can be determined accurately by using a broad-band acoustic Doppler current profiler (BB-ADCP). A suite of instruments, including two BB-ADCPs and four electromagnetic (EM) current meters was deployed in San Francisco Bay, California in an investigation of resuspension and transport of sediment during March 1995. The velocity measurements obtained in the bottom boundary layer by BB-ADCP were highly coherent (r2>0.94) with the velocity measurements obtained by EM current meters. During early March 1995, both BB-ADCPs and EM current meters recorded a very unusual flow event. Agreement among independent measurements by these instruments in describing such an atypical hydrodynamic occurrence further validates the velocity measurements obtained by BB-ADCP in the bottom boundary layer.