Observations of Nepheloid Layers in the Yangtze Estuary, China, Through Phase-Corrupted Acoustic Doppler Current Profiler Speeds

Abstract

A bottom quadrapod was deployed from March 29 to April 5, 2009 to measure bottom boundary layer (BBL) flows and nepheloid layer properties in the Deepwater Navigation Channel in the North Passage of Shanghai Port in the Yangtze estuary. Using a downward-looking acoustic Doppler current profiler (ADCP) and acoustic Doppler velocimeter, detailed measurements of mean velocity and turbulence distribution within 1 m above the seabed were obtained. It appears that corrupted speeds measured for the deeper bins are caused by formation of the nepheloid layer at the seabed, implying that the ADCP is not a suitable instrument to measure current velocities in the bins nearest the seafloor. A statistical clustering method was used to characterize the current profiles in the BBL. The majority of current profiles within the BBL had a simple shape with current speed monotonically decreasing with depth, reflecting a logarithmic boundary layer. Phase-corrupted ADCP speeds measured for bins close to the bottom are shown to be useful as proxies to indicate the presence of primary and secondary lutoclines/nepheloid layers. A lutocline is a sediment-induced density gradient or pycnocline. The primary lutocline is closest to the bottom, and below it is the nepheloid layer, which is commonly composed of fluid mud. The proxies indicated that a nepheloid layer formed in the neap tide when the current velocity 1 m above the seabed dropped below a threshold of 0.65 m/s. The lutocline height was indicated to be about 0.2 m above the seabed. A secondary lutocline in the water column was also observed in the second half of the record, when the lowest maximum currents occurred.