On the links between a river’s hyperpycnal plume and marine benthic nepheloid layer in the wake of a typhoon

In 2010 two moorings each configured with a CTD and an ADCP, one with an additional non-sequential sediment trap (NSST), were deployed in the head region of the Gaoping Submarine Canyon 3 days after the typhoon-induced peaks of the runoff and suspended sediment concentration (SSC) of the Gaoping River in southern Taiwan. Our data show a demarcation between tidal and hyperpycnal regimes in the temperature, salinity, and flow fields. The latter existed in the first 5 days out of the 18-day deployment, as defined by higher water density due to high SSC. Several lines of evidence indicate the presence of the tail end of a hyperpycnal turbidity current (HTC), including the retention of warm water near the canyon floor, high SSC, down-canyon directed residual flow and its vertical structure, and high terrestrial fraction (larger than 70%) of the organic particles carried by the flow. The decreasing mass flux during the deployment is also an indication of a waning HTC. Our findings also show that the vertical flow structure and the direction of the gravity-driven down-canyon HTC were retarded by the instantaneous up-canyon-directed tidal oscillations in the submarine canyon.