Dynamic evolution of a secondary turbidity maximum under various forcing conditions in a microtidal estuary

Axial survey and in situ mooring in microtidal Masan Bay were conducted to reveal the mechanism for movement of secondary turbidity maximum (STM) and sediment resuspension. The Bay is characterized by sluggish water circulation with a vertically well-mixed water column. The strength of the residual estuarine current (Δu¯) was enhanced by the combined effects of decreased tidal currents, increased precipitation, and down-estuary (northerly) winds, whereas Δu¯ was weakened by increased tidal currents and up-estuary (southerly) winds. The variability of current asymmetry originated from Δu¯ contributed to the entrapment of suspended sediments, creating a mobile sediment pool in central regions of the Bay. The sediments resuspended from the mobile sediment pool were more influenced by residual currents than the tidal currents, and they remained in suspension near bed. When the down-estuary winds were applied to the mobile sediment pool, the near-bed sediments were readily resuspended to form an STM > 40 mg l−1. The STM moved up-estuary by persistent high Δu¯ and down-estuary by an intermittent low Δu¯. Over the entire measurement periods, the STM moved by at least 450 m toward the up-estuary. As the consolidated sediment layer was exposed by the movement of the STM, near-bed currents were ineffective in resuspending sediments from the consolidated layer even though the winds provided sufficient current strength. Due to the depletion of available sediments for resuspension, the STM was not fully generated. This study highlights that the STM in a microtidal estuary can be determined by the combined function of various forcings such as tide, wind, and precipitation.