The impact of salt and sediment density stratification on vertical turbulent mixing in the Yangtze River Estuary

This study examines vertical density stratification of salt and sediment in estuaries. Data were analyzed from the south side of a 12.5 m deep channel along the north channel of the Yangtze Estuary, including water, salt and sediment measurements. The overall distribution characteristics of salt and sediment content in the north channel waters were analyzed. The vertical turbulent diffusion coefficient was calculated using the Richardson number method and PP81 zero-equation model. This investigated the effect of vertical salt and sediment density stratification on vertical turbulent diffusion in the north channel waters. Results show the diffusion coefficient increases from near-bottom to near-surface layers with stratification. Stratification has a more pronounced inhibitory effect in the middle and lower reaches. Salt stratification results in less inhibition during spring than neap tides, and less during flood than ebb tides. Sediment stratification causes more inhibition during spring than neap tides, and more during flood than ebb tides. Salt stratification is the primary inhibitory factor, with coefficients up to 5 times lower under salt versus clear water. Inhibition by sediment is weaker, with coefficients up to 2 times lower under mud versus clear water, and strongest inhibition in near-bottom layers. Combined salt and sediment stratification more markedly inhibits material diffusion, with maximum coefficients of around 22 times lower in near-bottom mid-channel areas.