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Traditionally, vertical circulation (induced by gravity circulation and tidal straining), tidal pumping, and resuspension are suggested as the major processes for the formation and maintenance of the estuarine turbidity maximum (ETM). Due to strong mixing, tidal pumping is considered as the dominating process in macrotidal estuaries. To analyze field observation data, the classical empirical decomposition method is commonly suggested, but the tidal pumping flux (TPF) based on this method may lead to erroneous conclusions about the mechanisms of ETM formation because the effects of advection induced by the horizontal SSC gradient and fine bed sediment supply are ignored. If these effects are included, the TPF clearly reproduces the convergence patterns and thus demonstrates its role in the formation of the ETM. By a simplified analytical solution, the TPF is the result of the competition between the downstream flux induced by the river current together with the lag in sediment response and the upstream flux induced by tidal asymmetry and the lag. Field observations in the well-mixed macrotidal Yalu River estuary (located between China and North Korea) were analyzed. Tidal pumping is identified as the dominant mechanism of its ETM formation, and the position of the ETM for different river discharges and sediment settling velocities can be predicted by the concept of tidal pumping by numerical and analytical procedures. The present study provides a typical example of how to evaluate the tidal pumping contributions on ETM formation using the combined information provided by field data, numerical modeling results, and analytical solutions.

[1] Estuarine boundary layer and water column in situ measurements of hydrodynamics, sediment resuspension, and sediment particle size distribution are presented for a macrotidal environment in SE China. Vertical and tidal variability of sediment size and its relationship with turbulence and hydrodynamic forcing are examined using time series from two week long experiments after they are phase-averaged to reconstruct typical neap and spring tidal cycles. In situ particle size distributions obtained using laser diffraction show clear evidence of flocculation processes that change dynamically during the tidal cycle. Mean particle size of particles in suspension is found to be one order of magnitude larger than the primary size of the sediment. The coarser particles in suspension were present in the upper water column, whereas the finer particles were confined predominantly within the bottom boundary layer. Correlation analysis indicated that aggregate size appears to be controlled by turbulence more than any other parameters with floc size being inversely related to turbulence dissipation, while settling velocity of aggregates being proportional (on a log scale) to turbulence dissipation. Simple statistic and dynamic models incorporating the turbulence parameter are adopted and compared with previously developed models ; the comparative study using our data sets shows that the dynamic model of Winterwerp (1998) as modified by Law et al. (2013) to include advection qualitative captures both the tidal and vertical variability of aggregate size.

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