Particle trapping in stratified estuaries: Application to observations

Estuarine turbidity maxima (ETM) retain suspended particulate matter (SPM) through advection, settling, aggregation, and nonlinearities in bed processes, but the relative importance of these processes varies strongly between systems. Observations from two strongly advective systems (the Columbia and Fraser Rivers) are used to investigate seasonal cycles of SPM retention and the effects of very high flows. Results for the Fraser and Columbia plus literature values for 13 other estuaries illustrate the applicabitity of scaling parameters and the response of ETM phenomena to a range of river flow (U-R) levels and tidal forcing. The most efficient trapping (represented by Trapping Efficiency E, the ratio of maximum ETM concentration to the source SPM concentration) occurs for low ratios of river flow to tidal current amplitude (UT), represented by low values of the Supply number S-R. E in the Columbia is found to be maxima] in a null zone where advection or tidal asymmetry (represented by Advection number A) is weak (A similar to 0). The ratio of aggregation to disaggregation (the Floe number Theta) is maximal on neap tides, while the ratio of erosion to deposition (the Erosion number Pi) is maximal on spring tides. The ratio of settling velocity to vertical mixing (Rouse number P) is relatively constant in the Columbia ETM (P similar to 0.7), because particle settling velocity and turbulence levels adjust together. Assuming that this result applies broadly, scaling variables and data are combined to express ETM properties in terms of the friction velocity (U-*), UR, and UT, allowing a considerable simplification of the parameters used to describe ETM.