Particle trapping in stratified estuaries: Consequences of mass conservation

Estuarine turbidity maxima (ETM) can retain suspended particulate matter (SPM) through advection, settling, aggregation, and nonlinearities in bed processes. We define a parameter space descriptive of ETM water column particle trapping processes through a scaling analysis of the local and integral SPM balances. There are six primary non-dimensional parameters for the large particles or aggregates that are typically trapped in an ETM. Rouse number P, the ratio of settling velocity W-S to the shear velocity U-*, describes the material trapped in the ETM in terms of the local vertical balance between vertical mixing and aggregate settling. Advection number A = P Delta U/U-T scales the landward transport of SPM in terms of flood-ebb velocity difference (Delta U; the internal asymmetry) and maximum tidal current (UT). Supply number S-R = PUR/U-T defines SPM supply and removal (U-R is river flow). Changes in the estuarine inventory of SPM are described in terms of a Trapping Efficiency E, a ratio of peak ETM concentration to fluvial or marine supply concentration. The effects of aggregation and disaggregation in the integral dynamic balance are quantified by a Floc number Theta = Phi/Gamma that describes the balance of aggregation (Phi) and disaggregation (Gamma). The balance between erosion and deposition at the bed is described by the Erosion number Pi = Psi/Omega, the ratio of erosion (Psi) to deposition (Omega). The non-dimensional, integral SPM conservation equation is then used to examine steady and unsteady particle trapping scenarios, including adjustments to a change in river flow and to a neap-spring transition in salinity intrusion and stratification.