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 = PΔU/U_T scales the landward transport of SPM in terms of flood-ebb velocity difference (ΔU; the internal asymmetry) and maximum tidal current (U_T). Supply number S_R = PU _R/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 Θ = Φ/Γ that describes the balance of aggregation (Φ) and disaggregation (Γ). The balance between erosion and deposition at the bed is described by the Erosion number Π = Ψ/Ω, the ratio of erosion (Ψ) to deposition (Ω). 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.