Particle trapping in a stratified flood-dominated estuary

Observations in the Navesink River estuary in northern New Jersey demonstra te that buoyancy augments the particle trapping tendencies of hood-dominate d systems because these estuaries heighten tidal period asymmetries in stra tification. During the long and slow ebb which typifies flood-dominated sys tems, a positive feedback between tidal straining and weak vertical mixing stratifies the estuary. In contrast, during hood, turbulence generated by t he stronger tidal currents augments overstraining of the density field and the water column becomes well mixed. The tidal period asymmetries in strati fication have profound effects on the vertical structure and transport of s uspended matter. During ebb, weak vertical mixing allows suspended material to settle downward. In contrast, strong turbulence during hood mixes suspe nded matter into the water column where it is transported up estuary. Furth ermore, observations reveal that resuspension events are marked by multiple turbidity spikes, suggestive of multiple, limited layers of erodible mater ial. The transport of the turbid waters is consistent with horizontal advec tion modified by horizontal dispersion. Periods of enhanced stratification are also marked by relatively low levels of turbidity during the ebb, consi stent with more complete settling of suspended material following times of high river discharge. The interplay between buoyancy and tidal asymmetries are further elucidated with a one-dimensional numerical model featuring a turbulent closure schem e and a passively settling tracer. Model results are generally consistent w ith the field observations, both emphasizing the robust particle trapping t endencies of a stratified flood-dominated estuary. We speculate that enhanc ed particle trapping following times of high river discharge may have impor tant biological consequences.