Suspended Cohesive Particle Characteristics in the Connecticut River Estuary

Abstract

To determine the role of cohesive suspended particle characteristics on sediment transport patterns in an energetic estuary floc size, density, and settling velocity were investigated in the Connecticut River estuary over three years spanning varying fluvial discharge regimes. Concurrent measurements of in-situ floc size, flow, bed stress, salinity and suspended-sediment concentration (SSC) were used to identify primary influences on floc size variability. Water discharge ranged from 202 to 910 m³/s between the three sampling campaigns, and the timing of major sediment discharge events preceding measurement periods from 23 to 162 days. Two distinct particle populations were observed under high and low sediment discharge regimes. With abundant fluvial sediment input, flocculation occurred resulting in large, loosely-packed flocs dominating the suspended signal (median sizes of 194-209 µm; median excess densities of 13-17 kg/m³). Following an extended period of low sediment discharge, small, dense aggregates resuspended from the bed were observed throughout the water column (median size of 171 µm and excess density of 60 kg/m³). The timing of and partial decoupling of water and sediment discharge led to inter-annual patterns of cohesive particle characteristics controlled by fresh sediment supply. The large, light flocs with lower settling velocities characteristic of high sediment supply regimes likely bypass the estuary. Smaller compact aggregates dominated the low-sediment discharge regimes. However, the similar disaggregated size distribution of the two regimes suggests the same fine source material is reintroduced to the estuary with the intrusion of the salt wedge, which extends farther up-estuary during low discharge regimes and ultimately supplies the off-channel bays and coves.