Tracer study of mixing and transport in the Upper Hudson River with multiple dams

Abstract In October 2001, ca. 0.2 mol of SF 6 was injected into the upper Hudson River, a modified natural channel with multiple dams, at Ft. Edward, NY. The tracer was monitored for 7 days as it moved ca. 50 km downriver. The longitudinal evolution of the tracer distribution was used to estimate 1-D advection (9.0 ± 0.2 km d -1 ) and dispersion (17.3 ± 4.0 m 2 s -1 ) along the river axis. Comparison of these results to tracer studies on channels without dams suggests that dams reduce longitudinal dispersion below the value expected in a natural channel with the same discharge. SF 6 loss through air-water gas exchange along the river and at two dams (10.7 m combined height) was estimated by observing decay in peak concentration. Losses at dams (approximately 50% per dam) were dominant. The estimated gas exchange at dams was compared to a simple model adapted from those available in the literature. Small amounts of tracer were trapped in a canal segment (ca. 5 km long) that parallels the river, where advection and dispersion were sharply reduced. Caplow, Schlosser, and Ho

Transport in the Hudson estuary: a modeling study of estuarine circulation and tidal trapping.” Estuaries 27

ABSTRACT: The effects of estuarine circulation and tidal trapping on transport in the Hudson estuary were investigated by a large-scale, high-resolution numerical model simulation of a tracer release. The modeled and measured longitudinal profiles of surface tracer concentrations (plumes) differ from the ideal Gaussian shape in two ways: on a large scale the plume is asymmetric with the downstream end stretching out farther, and small-scale (1-2 km) peaks are present at the upstream and downstream ends of the plume. A number of diagnostic model simulations (e.g., remove freshwater flow) were performed to understand the processes responsible for these features. These simulations show that the large-scale asymmetry is related to salinity. The salt causes an estuarine circulation that decreases vertical mixing (vertical density gradient), increases longitudinal dispersion (increased vertical and lateral gradients in longitudinal velocities), and increases net downstream velocities in the surface layer. Since salinity intrusion is confined to the downstream end of the tracer plume, only that part of the plume is effected by those processes, which leads to the largescale asymmetry. The small-scale peaks are due to tidal trapping. Small embayments along the estuary trap water and tracer as the plume passes by in the main channel. When the plume in the main channel has passed, the tracer is released back to the main channel, causing a secondary peak in the longitudinal profile