ADCP MONITORING OF SHORT-DURATION TRANSPORT IN THE UPPER HUDSON RIVER

The transport of suspended sediment in the upper Hudson River is very closely tied to hydrologic events in the many tributaries and sub-basins of this area of the river. Transport events are typically of shorter duration and have less attenuation than transport in the lower, tidal Hudson. We have recently explored the use of Acoustic Doppler Current Profilers (ADCPs) to monitor these transport events in "real time" using acoustic backscatter as a proxy for suspended sediment concentration.

We deployed ADCPs at three sites near Albany. Our ADCP on the Champlain Canal near Mechanicville has been active since 2004, and has been used to monitor transport driven by storms, snowmelt, and the operation of navigational locks and power plants in the non-tidal, upper Hudson. A second ADCP was deployed in April 2006 on the Erie Canal on the Mohawk River near Waterford to measure sediment transport from the Mohawk River basin. An ADCP in the tidal Hudson at Albany, approximately 10 miles downstream from the confluence with the Mohawk River, reveals net transport in the tidal portion of the river.

Water samples collected and filtered from all three sites provide a time series of total suspended sediment concentrations (TSS) for calibration of the ADCP-derived acoustic backscatter measurements. Calibrations for March and April of 2005 at Lock 2 show especially good correlations between TSS from discrete water samples and the backscatter from the ADCP. Preliminary calibrations indicate the utility and sensitivity of ADCP monitoring techniques in the upper Hudson, and point out the importance of post-freshet, event-driven transport.

While there are limits on the dynamic range of the backscatter signal, the temporal resolution of the ADCPs provides excellent detail of short-duration transport events in the upper Hudson. Multiple short-duration events in the spring and summer of 2005 and the spring of 2006 show that an increase in backscatter intensity often precedes a change in stage, suggesting that stage the carrying capacity of the river has a faster response than what would be predicted from stage alone. The resolution afforded by ADCP may be especially relevant to monitoring as sediments are dredged from the upper Hudson as part of the proposed PCB cleanup effort.