The transport of suspended sediment in the Hudson River is very closely tied to hydrologic events in its tributary drainage basins. In addition to seasonal runoff from Spring snowmelt, short-duration storms in the tributary basins can lead to an increased amount of fine particle transport.
We are using Acoustic Doppler Current Profilers (ADCPs) to study the importance of short-duration events in transporting sediment. This is of great concern in the upper Hudson, where short-duration events are likely to transport particle-associated contaminants farther downstream.
ADCPs are typically used to measure the velocity of a river or stream current. The instrument sends out a pulse of acoustic energy (a "ping") from a drum-like transducer. The acoustic signal travels through the water column and reflects off suspended particles. The difference in time between transmission of a series of pings and the detection of the return signal yields a velocity.
In our Upper Hudson work, we use the strength of the return signal (the "backscatter intensity") as an indicator of suspended sediment concentration. We use horizontal ADCPs (H-ADCPs) at mid-channel depth, configured to measure backscatter intensity every 15 minutes.
We calibrate the acoustic backscatter signal by collecting daily whole-water samples and filtering them in the lab for a direct measurement of total suspended solids.
In the Upper Hudson Riverscope project, we have two H-ADCPs deployed in the Hudson River – one in the non-tidal, upper Hudson near Waterford, and one in the tidal Hudson near Albany. A third H-ADCP was deployed on the Mohawk River near Halfmoon in March 2006.
We have completed three field seasons for whole-water sample collection and H-ADCP monitoring at each of the Hudson River sites, and have been collecting whole-water filtered samples at the Mohawk River site during the March-November 2005 field season.
Acoustic backscatter and filter data in the non-tidal areas indicate a strong link between sediment transport and short-duration hydrologic events. In both field seasons, significant transport has occurred in response to late Summer / early Fall storms. We are able to resolve fluctuations in sediment transport in the upper Hudson due to the operation of navigational locks and hydropower plants. In the tidal Hudson, we are able to resolve tidally-driven transport in addition to a more attenuated, less "spiky" response to hydrologic events in the upstream tributaries.
The high resolution of the H-ADCP data allows us to study individual storm events in great detail, and estimate a time-of-response to events in the tributary basins. While final calibrations to derive a net sediment flux are ongoing, preliminary calibrations are quite promising.