ACOUSTICS AND SUSPENDED SEDIMENT TRANSPORT IN THE TIDAL HUDSON RIVER, NY

The U.S. Geological Survey (USGS), in cooperation with the New York State Department of Environmental Conservation, has deployed and maintained an upward looking acoustic Doppler current profiler (ADCP) on the Hudson River bottom near Poughkeepsie (river mile 72) since the spring of 2002 to better understand the movement of suspended sediment through the tidal Hudson River. The Hudson generally is considered the largest terrestrial source of sediment to New York Harbor, where continual dredging of sediment is required for navigation.

ADCPs are designed to measure water velocity by transmitting an acoustic pulse through the water and listening for an echo returning from particles suspended in the water that are assumed to be moving at the same velocity as the water. When these particles are moving closer to or further away from an ADCP transceiver, the return echo is Doppler shifted. The ADCP uses the Doppler shift recorded in each of the ADCP’s 4 transceiver beams to resolve the velocity vector of the particles. The USGS has developed a relation between ADCP-derived river velocity and discharge resulting in the first and only continuous record of instantaneous and net discharge at a tidal Hudson location.

In addition to velocity, the ADCP records the intensity of the return echo in each of the transceiver beams. Once adjusted for sound absorption, beam spreading, and other instrument-specific variables, echo intensity can be directly related to instantaneous suspended sediment concentration (SSC). Discharge and SSC where used together to estimate the net suspended sediment load over the study period.

The long-term temporal pattern of suspended sediment load observed at the site is very similar to that of the combined load of the Upper Hudson and Mohawk Rivers, which enter the tidal Hudson at the head-of-tide. These patterns suggest that sediment entering the tidal Hudson from the head-of-tide and tributaries moves past the site fairly quickly. Seasonal patterns of sediment load at the site indicate that sediment transport during late fall can be at least as significant as in the spring during snowmelt runoff. The estimated total suspended load compared to that entering at the head-of-tide suggests tidal Hudson River tributaries may be more significant a source of sediment than previously assumed.