Motivation

The Hudson River is the largest terrestrial source of suspended sediment to New York Harbor and millions of dollars are spent annually maintaining navigation channels and disposing of sediment removed during dredging. Understanding the amount of suspended sediment moving into the Harbor and the timing and mechanisms associated with that movement is the first step in reducing or eliminating their contribution to the harbor.

Challenge

The Hudson River is tidal 153 miles from New York Harbor to the head-of-tide at Green Island (map). Conventional methods such as the development of a stage-discharge relation for computing discharge and periodic sample collection to determine suspended sediment concentration (SSC) cannot be used in tidal systems because stage is independent of discharge and the number of samples required to accurately depict SSC over time is problematic.

Approach

Our approach is to use an acoustic Doppler current profiler (ADCP) to 1) measure the velocity of the river at a particular location and relate it to the river discharge and 2) use the echo intensity recorded by the ADCP as a surrogate for suspended sediment concentration. By multiplying the river discharge by the suspended sediment concentration we can compute the mass of suspended sediment moving past the study location.

Goals

1) To quantify the mass transport (flux) of suspended sediment moving through the tidal Hudson River.

2) To provide suspended sediment and river discharge data to the public in near-real time.

3) To better understand the mechanisms responsible for sediment transport in the tidal river.

Instrumentation

Several types of instruments are used in addition to the ADCP, including an acoustic modem, an optical backscatter sensor, a locator pinger and a submersible compass. Acoustic release buoys were also used for a short time.

* Buoys removed 7/31/02

Equipment Manufacturers and Product Specifications:
ADCP - RD Instruments - 600kHz Sentinel
OBS - D&A Instruments - OBS-3A
Acoustic Modem - LinkQuest - UWM2000
Pop-up Buoys - Benthos - 875-PUB
Locator Pinger - VEMCO - V16-6HR-01
Tripod - Oceanscience - Sea Spider
Compass - Advanced Orientation Systems - EZ-Compass Dive