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Dredge Plume Mapping Through Acoustic Backscatter Surveys
Vessel-Mounted ADCP Used to Monitor Environmental Effects Of Dredging Activities at US Naval Base Guam's Kilo Wharf

Christopher Goody
Ocean Engineer
Marc Ericksen
Vice President
Sea Engineering Inc.
Waimanalo, Hawaii

As part of an effort to document the environmental effects of dredging activities at the Kilo Wharf Extension project, Sea Engineering Inc. (Waimanalo, Hawaii) under subcontract to Cardno TEC in Honolulu, Hawaii, was requested by Naval Facilities Command Pacific to develop and implement a methodology to monitor suspended sediment generated by dredging at the site. The project, located within Apra Harbor at U.S. Naval Base Guam, had two investigative components: boat surveys that utilized an acoustic Doppler current profiler (ADCP) to measure backscatter intensities from suspended sediments and the installation and maintenance of two bottom-deployed control stations on either side of the wharf. The project fieldwork commenced in September 2008 and finished in December 2009.

An important factor in determining dredge-related turbidity impacts is the amount and nature of suspended sediments introduced into the water column and how those suspended sediments are transported and deposited on the seafloor. Existing data documenting how sediment is released into the water column is limited, and the data that are available are primarily derived from U.S. mainland harbor dredging projects, which are significantly different environmentally than tropical reef environments such as those in Guam.

A 3D visualization of the actual dredge plume during active dredging and typical tradewind conditions. Lower concentration areas are blue, and higher concentration areas are red.

This investigation therefore focused on measuring and characterizing the sediment introduced into the water during dredging with a vessel-mounted, downward-looking ADCP.

Backscatter Surveying
In order to characterize the dynamic dredge plume, acoustic backscatter surveys were conducted on 13 days during five separate site visits over a one-year period. The intensity of the ADCP backscatter is dependent on the amount and character of particulate matter in the water column. It is possible to measure suspended sediment plumes if a relationship can be established between acoustic backscatter intensity and the total suspended solids (TSS) concentration in the water. The technique requires careful correlation of the measured acoustic backscatter intensities with corresponding water samples, which must be analyzed to determine TSS concentration along with other water column properties such as temperature and salinity.

Backscatter surveys were conducted from a 23-foot work skiff using Teledyne RD Instruments' (Poway, California) downward-looking, 600-kilohertz Workhorse Monitor ADCP to record acoustic backscatter intensities throughout the water column along survey tracklines. During surveys, a grid-line strategy was employed with differential GPS while backscatter intensity profiles from the ADCP were recorded continuously.

Backup positioning was acquired simultaneously on a secondary computer using Hypack Inc.'s (Middletown, Connecticut) HYPACK hydrographic survey software, which was also used for navigating on survey lines, marking events and time-stamping for calibration water samples. ADCP backscatter and positioning data were acquired using Teledyne's WinRiver software at a rate of 1 hertz, with a vertical bin size of 40 centimeters for maximum vertical resolution while maintaining sufficient range for deeper portions of the survey area. The instrument's bottom-tracking feature was critical for determining which cells were above the seafloor while traversing the project site's relatively complicated bathymetry and steep slopes.

Because the acoustic characteristics of sediments generally vary from site to site, water samples for calibration were collected daily during surveys at various depths, locations and times of the day. Additionally, the research team collected calibration data from various ranges of apparent TSS concentration in order to bracket the observed range of values. Water sample calibration data were collected using a 2-liter Niskin bottle, along with a model RBR Ltd. (Kanata, Canada) CTD plus turbidity profiler affixed beneath it, which was lowered into position while the ADCP was recording backscatter intensity.

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Christopher Goody is an ocean engineer with Sea Engineering Inc. He holds a master of science degree in ocean engineering from the University of Hawaii. His expertise includes applications of oceanographic modeling, coastal engineering, engineering diving and naval architecture.

Marc Ericksen is vice president of Sea Engineering Inc. He holds a master of science degree in marine geology from Pennsylvania State University. His expertise includes numerical modeling of circulation and suspended sediments, monitoring of harbor dredging, field measurement of currents and turbidity, and hydrographic and geophysical surveying.

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