Clamshell Buckets? Are they the Right Choice for the Project?
|In 2009 dredging began in the Upper Hudson River to remove sediments with PCBs. Read more.|
I’ve had many people ask me why the dredging is being performed with environmental clamshell buckets instead of using the hydraulic (with a hose and suction) method. These people point to the fact the clamshells are encountering a large volume of logs, sticks and wood shavings, which prohibit the jaws of the buckets from closing tightly and cause water to spill out as the buckets are raised from the river bottom.
Interestingly, that copious river debris (a casualty of the Adirondack logging trade) is one of the main reasons hydraulic dredging wouldn’t work well for this project. That debris – and the rocky nature of the river bottom – would clog hoses and greatly hinder dredging progress. Furthermore, the project spans 40 miles, so using hydraulic dredges would require an extensive infrastructure of pipeline and pump stations up and down the river corridor. But the most important factor is that engineers determined the resuspension of sediment is nearly the same using either method of dredging. Resuspension occurs when the river bottom is disturbed and dirt (in this case dirt with PCBs clinging to it) gets churned up. When this happens, the water-born sediments float downstream, so keeping resuspension to a minimum is an important project goal.
It is important to remember that PCB-tainted sediment dropping out of the clamshells typically settles to the bottom in a relatively short distance and gets removed in subsequent passes of the dredge. Moreover, PCB levels during all dredging operations are closely and continuously monitored to ensure compliance with EPA’s resuspension standard. And, so far, the monitoring has shown the sediment resuspension hasn’t caused an the drinking water standard for PCBs to be exceeded, and the first monitor is only one mile downstream of the dredging. Information about the monitoring for the project can be found at this site: www.hudsondredgingdata.com/. All things considered, the decision to use clamshell buckets was the right one.
About the author: Kristen Skopeck is originally from Cedar Rapids, Iowa. She is an 11-year Air Force veteran and was stationed in California, Ohio, Texas, Portugal, and New York. After working for the USDA for three years, Kristen joined EPA in 2007 and moved to Glens Falls, NY to be a member of the Hudson River PCB dredging project team. She likes to spend her time reading, writing, watching movies, walking, and meeting new people.
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