MFA Insists on Precision Dredging Techniques

Maul Foster & Alongi, Inc. (MFA), a Pacific Northwest professional consulting firm, last month wrapped up the Lake River dredging project, located in Ridgefield, Washington.

Sediment in Lake River was impacted by contaminants from a wood-treating operation that, from the 1960s through 1993, pressure treated poles and dimensional lumber with chemical preservatives.

The Port of Ridgefield and the Washington State Department of Ecology entered into a legal agreement in 2014 to collaborate on the cleanup efforts.

The cleanup scheme involved removing up to 100 old, treated wood pilings; dredging 10,000 cubic yards of contaminated sediment; placing a clean layer of sand in the river; stabilizing the bank with rounded cobbles; and revegetating 2.5 acres with native grasses and about 1,000 shrubs and trees.

MFA engineers insisted on applying “precision dredging” techniques in the design. This approach uses sophisticated global positioning system equipment to verify removal of toxic sediment, and a bucket design to limit disturbance and spreading of fine-grained sediment during construction.

The selected dredging method resulted in improved efficiencies during permitting; in a biological opinion, the National Marine Fisheries Service (NMFS) identified this approach as a desired method for minimizing the loss of sediment into the water column.

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