In 2003, a severe ice-jam event occurred in the part of the lower Grasse River. The ice jam formed directly over the 2001 Capping Pilot Study area, and was severe enough to scour the cap and some of river bottom material.
In response to this event, Arconic (formerly Alcoa) assembled an ice expert team and conducted a series of intensive investigations in 2003 and 2004 to understand what happened, why, and how often these events occur. Results of the ice investigations indicated that ice jams capable of scouring and redistributing PCBs buried in the river sediments can occur once every 7 to 10 years in the upper approximately 1.8 miles of lower Grasse River (between the Power Canal and approximately one-third of a mile upstream of the Route 131 Bridge). There is no evidence that ice jams occur downstream of this 1.8-mile stretch.
This ice jam scour was not previously known to occur in the Grasse River and the cap installed as part of the 2001 Capping Pilot Study was not designed to withstand the forces from these events. As a result, the company began to evaluate possible long- and short-term options for controlling ice jam related sediment scour for incorporation into a potential remedial solution for the river.
A number of different ice control options have been studied, including stand-alone piers, construction of a dam, and mechanical break-up of river ice. The Ice Breaking Demonstration Project was carried out in March 2007 and involved breaking the intact ice cover throughout a seven-mile stretch of the Grasse River.
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