KING CO. USES SERIES 370 "DRAGON" DREDGE FOR SEDIMENT TRAP IN INNOVATIVE DAM REMOVAL PROJECT

Learning from past mistakes, engineers in Michigan are trying to perfect the art of dam removal. By anticipating how a release of impounded sediments might affect the Muskegon River, they intend to minimize any negative consequences from the removal this week of the last remnant of Big Rapids Dam, 60 miles north of Grand Rapids.Sediment management is "the number one public issue on this project, other than safety," says James R. Hegarty, project manager at Prein & Newhof, Grand Rapids, engineer for the removal. "The project is designed to be a model for future dam removals. It is one of the Midwest's unique and trend-setting environmental projects," he adds."Everything they've done on the river has backfired on them," Hegarty says. In the late 1800s, lumberjacks built a 350-ft-wide timber-crib-and-rock dam across the Muskegon to float logs past a series of rapids. After a flood destroyed the original Big Rapids Dam, crews in 1914 built a 17-ft-high concrete replacement atop the remnant, and added a hydroelectric plant. In 1966, inspectors discovered cracks in the concrete, probably caused by dynamiting and drilling for gravel upstream. Blowing up the dam that year only hurt the river's health. The current carried fine sand impounded behind the dam and filled up virtually every fishing hole for eight miles downstream. The sediment also formed new islands that caused ice jams and flooding that damaged cottages in low-lying areas. Fishermen found it difficult to launch boats. Local residents got angry. To make matters worse, the contractor went bankrupt before finishing.Still, 30,000 to 90,000 cu yd of sediment remained behind the remnant of the 4-to-5-ft-high base of the powerhouse and spillway. Besides blocking fish, the remnant posed a safety hazard; the current sometimes pinned canoes there. One canoeist drowned there in 1997.The city intends to remove the cofferdam as well as the remnant dam, even though the plan makes downstream residents nervous about sedimentation ``because of what happened in the '60s when [crews] just let it all go," says City Engineer Donald J. Greiner. The city promised residents a staged removal of sediments and continuous monitoring.General contractor The King Co., Holland, Mich., dredged 17,000 cu yd upstream of the remnant using its Ellicott Series 370 "Dragon" dredge, which did a good job with minimum turbidity. Dredging work included construction of "traps" where future sedimentation will collect and be easily removable. At the same time, its crews erected sheet piles about 80 ft from the river's west bank, diverting flow from the old spillway. Demolition of the dam's remnant began in June with subcontractor Pitsch Companies, Grand Rapids, Mich., pulverizing concrete with vibrating drills mounted on backhoes.After removing the debris, crews incrementally lowered the sheet piles 6 in. to prevent any surge in flow from eroding the banks. As dredging continued, crews aimed to get the last of the dam's remnant out by month's end. "There's not a guarantee of 100% sediment removal but we'll make a good college try," Hegarty says.

Source: ENR Oct 2000