Dam breaching required a lot of preparation, planning on tight schedule
Salmon were captured, relocated above Condit
Wednesday, October 26, 2011
On June 13, after 12 years of delays, negotiations and regulatory hoops, PacifiCorp pulled the trigger. The Portland utility announced that it had reached agreement with federal regulators on all issues and would proceed in late October with breaching Condit Dam.
The window was tight. Threatened fall chinook salmon arriving in the lower three miles of the White Salmon River below the dam would have to be captured and transported above the dam, out of the path of a massive sediment surge and into their native waters. An estimated 2.7 million cubic yards of sediment had built up behind the dam in its 98 years. No one knew what lay beneath the sludge.
Fish biologists finished relocating the salmon earlier this month. They captured 679 fall chinook, of which 85 percent were wild. As of Monday, 299 spawning females had produced 180 egg nests, known as redds — the first new generation of wild fall chinook on the river in nearly a hundred years.
On Aug. 15, contractors for Pacif-iCorp began drilling and blasting the area immediately below the dam to allow workers and equipment to tunnel into its 90-foot-thick base. To maintain a minimum flow for fish, they diverted the river’s flow around a plunge pool and released it downstream. Equipment was lowered by cable from a tower at the top of the dam.
Engineers had spent a month at the site already, studying the logistics of dam removal and assessing the quality of the concrete in the dam. That would tell explosives specialists how far they could safety tunnel into the dam before the big day without threatening the structure’s integrity.
Crews worked in eight-foot increments. They’d drill a hole, pack it with dynamite, blast away the concrete and bore into the dam’s base. At the end, they left a 10-foot plug, which was blasted away Wednesday using 700 pounds of dynamite. Northwestern Lake, the reservoir behind the dam, burst through the 12-foot-by-18-foot hole and raced downriver to the Columbia River, draining the reservoir in an hour.
Breaching the dam is just the beginning of restoring the White Salmon River.
Actual demolition work won’t begin until next spring. Workers using concrete cutters will dismantle the concrete structure and bury the rubble along the route of the flow line, which until Wednesday carried most of the river south through a wood-stave pipe to a concrete powerhouse a mile away. The pipe, too, will be dismantled, but the powerhouse will remain.
Before demolition, PacifiCorp also had to replace a section of a water line at the top of the reservoir that supplies drinking water to the town of White Salmon.
In all, the dam removal project required federal approval of 18 separate plans, from vegetation management to monitoring of navigation impacts resulting from the sediment flush. It will cost the utility an estimated $33 million, nearly twice the price tag projected in 1999, when the company decided to breach the dam rather than install costly fish ladders as a condition of federal relicensing.
Two coffer dams built to reroute the river during the dam’s construction will be removed in May and the last of the structure should be removed within a year. Riverbank restoration will begin with the planting of native vegetation.
It won’t happen overnight and officials wouldn’t hazard a guess as to when it will happen, but as shrubs and trees grow up along the banks, as salmon reestablish their migration patterns, as bears and other wildlife return to feed on the fish, and as the river establishes a new course, the White Salmon will return to something approaching a natural state.