Northwest Newspaper Hydropower Articles

Draft Report Back Lava Tube Theory of Breach

By Erik Robinson
Columbian

Seepage into ancient lava tubes caused the spectacular collapse of the Swift power canal near Cougar in April, according to a draft report to federal regulators. The findings reiterate an initial report issued shortly after the canal's collapse on April 21.

The report, submitted to the Federal Energy Regulatory Commission in October and obtained by The Columbian through a Freedom of Information Act request, is being finalized by engineering consultant CH2M Hill on behalf of project owner Cowlitz County Public Utility District. Officials expect a final report by the end of next month.

Until the final report, Cowlitz official Ron Worthington said any discussion of the draft report is "probably premature."But the draft findings do raise questions about how much money and the kind of design that will be needed to safely rebuild the canal, which has undergone at least one other major repair.

At the root of the problem, according to the report, is the underlying cave created by lava flowing from Mount St. Helens 1,900 years ago.

The canal's builders apparently failed to detect a network of tubes and crevices in the black basalt covered by a 30-foot layer of natural sediment and fill.

The report concluded the 30-foot barrier of soil remained intact for 44 years, but gradually crumbled through a sinkhole into a large lava tube. Water poured into the lava tubes underlying the 3.2-mile-long canal early April 21, washing away the manmade embankment at the base of the canal in just four hours.

The 300-foot-wide breach wiped out a state highway spur and destroyed a 70-megawatt powerhouse owned by Cowlitz PUD and operated by PacifiCorp.

Despite the draft report's conclusions, Worthington said Cowlitz still intends to rebuild a canal and powerhouse that once supplied about 10 percent of the utility's electric needs.

Worthington, the utility's director of customer services, said Cowlitz's insurance companies are still waiting for the final report, as well as conducting their own investigation into the cause of the collapse, before paying out money to rebuild. Rebuilding the structure promises to be at least as expensive as the 1958 construction cost of $26 million.

The draft report prepared by CH2M Hill indicated the canal's builders may have overlooked the presence of tubes within the underlying basalt.

A review of project design records indicated trenches were dug prior to the canal's construction.

Although drillers discovered the presence of basalt, they apparently found no evidence that the basalt was anything but solid.

"No mention was made of voids in the cave basalt in any of the drill logs," according to the CH2M Hill report.

Workers did report digging and water-jetting to expose cracks and crevices in the black basalt as they prepared to build the embankment's foundation. They also reported shooting granular material into cracks and depressions inaccessible to compaction equipment.

But they apparently didn't search hard enough.

In 1973, engineers investigating a leakage problem discovered 65 sinkholes ranging in size from 2 to 20 feet in diameter.According to a CH2M Hill report dated May 1974, engineers discovered fine-grained material that was supposed to provide a watertight seal had instead "eroded into the underlying cavernous lava." Digging through the empty canal, investigators discovered two large caverns in the underlying cave basalt, both large enough to place a tractor-mounted backhoe inside."Concern was expressed that other tunnels might exist in unexposed conditions," according to the 1974 report.

Columbian
Vancouver, WA
January 2, 2003

Related Links:

Hydro News
News Archive

[News clippings posted on this site are copyrighted by the newspaper or magazine of original publication. These clippings were selected to further FWEE's not-for-profit mission of providing balanced and informative views concerning the sources, benefits and impact of using water as a renewable energy resource in the Northwest.]