Removing Oregon's Marmot Dam

Edited By Carl Molesworth | September 28, 2010

More than 2 tons of high explosives detonated last July ceremoniously shattered the top 10 feet of the Marmot Dam, located along the Sandy River approximately 40 miles east of Portland. The blast set off the state's largest dam removal project in decades and drew hundreds of spectators and dignitaries to witness the event.

The removal of the dam is the first step in the $17-million decommissioning of the Bull Run Hydroelectric Project. The work also will involve removal next year of a 16-foot-high dam downstream across the Little Sandy River, a nearly three-mile-long wooden box flume and the 160-acre Roslyn Lake above the 22-megawatt power plant. The infrastructure diverts almost all of the flow from the Little Sandy River, prime habitat for several species of migrating salmon, for power production. Portland General Electric and 22 government entities and conservation groups struck an agreement to restore free-flowing waters through the Sandy River Basin.

"Today, this partnership took a great step toward restoring a breathtaking river for fish, wildlife and people," said PGE CEO and President Peggy Fowler at the event. "We celebrate the future of a watershed that will provide unimpeded salmon and steelhead passage from the slopes of Mt. Hood to the Pacific Ocean."

The 47-foot-high concrete dam originated as a timber structure in 1913. The power plant downstream generated electricity for a trolley system that carried city dwellers out to the countryside. Following a flood, Portland-based Natt McDougall Co. rebuilt the dam in 1989 using Roller Compacted Concrete, a then relatively new no-slump material. Unlike conventional concrete, RCC consisted of a drier mix compacted by vibrating rollers that reduced labor and construction costs by eliminating the need for dowels, reinforcing steel, joints, forms, and finishing. The use of RCC as a concrete mass core protected by a conventional concrete shell on the faces translated into a simpler, faster and more economical construction method.

Ironically, the same contractor that built it was hired to remove the dam.

"It's a little tough" emotionally to go back and destroy it, reflected Natt McDougall, CEO of the company, which specializes in dams and other heavy infrastructure.

Following the detonation, heavy equipment operators advanced across the dam with two Model 460 and two Model 290 Link-Belt track excavators fitted with breaker heads and shovels. The project was running six weeks ahead of schedule, using the heavy equipment and four additional blasts, by the end of August when work ceased at the site as part of the removal plan. The bottom 20 feet were left in place until October when the last of the 20,000 cubic yards were to be removed, along with a lined diversion channel and a coffer dam breached so that river flow would flush out the streambed to the original Elevation 685. The stockpiled chunks of concrete from the dam will be recycled into road surfacing.

10,000 Holes

The contractor started at midnight on July 1 to build "in the dry" the 18-foot-high earthen coffer dam, a temporary bridge and diversion channel. Thereafter, the dewatering plan for the coffer dam was critical because the 10,000 cubic yards of borrow used to create the coffer structure rested only on sediment without any reinforcement or significant compaction.

In another ironic twist, it was here during construction of the Marmot Dam along the Sandy River that Natt McDougal Co. originally mastered its dewatering methods. While building the replacement structure for the timber dam in 1989, the well drilling subcontractor discovered that the layout of bore holes specified by an engineering firm had inadequate capacity.

"That forced us to install an unforeseen second row of wells on our own," he said. "Ever since, we've developed expertise in dewatering our projects. When this job came out, the engineers had copied the same dewatering scheme with the deeper 12-inch dewatering wells originally called for in the 1989 plans. Knowing beforehand that wouldn't work, we convinced them to let us substitute 36-inch-diameter wells with 18-inch HDPE casings packed in 1/2-inch gravel instead of 12-inch holes."

The crew drilled 10,000 holes in a pattern on the well casings, which were inserted into the 10 wells spaced at 30-foot intervals atop the coffer dam. Working with Rich Epperly, a pump specialist with the Portland office of ITT Flygt, another revised plan emerged that initially called for pumps capable of 288 gpm at 70 feet TDH to dewater the coffer dam. Epperly recommended that each well receive an ITT Flygt Model 2640 submersible pump to collectively deliver approximately 3,500 gpm. Model 2640 is one of the six new 2600 series submersible dewatering pumps that ITT Flygt has recently introduced.

The pumps feature innovative hydraulics, fewer components, new materials, and an ergonomic slim-line design. The patented DuraSpin hydraulic system improves wear resistance and sustains performance over time three to six times better than traditional open type impellers. A single screw enables easy adjustment for maximum pump performance. The results of all of these features are unmatched wear resistance, consistent long-term performance and thus the lowest cost of ownership.

The plan finalized in May originally envisioned 20 wells but was amended to 10, after the drilling crew reached bedrock beneath the channel at only 40 feet. The shallower wells, only one of which required two pumps, increased the discharge volume of each unit. The pumps worked well, helping the McDougall crew remove the RCC much faster than anticipated and significantly outpace the schedule.

After moving back onto the project in October, the crew would dig out the remaining 20 feet of the Marmot Dam, pull the diversion channel liner, grade the banks, and remove the pumps and well casings across the coffer dam. This allowed the river to saturate the temporary structure at the current breach and flush it downstream in a few days.

More Cost Effective

The restored rivers will enhance whitewater rafting and kayaking along the streams and numerous other activities along the banks. PGE announced in 1999 that it would remove the hydroelectric infrastructure after the company concluded that demolition was more cost effective than stricter measures to protect the fish migrations. The power sacrificed for the equivalent of 12,000 homes will be made up by wind farms and other environmentally friendly methods.

Headquartered in Portland, PGE also will donate 1,500 acres of its Sandy River Basin land to the Western Rivers Conservancy, which will convey most of the property to the U.S. Bureau of Land Management for permanent protection. The land will form the foundation of a planned 9,000-acre conservation and recreation area. The Sandy is expected to revert to one of the top Chinook salmon and steelhead fishing destinations in Oregon. PGE is surrendering its rights to the water "in stream," meaning no one can remove that amount of water in the future.

The removal of Marmot Dam unleashes the initial 10-mile stretch of a planned 100 miles of Sandy River — designated as "Wild and Scenic" — for recreation usage and unimpeded runs for winter and spring Chinook, Coho salmon and steelhead. The waters feed off the northwest slopes of Mount Hood and eventually flow to the Columbia River, which reaches the Pacific Ocean where the fish mature before returning to spawn. All are listed as threatened under the federal Endangered Species Act. The state of Oregon lists Coho as an endangered species.

Story and photos courtesy of Steven London Associates, Lansdale, Pa., for ITT Flygt Corp.