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DAM DECOMMISSIONING: Removing Marmot Dam

As part of the ongoing work to decommission the 22-MW Bull Run project, Portland General Electric completed removal of the 47-foot-high, 195-foot-long Marmot Dam, a roller-compacted-concrete structure, in 2008. This challenging undertaking yielded valuable lessons about dam removal.

By Tim M. Keller

Portland General Electric’s (PGE) 22-MW Bull Run project on the Sandy River in Oregon began producing electricity in 1912. When this project was built, site access was by horse-drawn stage. Standard construction equipment included horse-powered trucks, scrapers, and loaders.

In 1912, the company’s hydro projects formed the backbone of the area’s generating system. But over time, generating portfolios changed. Small hydro projects, such as Bull Run, typically were superceded by coal-fired and gas-fired projects and large hydro projects. In this situation, not all small hydro projects survive.

In 1999, PGE decided not to proceed with renewal of the Federal Energy Regulatory Commission (FERC) operating license for Bull Run and instead to decommission the project. This decommissioning involved removing: remains of an old timber crib dam at the site; the 47-foot-high, 195-foot-long Marmot Dam; the fish ladder; and other structures. Work to remove these structures was accomplished in just five months during 2008. All the work needed to return the site to its natural conditions will be complete during 2009.

Deciding to decommission Bull Run

The FERC operating license for Bull Run was scheduled to expire in 2004. In the late 1990s, PGE began working to renew this license. To determine how to approach the relicensing process, PGE performed a financial analysis of the project. PGE determined that operations and maintenance costs would continue to increase. The utility also determined that the likely cost of providing the necessary level of protection, mitigation, and enhancement for the resources affected by the project — such as threatened chinook and steelhead salmon and bull trout — would outweigh the economic benefit of generation at the project over the life of a new operating license.

Because of these findings, in 1999, PGE filed with FERC a notice of its intent not to seek a new license for the Bull Run project. Instead, the utility decided to decommission the project and return the river to its natural ecosystem.

Description of the project

The Bull Run project is on the Sandy River and its tributaries, the Little Sandy and Bull Run rivers, about 30 miles east of Portland. The Little Sandy River flows into the Bull Run River, which empties into the Sandy River.

Main project features are:

— Marmot Dam on the Sandy River, a 47-foot-high, 195-foot-long roller-compacted-concrete (RCC) structure. The dam had a spillway crest length of 345 feet. Construction of this dam was completed in 1989, to replace a timber crib dam that was built in 1913. The south end of the dam featured a ladder to provide upstream fish passage. The north end featured a concrete gravity wing dam that provided an additional 140 feet of spillway and directed water to an intake structure. The intake structure had two tainter gates that regulated the diversion flow into a canal system. Minimum flows were maintained on the Sandy River downstream of Marmot Dam;

— A 7,600-foot-long concrete-lined trapezoidal canal that conveyed water from Marmot Dam through three tunnels (with a combined length of 8,460 feet) to the Little Sandy River;

— Little Sandy Dam, a 15.75-foot-high diversion dam on the Little Sandy River. All of the flow from this river was diverted to the powerhouse, except when flows exceeded the diversion capacity of 800 cubic feet per second (cfs);

— A 16,800-foot-long timber flume that took water from the Little Sandy River to Roslyn Lake, the project forebay;

— Roslyn Lake, a 160-acre forebay located 320 feet above the powerhouse and surrounded on three sides by earthen dikes;

— Two 1,400-foot-long penstocks that took water from the forebay to the powerhouse; and

— The powerhouse northeast of Roslyn Lake, from which water was discharged into the Bull Run River.

Planning to remove Marmot Dam

PGE’s first step in the decommissioning process was to convene a working group in early 2000. This group was composed of representatives of all 22 governmental and non-governmental stakeholders in the project. The task of the group was to develop a decommissioning plan that would maximize benefits to the resources affected by the project (i.e., endangered species and public and private lands), consistent with PGE’s obligations as a regulated public utility.

These efforts were successful. In October 2002, 23 parties signed the resulting settlement agreement and decommissioning plan. The agreement describes the legal context, regulatory authorities, and related obligations of each of the parties. The agreement establishes PGE’s obligation to file with FERC an application to surrender the project license and other associated documents and requires that PGE decommission and remove the project according to a methodology contained in the decommissioning plan.

In accordance with the settlement agreement, in November 2002 PGE applied to FERC to amend the current license to allow project operation until 2008; to require pre-removal geomorphological and water quality monitoring; and to implement interim protective measures for endangered and threatened species. In May 2004, FERC granted PGE a surrender order for decommissioning Bull Run. PGE then submitted other permit applications to the Oregon Department of Environmental Quality, Oregon Division of State Lands, and U.S. Army Corps of Engineers. The final permit was signed in May 2007.

The decommissioning plan for the project indicates that PGE will completely remove: the RCC dam to the level of the original river bed, older timber crib dam just upstream, canal inlet and headworks, and fish ladder. This work was to be performed within one construction season, from July 1 to October 31, 2007. The other project features were to be removed after the Marmot Dam removal, based on the schedule in the decommissioning plan. Preparatory work, such as excavation and stockpiling of borrow material for construction of the cofferdams, was to be completed in June 2007.

One significant issue PGE and other stakeholders had to deal with during project decommissioning was the about 980,000 cubic yards of sediment accumulated behind Marmot Dam. During the fall of 1999, Squier Associates drilled a series of cores into the sediment to chart the various sediment units. Results indicated that the sediment consisted of two main units. The upper unit ranged from 6 to 18 feet thick, becoming thicker toward the dam, and was composed of sandy gravel with cobbles and boulders. Below this was a layer of predominantly fine sediment (silty sand to sand with gravel) ranging from 13 to 35 feet thick.

Under the terms of the settlement agreement, only the sediment in the immediate vicinity of the RCC and timber crib dams would be removed before demolition. This involved about 30,000 cubic yards of sediment, which contractor Natt McDougall Company (NMC) excavated using track-mounted excavators, rubber-tired loaders, and off-highway end dump trucks. NMC placed this sediment on Bureau of Land Management (BLM) land (a portion of the Marmot Dam site is on BLM-owned land) to fill borrow areas that were excavated for cofferdam material. The rest of the sediment would wash away once the upstream cofferdam was breached.

During the decommissioning work, stream flows were to be diverted through the existing approach channel and canal. Until the dam and appurtenant structures were removed, up to 600 cfs of river flow were to be conveyed to Little Sandy Dam and used for power generation. A requirement of the settlement agreement was that minimum flows be maintained below Marmot Dam during the decommissioning work.

Performing the work

There were many steps involved in decommissioning the Bull Run project. Cornforth Consultants Inc. prepared the final design, permit documentation, and contract plans and specifications for the entire project, including Marmot Dam. The work to remove Marmot Dam began in March 2007 and involved selecting a contractor, mobilizing equipment and selecting borrow areas to provide materials for cofferdam construction, building the upstream and downstream cofferdams, removing Marmot Dam, removing the fish ladder, breaching the cofferdams, and completing final site work of contouring and revegetation.

Selecting a contractor

In April 2007, six pre-qualified bidders attended a pre-bid meeting for removal of Marmot Dam. Because of the unique nature and complexity of the Marmot Dam removal, PGE required that the proposals be submitted in a predetermined format. PGE provided 24 forms the bidders had to complete. These forms encompassed commercial (i.e., pricing, labor and equipment rates, etc.) and technical (i.e., hazardous material plan, demolition plan, project schedule, etc.) items. Three bidders declined to bid, and two formed a partnership for this project. As a result, PGE received two bids in May 2007.

Use of the 24 forms PGE provided facilitated a concise and expeditious evaluation. PGE evaluated the submitted proposals based on compliance with the specifications, cost, and qualifications.

One of the bidders, NMC, had worked on construction of the RCC dam in 1989. NMC built the cofferdam, maintained the dewatering system, and constructed the dam. Interestingly, the cofferdam and dewatering system NMC specified in the bid for the Marmot Dam removal was based on the 1989 design. Based on both cost and experience, PGE awarded NMC the contract, at a value of $5.86 million, in May 2007.

Preparing for the work

During June 2007, NMC mobilized field office trailers and heavy equipment for the project. The company supervised delivery of well casings and pumps, filter pack rock, “Fabriform” revetment, geo-grid fabric, construction bridges, and sediment fence material to the site. Malcolm Drilling Inc., a subcontractor to NMC, mobilized drilling equipment, which consisted of a Soilmec 825 drill rig.

Hampstur Corporation, a subcontractor to NMC, performed surveying of the borrow areas and the cofferdam alignment.

In 1989, materials for construction of the cofferdam needed to replace Marmot Dam were borrowed from the right bank alluvial terrace. Subsequently, the excavated river sediments, old timber crib rockfill, and cofferdam soils from the 1989 work were loosely dumped and spread in this area. In June 2007, Hampstur Corp. dug test pits in this location to determine if the material would be suitable for cofferdam embankment fill.

Similar test pits were dug in four other locations. The company identified two borrow areas that would consist primarily of granular terrace alluvium (i.e., the sandy gravel layer) needed for cofferdam embankment fill. However, once the borrow areas were opened up the company discovered that Borrow Area A had excessive amounts of unusable material. This borrow area was abandoned for another area that had been identified. Borrow Area B also contained unusable material, but NMC expanded and deepened the area to obtain most of the initial material that was used to construct the upstream cofferdam.

The supplemental borrow area provided sand, gravel, and cobble material that was slightly higher in fines content. Materials from this borrow area were used to complete the upstream cofferdam adjacent to the approach channel, which leads to the canal system. This approach channel was lined with Fabriform revetment to seal and protect it from river flows. The lining was designed to reduce the pumping requirements for the dewatering wells to be installed in the upstream cofferdam.

Beginning June 25, NMC excavated about 4,500 cubic yards of material from Borrow Area B and stockpiled it along the right bank, near the upstream cofferdam location. Stockpiling was completed by June 29.

Building the upstream cofferdam

The upstream cofferdam was needed to perform demolition of the instream structures, such as the portion of the old timber crib dam that was abandoned in place during installation of the RCC dam.

NMC used hydraulic information from several sources to build the upstream cofferdam to the necessary elevation. These sources include a 2005 design report prepared by Cornforth Consultants and their sub-contractor ENSR International, flow data from the U.S. Geological Survey (USGS), and bathymetry data supplied by PGE.

Demolition of Marmot Dam, a roller-compacted-concrete structure, was completed in stages.

The upstream cofferdam had a top elevation of 741 feet. It was designed to withstand flows up to about 2,500 cfs and to fail during high flows after dam removal was completed. The upstream cofferdam required dewatering wells to maintain cofferdam stability.

At 10 p.m. on June 30, 2007, PGE closed the head gates for the power canal to reduce flow into the approach channel. At 12:10 a.m. on July 1, 2007, NMC started the upstream cofferdam construction. NMC used tracked excavators to place the material and a dozer to push, level, and supply material from the stockpile. By placing the material using the excavator bucket and tamping the underwater portions, there was essentially no lateral spread or loss of material from river flow during the placement operation. By the end of the day on July 2, the upstream cofferdam was 90 percent complete.

It was important to maintain the work area in a dry condition during excavation and demolition. Based on the 1989 cofferdam work, NMC estimated that a total discharge of 4,000 to 5,000 gallons per minute (gpm) was necessary. On that basis, dewatering wells, well points, and sumps likely would be necessary to dewater the excavation. The contract specifications required NMC to “design, install, and operate temporary dewatering measures to lower groundwater at least 3 feet below working level to permit Marmot Dam excavation and demolition operations to proceed under dry conditions.”

NMC submitted an approved design for dewatering wells different than that contained in the contract drawings. The design was based on the lessons learned during NMC’s experience from the 1989 cofferdam dewatering and its subsequent dewatering projects. Approved changes in the dewatering design and construction consisted of larger-diameter dewatering wells with coarser-grained filter packs installed to the top of the bedrock, with submersible pumps installed near the bottoms of the wells.

Malcolm Drilling began drilling the nine dewatering wells on July 2, using the Soilmec 825 large hydraulic rotary drill rig. Wells were drilled using precut steel casings: 11 feet of 54-inch diameter, 24 feet of 48-inch diameter, and 40 feet of 36-inch diameter. Malcolm Drilling removed the steel casings as the well casings and filter packs were installed. Well casings were 18-inch-diameter high-density polyethylene (HDPE) pipe with 0.5-inch perforations in the bottom 30 feet. The wells ranged in depth from 20 feet to 75 feet. The submersible pumps installed were capable of pumping an estimated 400 gpm at 50 feet of head. Eight of the nine wells pumped continuously from July 17 to October 19. The ninth well went dry after the other eight were fully developed. Work to install the dewatering wells was completed on July 7.

The dewatering wells were effective in drawing the water level down and maintaining a stable and dry excavation. The dewatering system was so effective that there was never a need for additional wells or pumps for the entire removal project.

Construction of the approach channel portion of the cofferdam that tied into the wing wall also went well. NMC asked PGE and was allowed to change the crest width and flatten the slopes to avoid constructing a compound slope with geo-grid reinforcing in the upper part of the embankment. NMC also requested from PGE and was allowed to widen the approach channel by excavating about 10 feet further into the right bank. NMC covered the entire upstream face of this portion of the cofferdam with Fabriform revetment to protect against erosion. The Fabriform was anchored in a trench excavated at the toe of the embankment slope. Grouting of the revetment was completed July 9.

PGE opened the head gates at 9 a.m. on July 10 (three days ahead of schedule).

The upstream cofferdam was stable, water tight, and resistant to erosion. Blasting vibration from the first concrete structure removal shot was measured by a seismograph placed on the crest of the approach channel portion as 2.28 inches per second. This is well within the tolerable limits for a soil embankment constructed from sand, gravel, and cobbles.

Building the downstream cofferdam

Starting July 13, NMC used a sump pump to dewater the area between the dam and cofferdam. Biologists from the Oregon Department of Fish and Wildlife (ODFW) and PGE salvaged fish (salmon and steelhead) from the cofferdam area and transported the fish upstream of the upper cofferdam.

NMC built the downstream cofferdam primarily using materials excavated between the upstream cofferdam and RCC structure. The company built a shallow cutoff trench of more impermeable soil to minimize seepage through the cofferdam. NMC used boulders to prevent erosion at the bridge crossing, instead of Fabriform as shown on the original plan drawings. No erosion occurred, and the cofferdam and bridge crossing performed as intended.

Removing Marmot Dam

Before dam removal work could begin, NMC had to excavate sediment from behind the dam and haul it to refill the borrow areas. Material gradation and distribution of the sediment was close to what was expected.

Superior Blasting, a subcontractor to NMC, performed drilling and blasting operations for dam removal. The company completed hole drilling and preparations for the first blast on July 20. To accommodate a PGE-designated “media event,” the company loaded the explosives on July 23 and performed the first blast on July 24. This blast demolished the top 10 feet of the dam and separated the reinforcing steel from the concrete. Immediately after the blast, NMC began clean up of the material, which was completed on July 26. The company stockpiled concrete rubble in the alternate borrow area for processing at a later date to use to resurface roads or as structural fill material to remediate the borrow areas.

Superior Blasting performed the second blast on July 30 and the third blast on August 3. The second blast produced results similar to those obtained during the first blast, but the third blast produced larger rubble that required additional mechanical breaking. NMC delayed the fourth and final blast to September 19, to reduce the time the cofferdam would be exposed to river flows in its most unstable condition. As a result of this blast, the RCC was rubblized and the reinforced concrete was separated from the rebar. Only a small amount of secondary breaking was required, using excavators fitted with hydraulic hammers. NMC completed the concrete rubble removal on September 29. The remnants of the timber crib dam were removed in parallel with the last concrete rubble from the dam.

Removing the fish ladder

PGE included provisions for fish protection in the settlement agreement and decommissioning plan. Per the agreement, the existing fish ladder was to remain in service until August 1. However, NMC proposed shutting down the ladder earlier to reduce the scope of work required to install temporary entrance and exit piping/components. Installation of the piping/components would have required the ladder to be shut down for four to five days.

The settlement agreement called for installation of a temporary fish trap and haul facility. PGE received approval from fish agencies to install this facility early and to provide a minimum of five days of overlapping operation before shutting down the ladder. PGE installed the trap and haul facility on July 13. On July 18, the fish ladder trap contained three salmon over 24 hours of operation. The shutdown of the existing ladder was performed jointly by PGE and ODFW. PGE and ODFW performed fish salvage operations immediately after ladder shutdown and continued to salvage fish two to three times a week thereafter.

NMC started fish ladder demolition on July 23 by removing components and miscellaneous metals. Demolition was accomplished by August 25 with excavators fitted with hydraulic hammer and concrete crushing jaws.

Breaching the cofferdams

The design report and contract documents specified the location for the breaching notch in the upstream cofferdam. PGE also funded development of a model of the Sandy River, Marmot Dam, and the retained sediments in early 2007. Engineers at the National Center for Earth-surface Dynamics developed this model. The center is a National Science Foundation Science and Technology Center at St. Anthony Falls Laboratory at the University of Minnesota.

The location chosen for the notch was near the left bank, between dewatering pumps 1 and 2. NMC created a swale in the down slope portion of the cofferdam to channel the breach flow along the left bank. The goal of this work was to encourage maximum sediment movement during the flood event. NMC contoured the downstream cofferdam along the left bank for the same reasons.

Flows in the Sandy River needed to reach about 2,500 cfs before the cofferdams could be breached. According to the National Weather Service’s Advanced Hydrological Prediction Service (AHPS), flows would be about 1,500 cfs on October 19 and rise above 2,000 cfs the following day. When PGE and NMC met at the site to determine breach implementation timing, flow was 1,900 cfs and rising. NMC contacted the AHPS office for the revised forecast. This forecast predicted flows increasing to 2,500 cfs by October 19 and cresting at about 3,000 cfs October 20. NMC mobilized crews to initiate the cofferdam breach.

The high flows on October 19 undermined the temporary bridge abutments. This made it unsafe for a heavy excavator to cross. After evaluating the stability of the bridge, NMC personnel crossed the bridge with a rough terrain forklift. The forklift could be used to remove the pumps but not the well casings. Using the forklift, personnel removed the dewatering pumps (starting from the far side of the cofferdam). About an hour after the first pump was pulled, water began to bleed through the cofferdam and caused localized sloughing of material. While NMC was pulling pumps, PGE and ODFW trapped and hauled 209 salmon from below the cofferdam and released the fish above the cofferdam.

After all pumps were removed from the cofferdam, NMC started scraping a notch in the cofferdam (using the forklift) down to water level. When water started flowing through the notch, NMC partially closed the head gates at the canal to divert more river flow to the notch. As flows increased, the sediments began to incise down, and this incision progressed up river. From notching to full flow diversion took less than 30 minutes. The approach channel was filled (to build a rough road to the approach cofferdam) after all flow was diverted into the river.

The 4-foot-high downstream cofferdam was located 50 feet below the upstream cofferdam. It was designed to fail when the upstream cofferdam was breached during high flows.

The following day, NMC removed the well casings from the river. One well casing was not recovered because of the high river flows.

After the storm event, the sediment movement was extensive enough to provide unrestricted migration of the salmon upstream of the former cofferdam location. Since the breach, PGE, research groups, and federal and state agencies have used aerial and ground surveys to monitor the movement of the sediments and fish migration. The consensus is that the recuperative power of the Sandy River is more than what was expected in such a short time. Through the entire winter season, no obstructions to fish passage were formed.

Final site work

After the breach, PGE worked with BLM to determine the final contours and condition of the site. PGE wanted to make sure that the site conformed to the future recreational area plan developed by BLM. By working together, both BLM and PGE reduced the restoration and redevelopment costs by as much as $200,000. NMC continued working after the breach to complete the remainder of the project. This involved filling in canals, plugging tunnels, regrading the site, crushing the concrete, and revegetating the area. Phase 1 of the Bull Run decommissioning was completed in May 2008.

The road to Marmot remains in place and will be transferred to BLM. NMC re-rocked the road using rubblized concrete from Marmot Dam.

As a part of the removal of Marmot Dam, PGE removed the fish screens and associated structures from the canals. The fish screen areas were filled, regraded, and replanted with appropriate species to blend with the surrounding areas. The borrow areas and other areas disturbed by the removal of Marmot Dam were revegetated, and invasive/exotic plants were controlled pursuant to the Revegetation and Noxious Weed Control and Site Restoration Plan. Erosion and sediment control measures were implemented as necessary to protect the environment.

A notch scraped in the upstream cofferdam using a forklift initiated breaching of the structure.

The original bid amount for Phase 1 of the Bull Run Decommissioning Project was about $5.86 million. By working closely with the contractor and all project stakeholders, final costs were reduced to $4.81 million, a savings of more than $1 million.

Phase 2 of the Bull Run decommissioning

Work on Phase 2 of the Bull Run Decommissioning began in June 2008. This work consists of removing the remaining project features — such as the Little Sandy River Dam, the wood flume, and the Bull Run powerhouse — and draining and reclaiming Roslyn Lake. JR Merit received the $7.5 million contract for this work. Completion is scheduled for late 2009.

Lessons learned

Work on this challenging undertaking yielded valuable lessons about dam removal. They include:

— Allow adequate time for permits. With biological opinions in place from state and federal fish agencies, the joint application for the 404 permit to the Oregon Department of State Lands and the Corps required additional studies to be performed, which resulted in a one-year delay of the start of decommissioning.

— Choose a contractor that is trustworthy and will work closely with the owner to develop efficient work strategies. For the Marmot Dam removal, NMC’s experience, knowledge of the site, and project history provided valuable insight. NMC was able to work with PGE to minimize the overall project cost while meeting the commitments contained in the decommissioning agreement.

— Demolition is not an exact science. When a structure is built, the engineer’s design is conservative, based on the allowable strength criteria for the materials and the maximum loads the structure needs to endure. To demolish the same structure, the contractor has to use his knowledge, experience, and “enough but not too much force.”

— It is difficult to accurately predict river flows. The Sandy River has more than 90 years of historical river flow data collected by the USGS. The AHPS has computer programs and satellites to predict future flows. ENSR produced a study predicting river flows. Even with all the information on the Sandy River, flows could not be accurately predicted more than 12 hours in advance. NMC had to be able to respond on short notice to initiate the breach procedure.

— Working early and cooperatively with stakeholders is invaluable. BLM laid out its plans and helped prevent PGE from doing earthwork that would be “undone” later. Through design coordination and multiple site visits, PGE avoided having to move the rubblized concrete twice and was able to concentrate on BLM’s plan, which required less material transport than PGE’s original plans. BLM also wanted to perform the revegation with its labor, which was a cost savings for PGE.

References

 


Tim Keller, construction coordinator for Portland General Electric, was responsible for contract management and demolition oversight during the removal of Marmot Dam.

Envirosight introduces inspection camera

Envirosight LLC announces availability of a new version of its patented QuickView pipeline zoom inspection camera. The company says the new version offers users double the zoom capability, greater illumination, and improved camera sensitivity for low-light viewing.

QuickView 3.5 allows users to see 400 feet down pipes ranging from 6 to 60 inches in diameter while capturing sidewall detail. Envirosight says the new model delivers double the zooming capability of previous models with 400 times magnification.

Its new lamp technology casts focused illumination 65 percent brighter, without draining the rechargeable battery before a full shift of inspection work is completed, the company says.

Zoom inspection is an alternative to inspections that require remotely operated vehicles. Contractors can use it to survey pipe condition before bidding services and to document completed work.

Project owners can use the camera to identify and prioritize maintenance issues, avoid confined-space entry, and inspect hard-to-reach infrastructure.
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Eaton offers online product demonstrations

Eaton Corp. announces interactive online demonstrations of many of its products, including controllers, switchgear, and protective relays.

The 13 interactive demonstrations explain the features and benefits of each product line, the company says.

For example, the protective relays demonstration features five sections: introduction, benefits, features, applications, and documents. The introduction highlights products available to protect the motor, bus, transformer, generator, and feeder zones of the power system. The documents section features links to instructional literature and technical data sheets for the ten products included in the demonstration.

Demonstrations are available on the website at:

www.eaton.com/EatonCom/Markets/Electrical/BusinessPartners/ConsultantsEngineers/InteractiveProductDemonstrations/CT_203352.
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