After several years of public engagement and technical exploration, the Mirror Pond Ad Hoc Committee is proposing a concept for the future of Mirror Pond for public input. The concept detailed in this summary satisfies stakeholders who wish to retain Mirror Pond and those who wish to see a more free-flowing river with enhanced wildlife habitat. This concept, called the Mirror Pond and Downtown Redevelopment Concept also offers a vision for the renewal of a critical area in Bend’s downtown core, with opportunities for new parks, restaurants and mixed-use development. The concept will not increase taxes and would be funded and managed by a partnership between the City of Bend, Bend Park and Recreation District, PacfiCorp and the private sector.
UNDERSTANDING THE PROBLEM
Mirror Pond is a collector for sediment flowing through the Deschutes River, which backs up into the pond behind Newport Avenue Dam, owned by PacifiCorp. This sediment build-up, if left alone, would eventually create a wetland, picturesque views would be diminished and river recreation would be impacted.
The pond was last dredged in 1984 to remove sediment. In recent years, a need to address the sediment build-up became increasingly acute.
Stakeholders have been divided on the best way to address sediment build-up, leading to an extensive community-wide debate on the future of Mirror Pond.
Factors influencing the debate include:
The PacifiCorp-owned dam is 100 years old and no longer produces adequate power to justify continued corporate investment.
When dredging last occurred in 1984 there were fewer regulatory requirements. Today, regulatory requirements make dredging challenging and more costly.
The dam is in poor condition; repairs will be costly.
The land under the pond is owned by a family trust of one of the founding families of Bend. Two local citizens have an option to purchase the land in order to help shepherd a solution.
Neither the City of Bend nor the Park and Recreation District (BPRD) have ownership of the pond.
BPRD owns approximately 60% of land adjacent to Mirror Pond between Newport and Galveston Avenues.
DEVELOPING A SOLUTION
The Mirror Pond Steering Committee was created to oversee the development of a series of scenarios to address the sediment build-up in Mirror Pond.
A Mirror Pond Technical Advisory committee provided scientific input and data from which to base the alternative scenarios.
Scientifically based illustrations were developed to depict the scenarios.
The scenarios were taken to the public for input.
Community meetings and on-line surveys resulted in input from over 4,000 people.
Community input indicated a division between those who preferred the river to flow in a more natural-like manner versus those who preferred the current look of the pond be maintained.
While respondent first choice interests were divided between keeping the pond and returning the river to a natural-like path, there was a second choice scenario that satisfied most respondents. It maintained the pond while improving wildlife habitat and providing fish passage.
Results also indicated seven community interests that were shared respondents, including:
Maintaining the historic character and picturesque appeal of Mirror Pond.
Maintaining or improving public spaces.
Enhancing natural habitat.
Providing fish passage.
Reducing the quantity of sediment deposited in the river/pond.
Reducing the frequency that the pond needs to be dredged.
Identifying funding with minimal burden on taxpayers.
Following extensive community input, the Mirror Pond Ad Hoc Committee, made up of representatives from the City of Bend, Bend Park and Recreation District and citizens, was tasked with reviewing public input and working with PacifiCorp regarding the future of the dam and exploring possible solutions that would address shared community values. The Mirror Pond and Downtown Riverfront Redevelopment concept resulted from this work.
THE MIRROR POND AND DOWNTOWN REDEVELOPMENT CONCEPT
The Mirror Pond Ad Hoc Committee is proposing for community consideration a project that preserves Mirror Pond, allows for a more free-flowing river, creates wildlife habitat and will encourage dynamic riverfront mixed-use development in downtown Bend.
PacifiCorp would divest from their power production interests at the site by relinquishing ownership of the dam and moving the substation to another Bend location. A sub-committee of the Mirror Pond Ad Hoc Committee is currently in communication with PacifiCorp regarding this scenario.
PacifiCorp would gift the dam to a public entity (City and/or Park District) who would then oversee conversion of the dam into a water impoundment constructed as a series of pools and riffles in the river. This new impoundment would cause a rise in the river water level behind it, resulting in the preservation of Mirror Pond near its average historic level.
The new pools and riffles would provide fish passage where none exist today.
The banks along the river would be reshaped to help to reduce sediment buildup and enhance habitat.
Pacific Park, the two Mirror Pond parking lots, and PacifiCorp’s powerhouse, parking lot and substation would be repurposed into new mixed use development including public spaces, plazas, restaurants, small businesses, housing and public parking.
Private property owners in the area would see value in redevelopment as a means to enhance their investments and support the community’s economic vitality.
The Deschutes River Trail would wind through downtown Bend, connecting people to parks, schools and business.
A public-private partnership involving the City of Bend, Park District, PacifiCorp and private sector interests would lead and fund redevelopment.
The Park District’s role would include selling what is now Pacific Park and using the funds generated to create a new Pacific Park near what is now the substation, improving the Deschutes River Trail through Bend’s urban core, and creating new public places.
The City’s role would include encouraging development on what are now the two Mirror Pond parking lots, updating storm water systems, and building a new parking structure. Funds would be generated by a combination of proceeds from land sales/rent, the formation of an urban renewal district, and/or development fees.
PacifiCorp’s role would be to divest in the dam and substation.
Private development’s role would be to invest in new mixed use development, including retail, commercial/office and housing.
The Mirror Pond Ad Hoc committee is inviting the community to give input on the Mirror Pond and Downtown Redevelopment Concept. Should the concept receive widespread support, an independent consortium would be formed to oversee a development plan. The City and Park District would lead the elements of the project on public lands and make improvements to the dam and pond.
A public input period on the proposed concept will be open from November 2014 to January 2015. The public may offer input through the following opportunities:
Bend, Ore. — PacifiCorp will lower the Mirror Pond water levels next week to facilitate another inspection of the dam.
The inspection will be conducted by Gannett Fleming, an engineering firm from Phoenix, for the Bend Parks and Recreation District as part of deliberations around the potential acquisition of the dam from PacifiCorp to maintain Mirror Pond after the facility is no longer used to generate electricity.
The drawdown will begin Tuesday morning and will continue gradually until about midday on Friday. The pond will be allowed to refill starting Friday afternoon and should be full again by the Sunday morning. The inspection itself is scheduled to occur on Thursday and Friday. According to BPRD employee Jim Figurski, for safety and security reasons, access to the inspection will be limited to BPRD personnel, the inspectors and PacifiCorp.
Because of increased flows compared to last October when the Mirror Pond was drawn down to facilitate a PacifiCorp inspection, water levels are not expected to drop as dramatically this time, although it will likely be noticeable.
As was the case during the October drawdown and refilling of Mirror Pond, PacifiCorp will monitor water quality and conduct fish surveys consistent with state regulatory requirements.
PacifiCorp inspected the dam on October 31 after a leak developed in one of the structure’s wooden panels. The inspection concluded that while the facility was safe, it would not be cost effective to rebuild the entire facility to generate power for current and future generations of customers across PacifiCorp’s six-state territory. Since then, the company and representatives of the Bend Parks and Recreation and the City of Bend have been in discussions around potentially transferring the dam to a local entity so the community can realize its vision for the future of Mirror Pond.
PacifiCorp has announced plans to install sheet pile reinforcement in front of the leaking panel. The reinforcement work is planned to take place in April, or earlier if permitting is complete and a contractor is in place. The company does not anticipate needing to lower Mirror Pond levels again for that procedure.
BEND, Ore. – PacifiCorp will reinforce a section of the company’s dam in downtown Bend to address a leak in one of the structure’s wooden panels that developed in October of 2013. The installation of steel sheet pile upstream of a leaking panel is planned to begin in April.
The company stopped generating power at the dam following the leak, and announced in November that it is not cost-effective to rebuild the entire facility to generate power for current and future generations of customers across its six-state territory. PacifiCorp has since been in formal discussions with Bend Parks and Recreation and the City of Bend regarding a potential transfer of the dam to a local entity for the purpose of maintaining the Mirror Pond impoundment.
“Though the dam remains safe, we fully understand the community’s concern about the potential for low water levels during summer recreation months,” said Mark Tallman, PacifiCorp’s vice president for renewable resources. “It’s possible Mirror Pond would have remained full this summer without this fix, but in our view this is the right action to take at this time.” Tallman said. “This reinforcement is also anticipated to restore hydro generation for the year, and should provide the community with more certainty about the structure and Mirror Pond during the summer season and going forward.”
The company used the sheet pile technique previously to address similar leaks in other wooden panels, known as bays. The work involves driving long pieces of interlocking steel sheets into the river bed to create steel facing on the upstream side of the leaking panel.
“This action will also facilitate further discussions with community leaders to determine if an agreement can be reached that enables PacifiCorp to clearly demonstrate that placing the dam under local control is better for all our customers than dam removal or other alternatives,” Tallman said.
“The company is very committed to trying to find the best possible outcome regarding this facility that balances the community’s priorities for Mirror Pond and our regulatory obligations” said Scott Bolton, PacifiCorp’s vice president for community and government relations. “That includes taking steps like this reinforcement as part of our community commitment, and our support of efforts by community leaders to preserve Mirror Pond’s future.” Bolton added, “We are hopeful an agreement can be reached that allows this to happen and also protects the interest of our rate-paying customers in Bend and throughout our six-state service area.”
Mirror Pond will make its return before the start of spring despite the leak in the dam that creates Bend’s iconic lake.
Considering the current dry weather and taking in account the current water levels at the Wickiup Reservoir, the Oregon Water Resource Department is expecting Wickiup to be full around March 10-20. This could change by a few weeks if the weather changes.
Local Watermaster Jeremy Giffin said he will likely open Wickiup around the first or second week of March to where the outflow of water equals the inflow (from the current 36 cubic feet/second up to around 600 CFS). This will increase the flow through Mirror Pond from around 450 CFS to over 1000 CFS. It is estimated that the pond should fill and spill over the dam at around 600 CFS.
Giffin said that he will be releasing roughly the same amount of water this summer as in years past. “For the peak part of the summer (June through August) the flow through Mirror Pond should range between 1300 and 1600 CFS, again depending on the type of weather we are having and the type of crops in North Unit Irrigation District,” he added.
Bend Hydroelectric Project Timber Crib Dam Spillway Inspection
December 10, 2013
Roger Raeburn, P.E – Chief Dam Safety Engineer
Nathan Higa, P.E. – Senior Engineer
On October 1, 2013 a change to the leakage pattern in the timber crib dam spillway structure at the Bend Hydroelectric Project (Project) was reported by the Bend plant operator at Needle Bay 11. The new leakage pattern appeared very similar in nature to events experienced in 2008 and 2009 which resulted in remedial action. The cause of the leakage was attributed to failure of deteriorated facing boards on the upstream side of the dam in the vicinity of the original needle bay.
The increase in leakage through the timber crib dam, combined with the seasonal decrease in releases from Wickiup Reservoir into the Deschutes River system, as a result of cessation of the irrigation season, resulted in the inability to maintain the water surface elevation behind the dam at the normal operating level. The result is the reduction in the water surface elevation of Mirror Pond by approximately two feet. There is no public safety issue associated with the recent change to the condition of the dam.
This leakage event and the associated change to the operating condition prompted an engineering inspection. This report provides documentation of an engineering inspection conducted for the water retaining structures associated with the Project, with an emphasis on the timber crib dam spillway.
The objective of the inspection was to ascertain the condition of the water retaining structures, and in particular the timber crib’s structural members, rockfill, and foundation connection to the extent possible, given the limited access to the timber crib’s downstream toe, face and interior due to a limited ability to drawdown the reservoir behind the dam. To fully isolate the timber crib would have required placing fill in the river, which would require additional permitting. The factors associated with the timber crib’s condition are considered to be the most influential in the development of judgments and conclusions regarding the Project’s overall condition and suitability for continued safe operation.
The Bend Hydroelectric Project (Project) is located within the city of Bend, Deschutes County, Oregon on the Deschutes River, at river mile 160. The impounded water creates Mirror Pond, which is surrounded, in part, by Drake Park. The Project operates on a non-expiring water right of 1325 cfs for power production with a priority date of 1905.
The Project was constructed and placed in service in 1916. The Project’s water retaining structures have remained essentially unchanged since that time. The generating assets and water retaining structures are approaching 100 years of operation and their condition reflects their age. While still operable if upgrades are performed, the Project is nearing the end of its expected service life.
The Bend Project is currently on a tri-annual inspection schedule by the Oregon Water Resource Department (OWRD) Dam Safety Section. The last inspection by OWRD was conducted July 13, 2012. Recommendations contained in the OWRD’s October 16, 2012 report (Attachment 1) included:
Continue with good maintenance and operations, including security and vegetation control.
Evaluate Deschutes River flow, and accompany OWRD on an inspection of the spillway structure at very low water. The timing of this inspection would be coordinated by OWRD and the local watermaster for the Deschutes River.
The OWRD inspection reports include an assessment of the condition of a project’s water retaining structures. The rating system utilizes five categories: 5 – Very good, 4 – Adequate, 3 – Maintenance or minor repair needed, 2 – Serious repair needed and 1- Urgent dam safety issue. The 2012 report identified no category 1 or 2 items, two project features rated category 3, and all remaining features rated category 4 or 5. The two category 3 features were:
Wooden boards at the toe of the timber crib dam section.
Gate leakage through the regulating outlet sluiceway. The next regularly scheduled inspection is due in 2015.
The water retaining structures associated with the Bend Hydroelectric Project comprise several components as depicted in Figures No. 1 and 2 below.
Timber Crib Dam Spillway – The timber crib dam spillway (timber crib) is comprised of square log timbers placed side by side and stacked to form open box sections that were then filled with large basalt rock. The design of the timber crib initially included 21 crib sections (bays), of which 14 were configured as “needle bays” incorporating vertical wood needles that allowed for the controlled passage of water directly through the timber crib by the adjustment of the needles. The timber crib section was later modified with the addition of a concrete sill on top of the crib that allowed for water to spill over the top of the dam and isolated the needles, rendering them non-functional. Following the timber crib modification, the release of water was accomplished over the top of the timber crib and controlled with wooden stoplogs that remain in service today.
Thirteen of the original needle bays remain exposed, with the 14th upstream-most partially covered with riprap from the abutment. The seven other bays were configured without needles, but do include stoplogs and are capable of spilling water over the timber crib.
Ice/Debris Sluiceway – The ice/debris sluiceway is located at the downstream abutment of the timber crib. It is constructed of grouted basalt rock and incorporates wood stoplogs that can be pulled to allow the passage of water, ice, and debris. The ice/debris sluiceway has not been used for a considerable time.
Non-Overflow Dam Section – The non-overflow dam section is located between the sluiceway and the powerhouse. It is comprised of a buttressed concrete wall approximately 12 feet high and founded on basalt rock. This section of the dam is considered a non-overflow section, with no provision for the passage of water.
Powerhouse – The powerhouse consists of a concrete foundation with a brick masonry superstructure and truss roof. It contains three James Leffel horizontal Francis turbine generator units operating at a gross head of 14 feet with a combined capacity of 1,110 kW.
Regulating Sluiceway Outlets – Two low-level sluiceway outlets are located to the right of the powerhouse. The original structure was designed with wooden stoplogs to control the release of water. The original stoplogs have been modified with the addition of two 4-foot wide by 5-foot high slide gates operated by an electrically powered manually operated gate actuator.
In response to the report of increased leakage through timber crib Bay 11, an engineer from Hydro Resources familiar with the 2008 and 2009 leakage mitigation work conducted an initial site visit to the Project on Friday, October 4, 2013. The site visit was conducted to assess the potential for the development of any dam safety concerns, observe the new leakage event, and assist with the preparation of plans for an engineering investigation/inspection of the timber crib.
At the time of the site visit the reservoir level staff gage reading was 16.3-foot, which is within 0.2-foot of the normal maximum level of 16.5-foot. The spillway was traversed back and forth, with photographs taken of each needle bay. Needle Bays 1 through 4 had no noticeable leakage flow. Needle Bay 5 had spillway overflow falling directly into the outlet, but it did not appear to be leaking. Needle Bay 6 had minor leakage flow. Needle Bay 7 and 8 had spillway overflow falling directly into the outlet, but did not appear to be leaking. Needle Bays 9 and 10 had a very small amount of leakage flow. The leakage flow through Needle Bay 11 was significant, with sufficient water being released to result in flow discharging through the timber crib above the needle bay. It appeared that some of the timber facing boards on the downstream toe of the dam had been dislodged due to the leakage flow. A 12-inch diameter vortex was noted on the upstream side of the dam directly in front of Needle Bay 11. Needle Bay 12 (upgraded with sheet pile in 2009) had no discernible leakage flow visible. Needle Bay 13 (upgraded with sheet pile in 2008) had a minor amount of leakage flow. Needle Bay 14 is partially covered with riprap materials from abutment erosion.
The performance of the dam since its construction has been satisfactory and the lack of any significant visual evidence indicating deformation of the timber crib structure makes it likely that the internal components are in satisfactory condition. After the initial site visit, it was concluded that confirmation of assumptions relative to the condition of the timber crib’s internal components would be performed. Actions identified include:
Perform a drawdown of the reservoir (Mirror Pond) to minimize leakage through the timber crib.
Conduct a visual structural inspection of the downstream face, needle bays, internal components and the dam’s foundation.
Develop a monitoring and surveillance program for the timber crib
Install horizontal and vertical alignment monuments on the timber crib,
Conduct a baseline alignment survey of the timber crib,
Establish an alignment survey schedule to monitor for any changes,
Develop a visual inspection check list for water retaining structures,
Initiate a monthly inspection of all water retaining structures by the Bend Operator,
Perform a visual inspection of all water retaining structures by a PacifiCorp Energy Dam Safety Engineer on a bi-annual frequency.
Drawdown & Refill
The drawdown process began at approximately 7:00 AM on October 28, 2013 at a rate of two inches per hour. Water released from Wickiup Reservoir was reportedly 25 cubic feet per second during the drawdown period, with flows at the Project estimated to be 500 cubic feet per second. On October 31, 2013 (the day of the inspection), the level of the reservoir as measured on the temporary staff gauge was 9.56 feet, approximately 7 feet below the normal full operating level at 16.5 feet (Figure 3 & 4). Refill of Mirror Pond began at approximately 7:00 AM on November 1, 2013, at a rate of four inches per hour. The reservoir was restored to the starting elevation by the evening of November 2, 2013. All ramping was performed during daylight hours by manual operation of regulating outlet sluice gate aperture at one-hour intervals. The operator responsible for ramping procedures made conservative changes to the regulating outlet sluice gate aperture, and was thus able to maintain compliance with the prescribed ramp rates. All drawdown activities were performed in accordance with a plan approved by the Oregon Department of Environmental Quality (ODEQ) and in coordination with the Oregon Department of Fish and Wildlife (ODFW).
Engineering Investigation & Inspection
Design information and construction record drawings for the timber crib are limited. In preparation for the detailed engineering inspection, PacifiCorp Energy engineering personnel conducted investigative work into the history of the dam’s construction and modification. While the exact timing of various modifications to the timber crib remain obscure, it was clear that the available drawings do represent what is currently in place. Figure 5 represents a depiction of the timber crib, with descriptors for the various elements of the structure used in this report.
The investigation conducted into the dam’s history also revealed additional information about the dam’s history that was previously not readily available. The present concrete spillway sill was constructed in approximately 1919, earlier than previously thought. Bits and pieces of evidence were identified that would indicate the upstream and downstream facing boards on the dam have been replaced more than once in the dam’s history. No record of the last replacement of downstream facing boards was found, and the most recent replacement of upstream facing boards was completed in the early 1990’s on a selected basis. The access walkway was most recently replaced in 1975.
No record of leakage through the timber crib of the type experienced in 2008, 2009 or October 2013 was found in the records (Figure 6 & 7). It would appear from available records that previous work to address leakage was implemented to address general leakage issues through the replacement of upstream facing boards with various materials.
The work completed to address the leakage in 2008 and 2009 consisted of the installation of
sheet piles across the upstream face of the dam, immediately in front of the leaking needle bay. A drawdown of approximately 3 feet was required to allow the cutting of an upstream ledger board, allowing the installation of a wide flange steel section at each end of the sheet pile section, tying them to the dam.
Inspection of Needle Bays 6, 12 and 13 confirmed that the installation of the sheet pile was a viable method for controlling the leakage through the needle bays. The effectiveness of the sheet pile in controlling the leakage varied in each case, from virtually no leakage to leakage judged to be less than one cubic foot per second (Figures 8 & 9).
Inspection of Timber Crib Spillway
A detailed inspection of the timber crib was conducted on October 31 and November 1, 2013, by this report’s authors. The Hydro North safety administrator assisted with the logistics of access and provided a safety watch during the engineering inspection activities. The OWRD Dam Safety Engineer participated in the inspection for a couple of hours. An overall visual assessment of the timber crib was completed as part of the inspection with specific attention given to the 14 needle bays. The needle bays are considered to be the design section most vulnerable to damage/deterioration, and associated directly with the last three significant leakage events.
In addition to the visual inspection and documentation of conditions with photographs, methods involving the use of an impact drill/driver, 18-inch straight blade screw driver and 3-pound hammer were used to assess the condition of wooden members and connectors. Holes were drilled through wooden facing boards and into timber crib members using the drill/driver and a 6-inch long 0.375-inch diameter lathe bit. The condition of the steel drift pins and foundation bolts used to connect the 12-inch by 12-inch timber crib members, and the 60d nails used to attach the facing boards were checked using the 3-pound hammer. When accessible, the ends of drift pins and the heads of foundation bolt heads were struck with the hammer in an effort to judge their condition as well as their effectiveness in continuing to provide a credible connection. The hammer was also used to drive selected exposed 60d nails into the timber crib members and through facing boards. The ease with which the nails were driven was judged and the extent to which they would pull separated boards together and hold was observed. The 18-inch screwdriver was used as a probe and pry bar to assist with the evaluation of a wood member’s condition and to a limited extent, the condition of the rock fill.
Inspectors accessed the concrete spillway sill by climbing through the space created by flipping the hinged 2-inch x 12-inch walkway board located on the upstream edge of each 12-foot crib bay. Access was gained to the toe of the timber crib using a 20-foot fiberglass extension ladder that was extended from the spillway sill to the floor of the needle bays. Representative photographs were taken of each needle bay to document the leakage observed, condition of exposed timber crib members and the ceiling, side and floor facing boards. The presence of the concrete foundation sill was confirmed by touch as the water beyond the needle bay floor was most commonly too turbid to see it. The depth of the water beyond the toe of the concrete foundation sill was determined by stepping off the sill to the river bottom directly downstream of the sill. The extent of undercutting at the downstream toe was judged using the engineer’s boot and feeling with the toe.
For Needle Bays 1 through 3, one bay was accessed at a time, moving the ladder to each individual bay and wading to each side of the needle bay to access the adjacent full timber cribs. As the inspectors became more comfortable with the access and footing in and around the toe of the timber crib, the ladder was moved to multiple bays and a tether line was used to allow access to one bay to either side of the bay in which the ladder was secured.
Access Walkway – The wooden access walkway structure is founded on and anchored to the concrete spillway sill. It is constructed of pressure treated members of varying sizes. Construction drawings of the walkway structure are available. With the exception of a couple downstream vertical 4-inch x 4-inch center supports that are missing or unsecured at the bottom connection point, the walkway support structure members are in good condition and sound. The parallel 3-inch x 8-inch members that form the lower cords for the walkway structure at each end of the 12-foot bay are founded on an original 12-inch x 12-inch timber that lays between the concrete spillway sill slab. In a number of locations the 12-inch x 12-inch timber member has deteriorated as much as 4 inches, causing a loss of support for the 3-inch x 8-inch members and the front vertical 8-inch x 8-inch timber member. There were also a few locations where the downstream anchorage of the 3-inch x 8-inch member to the concrete sill was loose, damaged or missing. Concrete Spillway Sill Slab – The concrete caps were confirmed to be as shown on the design drawings, approximately 10-foot long, 6-foot wide and 20 inches thick. Other than the sill slab in Bay 21, they appear to be level and are in good condition, with a few minor pockmarks noted where aggregate most likely was ejected. All the sill slabs showed some erosion/spalling of the downstream nappe with exposure of aggregate. It appears that the upstream 12 inches of the present sill slab was placed at a later time than the main sill slab section. The construction joint between this upstream 12-inch wide section and the larger downstream sill slab varied in width. The sill slab in Bay 21 has failed (Figure 10).
Figure 10 – Bay B21 Failed concrete spillway sill.
There was no obvious deflection or settlement noted that could be attributed to the failure. Based on the condition of the cracks that extend the entire length of the slab, the failure is not recent. A condition found to be common throughout the timber crib was visible separation between the bottom of the concrete sill slab and what remains of the original wooden spillway sill boards upon which it appears the sill slabs were placed. This is evidenced by 4 inches of the 60d nails anchoring the original spillway sill boards to the timber crib structure being visible. (Figure 11)
Figure 11 – Bay B21 Failed concrete spillway sill.
Needle Bays –The nomenclature used to describe the bays on the dam is described as follows. The 12-foot timber crib bays are numbered B1 to B21 from Downstream (D/S) Upstream (U/S). The first two characters of a bay description refer to the 12-foot wide bay. The second two characters refer to the needle bay (6-foot wide bay section) located in the respective timber crib bay. (i.e. B9N6 – Needle bay N6 located in 12-foot timber crib bay B9). If only the first two characters are shown, this refers to a timber crib bay that does not contain a needle bay.
The condition of various components within the needle bays varied considerably from bay to bay. A brief summary of each needle bay’s condition is provided in the following table:
Bend Timber Crib Spillway Inspection Summary Results
Downstream Facing Boards – The boards on the downstream face of the timber crib are fir. This is in contrast to the crib timbers being pine. The condition of the downstream facing boards varies significantly, with some in fair condition and experiencing less than 10 percent section loss and some surface weathering, to locations where the boards are absent, exposing the crib timbers and rockfill material. The material in all the remaining facing boards was found to be solid. There was significant erosion and weathering, but drilling of the boards identified no areas could be described as soft or rotten. A common condition for the boards was cracking and splits in the end 25 percent of the boards. Boards near the tailwater did not show any significantly different signs of deterioration (Figure 12).
Figure 12 – Bay B2 D/S face of timber crib looking from Bay B6N4 showing condition variability of D/S facing boards.
Crib Timbers – The crib timbers were found to be composed of pine. This is in contrast to the facing boards composed of fir. The condition of the crib timbers was generally consistent throughout the timber crib section where it was exposed and available for inspection. The exceptions to these observations were the timbers that had been exposed to the leakage events in 2008 and 2009, with those timbers exhibiting the greatest amount of section loss. Section loss was also noted to be accelerated at locations where the crib timbers were exposed due to the loss of a facing board (Figure 13). The condition of the remaining portion of the crib timbers was found to be solid and sound. The wood shavings provided by the drilling confirmed that wood material to be free of rot, and damp to cores. Where the 60d nails were driven into the timbers, the effort required was similar to what would be expected to drive a nail into green timber. There were no signs of any treatment products in the wood. It was noted that the knots in the timbers were much more resistant to erosion, leaving the timbers to appear as round sections (logs) in many instances. Splices to horizontal members within the crib were also noted.
Figure 13 – Bay B4N3 Erosion of crib timber due to lack of cover by D/S facing board.
Connectors – The timbers in the crib were connected using drift pins at the interfaces where they crossed at 90 degree angles. Splices were composed of fitted ends with two bolts. The foundation timbers were set directly on a bed of concrete and fixed with anchor bolts. The facing boards on the interior of the needle bays and the D/S face of the timber crib were attached using 60d nails. All metal connectors were found to be in good condition. The drift pins showed very little corrosion loss, and in every case where a pin was available, and struck with the 3-lb hammer, it proved to ring solid and did not move. All foundation anchor bolts that were noted appeared to be in good condition. Bolt bodies, washers and nut exhibited little corrosion damage. Exposed threads were commonly still visible. All anchor bolts sounded with the 3-lb hammer rung true and did not move (Figure 14). The 60d nails were generally in good condition. A few nails that were exposed in a location that experienced a continuous flow ofwater did show some signs of section loss. Nails that were a minimum 50 percent embedded in a wood member could be bent without damage to the wood or significant loosening of the nail. All exposed nails that were driven into wood members held tightly.
Figure 14 – Bay B2 Absence of rockfill (typical).
Rockfill – The rockfill is comprised of basalt rock, relatively well graded for D50 sizes from 3 inches to 15 inches. The rock shows no signs of deterioration or damage from erosion. Consistently across the entire timber crib section, the rockfill has settled, leaving a space of 3 to 6 inches between the top of the rockfill and the next horizontal level of the timber crib (layer defined by the 12-in x 12-in timbers). In a number of cases where the rockfill is no longer confined in the crib by a needle bay facing board, the gradation of the material has caused it to remain in place. Observations of the lower and D/S-most 2-3 feet of the crib made it appear that rockfill was never placed in this zone, or if it was, it is missing. Where there are missing D/S facing boards the rockfill is generally still in place (Figure 15 & 16).
Figure 15 – Bay B2 Absence of rockfill (typical).
Figure 16 – Bay B10 Rockfill settlement.
Abutments – There are no design or construction drawings for either abutment. The left abutment of the timber crib section is a former Newport St. bridge pier (Figure 17). It is massive concrete and appears to be in excellent condition. Access to the U/S or D/S toe of this abutment was not available.
Figure 17 – Left abutment from U/S
The right abutment (Figure 18) is a grouted masonry structure most likely constructed at the same time as the timber crib. Based on inspection of foundation of the adjacent stoplogged sluiceway constructed in the same manner with the same materials it appears to be founded on basalt at the river bottom. The walls were plumb and the top was level to the eye. There were no signs of distress in the grouted joints.
Figure 18 – Right abutment on D/S side
There is no public safety issue associated with the recent change to the condition of the dam. The timber crib dam spillway section is in good condition considering it is approximately 100 years old. The primary components consisting of the crib timbers and the basalt rockfill have experienced varying degrees of deterioration. Based on the visual observations made during this inspection they are judged to be in satisfactory condition. No conclusion can be made for Needle Bay B16N11 at this time, however it is anticipated that its condition is similar to what was observed in Bay B18N12 and B19N13.
The observations provide better knowledge than previously available regarding the mode of the severe leakage events experienced in 2008, 2009 and most recently in October 2013. The needle bay sections of the timber crib are the most vulnerable to deterioration, in particular the ceiling boards. As the internal components of the needle bays age and deteriorate, their ability to provide adequate support/restraint for the rockfill material is gradually lost. The loss or failure of a needle bay’s side facing board does not appear to be as critical as the ceiling boards. A ceiling facing board failure results in a release of the rockfill into the bay. The associated loss of support to the upstream facing boards due to the release of the rockfill results in greater stress loads in those upstream facing boards. As individual upstream facing boards fail, the loads are shifted to adjacent boards, increasing the stress level in them, causing them to deform further, and eventually fail. As this cycle of aging and deterioration continues, larger pathways for leakage are created leading to the loss of more rockfill (and its associated support), resulting in an increased number of upstream facing board failures.
While there has been settlement of the rockfill in the timber crib sections that do not support a needle bay, the rockfill remains sufficiently confined to prevent the complete loss of the upstream facing board support that leads to overstressing. In these bays the increase in leakage is most likely due to the erosion of joints/seams between the upstream facing boards and the slow progression/growth of holes between the upstream facing boards.
The observations made during this inspection confirm assumptions supporting the conclusion that the dam is not a threat to the environment or public. There is no reason to suspect that a failure of the dam will occur in the near term. Leakage through the timber crib can be expected to increase on a global basis. Absent action taken to address the degradation of the upstream facing boards with their replacement or the installation of a bulk head system (equivalent or equal to the sheet pile), leakage events similar to the type experienced in 2008, 2009 and most recently in October 2013 can be expected to continue. Increased leakage does not present a threat to the stability of the dam in the near term. In a longer time frame, the rate of leakage will continue to increase, and with it the deterioration of the timber crib’s structural components, eventually leading to their failure. It is anticipated this process will evolve slowly, and it is likely that leakage through the existing timber crib would become so pervasive that the dam would be unable to maintain normal water levels prior to the development of a structural failure mode. The progression of leakage through the upstream face of the timber crib leading to a loss of its ability to maintain the reservoir level is the most likely failure mode for the dam, and not a catastrophic failure.
Attachment 1: State of Oregon Water Resources Department – Inspection Summary Letter dated October 16, 2012.
PacifiCorp has determined that it would not be cost-effective for its customers to make the investments needed to continue long-term operations at the company’s Bend hydroelectric generating project.
The decision follows a thorough engineering inspection of the project dam after a leak in the dam developed in early October, the third such leak in approximately five years, which also prompted a broader analysis of the facility.
“First and foremost, the inspection confirmed what we expected; the dam remains safe and is in overall good condition for a 100-year old facility, but further investment would be required for the hydro project to operate long term,” said Mark Tallman, PacifiCorp’s vice president for renewable resources.
“After a century of producing clean, emission-free and affordable power for customers, it’s time to divest or retire our Bend hydroelectric plant,” Tallman said. “It simply isn’t cost-effective or in the best interests of all our customers throughout six western states for PacifiCorp to rebuild the facility and generate power to serve current and future generations of customers.”
The project is located near the Newport Avenue Bridge. In addition to diverting water for the company’s hydroelectric project, the dam creates Mirror Pond on a section of the Deschutes River in downtown Bend. The Bend community has been engaged in extensive public discussions of its desires and priorities for the future of Mirror Pond.
“We’ve known and have been candid with the community that the facility was reaching the end of its useful life as a generating facility for PacifiCorp’s customers. That time is now here, accelerated by the recent new leak. This converges with public conversations and comparison of options the public has participated in,” said Pat Egan, PacifiCorp’s vice president for customer and community affairs.
The company will pursue discussions with the Bend Parks and Recreation District and the City of Bend to determine if an agreement can be reached that places the dam under local control in a way that PacifiCorp can also demonstrate to regulators that the outcome is in the best interest of PacifiCorp’s customers.
“The company wants to be supportive of community efforts to preserve its vision for the future of Mirror Pond as long as PacifiCorp can also meet its regulatory obligations,” said Egan. “Among the various options we must responsibly explore, we are hopeful an agreement can be reached that allows this to happen and also protects the interest of our rate-paying customers in Bend and throughout our multi-state service area.”
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