Personnel safety, cost, efficiency and effectiveness were all determining factors when the operators of Lake Lynn Dam and its associated 51.2-MW powerhouse sought a solution to improve debris management practices. The new solution has prevented mechanical failures and reduction in spillway capacity.
By Stefan Schadinger, Bryce Mochrie, Andrew Datsko and Jacob Vozel
Lake Lynn Dam in West Virginia has a history of debris buildup around the spillway gates dating back to initial operation. A particularly telling incident was a 1985 flood, when almost all of the 26 spillway gates were partially blocked by debris. The significant amount of large debris reduced the spillway capacity, raised the reservoir level and flooded the powerhouse. The accumulation of debris during high flows is a potential failure mode for this facility, reducing spillway capacity and leading to higher flood elevations, potentially causing stability problems for the dam.
Lake Lynn Dam is a 125-foot-high, 1,000-foot-long concrete structure located on the Cheat River in West Virginia that consists of an integral intake/powerhouse, a gated spillway, and two concrete bulkheads. The dam is presently owned and operated by FirstEnergy Generation Corporation and has been generating hydroelectric power since construction was completed in 1926.
As a result of the 1985 flood, Allegheny Energy (now FirstEnergy) determined that improved debris management was required. The existing practice was to let debris buildup behind the dam and pass it after the high water event. This practice resulted in the flood gates being partially plugged during high water and debris flows events. In addition plant personnel had to manually guide large debris through the existing debris gates due to the small size of these gates.
Several potential debris management options were studied by the team of PB Power/Allegheny Energy (now FirstEnergy) engineering and Lake Lynn station operating personnel using the Six Sigma format, including installing debris control measures upstream of the project and revising gate operation procedures to redirect debris. This article discusses the results of these studies.
The two existing trash gates were replaced by a single larger hydraulically operated hinged crest gate (supplied by SteelFab Inc. of Charlotte, N.C.), with an increased ability to flush trash and increased spillway capacity. A 1,000-foot-long Tuffboom log boom (supplied by Worthington Products Inc. of Canton, Ohio) was installed to direct debris to the new trash gate. The improved trash gate and installation of the log boom will mitigate the potential for upstream flooding due to the accumulation of debris at the spillway.
Flood of 1985
The flood of record since the construction of Lake Lynn Dam occurred on Nov. 4-5, 1985, as a result of a significant rainfall event caused by tropical storm Juan passing through the watershed. The maximum headwater elevation associated with that flow was 876 feet, 2 feet above the powerhouse floor at the head gates and about 1 foot below the top of the parapet wall of the east bulkhead. Debris buildup reduced the dam’s spillway capacity during this high-water flow period by about 35% from the theoretical flow, and 13 of the 26 tainter gates were almost fully blocked.
Even though the dam did not overtop the the forebays, which are at a lower level than the top of the dam and were flooded, the flood still caused significant flood damage to the powerhouse. The entire powerhouse was flooded. Records show that it was out of service for four to six weeks before returning to limited operation, but the debris clean-up effort after the event lasted almost six months. Both the reservoir level and generation from the powerhouse generation were decreased during this time.
This flood event at that time triggered Allegheny Energy Supply, now FirstEnergy, to perform a new probable maximum flood (PMF) analysis, which resulted in a higher PMF elevation for the structure. This subsequently required a revised stability analysis of all the structures to ensure that all Federal Energy Regulatory Commission (FERC) requirements were met for Lake Lynn Dam, which is classified as a high-hazard-potential project.
The 8th Part 12 Safety Inspection Report potential failure mode analysis (PFMA), a dam safety inspection performed every five years that the owners and their independent dam safety consultant submits to FERC, identified failure due to debris reducing spillway capacity as a potential failure mode for the Lake Lynn facility. This potential failure mode is described below:
This potential failure mode involves debris, which accumulates during high flows. The debris blocks the gate openings reducing the gate discharges causing a reduced spillway capacity, leading to higher flood elevations (sooner and more severe overtopping), causing stability problems, leading to failure (PFMA report).
As a result of the PFMA, it was determined by the Lake Lynn Part 12 review team that the debris management plan at Lake Lynn needed to be improved.
|Structural stress and damage, mechanical failures, and reduction in spillway capcacity have been avoided at Lake Lynn Dam and the connecting 51.2-MW powerhouse by improving the debris management system.|
Debris management plan
Several debris management options were considered during the Six Sigma evaluation by the review team. The alternatives mostly focused on installing debris control measures upstream of the project and revising gate operation procedures.
The proposed debris control measures upstream of the project considered an interceptor boom across the entire width of the reservoir upstream of the dam. The intent of this boom is to capture and retain all debris coming down the river at some upstream location, where it is then removed from the river and properly disposed. The disadvantages to this option were the potential for a concentrated raft of debris to be swept into the dam should a boom failure occur. Also, removing debris trapped upstream of the project at the boom location would be a large maintenance expense (estimated at about $200,000/year).
Further, the location of the boom, potential flooding caused by floating debris from the reservoir at upstream locations, state environmental approval needed and public relations issues (since it would impact recreational use of the reservoir) in addition to the location of temporary disposal areas for collected debris were also disadvantages.
Continuing to pass trash and debris through Lake Lynn Dam was considered, with alternatives to modify the existing trash gates. These were considered in addition to a wide range of options, including rebuilding the existing trash gates, replacing the trash gates at the present location or relocating the trash gates.
The two existing gates are limited in their effectiveness for debris management because debris larger than 12 feet in length (much of the debris and debris rafts being passed is longer than this) which blocks the opening. Furthermore, station personnel cannot pass trash during high water events due to personnel safety hazards (such as being pulled off the work platform into the floating debris and reservoir and carried through the trash gate).
Finally, debris often needs assistance to be redirected or guided such that it passes through the gates, and this requires personnel to work just above the water from a walkway on the upstream side of the deck. As you can imagine, this is a significant personnel safety concern.
Further, the dam is suffering from the long-term effects of alkali-aggregate reactivity (AAR), meaning the trash gates cannot be lowered for about six months of the year due to binding. This condition, which affects both trash gates, is getting progressively worse with time and is the worst in the warmer summer season. When the trash gates cannot be used as required, plant personnel may have to attempt to pass trash through spillway gates.
An extensive AAR evaluation – including available project information, especially recent field observations, measurements, and input from Lake Lynn personnel – was performed by engineering personnel. This evaluation extended to the powerhouse, spillway and abutments. The AAR growth at Lake Lynn results in equipment misalignment problems, including the turbine-generator units and the intake gates and trash gates, as well as significant concrete cracking and movement of the deck/piers and the powerhouse. To remediate some of the trash gate difficulties and inclination of the piers of the spillway, seven expansion joints were re-established by full deck depth slot cutting.
|Due to the build-up of sediment behind these existing flood gates, the gates became partially clogged during high water and debris flow events.|
Since this was implemented, significant relaxation (many inches) has been observed at the expansion joints and other monitoring points on the dam and powerhouse. Any new trash gate work was planned to begin at least two years after the expansion joint effort to allow for most of the expected acute relaxation to occur.
The location, size and configuration of different trash gate options were considered.
Rehabilitating the existing trash gates at the present location was eliminated as an alternative because of the poor condition of the existing gates. Replacing the existing gates with new gates at the present location was a feasible option, with two possibilities for gate configurations. The trash gates could be replaced with the same models, but this would not eliminate issues with the trash gate becoming blocked. This alternative also would not eliminate the personnel safety hazard. A single gate could be used to replace the existing trash gates, which would effectively double the width of the gate.
Another option was to relocate the trash gates to the west side of Lake Lynn Dam, which is where the debris is pushed by the prevailing wind. This was the most expensive alternative of those explored. An issue with this alternative was the potential loss of some tainter gate flow-passing capability because two of these gates would be converted to a trash gate.
As a result of the study, the review team determined that the best alternative for debris management at Lake Lynn Dam includes replacing the existing trash gates with a new larger crest gate at the same location and installing a debris boom across the spillway section to help keep the tainter gates free of debris and maximize discharge capacity.
The existing gates consist of two leaf slide gates, with the upper portion sliding down behind the lower portion, so at full open only half of the height to the crest allows water to discharge. The crest gate is hinged at the bottom and rotates downstream, allowing water to flow over it. It is operated using a hydraulic power unit that controls one cylinder position at one side of the gate. The gate has the ability to fully open (be positioned horizontally), which maximizes discharge.
The construction, which was contracted to the Tarentum, Pennsylvia-based Joseph B. Fay Co., involved removal of the two existing 12-foot-wide trash gates, gate house, 3-foot-wide concrete piers between the gates, and concrete deck spanning between piers one and three. The two trash bays were separated by a center wall running the length of the spillway. An engineering evaluation was performed by the PB Power and the FirstEnergy project team that showed that although its more expensive, it was beneficial to remove the wall over the full length versus a partial removal to ensure that debris did not back up behind it.
The two existing trash gates were replaced by a single 27-foot-wide hydraulically operated crest gate fabricated by SteelFab. The wider crest gate has additional capacity to pass trash, can better pass oversized debris than the existing narrower gates, and has additional hydraulic capacity.
|The new sediment solution included the installation of a single trash gate with an increased ability to flush away debris, resulting in increased spillway capacity.|
During removal of the existing gates, demolition of the concrete pier, installation of the crest gate, and installation of new stoplogs was required to span the 30-foot center-to-center width between the upstream faces of the piers on either side of the trash bay to dewater the area and provide safe access for construction personnel. In addition, new stoplogs were also required once the crest gate was installed that would span the 27-foot span inside of the piers within the stoplog slot. The stoplogs were designed to be used for both the temporary and permanent conditions and were built to fit within the stop log slots between pier 1 and 3. Temporary stoplog extensions were bolted to the permanent stoplog to seal against the upstream concrete face of the piers.
However, the upstream faces of the concrete piers were not aligned, and the concrete bowed upstream by many inches below the crest. This rendered conventional gate seals or compressible rubber problematic due to their inability to contour to the very irregular surface. The project used grout socks, which were installed by divers after the stoplogs were positioned to provide additional sealing surface. This method resulted in stoplogs that sealed well. Steel brackets were installed on the upstream face of the piers on which the stoplogs sit. At the completion of construction, the extensions were removed and the stoplogs stored for permanent use at Lake Lynn Dam.
A structural steel storage rack on the deck was designed to support the stoplogs when not in use. A steel structural frame is located on the deck to support two hoists to maneuver the stoplogs from the storage rack into place in the stoplog slots, as well as the storage rack. The storage rack and frame will also support grating to replace the concrete deck that will be removed. The access to the upstream side of the dam will also be revised based on the new structural frame and deck.
The frame consists of steel beams beneath columns spanning between the concrete of piers 1 and 3 and steel columns to support the hoists. Because of the known AAR expansion problem at Lake Lynn Dam, the frame was designed with oversized slotted holes at the base plate connection of all beams on the powerhouse side. Thus the frame will be able to accommodate horizontal movement of the concrete piers due to AAR growth. During the demolition effort, the top of pier 1 separated from the powerhouse wall by about ¾ inch as soon as the deck moved due to the relaxation of built-up AAR forces.
A 1,000-foot-long Tuffboom trash boom was installed, spanning from the spillway back to the west shoreline. This trash boom was anchored into a massive concrete block supported on a drilled concrete caisson foundation. The trash boom will help deflect debris toward the new trash gate and away from the tainter gates.
Based on operating experience learned from the CEATI HPLIG (Hydraulic Plant Life Interest Group), a deflector shield was also included as part of this project, because the trash gate is located directly adjacent to the side wall of the powerhouse, which consists of a brick wall with some windows on the dam spillway side of the new trash gate. The deflector shield protects the powerhouse from the spray of water coming off the trash gate and from the anticipated large trees with limbs that will be passing as part of the debris during high flow events.
Similarly, the new upstream walkway, which provides access to the upstream side of the new trash gate, was designed as a debris barrier to protect the deck, stoplogs and hoist frame from large trees and other debris that had previously caused damaged to the former trash gatehouse.
After a careful study of many options to improve debris management at Lake Lynn Dam, an alternative was chosen based on personnel safety, total life cycle construction and operating cost, effectiveness to pass trash and debris, and licensing and environmental concerns. The chosen alternative reduces the risk of the potential failure mode related to the trash gate and tainter gates becoming blocked by debris and therefore resulting in a reduction in the spillway capacity.
The project was completed during the fourth quarter of 2012. Initial operating results have found that all of the objectives of this projects have been achieved. The successful execution of the project required extensive coordination and cooperation among Allegheny Energy Supply (now FirstEnergy), FERC and PB Power.
The new gate should reduce the upstream flood potential due to the additional capacity to pass trash and larger debris. The addition of the trash boom also should prevent the tainter gates from becoming blocked. The new trash boom deflects debris towards the trash gate so that it will be passed through the dam.
Stefan Schadinger is lead engineer and Bryce Mochrie is senior project engineer at PB Power, which played a role in engineering evaluations. Andrew Datsko is manager and Jacob Vozel is an engineer at FirstEnergy, which owns the 51.2-MW Lake Lynn project.