Replacing an antiquated and failing flood control system at Wisconsin's Hatfield Dam proved to be challenging. The selection and installation of hydraulic crest gates was cost-effective and withstood the test of a major flood that threatened downstream residents.
By Scott Klabunde and Kim Hanson
Many early 20th century hydroelectric projects were built with wooden boards (known as flashboards) placed atop the associated dams as a means to pass floods. Flashboards were also installed to raise head at the dam and, correspondingly, increase power output.
Some such systems were designed to automatically release the flashboards when the headwater reached a certain elevation. Others were manually tripped by an operator, who physically removed flashboard systems as the water flow increased. All of these systems required an ongoing program to seal the inevitable leaks between the boards.
Flashboards were economical and easy to construct. However, engineering a system for safe and reliable operation was another matter.
Today, project owners are faced with the task of operating and maintaining systems that are a legacy of the engineering priorities of an earlier era. Hydroelectric projects now must adhere to strictly enforced state and federal regulations for water level, flow water level and flow regulations and respect the environmental and recreational interests upstream and downstream. Project owners must meet ever-higher dam safety and reliability standards and comply with Occupational Safety and Health Administration regulations.
As a result, many wooden flashboard systems have been upgraded. There are many alternatives available today for replacing outdated systems. North American Hydro, owner of the 7.2-MW Hatfield project on the Black River in Wisconsin, was faced with upgrading the outdated wooden flashboards that were part of the original project to a safer and more reliable system.
The Hatfield project is in Hatfield, Wis., on the Black River, a tributary of the Mississippi River. The project's impoundment is named Lake Arbutus and comprises about 1,000 surface acres and 11,000 acre-feet of storage. Lake Arbutus is home to about 200 permanent residents, and its population swells into the thousands during the summer months. The Federal Energy Regulatory Commission issued a new 40-year federal license for the Hatfield project in 1997, with guidelines requiring tighter reservoir surface elevations with stricter flow management goals, including minimum flows and recreational whitewater boating flow releases. Numerous additional regulations governing dam safety and structural performance are also part of the new license.
The project was originally commissioned in 1908. It was configured with a principal spillway consisting of a 52-foot-high cyclopean concrete gravity rollway dam about 500 feet in length, topped with a wooden plank system and boardwalk. A major flood in 1911 resulted in the addition of a new gated spillway section at the project. The spillway consisted of 10 tainter gates, each 20 feet wide by 10 feet high. Both spillways divert the river into a 3-mile-long open earthen bank power canal to the generating station.
The newer gated section is normally used first to pass high flows. However, removing the wooden flashboards atop the gravity spillway was still necessary because the gated section's capacity was less than the 10-year reoccurring interval flood. Removal of the flashboards atop the gravity spillway required pulling about 5 vertical feet of horizontal boards of 8-foot lengths. As might be imagined, this is a slow, physically demanding and labor-intensive task. Reinstalling the flashboards often required a substantial lowering of the level of Lake Arbutus. At Hatfield, both the removal and installation process would be considered unsafe by today's standards.
The gravity overflow spillway retained its original wooden flashboard system through several ownership changes, as well as a project abandonment period.
The glacial moraine watershed of Lake Arbutus is conducive to periodic high flow conditions and flash flooding. Over the years, there were many instances when it was practically impossible to safely remove the wooden flashboards. Even if the flashboards could be removed, the vertical steel supports spaced for the 8-foot-long planking quickly became plugged with debris.
In 1993, the adjacent town of Hatfield was flooded because the dam operators were unable to safely remove the flashboards in time. High flows, with significant amounts of debris, backed up along the wood and steel-framed system and boardwalk.
The resulting 7 feet of reservoir surcharge overflowed the rim, flooded Hatfield and ultimately breached the project's power canal.
In 1999, after several million dollars were spent repairing the power canal, the Hatfield project was again generating electricity. However, due to concerns over the safety and reliability of the overflow spillway system, FERC required all flashboards to be removed and the reservoir to remain below the crest of the spillway. The lowered reservoir elevation curtailed both the recreational use of the reservoir and power generation. FERC also required the owner to improve the safety and reliability of the overflow spillway system.
North American Hydro purchased the project in 2005, accepting the responsibility of upgrading the spillway under the commission's direction.
|This upstream view shows the completed curb design at Hatfield Dam.|
The need to provide a safe, reliable flood control mechanism atop the gravity dam was obvious. But at a 7-MW plant, addressing this challenge economically was not so obvious. The system would have to be economically viable and would need to meet the following design criteria:
- Fail-safe operation. The system could not contain any failure modes that would restrict or prevent the gates from opening in the event of equipment failure or access issues.
- Minimum 40-foot-wide unrestricted discharge openings. This was required to pass large woody debris, even whole trees.
- Vertical clearance. Large trees tipping over the spillway crest require a large vertical clearance to overhead walkways or utility feeds.
- Safely operated. Operation from an overhead operating deck or walkway was considered unsafe for operating personnel due to the heavy and dangerous debris loading issues.
- Ability to close under full head. Lowering the reservoir to close the gates was unacceptable.
- Used only for flood flows. The project's adjacent tainter gate spillway would continue to be used for flow and level regulation purposes.
- Increase spillway capacity. Even if full utilization of the existing flashboard system could be achieved, the project's spillway capacity was marginal under current dam safety standards.
- Water tight. The system would need to seal tight without excess leakage.
Four options explored
To meet these requirements, several options were considered. They included:
- Stanchion gates: Initially, a system with vertical steel stanchions holding bays of horizontal wood was prepared. The system could be manually tripped by an operator, releasing adjacent sections simultaneously to achieve wide spill openings. However, this option was quickly abandoned. While it was economical, the manual tripping mechanism(s) contained several failure modes. It would be difficult to design a safe overhead operating deck high enough to be used effectively. The system could not be re-installed without lowering the reservoir and would leak.
- Rubber dam: This option was considered desirable for several reasons. Clear-spanning the entire 450 feet would have provided the widest possible uninterrupted discharge passage for debris and maximum flow capacity. Additional spill capacity could be achieved by cutting the gravity dam crest down to accommodate the tallest and deepest rubber bladder design available. But the inability to control or direct the flow was a problem. Two siphon-set generating units were installed at the base of the right side of the gravity dam in 2000 to convey the license-required minimum flow to the main river channel. Subdividing and sectioning the rubber dam to accommodate these units would have added substantial cost and was counterproductive to maximizing spill capacity. Concerns were raised regarding the longevity, warranty and source of a suitable rubber bladder.
- Obermeyer gates: The Obermeyer design consists of a steel gate system that is hinged on the bottom that overhangs an inflatable air bladder. The system could be sectioned for discharge control and appeared to be a more reliable and/or robust choice than the rubber bladder alone. It could also accommodate a deeper and taller gate to maximize spill capacity. The potential for ice build-up on the seals or individual panels was not a concern because the local community was endorsing a seasonal reservoir drawdown, meaning the gates would not be used in the cold weather months. But the higher cost of the Obermeyer design became a factor.
- Hydraulic crest gates: Hydraulic systems are well-known and understood. A hydraulic system could keep costs down by allowing the use of local contractors for construction. Each gate could be operated independently and remotely, without an overhead operating deck. The intermediate piers needed to support these gates could be kept relatively small in width and not unduly reduce the usable spillway width. The system would have good sealing mechanisms. The hydraulic system could be designed to "fail-safe" with an inherently low risk of failure to open during a major flood. A simple valve located in the remote hydraulic power unit control house could be manually operated to redirect the flow of hydraulic fluid and allow the weight of the water to lower the gate. The steel crest gate height could be designed to accommodate any depth cutoff of the gravity dam crest for additional spill capacity.
During the design phase, North American Hydro determined that saw-cutting and reconstructing the upstream approach edge of the gravity crest would allow the engineer to design the most efficient design discharge configuration.
Also contributing to the decision-making process was the fact that North American Hydro has operated a hydraulic crest gate system at another high-hazard dam since 1988. During the intervening years there have been few, if any, operational problems. In the end, the hydraulic system was chosen for Hatfield.
|A rubber compression sill seat at Hatfield Dam was built to ensure a tight seal when the hydraulic crest gates need to be closed.|
The fabrication and installation of the gates were contracted to Gerdes Manufacturing, a local contractor. The 450-foot crest of Hatfield Dam was configured with nine gates, each 45 feet wide and 5.5 feet high and having a curved skin plate radius of 17.5 feet. The gates are separated by concrete piers, each of which is 3.5 feet thick. Between the concrete piers, four cast iron supports or saddles were anchored to the gravity dam. The crest gate torque tubes are 12 inches in diameter and have raised machined stainless surfaces riding in the cast saddles.
The gate side seal material is reinforced rubber that is permanently attached to the concrete piers and field-matched to the curved gate skin. The hydraulic rams pull the gate into the seal and maintain sufficient pressure to seal each side of the gate. The bottom seal consists of a reinforced rubber compression seal between the upstream curb and the bottom edge of the gate as it closes.
The system is designed to be used for flood control with only two gate positions: fully opened or fully closed. Each gate is driven by two hydraulic rams, one on each end. The curved gate skin and supports are designed to withstand the failure of a single ram without damaging the gate. A counterbalance valve is provided, and a proximity switch is installed at each end of the ram's stroke for position feedback to the hydraulic power unit (HPU) and programmable logic controller (PLC). The PLC continuously monitors the position of each ram and makes adjustments to assure equal movement and maintain proper alignment of each gate. This corrects any drifting and adjusts for gate misalignment that could twist and damage the gate. The system operates on 1,000 psi.
The HPU, PLC and controls are located in a control house adjacent to the gravity dam. The HPU cabinet panel has individual controls for operation of each gate. Backup power is provided by a diesel generator and automatic transfer switch. Additionally, a fail-safe feature allows the operator to manually defeat each individual gate's counterbalance valves. In the event of a malfunction, the hand pump, located at the HPU, can be used to defeat the counterbalance valve pressure (about 200 psi). The weight of the water will allow the gate to close. The HPU was designed with the appropriate heaters and water/condensation separators and filters.
A mineral-based hydraulic oil was initially chosen for the system's fluid. Considerable design effort was spent on the layout of the fluid lines, the choice of material for the lines and connection method to be used.
The hydraulic lines running from the HPU to each of the gates were routed along the crest of the dam, downstream and behind the gate torque tube support hinges (or "saddles"). Tucked away under the steel gate, the lines are protected from water and debris. Intermediate gate piers were formed atop the gravity dam to allow the lines to be routed from pier to pier. The hydraulic lines are further protected by being run through the interior of the piers to their respective hydraulic rams. The ¾-inch-diameter hydraulic lines are made of stainless steel to preclude any failure modes associated with corrosion. Additionally, the lines were fabricated in continuous runs of welded construction. The number of connections was kept to a minimum to reduce the potential for leaks. The hydraulic line runs located atop the crest, immediately downstream of the gates, were covered with a removable aluminum plate to provide additional protection. It was later decided to fill the system with a more environmentally-friendly biodegradable blend of high oleic vegetable and synthetic polyester-based oil commonly used in food-processing plants. The system was started up and tested using the biodegradable oil.
|The Hatfield Dam crest gate project was completed in 2008, to improve safety of the dam during high flow events.|
In an effort to keep the costs down, North American Hydro provided its own management of the contractors and vendors involved with construction of the system. In the autumn of 2007, and in anticipation of construction scheduled to start in the following spring, the level of Lake Arbutus was lowered even further. Additional lowering of the reservoir was necessary to facilitate removal of the old steel flashboard support structure and the saw cutting of the crest and removal of the sawn concrete. The saw cut would allow maximum room for the gate hinge mechanisms without unduly sacrificing spillway capacity.
The saw cut would also provide a clean, level surface on the dam crest in preparation for installation of the concrete piers and the hinges (or "saddles") on which the gate torque tubes would rotate. A diamond wire rope saw-cutting process was used to remove almost 3 feet from the height of the crest. A series of 1.25-inch holes were drilled horizontally through the dam at the 3 foot depth, every 10 feet along the crest. The wire saw-cutting apparatus was then anchored to the upstream side of the dam, and a horizontal cut was made every 10 feet, followed by a vertical cut. A barge-mounted crane was used to remove the cut concrete pieces from the dam.
The saw cut revealed a valuable structural feature that had previously been unknown and did not appear in any construction reports or on any construction drawings. The gravity spillway is designed with very strong and large vertical keyway control joints located at 50-foot intervals.
Over the winter months, local contractors began fabrication of the gate torque tube supports (or "saddles") and the nine crest gates, as well as the assembly of the HPU and control system. A local contractor was also chosen for the civil work related to forming and placing the concrete piers. Construction began soon after spring runoff.
The pier design utilized the contractor's standard bridge forms, eliminating additional expense associated with (what would have otherwise been) custom-designed forms. Spillway discharge efficiency was maximized by designing the upstream approach curb with an 18-inch radius, utilizing the optimum design coefficient.
This was a critical aspect in achieving an additional 20% spillway capacity. After the upstream steel curb was placed, the piers and saddles were anchored in the gravity dam as per the design specifications. The brackets for attaching the hydraulic rams to the piers were set and aligned, and the hydraulic rams were mounted.
The steel crest gates were delivered on a barge-mounted crane and placed on their respective saddles, with each side connecting to the hydraulic rams. Simultaneously, the stainless hydraulic lines were being installed along with the HPU. The system was tested and placed in service in 2008.
|Hatfield Dam and its new hydraulic crest gate saw a major real-world test in 2010, when a historic flood struck the area. Although downstream residents prepared for the worst, the dam and its crest gates performed as designed and kept the town of Hatfield flood-free.|
The test: The historic flood of 2010
The new hydraulic system did not have to wait long for its first major operational test. In 2010, Hatfield experienced a historic flood - the second largest on record. The 500 Hatfield residents who were evacuated from flooded residences in 1993 were preparing for the worst.
However, this time the results were much different. The system operated exactly as designed, using all nine of the crest flood gates. Community residents breathed a sigh of relief when no flooding occurred there. All involved in the spillway project upgrade were rewarded with the satisfaction of knowing they had produced a reliable product that delivered exactly what was promised.
On March 28, 2012, the majority of North American Hydro was purchased by Eagle Creek Renewable Energy, which is based in Morristown, N.J. At this time, additional wood flashboard system upgrades are being planned within the North American Hydro/Eagle Creek Renewable Energy portfolio of projects.
Scott Klabunde is vice president of operations for North American Hydro. Kim Hanson, PE, was design engineer with Mead and Hunt for the Hatfield Dam hydraulic crest gate project and is now with MWH Global.
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