The 2.6-MW Rainbow Falls powerhouse and dam in New York suffered significant damage during Hurricane Irene. This article details the work performed to bring the hydroelectric project back to full generating capacity.
By Caroline R. Wheadon, PMP, EIT
In late August 2011, Hurricane Irene battered eastern New York State with torrential rains and high winds, which flooded the Rainbow Falls hydroelectric powerhouse. The facility is located on the Ausable River, 5.5 miles upstream of Lake Champlain and in the northeast corner of Adirondack Park. It is operated by a local utility company. The powerhouse, dam and other surrounding infrastructure was heavily damaged, and this set off a multi-year recovery and reconstruction effort that began shortly after the hurricane. Multi-disciplinary teams worked across technical boundaries and company lines to complete rehabilitation efforts in 2020, resulting in new, modernized equipment that will perform for decades to come. Today, the powerplant has a generating capacity of 2.6 MW, which produces enough clean, local, renewable energy to power about 1,775 average homes.
LaBella Associates became involved in 2014, when the owner assembled a team to oversee and manage the plant restoration project. LaBella provided comprehensive project management services, developing detailed schedules and cost forecasts and providing technical design oversight and field construction and commissioning coordination. LaBella also designed several plant support and auxiliary systems. In parallel, LaBella was retained to design a dam spillway resurfacing solution in 2014 and 2015. From 2016 to 2018, LaBella provided project management, field construction oversight and engineering administration.
River hydrology
The Ausable River watershed covers 512 square miles, includes 94 miles of river channel, and is fed by more than 70 streams, including its two major tributaries, the Chubb River and Black Brook. Seven towns, eight hamlets and one incorporated village are located in the watershed, which covers portions of two counties. Except for a small area at the river’s mouth on Lake Champlain, the entire watershed is located within the boundaries of the 6 million acre Adirondack Park.
The Ausable River includes three major sections: the East Branch, West Branch, and Main Stem. The headwaters of the East and West branches are nestled some 4,000 feet above sea level in the high peaks of the Adirondack Mountains. The East and West branches join forces at Au Sable Forks. From there, the river’s main stem meanders through gently sloping lowlands before tumbling through the spectacular bedrock gorge of Ausable Chasm and entering Lake Champlain 100 feet above sea level. This rapid descent from its headwaters to the lake makes the Ausable the second steepest river in New York State.
Intervals of peak flow exceeded the 500-year flood at stream gauges throughout the Ausable River Basin. The river is very “flashy,” meaning that the magnitude of flow spikes happens quickly because the watershed area is steep. The spillway, located at the top of Rainbow Falls, handled flow estimated at 45,000 cfs during the hurricane event.
Rainbow Falls powerhouse restoration
The Rainbow Falls plant was constructed in the early 1900s and consists of a dam, intake works, two 6-foot-diameter steel penstocks, two 1.3-MW generating units, auxiliary systems and a powerhouse structure. In the aftermath of Hurricane Irene, recovery response started with stabilizing and securing the building, installing temporary utilities and performing damage assessments.
Assessing the damage
The two generating units were completely submerged during the storm’s highest flows, about 10 feet above the generating floor. The switchgear and controls were also inundated, to the point where they were no longer salvageable. Turbine-generator auxiliary systems were rendered unusable.
The owner worked with a team of engineers from internal departments and as external contractors to establish a scope and basis of design to restore the facility to full capacity. Plant communications, mechanical components and electrical technologies have evolved and modernized over the past several decades, so it was important to identify to what extent these systems would be upgraded. Ultimately, it was more reliable and a better financial investment to fully upgrade all powerhouse systems, with the exception of the turbine-generator units.
Rehabilitation efforts
The generators were disassembled, dried out, reassembled and restored to original operating conditions. However, the systems that support reliable generation were all newly designed with up-to-date technology. The powerhouse devices were previously connected to a remote terminal unit (RTU) that communicated to the supervisory control and data acquisition (SCADA) system. The control center was upgraded to a real-time automation controller and a human machine interface (HMI) screen. Fiber optic cable was also installed from the rack house to the powerhouse to integrate a secondary RTU and HMI at the rack house. This configuration allows operators to check on plant alarms and component status from multiple locations.
The control system, switchgear, metering cubicles and house and equipment transformers were elevated off the turbine-generator floor onto a newly constructed structural steel mezzanine level. Steel members actually penetrated the existing powerhouse floor to bear on the bedrock below. Should another storm event flood the powerhouse (hopefully not in our lifetimes!), in theory, the switchgear and control cabinets will stay dry and preserved. However, the powerhouse floor real estate did not go unused. The engineering team strategically placed systems that would be cheaper and easier to replace turnkey in the event of another flood, such as a new motor control center, with local mechanical breakers for each powered device. A new battery bank was installed in a new room with code-compliant ventilation, fire protection and lighting.
Areas of the powerhouse floor were also modified with additional steel reinforcing to adequately support generator components as they were staged and reassembled. In other areas, several diamond plate hatches were cut in to allow equipment to be lowered onto the basement level. An automated ventilation system was implemented to turn on when the generators were in use and temperatures in the powerhouse rise. The service water system screens river water through a custom-designed filter and pump system to act as a coolant for the turbine bearings. Other upgrades to the powerhouse components include a new compressed air system for turbine brakes, AC and DC electrical distribution systems, a medium-voltage system for generator output and protection, a lubrication oil system, and several other upgrades improving operations, safety and generation reliability.
As the inside of the powerhouse was undergoing major upgrades, the outside remained largely undamaged. These facility buildings are listed on the New York State Register of Historic Sites due to their stone masonry construction, vaulted copper roofs and timber trim details. The windows were replaced in kind to preserve the architectural character of the original early 1900s construction.
Challenges and lessons learned
It is rare in the northeast U.S. for an entire facility to undergo a comprehensive rebuild. Generally, units are overhauled one at a time, structural concrete is improved, or a single system is upgraded with the latest technology. Replacing every system with something new presented our project team with several challenges along the way, the biggest of which was coordinating dozens of different designs, equipment installations and vendors. The overall project required significant management and coordination efforts, team motivation tactics and positive communication. While the team was anxious to get the plant back up and running, in hindsight, project requirements needed to be established at the onset of the system upgrades. The integration of all of these separate systems was challenging, but commissioning was ultimately successful because of a determined project management and engineering team.
Rainbow Falls spillway resurfacing
Structural deficiencies observed after Hurricane Irene prompted the plant owner and regulating agencies to evaluate the condition and stability of the spillway. That evaluation confirmed that resurfacing of the dam was required to establish a uniform ogee shape for the spillway, and resurfacing of the abutments was required for efficient and stable operation of the plant.
The spillway is an ogee shape that is 10 ft to 15 ft tall on the east side and 20 ft to 25 ft tall on the west side. It is listed as a Class B Intermediate Hazard Dam on the New York State Department of Environmental Conservation Inventory of Dams. The crest of the dam is at elevation 307 ft MSL, about 71 ft above the operating floor of the powerhouse at elevation 236 ft MSL.
The dam had significant concrete loss on the face of the spillway and significant delamination on both the east and west abutments. Although the dam had not exhibited any major leakage or undermining, the owner committed to performing upgrades to prevent any further deterioration of the concrete. All flows for the restoration needed to be managed with the only existing spillway and other intake gates because the plant was offline due to the hurricane.
Resurfacing of the spillway presented unique dam stability and flow management challenges, which required custom design solutions by both the engineer and contractor to maintain dam safety. To overcome these challenges, LaBella designed a two-phase construction sequence, in which flows were diverted to the “non-work” side of the spillway. A cofferdam system was placed on the crest of the dam to protect the work area but was also designed to fail under flood conditions to relieve loads on the dam.
Existing conditions assessment
To assess the existing condition of the dam, the owner had to draw the pond down through a gate located downstream in the intake canal. The inspection indicated that the spillway was divided into 13 monoliths, with construction joints between each section. The largest spalls were located on monoliths 11 to 13, in some spots exceeding 24 in deep. Concrete cores of the existing dam showed high-strength concrete, some breaking at over 7,000 psi.
The loss of concrete on the ogee face, in addition to sediment buildup behind the dam to about 5 ft below crest elevation, prompted a stability analysis to confirm the loss of concrete section didn’t create an unsafe overturning or sliding stability concern. The stability analysis results showed no immediate stability concern. However, without restoration, compromised flow character could have impacts on spillway performance in the future.
Design development and details
The primary scope of the project was to restore the original ogee shape of the dam, key the toe into the founding bedrock to prevent undermining, and install an underdrainage system to alleviate hydrostatic pressure and seepage through the original dam structure. Flashboards and pins were also reanalyzed and replaced due to the new crest concrete and socket construction.
Each of the 13 monoliths was analyzed for its resistance to overturning and sliding under hydrostatic, ice, silt and seismic load cases. The analysis was performed for both normal and flood flow conditions. The results of the stability analysis provided the maximum allowable depth of concrete removal without the requirement for temporary stabilization measures.
Demolition plans were developed by sectioning 16 total profiles across each monolith, showing rock removal for the toe keyway and concrete removal on the spillway. The demolition limits were established with a combination of existing conditions visual inspection, survey and field engineering.
The original documents specified localized patching to fill deep voids on the existing spillway surface before placing the overlay, in order to create an even surface for placement of an underdrain system. MiraDRAIN strips in a 3-ft horizontal by 10-ft to 15-ft vertical grid pattern were used between the spillway and spillway surface overlay concrete to channel any seepage and relieve hydrostatic pressure.
Due to the continuous river flow over the crest of the spillway, the height of the spillway had eroded between 2 in to 3 in below the Federal Energy Regulatory Commission-required crest elevation. The crest was demolished an additional 7 in to allow a minimum replacement thickness of 12 in. New red pine flashboards were installed to a height of 36 in above dam crest and spaced 2.5 ft apart along the length of the entire spillway.
The 12-in-thick concrete overlay consisted of 4000-psi concrete with varying bar sizes and embedment developments. Two large square portal openings were filled in with concrete, as they no longer provided a purpose. The upstream bulkheads were timber planks, where decades of use increased the risk of failure and uncontrolled flow and head loss.
Construction methods and water control
The face of the spillway was demolished using a milling head mounted to an excavator to grind and smooth out the undulating concrete surface. This method enabled the contractor to follow strict concrete removal guidelines, so the stability of the dam was not compromised in the event of a 100-year design flood. The new reinforced concrete overlay was poured with self-consolidating concrete on alternating monoliths using custom concrete forms provided by EFCO to restore the original 1925 spillway ogee shape in varying radiuses. The toe was keyed into the bedrock riverbed to prevent undermining, and the underdrain system was installed to relieve seepage through the existing dam.
With the plant offline due to Hurricane Irene, the turbine units could not be used to pass water from the upstream pond to the river downstream. There is an intake bypass gate located just before the water enters the rack house, which could be used to control the pond upstream of the dam. Flow curves were developed to show that the gate could pass about 650 cfs and normal pool elevation (top of crest at 307 ft MSL). A siphon capable of passing 30 cfs was also used to redirect flow over the falls for the FERC license requirement for aesthetic flow.
Due to historic flows and the cure times estimated for each toe and monolith pour, the design was based on two construction seasons. Custom steel EFCO forms were designed to act as a protective cofferdam upstream of the work area. The forms also had to function as a flashboard system for flood control and fail under specified flood conditions (100-year flood elevation). The forms were installed when flows were below 650 cfs, so the pond was drawn down within compliance below crest. When flows exceeded 650 cfs handled by the intake canal bypass gate, all flows were diverted to the non-work side of the spillway so construction could continue with the cofferdam/flashboard system EFCO barrier.
Toe demolition and spillway milling was conducted to an accuracy of 2 in to facilitate placement of the underdrain so it laid flat against the concrete base. The bedrock was excavated on average at the toe an additional 1 ft to 2 ft deep by 2 ft horizontally downstream. After the rebar and formwork was installed, the toe was poured first across the entire first half of the spillway. Implementing this sequence allowed the contractor to use the toe as a support for the custom EFCO ogee forms when the spillway monoliths were poured.
Permitting and environmental considerations
FERC, U.S. Army Corps of Engineers, and New York State Department of Environmental Conservation permits were required, as well as approval from the Adirondack Park Agency. The facility is operated under a 40-year license that requires all maintenance construction activities to be approved by the FERC New York Regional Office Dam Safety, with supporting consultations from other various resource agencies. An Army Corps Nationwide Permit was also required. A major consideration of the Corps was the volume of fill below the ordinary high water mark on the access roads, spillway and abutments. Endangered species were also evaluated and the Northern Long Eared Bat was identified in the vicinity. As such, a requirement of the construction dictated that no trees could be removed from April 1 to Nov. 1. To meet this rule, the contractor constructed an access road outside of the bat hibernacula window.
Other permitting conditions enforced the NYSDEC 401 Water Quality Certification requirements. These include twice-daily turbidity monitoring, 100 cfs minimum downstream flow to the Ausable Chasm Company for tourism and recreation purposes, and 30 cfs aesthetic flow over the falls during peak summer season.
Conclusions
The Rainbow Falls spillway resurfacing and powerhouse restoration projects were complex, multidisciplinary projects that required ingenuity and team grit. Customized methods were used to design, permit and construct the projects. The Ausable River is a force of nature that provides reliable renewable hydropower energy, after implementing contemporary technology and engineering to modernize the Rainbow Falls Hydropower Facility.
Caroline R. Wheadon, PMP, EIT is the hydropower discipline leader with LaBella Associates.
