Providing safe passage8 December 2006
A facility has been constructed at the St Lawrence-FDR power project in New York, US, to provide safe passage upstream for the American eel
The St. Lawrence River is the one of the largest rivers in North America, having an average annual flow of approximately 7050m3/sec and draining nearly 777,000km2 of the US and Canada, including the five Great Lakes. Development of the potential for hydroelectric power and deep-water navigation between the Atlantic Ocean and the Great Lakes had been goals of the Canadian and US governments since the early days of the 20th century. These goals were realised with the construction of the 1800MW international St. Lawrence Power Project (Figures 1 and 2) and the St. Lawrence Seaway in the 1950’s.
While the production of reliable hydroelectric energy and the establishment of a deep-water seaway provided many benefits, they also brought consequences to the environment, including blocking migration for young American eels moving from the ocean to the upper river and Lake Ontario. This impediment to migration was mitigated to some extent when Ontario Hydro, the owner of the Canadian half of the power project, installed an eel ladder on its portion of the dam in the 1970’s. The need for additional passage for young eels at the dam was raised as an issue in the late 1990’s when the New York Power Authority (NYPA), owner of the US portion of the project, began the process of obtaining a new 50-year licence from the Federal Energy Regulatory Commission (FERC).
The initial focus with respect to eel-passage facilities on NYPA’s part of the dam involved assessing: a) whether an additional ladder was needed; b) where such a ladder would be installed, if it was needed; and c) the appropriate configuration for the passage facilities. The answers to these questions were provided by a series of studies between 1996 and 2001 and led to an eel-passage facility with several unique elements that ensured the effective passage of eels over and safely upstream of the dam.
The need for additional passage facilities was affirmed through studies that examined the distribution of eels across the 1000m of the downstream face of the dam. These studies demonstrated that there were a substantial number of eels on the US side of the dam and that many of these eels did not move across the face of the dam to use the eel ladder in Canada. Thus, it was concluded by NYPA, the US Fish and Wildlife Service (USFWS), and the New York State Department of Environmental Conservation (DEC) that a new fishway would be a benefit and would provide an opportunity to increase eel passage. Based on the relative abundance of eels at the various collection locations, it was concluded that this passage facility would be located approximately 50m from the shoreline, in an existing, decommissioned sluiceway.
Beyond the question of location, NYPA began to examine other eel-passage facilities to determine what worked well and where improvements could be made. Information on these topics was close at hand in the eel ladder installed on the Canadian side of the dam and in an eel ladder recently installed at Hydro Québec’s Beauharnois power project 100km downstream of St Lawrence-FDR. The Hydro Québec eel ladder provided valuable information that updated the design data on which the earlier Ontario Hydro ladder was built. Specifically, the Hydro Québec ladder demonstrated that the angle of the ladder (40 degrees) could be steeper than that of the Ontario Hydro ladder (12 degrees), thereby providing a shorter distance for eels to climb. Hydro Québec’s ladder also demonstrated that the climbing substrate, designed by Milieu Inc. (La Prairie, Québec, Canada), consisting of a series of staggered plastic tubes extending upward from bottom of the ladder, somewhat like large scale children’s ‘legos’, provided an efficient surface for eels to push against as they climbed the ladder.
Experience with Ontario Hydro’s ladder provided valuable information on the timing of the seasonal and daily periods of eel migration. But the most surprising information that the studies provided was that almost half the eels that exited the top of the ladder into the forebay above the dam fell back below the dam to the tailwater, presumably through the turbines. This unexpected result indicated that a ladder to carry eels over the dam was not enough. It would be necessary to find a way to release these eels at a location where they would be less likely to fall back through the turbines.
The answer to finding a safe release point was provided by a study in which tagged eels were released at various locations upstream of the US portion of the dam and were then trapped downstream of the dam to see what proportion fell back through the turbines. The study confirmed that almost half of the eels released into the forebay directly adjacent to the dam would fall back to the tailwater. The fall-back rate decreased to approximately 6% at a release location approximately 300m upstream of the dam, with only slightly lower fall-back rates at release points still farther upstream. Based on this information, the 300m location was selected for the release location for the eel-passage facility.
Several methods were considered to move the eels to this upstream location, including a trolley, a gondola, and a pipe. The pipe was considered to be the simplest and most cost-effective alternative. A pipe had never before been used to move eels a distance of 300m, so this approach provided a unique challenge. NYPA conducted a study that assessed the ability to move eels through a test section of 15cm inside diameter, 100m long pipe by flushing them with high flows. Despite the scientic literature suggesting that juvenile eels are not thought to be strong swimmers, the eels were readily able to swim against all but the highest velocities (1.8m/sec). So instead of being flushed, the eels were swimming into the flow or struggling to maintain position in the flow. Undeterred, the biologists realised that they could take advantage of the eel’s natural inclination to swim into the current by reversing the flow pattern. When the direction of flow was reversed, 100% of the test eels swam against a 0.25m/sec flow and successfully passed through to the pipe.
With the pertinent information in hand, NYPA, working with the design team of engineers from C&S Engineers (Syracuse, NY, US) and eel biologists from Milieu Inc. as well as staff members of the USFWS and DEC, designed the eel-passage facility. The facility consists of four distinct elements: a ladder that provides the pathway for the eels to ascend to the top of the dam, a collection hopper that provides a transition to a passage pipe, the passage pipe that provides a pathway for eels to swim upstream of the dam, and a receiving basin that provides a safe haven for eels after passage through the facility before they return to the river. A detailed description of the four components and associated equipment follow.
The ladder is 55m long and 0.37m wide (Figures 3 and 4). It is constructed of aluminum and has a hinged cover to provide a dark environment for the migrating eels. An aluminum trough supports the plastic climbing substrate. The climbing substrate consists of staggered plastic pipes (Figure 5) that are wetted by a layer of water approximately 6mm deep flowing at a rate of approximately 0.36 liters/sec. The water keeps the eels wet and encourages them to swim up the 35 degree incline. The eels are attracted to the ladder by an attraction flow of approximately 21 liters/sec released adjacent to the entrance to the ladder (Figure 6).
The collection hopper is an approximately 1000-liter fiberglass vessel that serves as a transition area from the ladder to the passage pipe. Eels slide down a ramp from the top of the ladder into the hopper, where they sense the flow from the passage pipe and quickly pass through the hopper into the passage pipe. The hopper also provides a point where eels can be collected for biological samples using a collection basket.
The 300m long passage pipe, with an inside diameter of 15cm, serves as a conduit for the eels’ journey from the hopper to the release location (Figure 7). The eels swim upstream in the pipe against a constant flow of approximately 0.30m/sec. The pipe is insulated to minimise warming during daytime. Since zebra mussels are a common fouling organism in the St. Lawrence River, the pipe is equipped with numerous cleanouts.
The receiving basin is a 1.2m diameter, 18m long ductile iron pipe that extends into the river (Figure 8). It is armored with large riprap to protect it from winter ice damage. The receiving basin has numerous 5cm diameter holes along its surface that provide numerous exit locations for eels while keeping larger predators out of the basin.
Another unique feature of the facility is the receiving tank which serves the dual purpose of supplying a steady flow to the passage pipe while at the same time allowing the eels to move from the end of the passage pipe to the protective receiving basin. These two purposes introduced the challenge of having two separate flows in the receiving tank; a 0.02m3/min flow released at the top of the short (0.8m long) ramp that eventually leads to the receiving basin (Figures 9 and10) and the 0.31m3/min flow introduced into the bottom of the tank for the passage-pipe flow. Since it was necessary for eels to find the much smaller flow from the climbing ramp, it was necessary to eliminate cues provided by the flow for the passage pipe. This was accomplished by delivering the greater flow through an inclined porous false plate in the bottom in the receiving tank. This porous plate physically separated the eels from this flow and dispersed it through a large enough area that it minimised any flow cues. This arrangement was so effective that on average eels spent less than six minutes moving from the passage pipe to the receiving basin.
The eel-passage facility was completed in June 2006 at a cost of US$2M. The facility began to operate on 1 July 2006 and continued operation through the end of October, the period of eel migration. The facility performed exceptionally well, with nearly 100% availability. During its first year of operation, a total of 8160 eels completed the journey through the facility. The eels averaged 379mm in length and generally ranged between 300mm and 500mm.
The facility is equipped with five passive integrated transponder (PIT) readers at strategic locations (Figure 8). Using the readers, biologists monitored approximately 300 PIT-tagged eels that entered the facility. Approximately 85% of the eels that entered the ladder portion successfully climbed the ladder, and it typically took them about 60 minutes to do so. All eels that climbed the ladder travelled through the passage pipe, and they did so rather quickly, in approximately 30 minutes. One eel moved from the entrance of the ladder into the receiving basin in 36 minutes! Preliminary analyses suggest that eel fallback through the project turbines was below 2%; less than the 6% estimated from the earlier release-location study.
Years of study and careful planning have resulted in a fishway that performed almost flawlessly during its first year of operation. Although some minor modifications for more efficient maintenance and operation and another year of monitoring are being planned for next year, this fishway is seen as an unqualified success by NYPA and the state and federal resources agencies. The facility will operate for the duration of NYPA’s licence to continue to provide safe upstream passage for an increasing number of eels.
For further information contact Kevin J. McGrath, Ph.D, Senior Environmental Scientist, Environment, Health & Safety, or Thomas Tatham, Licensing Manager, Public & Governmental Affairs, New York Power Authority, 123 Main St, White Plains, NY 10601, US. Email: [email protected] or [email protected]