Thin Air and Thick Ice Unusual Challenges at a High Elevation Hydro Plant

Transcription

1 Thin Air and Thick Ice Unusual Challenges at a High Elevation Hydro Plant By Ginger Gillin, John Pizzimenti, and Doug Foss, GEI Consultants, Inc., USA, and Jon Jourdonnais, PPL Montana, USA. Abstract The Mystic Lake Project is a high elevation hydroelectric project adjacent to a pristine Rocky Mountain wilderness. On December 17, 2007 the Federal Energy Regulatory Commission issued PPL Montana (PPLM) a new License to continue operating Mystic under FERC s new Integrated Licensing Process. This high head project diverts water from Mystic Lake via pipeline, surge tower and penstocks across steep and geologically active terrain. PPLM needed to demonstrate that it had historically protected the fishery in the two-mile bypass reach and tailwaters and would continue to do so in the future. Additionally, PPLM needed to demonstrate they could protect the aquatic resources in the event of failure of the flowlines. This paper describes how the utility (1) effectively studied the aquatic resources, (2) assessed past effectiveness of their operations and (3) improved the reliability of the project to protect those resources in the future. They accomplished this by building trust Figure 1. Mystic Lake Dam and Reservoir. and consensus with a large stakeholder group who amazingly achieved its collective goals without sacrificing any of the energy or capacity benefits nor the environmental attributes the project has provided since PPLM and stakeholders found ways to maintain the Mystic Project s renewable and peaking power resources and simultaneously meet other important public goals of protecting the natural resources and providing recreational opportunities. Overview Constructed in 1927 in an extreme environment not far from Yellowstone National Park, Mystic Lake is a remarkable example of early 20th century hydropower. Mystic Lake Dam at elevation 7673 feet is in the headwaters of West Rosebud Creek. It took tenacity to overcome the steep, dangerous topography and snowy winters at this high elevation Rocky Mountain site. The 10 Mw powerhouse is 2.3 miles downstream of the dam at elevation 6550 feet. This project develops 1100 feet of head and operates in a peaking mode during the low flow seasons. The elevated storage lake can be drawn down up to 60 feet for winter capacity and energy use. 1

2 On December 17, 2007 the Commission issued the Mystic Project a new license, the first under FERC s new Integrated Licensing Process, or ILP. The bypass reach of West Rosebud Creek between the dam and powerhouse contains a valuable rainbow and brown trout recreational fishery. Previous studies in the bypass reach established a minimum instream flow of 10 cfs between June 1 and September 1, and a 3 cfs flow the rest of the year. They also established a minimum flow of 20 cfs downstream of the Figure Mile Bypass Channel. Elevational profile of the West Rosebud Creek bypass reach. The yellow square at approximately 7,100 ft represents where Maxie Creek enters West Rosebud Creek. project and re-regulated power discharges to reduce peaking fluctuations in the lower river. The project is also a target destination of white water boaters for the June-July runoff. Using consensus based study approaches in the ILP and negotiated settlements based on data, the participants and FERC agreed, to maintain the same historic instream flows to protect the healthy fishery. The aquatic study team however also recognized a failure of the flow line could impact the fishery and aquatic habitats of both the bypass and mainstem river downstream of the hydro. Rock and Ice avalanches are known in the area and could rupture the penstock and cutoff flows to the powerhouse as well as the bypass reach and lower river. Should disruption occur at normal pool elevation the spillway would operate to protect the instream flows. In winter however, when Mystic Lake is below spillway elevation, the project has no mechanism to quickly restore instream flows to the bypass or tailwaters. PPLM s engineers have come up with a solution to bypass the flow line with a new valve at the outlet of the rock tunnel not far from the dam. A 60-inch flow line emerges from the tunnel onto a narrow rock ledge. The site and conditions create numerous Figure 3. Doug Foss inspecting Mystic Lake Penstock via Cable Car Rail Line. 2

3 challenges to design and construct a sufficiently large valve system that would operate under the extreme conditions of a runaway failure under sub-freezing conditions without power. Challenges include (1) potential need for the largest of air cranes (Sikorsky helicopters) for up to a 15 ton valve system; (2) need for erection and maintenance cranes; (3) need for protection from avalanches and rock falls and (4) a remotely controlled but automated system capable of closing under runaway mode. Instream Flow Concerns West Rosebud Creek-Mystic Lake hydrology has an early summer snowmelt driven peak with a low flow winter condition that can actually reach zero discharge due to total freeze up in the upper elevations. The hydropower operations reduce the peak discharge by refilling the reservoir after a gradual 60-foot winter drawdown. Downstream of the project there are two recreational lakes, West Rosebud Lake which has a re-regulating dam and spillway and Emerald Lake downstream of that. Under the previous FERC License PPL had committed to a minimum flow of 20 cfs all year downstream of West Rosebud Lake. Thus historically there was 20 cfs continuously from winter power operations when natural flows might otherwise drop to a fraction of this amount due to freezing conditions. The project was also diverting flows around a steep two-mile section of West Rosebud Creek known as the bypass reach. The key aquatic resource questions were then: what effects, positive or negative were these historic operations having on the resources in the bypass? Historic instream flows in the bypass in the previous license were 10 cfs summer (June 1 - August 31) and 3 cfs all other times. This is significantly less flow than would be present without the project. But there were times when the natural winter inflows may have dropped to zero due to extremely low temperatures. Therefore, two key questions were (1) what affects if any, was is the historic instream flow having and (2) had the project maintained its required flows during the past 30 years? In addition, concerns were raised about a lack of ramping rates in the bypass reach, particularly on August 31, when flows would be adjusted from 10 cfs to 3 cfs potentially stranding fish and dewatering macroinvertebrates. Study Plans and Results Hydrologic Records The consulting team digitized all the minimum flow, USGS hydrology and powerhouse discharge records from the past 30 years. When averaged, the project was diverting close to an average of 5 cfs during the low flow season and more than 10 cfs during the summer season. They identified that with the exception of about a total of 52 days during the past quarter century, the project had met its minimum 3 cfs flow bypass requirements. It met nearly all of its 10 cfs summer minimum flow records and any transgressions were minor. Detailed study of the 3 cfs flow violations occurred during two extreme events in two separate winters when the entire system froze and there were no flows entering Mystic 3

4 Lake. There was no evidence that flows had ever dropped to zero in the bypass at any time. Flows of 20 cfs downstream of West Rosebud Lake were likewise historically being met consistently by the project since the construction of the re-regulating dam. The automated re-regulating gate system enabled excess flows to pass. The storage in West Rosebud Lake was sufficient to maintain a minimum discharge of 20 cfs during periods when the powerhouse was not generating. Fishery Populations and Aquatic Habitat The Aquatic Resource Team debated the need for hydraulic models of the bypass habitat and concluded that assessment of the condition of the actual fishery would be the first indicator of the health of the fishery. In addition, the Team agreed to sample and assess the physical habitat of riffles and pools under low flow conditions to determine the extent of usable trout habitat in the bypass reach. Data showed that rainbow trout occupied lower, middle and upper reaches of the bypass wherever slopes were sufficient to create pool habitat (Table 1). Minimum flows appeared adequate to keep these pools full given the hydraulic gradients and geology of the streambed. Total Length Inventoried (ft) Width (ft) Pool Depth (ft) Crest Depth (ft) Riffle Depth (ft) Pocket Pool Depth (ft) Available Spawning Gravel ft^2 Pool to Riffle Ratio Lower Reach :1 Middle Reach :1 Upper Reach :1 Total NA NA NA NA NA NA NA NA NA Table 1. Summary statistics for the November modified R1/R4 habitat survey conducted in the bypass reach of West Rosebud. Creek. Rainbow trout inhabited Mystic Lake and presumably could and did colonize the upper bypass reaches via the spillway. Brown trout which were extant only downstream of the powerhouse were able to colonize the lower bypass areas up to the first major waterfall barrier. Impassable waterfalls occurred throughout sections of the bypass and restricted brown trout to the lower most reach of the bypass Sampling showed healthy populations of rainbow trout in Mystic Lake and rainbow and brown trout in the two lakes downstream of the powerhouse. They also showed densities among the highest known for the state for a stream of this size in the bypass reach. The presence of all life history stages from juvenile through reproductive size adults demonstrated that the system was self-sustaining. These fishery data led to conclusions that instream flows had been protective and that power operations including re-regulation had been prudent in protecting the fisheries around the project area since the last license conditions were imposed. 4

5 Downstream of the re-regulating dam in the mainstem river, habitat assessment showed good trout habitat at low flow conditions. Historic data on the fishery from Montana Fish, Wildlife, and Parks data indicated that this population was healthy, self sustaining and being protected if not enhanced by the 20 cfs minimum flows provided by re-regulated discharges. Herpetology experts conducted aquatic habitat surveys for herptile species around the project and determined no critical habitat exists for sensitive species. At the request of the U.S. Forest Service, the consulting team conducted an analysis of project affects on overall system hydrology using the Nature Conservancy method known as Indicators of Hydrologic Alteration or IHA (Smythe Scientific, version 7, 2005: www:weba.viawest.net/users/csmythe). The greatest change in hydrologic regime below the project was determined to be slightly lowered average peak discharge as a result of storage operations. Therefore, no change in the minimum instream flow was proposed. Riparian biologists conducted plant surveys downstream of the project also to determine if the altered hydrology was affecting habitat. The riparian habitats were found to contain healthy populations of native cottonwoods and other species. Those few altered areas were correlated with land disturbances from livestock and other ranching activities. Plans to monitor riparian conditions were included in the FERC License conditions. Figure 4. View of Mystic Lake and Absorkee. Conclusions The historic and existing minimum flow operations both in the bypass and the reregulated reach of the river were judged by the Aquatic Resource Team to have been protective of the natural aquatic resources. Due to the methodology of measurement and to historic records, it was clear that the Mystic Project had on average contributed closer to 5 cfs on average to the bypass during the minimum flow season. In order to maintain the actual historic flows that have supported the excellent fishery, PPLM agreed to maintain the historic levels which were judged to be typically 5 cfs with the allowance to reduce flows to 4 cfs 11 days each month (September through May). Although this appears to increase in the required bypass flow, in actually it codified continuation of PPLM s actual historic operation which was enhanced due to natural 5

6 inflows augmenting controlled releases. During summer months (June through August) the minimum bypass reach flow was kept to 10 cfs. This healthy fishery approach avoided the need to conduct costly and often debatable hydraulic habitat studies. This led to rapid resolution of what is often the most costly aspect of relicensing projects with bypassed instream reaches. PPLM also agreed to ramping the descending bypass minimum flow below 10 cfs to a maximum of 2 cfs per hour when the minimum flow is changed from summer to fall operations August 31 each year. Fish Valve Design PPLM showed good faith and stewardship of the river by recognizing the potential for dewatering the stream even though it might be an unlikely event. PPLM is continuing to work on the design elements of the emergency fish valve and has committed to installing the valve by As envisioned, the valve will provide a means of releasing a minimum of 5 cfs during the low flow season and 10 cfs during the summer season. Most importantly, the new valve will be ready to also maintain instream flows of 20 cfs to the entire river should any catastrophic event prevent water from flowing to the powerhouse. We will report at a future date on the successful implementation of this important safety element of the new Mystic Lake project. Figure 4. Existing configuration of the Mystic Lake flow line and penstock system Authors Ginger Gillin is a Senior Environmental Scientist with GEI Consultants in Sacramento, California. She works on FERC licensing and aquatic biological issues on regulated rivers in the western USA and served as lead aquatic scientist and project manager for this project. 6

7 John Pizzimenti is a Vice President at GEI Consultants in Portland, Oregon. John served as project manager and senior principal consultant for this project. Doug Foss was a Mechanical Engineer and lead engineer for this project from GEI Consultants Bozeman, Montana office. Doug passed away shortly before Mystic received its new license. We dedicate this paper to Doug s memory. He was instrumental in many of the successful elements in this and so many other hydro projects. He will be missed. Jon Jourdonnais is Hydro Licensing and Compliance Manager for PPL Montana in Butte, Montana. Jon led the entire Mystic Lake Relicensing effort and he and PPL Management empowered this team to be the first to successfully license a project under the ILP. 7