By Richard Grost, Matthew Prociv, Clint Smith and Scott Schevenius
Constructed in 1952, PacifiCorp’s 11-MW Soda Springs development blocked access to about 7 miles of available habitat for migrating fish such as Pacific lamprey, Chinook salmon, steelhead trout and Oregon coast coho salmon — the latter of which is considered an Endangered Species Act “threatened” species. Improving environmental conditions for these species was an integral part of Soda Springs’ relicensing process.
About the Soda Springs plant
Soda Springs Dam – 55 miles east of Roseburg, Ore., on the North Umpqua River – is the eighth and most downstream hydroelectric facility within PacifiCorp’s 194-MW North Umpqua project. The 309-foot-long, 77-foot-high concrete arch dam includes a gated concrete ogee spillway section. The eight developments range in capacity from 11 MW to 43 MW and use three reservoirs and five forebay ponds as the primary source of water that allows for shaping flow for daily peaking operation.
The North Umpqua River immediately downstream of the Soda Springs powerhouse is designated as a recreational Wild and Scenic River. Soda Springs Dam impounds a 32-acre reservoir used for re-regulation of flows from upstream developments and is operated with the goal of maintaining a relatively natural flow in the river below the powerhouse.
The need for improvements
Through the Federal Energy Regulatory Commission relicensing process, PacifiCorp entered into a settlement agreement for the entire North Umpqua project with the National Marine Fisheries Service, U.S. Forest Service, U.S. Fish and Wildlife Service, U.S. Bureau of Land Management, and Oregon’s Department of Fish & Wildlife, Department of Environmental Quality, Water Resources Department in 2001. Upon issuance of the 35-year FERC operating license in 2003, and completion of the appeal period in 2005, planning and design of new fish passage facilities at Soda Springs were initiated. All design work was performed by a team including PacifiCorp, its design consultant, MWH Americas, Inc., the fishery management agencies, and, because the project is located entirely on the Umpqua National Forest, the U.S. Forest Service.
New passage facilities were required to include volitional upstream passage in the form of a fish ladder for adult fish, exclusion screening of the powerhouse flow to safely pass downstream-migrating juvenile and adult fish, and spillway modifications to further improve downstream passage conditions for fish during spill events.
|This aerial view of the Soda Springs hydropower plant shows improvements completed in 2012 that added fish passage for a section of Oregon’s North Umpqua River.|
After considering many options, the preferred and most feasible design included a “half-Ice Harbor” fish ladder beginning at the base of the dam, V-shaped fish screen, powerhouse intake, and spillway constructed within one of the two spillway bays. The fish screen location was selected after other locations within the flowline on the opposite abutment proved to be infeasible to construct such a massive structure. Locations along the original flowline that would provide the necessary space and hydraulic conditions for juvenile fish passage were found to have inadequate foundation support and were subject to landslides within the steep canyon.
Conversely, the spillway bay and adjacent bank was a relatively solid location with the added benefit of being a surface intake rather than submerged in the reservoir.
Challenges to development
Located in a steep canyon about 1,000 feet below adjacent ridgelines with near-vertical side slopes in many areas, Soda Springs’ location provided a significant topographical obstacle, for PacifiCorp and prime contractor MWH Americas Inc. The challenge was increased by with the area’s variable geologic conditions, which include such as faults, poor-quality rock, and active landslide areas, andin addition to flashy hydrology during fall and winter storms.
The hydro project is also operated within strict instream flow and ramping requirements, including minimum instream flow and ramping rates both within a 0.5-mile-long bypass reach between the dam and powerhouse, and also downstream ofbelow the powerhouse. To meet achieve these regulation requirements while shaping load at upstream facilities, the reservoir water level can vary up to 14 feet, with normal daily fluctuations of as much as 8 feet. The maximum diversion to the powerhouse is 1,600 cubic feet per second, and minimum flow into the bypass reach is 275 cfs.
The consultant design team for the project was led by MWH Americas, Inc. who provided the planning and design services for the project starting in 2006. MWH Americas, Inc. (dba Harza) had worked with PacifiCorp to complete the licensing process and Settlement Agreement completed in 2003. Several subconsultants were instrumental in the project development including Cornforth Consultants who provided extensive geotechnical engineering support throughout the project and construction. Environmental and biological services were provided by Meridian Environmental and Normandeau Associates Technical expertise and peer review was provided by Kozmo Bates for fish passage, Kim de Rubertis for geotechnical and dam safety. ENSR (Alden Labs) provided hydraulic analysis and physical hydraulic modeling. Finley Engineering provided the surveying.
Fixing the problems
Soda Springs Dam was constructed in 1952 without any fish passage facilities and was a complete blockage to upstream migrating fish. The existing new fish ladder restoresed access to access to approximately seven miles of spawning and rearing habitat. The fish ladder entrance was located at the base of the arch section of the dam to take advantage of the natural fish holding pool, existing sluiceways, and best protect it areas and a limited amount of protection during projectfrom damage during spill events.
The dam creates a 60-foot rise between the tailwater pool and the reservoir pool upstream of the dam. The purpose of the fish ladder is to break that 60-foot rise into smaller manageable steps that migrating fish can navigate within their swimming capability. This required the ladder to include 59 pools that are formed by weirs of 1-ft rise at each. The shape of the weir was taken from the working fish ladder design at Ice Harbor dam on the Columbia River. This type of fish ladder weir allows fish to pass between pools ether by swimming through the 1.5-ft-square submerged orifice at floor level, or by jumping over the 4-foot-wide overflow weir section. The ladder’s floor is continuously-sloped, smooth concrete except where it is routed under the spillway over to the left abutment of the dam via a 100-ft-long level transport channel, and where it penetrates the dam and passes a fish counting station via another level transport channel.
|The Soda Springs hydroelectric plant, shown before fish passage improvements in 2004|
The majority of the overall vertical rise in the ladder is provided on the downstream side of the dam, but the last 14 fish ladder pools are in the reservoir upstream of the dam abutment and approximately 20 feet below the crest. To accommodate the 14 feet of reservoir fluctuation, the fish ladder was provided with modified pools and exit gates that are automated to open and close to provide the ladder design flow of 25 cfs, and fish access, at any reservoir water surface level.
To improve fish attraction to the ladder, the entrance at the base of the dam was sized to accommodate the required minimum instream flow of 275 cfs for the bypass reach. Routing the full river flow out of the ladder entrance improved its effectiveness by eliminating competing water sources that might create conditions where fish would be attracted away from the ladder.
Downstream passage – fish screens
Extensive work was invested in siting and selecting the appropriate fish screening and return technology for the project, with traditional passive and “experimental” in-conduit screens evaluated. The analysis concluded that the best balance of fish protection, project risk, operational complexity and constructability was achieved by placing the fish screen within the southern-most spillway bay to capture flow for generation, fish ladder attraction, and downstream fish transport while still retaining high-flow spill capacity.
The screen structure is designed to pass up to 1,875 cfs, including 1,600 cfs for generation plus the 275 cfs minimum instream flow which is also the fish ladder attraction flow. The fish screens are constructed in a V-configuration, with flow passing through about 4,900 square feet of screen. The average approach velocity normal to the screen face is below 0.4 feet per second. The screen’s V-configuration guides fish to the apex of the V and into the return system that transports them to the river below the dam. The primary, secondary, and finishing screens are cleaned by automated traveling brushes and backwash spray.
Screened water for generation is conveyed through a 12-foot-diameter steel pipe that free spans 130 feet above the river to the original flowline pipe benched into the right bank. Water used in the 275 cfs pipe is routed back upstream under the screen structure and under the spillway to the fish ladder entrance.
|The Soda Springs hydroelectric plant, shown before fish passage improvements in 2012 after the installation of a fish ladder, collecting screens and modified spillway.|
Physical site constraints coupled with the requirement for accommodating the full reservoir fluctuation made the use of a normal open channel fish return pipe system unfeasible. Through extensive hydraulic analysis, modeling and consultation with resource agencies, a closed-pipe fish return system was developed. It consists of three main elements: an inlet hopper, a pressure pipeline segment and a gravity pipe segment.
The inlet hopper is automated to raise and lower and thus accommodate the 14-ft reservoir range based on variations in the reservoir’s water level within the fish screen structure. The hopper serves as a transition to a fixed-pipe segment via a 40-foot-long ramp connected to a concrete floor on its upstream end and weir crest at the downstream end dropping into the hopper. This ramp gradually accelerates flow to a trapping velocity in which juvenile fish cannot reverse direction before transitioning into an 18-inch-diameter pipe.
This pressure pipeline segment is designed to minimize the potential for fish injury and debris blockage with 15-foot-radius sweep fittings, smooth interior coatings and specially designed flange joints. Hydraulically, the pipe is sized to dissipate the energy in the flow over its length. This pipeline segment extends downstream and across the river, dropping the hydraulic gradeline 31 to 45 feet over a distance of 600 feet. The pipe discharges at a fixed transition structure, where the flow is converted to open channel flow.
The lower gravity flow segment then transitions from full-pipe flow to open-channel flume. The flume can be diverted to the fish evaluation facility, or to the river through a 30-inch fish return pipe. Under normal operation, fish are routed directly to the river without passing through the evaluation facility. Flow from the evaluation facility and the fish return pipe recombine where the pipe is routed to an outfall structure in the river.
Downstream passage – spillway
The original construction of Soda Springs’ spillway included a short apron at the base of the spillway that terminated on the bedrock 31 feet above the normal tailwater pool. Under medium to small spill events during fish passage seasons, flow through the gates would flow off the apron and cascade down the rock face, creating potential impact to fish passing within the flow through the spillway.
To reduce the potential for contact with the rock face or with the bottom of the tailwater pool, the spillway apron was extended and shaped to a modified ogee curve. This reshaping, with modified gate opening procedures, is designed to discharge water and fish into the tailwater pool without plunging to the bottom, where they could possibly strike the bottom of the pool.
The North Umpqua project is a unique combination of eight small to medium hydroelectric developments. Maintaining the ability to operate them for peaking operation, if desired, is an important part of its economic viability and requires Soda Springs to act as a flow re-regulation facility. PacifiCorp made a substantial investment and overcame a number of physical challenges to modify Soda Springs dam to provide safe, volitional fish passage for fish. The new fish passage facilities were constructed in 2010-2012 and have been improved and operated since then.
Through a combination of hydraulic design, mechanical systems and automation, the significant challenge of maintaining the licensed reservoir fluctuation while also maintaining conditions that facilitate fish passage have been addressed on this project. Monitoring and evaluation of the facilities are ongoing, but initial results indicate that the facilities meet agency hydraulic criteria, and that fish are successfully moving past the dam in both upstream and downstream directions. Upstream passage has included about 600 to 900 adult salmon and steelhead annually, and should increase as the offspring from these fish return from the ocean to spawn in future years.
Richard Grost is senior aquatic scientist for PacifiCorp. Matthew Prociv was project engineer and fisheries engineer for MWH Americas. Clint Smith is principal project manager and fish passage engineer for MWH Americas. Scott Schevenius is lead senior engineer at PacifiCorp.
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