Water Online

November 2017

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

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spacing varied from 1.1 to 3.4 feet and expanded to roughly 3.5 feet at the downstream end. Vertical vane spacing expanded from approximately 1.6 feet at the upstream end of the vane array to 3.2 feet at the downstream end. A view of the installed array from downstream is presented in Figure 2. Increasing the settling capability of the existing basin is the most effective means of removing sediment from the system, as settled material in the basin is passively flushed out of the basin into the river by a low-level outlet. A three-dimensional (3D) computational fluid dynamics (CFD) model was used to develop the guide vane array design as well as quantify the expected improvement in settling basin performance. Particle sizes introduced into the model were based on sediment samples from the river, ranging from gravel size to fine silt. Model results showed that the overall amount of sediment making it past the sediment basin into the screening channel was reduced by a factor of 7 by the vane array for both the average and maximum intake flow conditions. The majority of the sediment expected to enter the intake was sand, which modeling predicted would be 85 percent removed with the guide vane array installed in the settling basin compared to only 14 percent removed without the guide vane array under average diversion flows. The array had little predicted impact on removal of silt-sized material. Sediment Eductor System Since some sand and the majority of silt-sized materials would still pass the settling basin and deposit in the facility behind the fish screens, an eductor system was designed to aid removal. The sediment eductor system (Figure 3) consisted of a network of pipes installed on the floor of the basin located behind the fish screens where flow velocities were lower and deposition occurred. This 120-ft by 20-ft area had historically been a maintenance problem, as it had very low velocities leading to accumulation of large sediment deposits as well as poor access for manual sediment removal via vacuum truck. As such, it was a high priority to try and reduce sediment deposition. The eductor collector pipes have crowns with narrow slits (Figure 3). The pipes were joined to collection headers that ran to a common discharge point along the river shore. The differential head from the water level in the eductor collector pipe basin to the free discharge point along the shoreline generated flow which entrained the sediment through the slits and transported it through the pipe network to the river. The eductor pipe system was split into four separate zones designed to be operated individually and controlled by isolation valves that cycled on and off on a timer. This eductor did not remove all of the sediment from behind the screens — only that which passed in close proximity to the pipe slits or settled out near the pipes. Corrugated floor plate sections between the collector pipes to assist in directing sediment to the pipe slits were designed, but were eliminated from the initial installation as they would make access more difficult during manual removal of sediment if the eductors were not completely effective. The corrugated plates may be added at a later time if found to be needed. Conclusions Given the facility site constraints, two novel sediment removal alternatives were developed and constructed to reduce the impact of sediment on headworks operations. The guide vane array substantially increased the efficiency of the existing settling basin within its current footprint, while the eductor system provided an additional passive sediment removal method. These passive removal methods along with other improvements greatly reduced downtime at the headworks facility, increasing operational resilience and saving the utility approximately 56 man-hours of maintenance over the last two years. n PLANTDESIGN About The Authors Isaac Willig, senior engineer at Alden Research Laboratory, has 10 years' experience in the hydraulic engineering field, specializing in numerical models to design and analyze hydraulic structures, including spillways, outlet works, and fish passage facilities. Figure 2. Downstream end of installed guide vane array Figure 3. Sediment eductor system installed behind the fish screens Chick Sweeney, senior technical fellow at Alden, has more than 40 years' experience in hydraulic engineering consulting and is a recognized expert in applying field data collection programs and both physical and computer-based hydraulic models to solve facility site selection, design, and permitting problems. Dr. Joe Orlins is the director of Alden's Redmond, WA, hydraulic engineering and modeling laboratory. He has over 30 years' experience in hydraulic engineering, higher education, engineering and project management, and facilities planning and construction. Clint Smith, principal engineer at MWH, now a part of Stantec, is a civil, fisheries, and water resource engineer with over 32 years of consulting experience. His background includes the planning, design, and construction of all aspects of fish passage. Greg Volkhardt is the environmental programs manager at Tacoma Water. He was the project manager for design of the Headworks Intake Modification Project and has over 30 years' experience working on natural resource issues and salmon recovery in the Pacific Northwest. 19 wateronline.com n Water Innovations

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