By Isaac Willig, Chick Sweeney, Joe Orlins, Clint Smith, and Greg Volkhardt
Sediment and turbidity can be enough to shut down a drinking water treatment plant if the headworks aren’t suited to the source water. Learn how one facility near Tacoma, WA, which incorporates fish-handling to further complicate intake operations, secured sustainability through masterful design.
The Green River Headworks is a water-diversion and fishhandling facility owned and operated by Tacoma Water. The headworks facilities consist of: a diversion dam, intake structure, settling basin, fish screen structure, backup auxiliary traveling water screen and bypass pipeline, juvenile fish bypass, fish ladder, fish trap and sort facility, and pipelines that convey the screened water to a spill chamber and treatment facilities. The project has been providing drinking water for the City of Tacoma since 1913. Numerous improvements have been made to the facilities over the years; however, construction of a new treatment facility for the Green River Supply (completed in 2015) required modification of the headworks to increase reliability and reduce the frequency of operational outages.
Figure 1. Aerial view showing intake structure and fish-handling facilities
Prior to completion of the new treatment facility, water was diverted year-round when turbidity levels in the Green River were less than 30 Nephelometric Turbidity Units (NTU). When turbidity exceeded 30 NTU or at times when the river carried an excessive debris load, the facility was shut down (the project relies on groundwater well supply during these times). Operating during high river turbidity periods caused significant sediment deposition in the facilities and pipelines, creating the periodic need to dewater the facility for manual removal of the sediment.
With the construction of the new 168-MGD filtration facility, Tacoma Water desired to operate the headworks over a much wider range of stream flows, debris loads, and turbidities than previously. possible. The goals of the headworks intake modification project were to screen water with river turbidities up to 600 NTU, provide passive removal of coarse sediment, and reduce settling material delivered to the filtration facility.
Typically, sediment is removed using large settling basins that provide low velocity over a long distance to allow suspended sediment to settle out. Due to site constraints, there was no room to increase the existing settling basin size or add a new one. A brainstorming session including all involved parties was held to develop innovative solutions. Alternatives were evaluated, and Alden was tasked with carrying the selected alternatives forward to design and construction. These included a guide vane array in the existing settling basin and a sediment eductor system located in the screened water basin on the downstream side of the fish screens.
Guide Vane Array
The guide vane array’s purpose was to more evenly distribute the flow exiting an approximately 13-feet-wide by 10-feet-high tunnel into the existing 28-feet-wide by 20-feet-high settling basin to improve settling basin efficiency. The tunnel supplying the settling basin included a curve directly upstream of the tunnel exit. Centrifugal force associated with the flow traveling through the curve produced an asymmetric velocity distribution at the tunnel exit, leading to inefficient sediment settling in the basin. Efficient settling was further impacted by the hydraulic boils that resulted from the lack of transition between the 130 ft2 tunnel outlet and the 560 ft2 settling basin.
The vane array consisted of horizontal and vertical vanes in which the spaces between the vanes expand in the downstream direction. The vane spacing at the upstream end of the array was designed to intercept equal amounts of flow exiting the bend and to distribute the flow uniformly over the cross-sectional area of settling basin entrance. This resulted in more uniform and lower flow velocities entering the basin, increasing its efficiency at settling out sediment. The upstream horizontal vane 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.
Figure 2. Downstream end of installed guide vane array
Figure 3. Sediment eductor system installed behind the fish screens
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.
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.
About The Authors
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.
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.
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.
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.