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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By Isaac Willig, Chick Sweeney, Joe Orlins, Clint Smith, and Greg Volkhardt T he Green River Headworks is a water-diversion and fish- handling 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. 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 ft 2 tunnel outlet and the 560 ft 2 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 18 wateronline.com n Water Innovations 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. Sediment Solutions Within Existing Intake Facility Footprint Figure 1. Aerial view showing intake structure and fish-handling facilities

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