Geomembrane Water Proofing of a 16.3 km Hydroelectric Structural Wood Flume

In 2015 the authors undertook to refurbish a 16.3 km (10.1 mile) elevated wood and metal flume that was owned by a private hydroelectric company near Mount Rainier, Washington. The hydroelectric facility was originally built in the early 1900s. Water is diverted from the Puyallup River at an intake diversion and carried by a wood flume structure downstream to the hydroelectric generating plant. The flume is supported by approximately 6,200 beams of which approximately 1,200 are wooden. In many areas of the flume, the wood floors and walls were badly decayed and leaking. The plant had an operating capacity of 26 megawatts (MW) however as a result of the structural decay the flume was experiencing significant water leakage resulting in the plant only operating at 8 MW. The project started with 165,000 m² of a high strength geotextile that was installed below the geomembrane to help structurally reinforce the deteriorating wood flume walls and flooring. The water proofing of the flume required a total of 140,000 m² of 2.5 mm and 2.0 mm HDPE liner material to be installed. The project faced many challenges as a result of poor access and tight space constraints. There was no roadway or vehicle access other than three staging areas. All project related materials and equipment needed to be deployed using a small rail car and track built into the top of the flume structure. Crews had to deploy, weld and mechanically attach geomembrane while working in a very confined working space.