A Case History of Scour Impacted Pipelines in River Crossings: Part I — Scour Evaluation

Two Water Transmission Pipelines owned and operated by the City of Everett, Washington, were installed in 1960 and 1966 using open cut methods. The pipelines cross under several rivers, creeks and sloughs, including the Snohomish River, Pilchuck River and Ebey Slough. The respective inside diameters of these pipelines are 48/42 and 51 inches [122/106 and 130 cm] and the estimated soil cover ranges from 2 to 8 feet [0.5 to 2.5 m]. Over time, changes have occurred in the rivers and slough that have resulted in the erosion of the soil cover above five pipeline crossings, resulting in the partial exposure and damage of the pipelines at these crossings. Since the City has plans for an additional 40 years of use for Pipelines, it was necessary to evaluate the scour potential and its impact on these pipelines. As a critical outcome, two of the five pipeline crossings were found to be in need of immediate repairs and were repaired as emergency projects, while other three crossings will be repaired as non-emergency projects. This paper describes the scour analysis approach, methods used, and results obtained for the two emergency projects.