The American Society of Civil Engineers Sacramento Section awarded PG&E and SAGE Engineers with the 2017 Outstanding Water Project of the Year for the South Yuba Canal Landslide at 8/2 Flume.
In February 2017, PG&E discovered that landslides had occurred adjacent to the South Yuba Canal (SYC) and damaged a 250-linear foot flume. The landslides had damaged the flume and partially undermined the upstream foundations and transition structure. The damage fully disrupted functionality of the canal. The SYC is a key part of PG&E’s hydroelectric operations, as well as a local irrigation districts main source of both domestic and irrigation water. The canal moves water from PG&E’s powerhouse, and is then used to deliver water to the local irrigation district, which in turn provides water to users throughout Nevada County, CA. With the onset of irrigation season, and water demand expected to increase, hydro crews from PG&E needed to work as quickly as possible to make the necessary improvements and bring the canal back into service.
PG&E approached SAGE within hours of discovering the failure to evaluate the stability of the site, assess the damage, evaluate alternatives, and design a permanent repair, with the goal to restore full water service as soon as possible while ensuring the safety of construction. SAGE performed a site reconnaissance, deploying a team of engineers and geologists on rope access to investigate the slope above and below the flume to assess the underlying conditions, specifically to identify a possible cause of the landslide and determine the condition of the flume as it was left in place. The extent of the landslide was mapped and presented to the project team to develop repair alternatives. The alternative selected was to mass grade the slope back to stable conditions and create a new wider bench for a timber flume.
The project was immediately determined critical by PG&E, and the project team was able to quickly obtain a Special Use Permit from the United States Forrest Service and enlist the services of experienced biologists and cultural resource specialists to expedite the permitting process.
The mass grading, bench construction, and flume restoration was a massive undertaking. This remote site is accessed via dirt roads roughly 3.5 miles from pavement. Most of these roads had been significantly damaged in the same storms which caused the slide, and needed to be rebuilt to allow access to the site. The repair consisted of reshaping the existing slope by removing 22,400 CY of soil/rock to construct a new, wider bench and constructing a timber flume based on PG&E standard designs. Construction commenced on March 28, 2017 and the canal was returned to service on June 1, 2017. Additional work continued on the final slope and site grading, off-hauling, and drainage and concluded on August 30, 2017. The unstable terrain, saturated soils, snow covered landscape, and extreme weather during one of the wettest years on record made for a challenging job, but the crews stayed diligent and used every resource to get the job done right.
Working collaboratively, and at a rapid pace, PG&E, their consultants, the construction team, and regulatory agencies successfully repaired the canal before the hot summer months arrived when water demand is at its peak. These joint efforts helped stave off potentially disastrous economic impacts to Nevada County residents, businesses, farms, and ranches.