Assessing powder cloud impact on electrical transmission lines at Snowslide Creek avalanche path in Southeast Alaska

Power transmission lines conduct electricity across avalanche terrain to communities in Alaska. Many of the towers along these lines have been redesigned, and are now strengthened, protected through relocation, or engineered with diverters in response to damage from past avalanche impacts. The challenge that remains in many areas is providing protection to the powerline conductors that are exposed to high impact pressure powder clouds generated from large avalanches. Alaska Electric Light and Power (AEL&P) provides hydro-generated electricity to approximately 17,000 users (meter count) within the City and Borough of Juneau in southeast Alaska. One critically exposed segment of the transmission line is located south of Juneau along Thane Road. The transmission line crosses numerous avalanche paths on Gastineau and Roberts peaks, of which Snowslide Creek is the most prominent, with frequent naturally and artificially generated avalanche activity. Avalanches here have destroyed power transmission towers and conductors many times in the past, and larger events often deposit debris on Thane Road, maintained by Alaska Department of Transportation & Public Facilities (ADOT&PF). A deflecting dam was developed at Snowslide Creek which redirects dense avalanche flow and catches debris in the early part of the season until it fills up. On March 4, 2021, ADOT&PF artificially triggered an avalanche in Snowslide Creek. Most of the dense portion of the avalanche stopped within the deflection dam with only a small portion spilling over; however, the powder cloud snapped the breakaway connections to the towers and the conductor was carried into Gastineau Channel, resulting in a costly repair for AEL&P. Here we present a back-calculation and reconstruction of this avalanche event using the dynamical avalanche run-out model Rapid Mass Movements Simulation (RAMMS) Extended to estimate the powder impact pressures along the transmission line. We collected airborne lidar data at Snowslide Creek pre- and post-avalanche mitigation that was used as input and for validation of the avalanche simulations. We also analyzed video footage from the event and extrapolated information from SNOWPACK model runs at Mount Roberts weather station as well as nearby snow pit information. We reviewed the engineering documents for the transmission line conductors and breakaway connectors, and compared these values to our reconstructed powder impact pressures and developed ideas for potential system improvements to increase the reliability of this exposed span. Future plans involve exploring instrumentation options for recording impact pressures at Snowslide Creek avalanche path and simulating snowpack scenarios and avalanches representing the changes we anticipate in future climatic conditions in this region.

Citation

Wikstrom Jones, K., Wolken, G.J., Janes, M., Wilbur, C., Glaus, J., Bartelt, P. and Christen, M.. 2023. Assessing powder cloud impact on electrical transmission lines at Snowslide Creek avalanche path in Southeast Alaska. International Snow Science Workshop Proceedings. 890 – 897. http://arc.lib.montana.edu/snow-science/item/2981.