The wind-speed dose-response of tree-falls impacting the transmission grid of British Columbia

Extratropical cyclones (ETCs) originating in the Pacific Ocean routinely bring strong winds to British Columbia (BC). The winds from ETCs can topple trees onto electrical transmission lines, depending on infrastructure proximity to wooded regions. As part of a much broader study focused on the mitigation of windthrow-related electrical disruptions, examination of power outage (from the BC Transmission Corporation) and official wind data (from Environment Canada) from 1990-2008 for a 50-km radius centered on the Vancouver International Airport and a 100-km radius around the Prince George Airport reveal a complicated blackout response from wind-induced tree-falls. There is no correlation between wind speed and the magnitude of power outages as measured by customers and circuits affected. For example, winds as low as 15-20 km/h have toppled trees (probably made vulnerable due to disease) onto circuits, whereas a strong 65+ km/h wind may not drop a single tree onto a transmission line. Many factors involve a given tree's wind-firmness, including height, taper and soil moisture conditions, and these variables likely contribute much noise to the analysis. Also, wind records are limited to a small number of stations, and the transmission grid occupies a wide and geographically diverse region. However, when the data are examined from the standpoint of frequency of outages for a given magnitude of windstorm, a trend can be identified. There appears to be a threshold, at least in the Vancouver data, where the probability of a tree-related transmission line interruption significantly escalates. This speed is around 70 km/h (with higher gusts). Below 70 km/h, transmission line outages only happen during a small percentage of storm events, 3-20% depending on magnitude of wind. At roughly 70 km/h and above, the outage probability exceeds 50%. Outage data from BC Hydro for the period 2005-2009 also indicate a critical tree-related outage threshold around 60-70 km/h.