Tornado hazard assessment for southern Ontario

Abstract

Southern Ontario experiences a number of natural hazards including tornado. However, quantitative tornado hazard assessment in terms of annual probability of exceeding a given wind speed has not been reported for this region. To carry out such a tornado hazard assessment, a statistical characterization of tornado parameters in southern Ontario has been developed using the tornado database of Ontario and that of the neighbouring regions in the United States. These parameters include the tornado occurrence rate, intensity, path length and width, and motion direction. Using the developed statistics and an existing wind field model, a probabilistic assessment of the tornado hazard in terms of wind speed is obtained for southern Ontario. The analysis suggests that for point-like structures such as residential structures, which are usually designed for a 30 year return period wind speed with a load factor of 1.5, the probability that a tornado wind speed exceeds the factored 30 year wind is very low. However, for line structures or elongated systems such as transmission lines, the tornado hazard becomes significant and increases with an increase in the length of the system. Key words: tornado hazard, wind speed, transmission line, probability, wind field, southern Ontario.