Electronic Thesis and Dissertation Repository

Thesis Format



Master of Engineering Science


Civil and Environmental Engineering


Najafi, Reza M.

2nd Supervisor

Souza, Camila P. E.


3rd Supervisor

Sills, David M. L.



Compound weather extreme events, such as tornadoes and flash floods, can significantly impact societies and infrastructure systems. Disaster response agencies provide instructions to the exposed communities to retreat to safety specific to the natural hazard. However, the instructions can become confusing if natural hazards demand conflicting responses. This study characterizes the compound tornado and flash flood (TORFF) events to assess and predict the simultaneous occurrence probability of such hazards across Canada in the long term. We quantify dependencies between the tornadoes and flash floods using ground-based and reanalysis datasets. Tornado data are available based on the recorded Fujita rating for each event, and the corresponding wind speed values are determined through a resampling approach. The TORFF events are clustered and the bivariate probability distributions of the resampled windspeed and precipitation are characterized based on Copula. The corresponding individual and joint return levels are investigated under different scenarios (AND, OR, and conditional) across Canada. Results show positive dependencies between resampled windspeed and associated precipitation in Saskatchewan, followed by Alberta, Manitoba, Ontario, and Quebec (least dependency) regions. Higher dependencies between tornadoes and flash floods over regions such as Saskatchewan suggest that analyzing these events in isolation can underestimate the associated risks. Higher precipitation is also expected during extreme wind speed, as observed in the conditional assessment of precipitation given windspeed. This study provides insight for more realistic recurrence interval estimation for tornadoes and flash floods to aid in the evacuation decision-making process.

Summary for Lay Audience

Tornadoes and flash floods are among the most dangerous natural hazards in Canada, especially in populated areas (Environment Canada 2017). Since the 1970s, they have caused a total of more than $CAD 10 billion in damage in Canada. More than 1,800 tornado events are recorded in Environment Canada Tornado Database (Environment Canada, 2020) between 1980-2009. Certain types of thunderstorms generate tornadoes that come with associated precipitation. Intense rainfall in a short period might cause flash floods in certain areas. The combination of multiple extreme events can cause more catastrophic consequences compared to individual extreme occurrences. Unique concerns arise when tornado and flash flood (TORFF) events occur simultaneously due to their specific evacuation instructions: seek underground shelter (tornado) and retreat to high ground (flash floods), which are contradicting with each other. Having a better understanding of TORFF risks and their impacts can help inform emergency response authorities in providing accurate evacuation instructions. Windspeed data were obtained through a unique resampling approach from Fujita rating windspeed interval records, and the corresponding precipitation data were extracted from a reanalysis product (i.e., NARR) based on the location of the tornado event. The dependency between resampled windspeed and associated precipitation is investigated after grouping tornado events into multiple clusters across Canada. The datasets are then analyzed using a multivariate probabilistic model, copula, and their joint variability are assessed under different return period scenarios. Lastly, the areas with higher dependency and subsequently TORFF risks are identified in Canada so that the appropriate response and evacuation instructions can be issued.