Tsunami Vulnerability Assessment of the Canadian Pacific Coast

Abstract

The North American Pacific coast, located within the Ring of Fire, is at risk of severe subduction tsunamis. This danger has pushed the United States to make a strong push in tsunami research. Recent advancement has resulted in the implementation of a new chapter in the ASCE 7-16 standards focusing on tsunami structural loads and effects. Within the scope of this new standard, the tsunami inundation hazard of the US West coast has been mapped. However, no such work has been completed in Canada, leaving the tsunami hazard and vulnerability for most of the Canadian West coast uncertain. The life safety vulnerability from the most hazardous source, the Cascadia Subduction Zone, is evaluated in terms of pedestrian evacuation capabilities. Using a static distance-only model, the ability of individuals to evacuate to safety in natural high grounds is computed for all communities in British Columbia and compared by Tsunami Notification Zone. A new variable – the available time – for tsunamis life safety vulnerability assessment is proposed. This variable considers both tsunami arrival time and time to safety, resulting in a life safety threshold value of 0. Zone E has the largest surface area and population within the hazard zone, even though it has the smallest probable run-up range because of the large number of communities in this zone and the low-laying areas of the Lower Mainland. All communities within Zones A, B, D, and E have low life safety vulnerability at the maximum probable run-up of their respective zone, suggesting that pedestrian evacuation should be possible. Zone C has the highest vulnerability of all zones, as it has the lowest available times. With a 9 m run-up and over 25% of its communities lying within the moderate- or high-vulnerability categories, it has an available time of below 30 min and 15 min, respectively. Zone C also has the highest percentage of its surface area inundated at its maximum probable run-up (39.7%). The most vulnerable communities are identified, including 45 First Nation and 5 non-indigenous communities: Tofino, Winter Harbour, Ucluelet, Port Renfrew, and Bamfield. The life-safety threshold is surpassed in Barlett Island 32, Grassy Island 17, Hesquiat 1, and Tofino. Delta and Richmond, in Zone E, also have a minimum available time below the life safety threshold at run-ups between 5 and 7 m, at or above the probable run-up of their zone, as they have large low-lying areas. As the tsunami arrival time is very large here, the merits of vehicle evacuation should be evaluated. Additionally, they are likely to be more vulnerable to landslide tsunamis, as the tsunami arrival time would be much shorter than one from a Cascadia tsunami. A more detailed vulnerability study using anisotropic path-distance modeling was performed in Tofino. This more complex model found lower available times than the distance-only model. Maximum differences ranging between 14.4 to 29.9 minutes were found for three pedestrian velocities. The minimum available time was found to be -29.0 minutes within the official municipality boundary and -40.1 minutes within one of the beaches. Two vertical evacuation structures are required to reduce the time to safety below the tsunami arrival time of 28.1 minutes for run-ups between 13 and 19 m. Run-ups above 19 m required three vertical evacuation structures. No configuration could be found to sufficiently reduce the time to safety on Frank Island.