Tsunami Hazards and Mitigation in Canada

Introduction

Tsunamis are not very common natural disasters, but the fact that they can occur without warning makes it worth to try and find out what are the hazards associated with them. This paper will try to first define tsunamis, determine what are the hazards associated with them (especially in Canada) and give some examples of mitigation that can be used to prevent life loss during tsunamis.

What is a Tsunami?

Tsunamis are series of waves, generated by earthquakes, landslides, volcanic eruptions, explosions, or even impact of cosmic bodies, that travel across the ocean and have extremely long wavelengths. A tsunami can be created when a disturbance displaces a large water mass from its equilibrium position. In earthquake-generated tsunamis, the water column is disturbed by the uplift or subsidence of the ocean floor. Submarine landslides, which often come with large earthquakes, as well as disintegration of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami (Ruff, 2003, Yalciner, 2003). Conversely, terrestrial landslides and cosmic-body impacts disturb the water from above, as energy from falling debris is transferred to the water into which the fragments falls (Murty, 1977, Bryant, 2001, Clague et al., 2003, Yalciner, 2003).

Hazards and Risks in Canada

A popular misconception about tsunamis is that it is only one large wave. A tsunami, however, is a series of waves separated by minutes to an hour or more. The second or third wave is normally the largest. Tsunami hazards include flooding, strong currents, bores (wall-like waves of water traveling up rivers or into estuaries), "seiching" (sloshing in a harbour or semi-enclosed area), sediment scouring and deposition, sandblasting and floating debris. Tsunamis flood low-lying coastal areas and are amazingly powerful. Even a tsunami 20cm high could knock somebody off his/her feet. During a tsunami, people may drown or be injured by floating debris. Floating debris are also dangerous during the return flow in which the wave draws back, bringing the debris back with it. Inundation causes saltwater contamination. This can damage vulnerable ecosystems, land productivity, buildings and structures. The effects of saltwater flooding may last long after the water has retreated. While destructive on their own, tsunamis are often associated with another natural disaster that has already caused much damage (an earthquake, for example) (Murty, 1977, Okal, 1993, Clague et al., 2000, Bryant, 2001, Clague et al., 2003, Yalciner, 2003).

Tsunamis occur mostly in coastal regions situated around the "Ring of Fire" of the Pacific Ocean. Canada has experienced many tsunamis, but few have caused lots of damage. The worst tsunami disaster that has occurred in Canada is the one triggered by the Grand Banks earthquake of November 18, 1929, which killed all the population (29 people) in Newfoundland's Burin Peninsula except for one individual (see figures 1 and 2). The tsunami caused by Halifax harbour explosion of December 6, 1917, also killed many people. There is also a third large destructive tsunami that occurred in 1964 on the west coast of Vancouver Island following a great earthquake in Alaska (Clague et al., 2003).

It is possible that much larger tsunamis than those could hit Canada in the future (Okal, 1993). An enormous tsunami could be generated by a great earthquake at the Cascadia subduction zone, which underlies the sea floor of the eastern North Pacific Ocean (see figure 3). A large landslide from the submerged flank of one of the volcanoes on the Hawaiian Islands could also produce a devastating tsunami on the west coast of Canada (Clague et al., 2003). The east coast of Canada is also at risk. Landslides occurring at the Atlantic continental margin, or collapse of the flank of a volcano on the Canary Islands coast might trigger a tsunami that would devastate the east coast (Liverman et al., 2001, Clague et al., 2003). The tsunami risk along Canada's Arctic coast is low in comparison to that of the Pacific and Atlantic coasts, mostly because of the presence of Atlantic sea ice. These are only possibilities and large tsunamis occur rarely. However, if they were to occur, they would have catastrophic effects on coastal infrastructure and might kill or injure large numbers of people (Okal, 1993, Clague et al., 2003).

Mitigation

Tsunamis cannot be predicted because they happen without much warning and can travel very fast. However, there is a possibility to reduce the damage they can cause. Hazard-reduction measures are of two types: non-structural and structural.

Non-structural measures include land-use control, emergency preparedness and public education. Emergency preparedness requires an effective system in place to warn people of an approaching tsunami and to provide time for evacuation. Of course, warning systems are useful only when the source of the tsunami is far from inhabited shores. Public education is crucial because people need to react quickly and efficiently to a warning. Education, such as evacuation routes (see figure 4), is especially important in schools and helps to control fear and panic. When people are ready to face a disaster, they react more calmly (Hebenstreit, 1997, Clague et al., 2003, Yalciner, 2003).

Structural measures comprise dyking, barrier construction (See figure 5), flood proofing, and tsunami resistant construction. Structural