Negative Storm Surges on Canada’s East Coast

Abstract

Negative storm surges in eastern Canadian water bodies are studied. These water bodies include the Atlantic Coast of Canada, the Labrador Sea Coast, Bay of Fundy, Gulf of St. Lawrence and St. Lawrence estuary. Using hourly observed and predicted sea level variations for a period of eleven years (1956—1975), statistical analyses were made to examine the extreme values, the return periods for negative surges of various amplitudes, the influence of an ice layer and the interaction between tides and surges. The analysis for the Labrador coast, Bay of Fundy and Gulf of St. Lawrence shows that negative surges with amplitudes up to 1.7 m have occurred during the 11-yr period of study at Pointe-du-Chêne. The negative surges generally are greater where strong and abrupt water depth changes occur. Largest negative surges occurred in winter, and smallest negative surges occurred in summer. This is due in part, to the presence of ice cover and, in part, to the fact that the weather systems that generate these surges are much stronger in winter than in summer. In these regions, the return periods of the extreme surges vary between 1 and 11 yr, but usually are less than 5 yr for most of the stations. Extreme values for negative surges in the St. Lawrence estuary are, in general, greater than for the other regions considered, with maximum value of 2.2 m observed at Québec. All other locations in the upper estuary have amplitudes with value of 1.4 m between 1.8 and 1.9 m. The smallest amplitude negative surges occurred at Pointe-au-Père and Baie- Comeau. In terms of frequency, the negative surges are most prominent at St-FranÇois and least prominent at Ste-Anne-des-Monts. Based on the amplitudes and frequencies for both positive and negative surges, we conclude that stations located at the lower part of the estuary (Pointe-au-Père, Baie-Comeau, Rivière-du- Loup) are reasonably safe from storm surges, while Québec City and St-FranÇois are extremely prone to storm surges. However, one reassuring feature at Québec City and St-FranÇois is that positive surges occur mostly at the time of low tide while negative surges occur at the time of high tide. On the other hand, at St- Jean-Port-Joli and St-Joseph-de-la-Rive positive surges occur at the time of high tide, which is a factor that makes the overall water level variations at these locations quite significant. Similar behaviour of the tidal wave propagation in the estuary and that of storm surge amplitudes is demonstrated.