Falling Rocks and Landslides: Gravity at Work
Between eleven and eleven-thirty on the morning of May 17, 1841, a 360-foot chunk of the Cap-Diamant promontory, opposite the King's stores, collapsed with a tremendous crash onto homes along Champlain Street, taking part of the wall with it. Thirty people were killed, and six homes destroyed in the incident, one of the most catastrophic ever in the Quebec City area.
The talus that accumulates at the food of most slopes demonstrates clearly the universal nature of rockfalls, landslides and mass movements. These geological phenomena occur at all scales, both on land and in the oceans, and they constitute an important erosion mechanism. Rocks are subject to the effects of weather as soon as they come into contact with the air. When we build at the foot of a cliff, we do so at our own risk.
Since the 1700s, rockfalls have made headlines in Quebec City on numerous occasions, with most of the damage occurring in the Quartier Champlain, at the foot of the Cap-Diamant promontory. This area is particularly vulnerable because of the interplay of several factors. The steeper the slope, the greater the risk of a rockfall. The higher the drop, the more speed the debris can acquire as it descends the slope and the more damage it will be able to cause at the other end. Undercutting the foot of the cliff to provide more space only enhanced the danger of rockfalls by destabilizing the upper part of the slope. In the absence of vegetation, there was nothing to anchor the overburden or the soil. On Cap Diamant, because the planes of weakness in the rock run parallel to the slope, large slabs of rock can become detached and slide down the cliff.
While gravity is the primary cause of such mass movements, water and air also contribute to their occurrence by acting as lubricants. It is not surprising that some rockfalls have occurred after heavy a heavy rainfall. Freeze-thaw cycles and earthquake vibrations are additional contributing factors. Some of the rockfalls at Cap Diamant were even associated with canon fire.
North of Quebec City, Jacques-Cartier Park offers several spectacular examples of another type of landslide called a skin flow. This type of landslide occurs on slopes steeper than 35° when the thin layer of debris or soil covering the rock begins to move because the soil in the upper part has become saturated with water. The vegetation is stripped along a narrow swath which can be up to tens of metres in length.
Effective methods exist of reducing the risk of rockfalls in places where human lives are threatened. The unstable material can be removed, the steepness of the slope can be reduced, rock bolts and grillwork can be installed. All of these techniques have been used along Champlain Boulevard and Sault-au-Matelot Street.
Liste des éboulis
October 2, 1775, Champlain Street | One house damaged | Canon fire |
June 22, 1779, foot of Cap Diamant | Two sheds damaged | Torrential rains |
May 14, 1789, Cap Diamant | One house damaged | |
May 17, 1841, Champlain Street | Twenty-seven killed, six houses destroyed | Heavy rainfall |
July 14, 1852 | Five deaths | Torrential rains |
October 11, 1864 | Four deaths, two houses destroyed | |
September 19, 1889, Champlain Street, below the terrace | Forty-five deaths | Torrential rains |
November 9, 1905 | Two houses destroyed | |
June 28, 1957, Champlain Street | Several houses damaged | |
March 28, 1958, Champlain Street | ||
April 24 or 25, 1979 | No damage | Freeze-thaw |
April 1, 1982 | No damage | Unknown |
September 10, 1982 | No damage | Heavy rains, clean-up of rockfall talus |
March 18-19, 1983 | One house destroyed | Heavy rains |
April 4, 1984 | No damage | Unknown |
February 20, 1994 | No damage | Rapid thaw |