11. Lemieux Landslide
Leda Clay landslide along South Nation River
County Road 16 at Concession Rd. 14, The Nations Municipality, Ont.
Park at the intersection of Regional Road 16 and Concession 14. Walk north along the open trail to the headscarp of the Lemieux landslide. The best views are from the opposite side of the landslide. It is easier to walk around the headscarp than to attempt to cross the scar.)
The 20 km stretch of valley of the South Nation River downstream of Casselman is the most active landsliding region in Ontario. This section of the river is entrenched in a deep (~25 m) valley incised through deltaic sand into the underlying sensitive marine clay informally known as Leda Clay. A large retrogressive earthflow occurs here, on average, every 20 to 25 years. Following a major earthflow in 1971, geotechnical testing along the South Nation River led to the identification of the town of Lemieux as lying within a zone of potential highly retrogressive failure. As a result, Lemieux was abandoned in 1991 and residents were relocated. In 1993, only two years later, a large earthflow consumed 17 hectares of farmland adjacent to the former town site.
The failure began at the river bank, possibly as a small slump. Significant loss of strength and liquefaction of the clay in the most sensitive zone (which lies 20 to 30 m below the surface and even with the level of the river) caused this initial failure to develop into a rapid earthflow. The initial liquefaction ultimately caused most of the overlying silt and clay to also liquefy and flow. The stiffer surface sediments were fractured and carried as rafted blocks on the flow of remolded clay.
In less than 1 hour (and in under 15 minutes for the most part), the Lemieux earthflow rapidly ate back 680 m into the surrounding plain, severing a highway. The landslide also widened, creating embayments into both the north and south sidewalls. The lateral extent of the failure seems to have been restricted by the presence of nearby deep ravines lying to the north and south of the landslide. These ravines probably allowed sufficient drainage of their immediate area to prevent failure.
About 2.8 million cubic metres of debris flowed into the river valley, extending 1.6 km upstream and 1.7 km downstream of the crater mouth and completely damming the South Nation River for 4 days. Initially, upstream water levels were raised 12 m above normal, and elevated levels were maintained for well over a year.
Since 1993, frost action, precipitation, and revegetation have subdued the scar surface. The river has cut down into the debris in the valley and a deep gully now extends from the river into the scar. The steep headwall at the top of the landslide has been graded and smoothed to increase stability.
The local township estimates that direct costs of this event are $4 million; if indirect costs are included, the estimate is $12.9 million.