SECONDARY EFFECTS OF LANDSLIDES IN NORTH AMERICA (UNITED STATES AND CANADA)

The secondary effects of landslides (landslide dams, landslide tsunamis, and impact-generated mudflows) extend potential damage of a landslide beyond the limits of the landslide debris itself. These processes have been responsible for some of the world’s major landslide disasters and are an important, sometimes delayed, element of landslide hazard. The damming of rivers by large-scale rock movements has occurred on a number of occasions in the North American Cordillera since 1903. Rock avalanches blocked the Gros Ventre and Madison Rivers in 1925 and 1959, respectively, impounding significant lakes. The Gros Ventre rockslide dam breached in 1927. The 1980 flank collapse of Mt St Helens created two major landslide-dammed lakes and mass flows in the Garibaldi Volcanic Belt blocked major rivers in 1975, 1986, 1998 and 2010. Bedrock slides have also blocked rivers in the Plains. Landslides in Quaternary sediments have created significant landslide dams in northern North America. These include a) landslides in glaciolacustrine sediments in the Cordillera and on the Plains (e.g, the Attachie landslide that blocked the Peace River in 1973), and b) landslides in glaciomarine sediments on the Pacific Coast and the St. Lawrence Lowlands (e.g, the massive St. Alban quick clay landslide that blocked the St Anne River, Quebec in 1894). Landslide-generated displacement waves (landslide tsunamis) are also an important secondary process associated with rapid landslides that enter ocean inlets and fiords, mountain lakes, and rivers. Rockslide-generated tsunamis have been documented along the Pacific Coast (e.g., Lituya Bay, 1958; Knight Inlet, 1999) and in mountain lakes (e.g., Mt. Colonel Foster, 1946; Chehalis Lake, 2007). Landslides in Quaternary sediments generated deadly displacement waves in rivers at Spences Bridge (1905) and Notre Dame de la Salette (1908). Lastly, the impact of rockfall or rockslide debris on valley bottom sediments may generate a high velocity debris flow that travels beyond the limits of the initial landslide debris. Most of the fatalties in the 1903 Frank Slide were associated with destruction caused by secondary mudflow beyond the limits of the rock debris. The complexity of secondary processes contributes to the difficulty of performing a predictive assessment of possible landslide impacts.