Landslides often follow flash flooding. On September 12, 2013, for instance, FEMA declared Boulder, El Paso, and Larimer counties in Colorado a disaster area after severe storms, flooding, landslides, and mudslides struck the area.
Colorado flash floods left many homes swamped, but Google’s Crisis Response released a Colorado Flood Map which provides location information, evacuation orders, warnings, and other data for residents. How could landslides affect you – and how can you prepare?
Landslides: Damaging, Dangerous, Costly
No matter the triggers – whether excessive precipitation and rushing water as in Colorado, quakes, volcanoes, melting permafrost, or human activity – a landslide is a ground failure caused by gravity. And, according to the USGS, landslides cause $1-2 billion in damages and more than 25 fatalities on average each year in the U.S.
Other areas of the world such as the Himalayas, Indonesia, and China, however, are much more prone to fatalities.
The USGS keeps a list of the large landslides that have occurred around the world in the 20th and 21st centuries, and which have had notable socio-economic impacts.
Although we think of the natural triggers of heavy rainfall, earthquakes, and volcanoes, some experts believe that the 2008 Al-Duwayqa rockslide east of Cairo Egypt resulted from sewage and decomposing garbage on top of a rock scarp that led to clay layers expanding, adding to destabilization already present due to construction activities.
What Are Landslides?
W.G. Moore calls landslides ‘landslips’ and goes on to define them as “A downward movement of a large mass of earth or rocks from a mountain or cliff; often caused by rain water soaking into the soil and earthy material on a steep slope.”
Any area composed of very weak or fractured materials resting on a steep slope can and will likely experience landslides. For example, I live on a relatively flat peninsula surrounded by lakes St. Clair and Erie and the Detroit River, with shorelines created by ancient glacial lakes. The cliffs of these shorelines, with the occasional ribbons of beaches below them, have been moving backward for 10,000 years; still, people choose to live above, over, and below them, sometimes with catastrophic results.
For simplification, the USGS includes a range of three ground movements in its ‘landslide’ classification:
- Rock falls. e.g. 2008 Al-Duwayqa rockslide (107 deaths);
- Deep failure of slopes. e.g. January 10, 2005 landslide that struck the community of La Conchita in Ventura County, California (10 deaths); and
- Shallow debris flows. e.g. they are frequent and widespread along Puget Sound’s shoreline, typically occurring during prolonged periods of heavy rainfall.
Landslides: Impacts on Civilization and Earth
One view of landslides is that, as in the words of Brenda Bell, “they play an essential role in the synchrony by which the uplifting continents are continually worn down, their sediments swept away and deposited somewhere else to start anew.”
Geertsema et al, writing in Disaster Risk Reduction, note that landslides, as natural disturbance agents, renew the environment. They then go on to explain that “the ecological role that landslides play is often overlooked. Landslides contribute to aquatic and terrestrial biodiversity. Debris flows and other mass movement play an important role in supplying sediment and coarse woody debris to maintain pool/riffle habitat in streams. As disturbance agents, landslides engender a mosaic of seral stages, soils, and sites (from ponds to dry ridges) to forested landscapes.”
On the other hand, nature and landslides, while more interested in equilibrium than humans, do continue to have a large impact on infrastructure, loss of life, and property. The Washington State Department of Natural Resources, for instance, an agency in a state which knows more than its share about landslides and its triggers, states: “Although we do not have an exact estimate of the damage landslides have caused in Washington, a rough estimate from only large storm systems and earthquakes is in the billions of dollars. The potential damage from landslides is tens to hundreds of billions of dollars… Nationally, landslides account for over $2 billion of loss annually and result in an estimated 25 to 50 deaths a year.”