A landslide is a sudden mass movement of soil, artificial fill, and/or rock down a slope. Landslides include many different kinds of mass movements, including falls, topples, slides, spreads, flows, or a combination of one or more of these movements. Slopes of almost any angle, from slight hills to steep mountains, can fail in a sudden landslide. Landslides can be small or very large, up to thousands of cubic feet, can travel incredibly quickly (faster than a person can run), and may recur multiple times in virtually the same location.
Because landslides of any size may threaten people and infrastructure, it is important to understand where and how landslides occur and how they may affect future development. Colorado experiences many landslides each year because of its steep terrain. Some of them occur in remote areas that are difficult to monitor, with most occurring west of the Front Range to the Western Slope. Damage from landslides in Colorado is estimated to be millions of dollars per year. A large rockfall or landslide can dam a river, cover or damage roads, knock bridges off their abutments, or crash into moving traffic. Landslides can also present serious threats to buildings and homes built in slide paths.
As a a widespread and active geologic hazard in many areas of Colorado, landslides that pose the highest risk to communities, areas, and infrastructure are carefully mapped and inventoried by the CGS.
Landslides are often activated by heavy rainfall, snowfall, or melting snowpack, but can have many other causes:
- Gravity acting on a too-steep slope
- Erosion or removal of soil and rock at the base (toe) of a slope from streams, rivers, or human activity like road-cuts, quarries, or trenches
- Soil or rock saturation from heavy rain or snowmelt
- Removal of vegetation due to a fire, drought, or human activity such as construction and logging, which can suddenly increase soil or rock saturation
- Earthquakes that shake and weaken slopes or rocks
- Excess weight at the top of an unstable slope, such as rain or snow, man-made buildings and landfills (earth embankments for buildings and roads, etc), or piles of rock, ore, or waste products
- Volcanic eruptions that produce loose ash deposits, increase rainfall, and debris or mud flows (also known as lahars)
The changes that instigate a landslide can occur quickly, such as an intense rainstorm or a large rockfall event that overloads a slope. They may also occur over a number of years, as with a wet cycle that saturates and weakens layer of rock and soil, or stream erosion that undercuts the base of a slope.
Planning for landslides
The CGS is required by statute to review geologic reports done for new developments in unincorporated parts of counties, and for all new school construction or critical facilities for geologic hazards, including landslides. The CGS scientists who evaluate a site for possible landslides ask three basic questions:
- Is there evidence of recent (or continuing) landslides?
- Is there evidence of past landslides?
- Could landslides be triggered by construction or by normal use of buildings after construction?
If the answer to any of these questions is “yes”, then the cost of trying to forestall future landslides at that site must be weighed against the benefits to be gained from the proposed new use of the land. Benching slopes, placing ample subsurface drain tile, or installing buttresses or anchors are types of mitigation efforts that can work in certain circumstances. However, all of these measures are expensive, and they may not be effective in preventing landslides in the long run. Generally, the best advice is to build outside of areas where landslides may occur.
CGS — video on landslides https://youtu.be/s8qW4IqJs0A
Colorado Government — Colorado Natural Hazard Mitigation Plan
Colorado Government — Natural Hazard Mitigation Plan https://www.colorado.gov/pacific/mars/colorado-natural-hazard-mitigation-plan
USGS — Landslides 101
USGS — The Landslide Handbook
Washington Department of Natural Resources and the Oregon Department of Geology and Mineral Industries — A Homeowner’s Guide to Landslides for Washington and Oregon