NEW: Landslide Map Viewer

NEW: Impact and Loss Estimation for 2010 (Note: Document published in 2012 – Latest year for which complete information is available is 2010.)

Following is a CGS video on landslides and the hazards they represent around the state.

Landslides (as opposed to rockfall) occur when masses of soil and rock move downward and outward from a slope. In Colorado, landslides are thought to be relatively infrequent, but many may occur in remote areas that are difficult to monitor. Landslides can be massive or they may disturb only a few cubic feet of material. Landslides can recur time and time again in virtually the same location.

Slopes of almost any angle can fail, even ones as gentle as 5 degrees!

Landslides occur when the stability of a slope changes and it loses equilibrium. The degree of steepness, soil moisture, soil thickness, the angle at which rock debris lies and the presence of vegetation combine to produce a slope that will tend to stay put until something changes.

Often the determining factor is elevated soil moisture or ground water pressure as a result of heavy precipitation or melting snowpack leading to soil saturation. Other mechanisms may also precipitate a slide: the loss of vegetation as after a wildfire, the erosion of the toe of the slope by rivers, or earthquakes. The excavation of roadcuts, quarries, or trenches may remove supporting earth and rock from the base of a slope. Conversely, the construction of mine tailing piles, landfills, or earth embankments for building sites and roads may overload a slope. In addition, construction activities, logging or the removal of low vegetation can alter natural drainage patterns and increase the infiltration of water.

The changes that instigate a landslide can occur quickly, such as a large rockfall event that overloads a slope or a run of several unusually wet months. 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.

Landslides threaten people and damage infrastructure in any number of ways. A large landslide can dam a river, creating an unstable impoundment which presents flood risks upstream and down. Slides can cover or alter the camber of roads, knock bridges off their abutments, or rumble into moving traffic. Landslides can also present serious threats to buildings and homes built in slide paths.

The Colorado Geological Survey 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 geologists who evaluate a site for possible landslides asks three basic questions: (1) Is there evidence of recent (or continuing) landslides? (2) Is there evidence of past landslides? (3) 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 benefit 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 the long run. Generally, the best advice is to build somewhere else.