Landslide, Mud/Debris Flow, and Rockfall

According to READY Colorado, it is estimated that there are thousands of landslides in Colorado each year, with varying degrees of frequency and severity. Most of these events do not result in casualties or property damage, though the annual damage in Colorado is estimated to exceed $3 million to buildings alone (Colorado Natural Hazards Mitigation Plan, 2013, p. 3-185). A massive landslide in a relatively unpopulated area of Mesa County near Grand Junction killed three people in 2014, leaving a swath of debris three miles long and ¾ of a mile wide. A deadly rockfall in September 2013 claimed five lives of a vacationing family following heavy rains near a popular hiking location near Buena Vista, Colorado (Shoichet, et al., 2013). Rockfalls are less frequent but remain a constant threat, particularly to Colorado’s mountain roadways. All of these geologic hazards may endanger Colorado’s built environment and can damage or destroy buildings, roads, and other infrastructure when proper land use or mitigation practices are not considered. 

In recognition of this fact, the Colorado Geological Survey (CGS) provides a range of services and resources to assist and advise local planners on geologic hazards, including the review of preliminary plans or reports for new development as well as conducting studies, collecting geologic information, and publishing maps, reports, and bulletins with regard to land use activities.

Still, while a variety of relevant national and statewide data exists to determine hazard risk in a very general sense (including geologic, topographic, and soil maps), most Colorado communities do not have readily accessible information or detailed maps necessary for implementing local regulations. Doing so often requires field surveys and even geotechnical tests by trained earth scientists to identify specific problems associated with land development and public safety. 

Consultation with geologists and other experts familiar with local conditions is an important first step for local planners seeking to assess the risk of their community and specific areas that are susceptible to geologic and soil hazards. The CGS and other official sources can provide map information on levels of risk, past hazard events, and the probability of future events. More site-specific data and mapping, however, will need to be obtained through technical studies for specific areas of concern. Communities may opt to hire a consulting geologist or geotechnical engineer to perform this work, or require such expert studies as part of the local development permitting process.

As summarized in the chapter, Planning Framework, there are several state statutes and regulations that specify requirements for the submission of geologic suitability reports in conjunction with land use applications to be reviewed by CGS. Other statutes address the manner in which geologic and soil hazards are to be addressed by developers and local governments, including but not limited to hazard analyses and site recommendations.

At a minimum, planners should have a general understanding of where geologic and soil hazards exist and what their implications are for safe development so that the viability of available planning tools and strategies to reduce their risk can be further evaluated. Ideally, using this information, most communities should be able to prepare a map of the entire community that distinguishes particular areas of concern. This type of map can help planners and decision makers identify areas that are generally less desirable for future development and may require further technical study, along with smaller-scale maps for implementing regulations or requiring closer examination during the review of development proposals.