REDUCING LANDSLIDE RISK THROUGH LAND-USE PLANNING IN COLORADO

The Colorado Geological Survey (CGS) has an online, statewide GIS landslide inventory that includes mapped landslides from traditional geologic maps and existing geologic hazard maps. In the early 1970s, geologic and natural hazards were defined as being a "matter of state interest" in Colorado, thereby providing the basis for regulation of geologic hazards in land-use planning. Concurrently, additional tools for local governments were funded, including geologic hazards maps and drafting hazard definitions, identification criteria, and land-use guidelines. Ideally, the approach to reduce the risk of landslides in land-use development is by avoidance through the use of master plans and no-build zoning. However, the push to develop marginally stable or potentially unstable slopes increases as available land decreases and property values increase. Ideally, land-use planning approval occurs only after the appropriate engineering geology/geotechnical investigations and analyses are completed and slopes have been shown to be sufficiently stable under developed conditions. CGS's role is twofold in this process. First and most significantly, Colorado planning laws requires Colorado counties to submit to the CGS for review all plans with subdivided lot sizes less than 35 acres. Many municipalities also voluntarily ask CGS to review development proposals. The CGS provides an unbiased technical review of the site plan and any submitted geological, engineering, soils, or drainage reports. CGS also conducts its own site inspection to ensure that the developer has avoided or mitigated all potential geologic hazards. Secondly, the public availability of landslide maps improves the standard of practice in Colorado by prompting more thorough investigations on the part of the consulting community in landslide-susceptible areas. The City of Colorado Springs has taken their land-use controls a step further. Within landslide-susceptibility overlay zones, they are now requiring stability analyses that show minimum 1.5 safety factors or the insertion of hazard statements into title documents.