LANDSLIDE SUSCEPTIBILITY MAPPING IN COLORADO SPRINGS, COLORADO

The city of Colorado Springs lies at the boundary of the Great Plains and the Rocky Mountains. The western portion consists of dissected foothills underlain by weak Cretaceous claystones prone to slope instability. Many of these areas have been mapped as landslide deposits. Early growth avoided the foothills; however, these areas are becoming increasingly built out since the 1980s. Rising land use risk and future problems may occur when land development is allowed on landslide-prone terrain.

During wet spring seasons in 1995 and 1999, several landslides occurred in Colorado Springs. In partial response to the 1995 events, the City enacted its first geologic-hazards ordinance in 1996. The 1999 event included flooding, and prompted a Presidential Disaster Declaration. Colorado Springs was provided funds from the Federal Emergency Management Agency (FEMA) to acquire landslide-affected properties.

Landslides remain a controversial topic in Colorado Springs. FEMA has spent $4.5 million in acquiring affected properties and public awareness of landslides has grown. However, the City and the business community are protective of property rights for new and existing land uses. Requirements for landslide investigations and slope stability analyses for land use planning must be based on scientific and engineering principles.

The Colorado Geological Survey continues to provide the City with landslide-hazard assessments, and has recently completed a GIS-based map of landslides and landslide-susceptible areas as a follow-up project to the emergency FEMA program. The landslide susceptibility map is based on geology, topography, and landslide history. Mapped landslides from published maps, consultant reports, aerial photography interpretations, and field investigations were used to evaluate the common geologic factors leading to slope failure. These factors were then compared to other areas of the City to determine potential landslide susceptibility. City planners, consultants, developers, and homeowners can use this data as a tool for future development and prompt a level of future geological and engineering investigations appropriate for the hazards and potential risks present.