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Chapter 1 INTRODUCTION BASIS FOR A COLORADO LANDSLIDE HAZARD MITIGATION PLAN Colorado has a long history of damaging landslide and debris-flow events. The community of Brownville (near Silver Plume) in Clear Creek County was engulfed and destroyed by a series of debris flows in 1912. The town of Marble in Gunnison County was nearly destroyed by debris flows in the 1930s and 1940s. A large landslide in DeBeque Canyon in June of 1924 temporarily blocked the Colorado River and resulted in the forced relocation of a small community, highway, and railroad. In recent years, losses from landslides have been extremely high. In 1984, 15 Western Slope counties were declared disaster areas by the President due to floods and landslides associated with the spring runoff. More than $6.6 million in federal, state, and local disaster assistance was administered to restore public facilities and services impacted by the disaster. In 1985, floods and landslides again caused $1.4 million in damages in two western counties. An emergency declaration by the Governor authorized $200,000 in emergency repairs from state and local funds. Addition- ally, millions of dollars in federal emergency highway funds have been committed to repair landslide-damaged highways at Douglas Pass, Muddy Creek and other landslide-prone sites in the western part of the state. In terms of lives, property, and damage to the state's infra- structure (roads, utilities, bridges, and buildings), losses are high enough to significantly impact the state's economy over the short term. It present loss trends continue, expected losses could eventually become high enough to disrupt the state's economic well-being and the quality of life of its citizens. It is estimated that losses from a major landslide event at Dowds Junction could run as high as $1.7 billion (Minturn Earthflows Task Force, 1985). The potential for higher losses is growing as urbanization and develop- ment increase in Colorado's vulnerable mountainous areas. FIgUre 1 indicates the extent of Iandsliding in Colorado relative to adjacent states. Most significant landsliding occurs in the mountainous areas in the western two-thirds of the state. The Landslide Hazard in Colorado There are thousands of landslides of various ages and degrees of activity in Colorado. A preliminary screening by the Colorado Geological Survey (CGS) has identified 12 large landslides with high potential for very large future losses. Thirty-seven additional areas have been identified that have high potential for future landslide or debris-flow events that could have serious local com- munity impacts. Although few lives have been lost as a result of lands1iding, the budgets of the Colorado Department of Highways (CDOH) and many counties and cities have been significantly impacted by costs related to lands1iding. Many private industries have also experienced high losses from landslides. These include railroads, mining companies, oil and gas production and transmission companies, and electrical transmission firms. Many individuals have also been seriously affected. Lands1iding in Colorado intensified during the period 1983-1987 as a result of higher than normal annual precipitation in many areas of the state (FIgUre 2). Although lands1iding has always occurred in Colorado, this recent cycle of increased precipitation combined with expanding population growth has increased the likelihood of damage and disaster. Some of the com- munities and areas believed to be at relatively high risk are listed in Thble 8. Other Hazards Related to Landsliding FIood'mg For many years flooding has been Colorado's most fre- quent and costly hazard. Landsliding is closely allied to flooding in that both are related to precipitation, runoff, and ground saturation. This was recognized by the Federal Emergency Management Agency (FEMA) and the State of Colorado when specific types of Iandsliding were integrated into the 1985 Co/oradQ Flood Hazam Mitigatiqn Plan (Colorado Water Conservation Board, 1985). Seismicity Most of the mountainous areas of Colorado that are vulnerable to landslides also have experienced a moderate level of seismicity in historic times. The coin- cidence of these hazards has the potential, under certain conditions, of significantly increasing the risk of serious damage. Dam failure Failure of man-made dams is also an important hazard in Colorado that may be in part related to landsliding. 7