GEOLOGIC HAZARD OCCURRENCE IN COLORADO DURING THE 1999-2002 DROUGHT

Colorado, like much of the western U.S., experienced drought conditions beginning during the autumn of 1999 and culminating in an exceptional drought during 2002. Major effects of the drought included decreased snow and rainfall precipitation and a negative water balance, drying of the near-surface soil, and stressing or loss of vegetation. In 2002, Colorado experienced 3,072 wildfires burning over 600,000 acres, the state's worst wildfire season on record.

During this period, the drought affected the occurrence and severity of various geologic hazards in different ways. Active landslides, such as the Slumgullion earthflow and the DeBeque Canyon landslide, deformed at slower rates in concert with the decreased precipitation, but did not stop. Few new landslide occurrences were noted during the drought. Rockfall incidents continued to occur around the state, although with somewhat less frequency and magnitude than during non-drought years.

Expansive soil deformations included both swelling and shrinking, depending on irrigation practices in response to the drought. Severe lawn-watering restrictions in Denver and several suburbs resulted in unusual shrinking-related settlement damage to foundation and porch slabs and building walls. Deep ground cracks formed in the dried clay soils, setting the stage for eventual future water infiltration and swelling. In one newer subdivision, the shrinkage of soils away from foundations was responsible for pulling underground electrical and utility connections away as well, shorting out the lines and causing house fires. Soil swelling and swell-related damage, which is typical for the Denver area, continued unabated in other suburban communities where watering restrictions were not imposed.

Debris flows were abundant during the 1999-2002 drought years as a result of the continued occurrence of local thunderstorms. The largest and most destructive were post-wildfire mudflows that occurred in the wildfire areas near Durango, Glenwood Springs, and South Fork. However, dozens of debris flows occurred in unburned areas across the state. In those areas, there was abundant source material due to the die-off and shrinkage of ground vegetation and the drying, cracking, and loosening of near surface soils.