POSSIBLE CAUSES AND IMPLICATIONS OF ENIGMATIC DROUGHT-YEAR LANDSLIDES IN NORTHERN UTAH

The landslide inventory in northern Utah contains a subset of recurrently active slides, some of which reactivated in dry years that occurred during a regional drought between 1999 and 2004. Several of these drought-year landslides caused damage to developed hillslope areas. Possible causes of recurrent landslide movement during the drought include a combination of climatic, hydrogeologic, geologic, and human-caused factors. Drought-year landslides reveal some misperceptions regarding the presumed stability of landslides in some geologic materials and the wet climatic conditions typically thought necessary to cause landsliding in northern Utah.

Climatic factors include the precedence of the recent drought by a wet period from 1995 to 1998 that locally persisted through 2000 in northern Utah. Ground-water-level data suggest historically high levels in some landslides in the initial year of the drought and only a gradual decline in levels during the subsequent drought years. In addition, whereas a hydrologic drought was sustained between 1999 and 2004, wetter-than-normal to near-normal years interrupted the meteorological drought. Periods of wetter-than-normal precipitation also occurred within dry years, particularly during the critical period prior to the months of March through May when landslide movement typically triggers in northern Utah. One monitored landslide remained dormant only in years that dry conditions were sustained during the critical period.

Hydrogeologic and geologic factors include variable fluctuations in peak and low seasonal ground-water levels in landslides during dry conditions, enhanced infiltration capacity of pre-existing slides caused by ground deformation, inherently low-strength geologic materials, and displacement-induced reduction of soil shear strength in recurrently active slides. Human-caused changes to hillslope areas that also contribute to landsliding during drought years include excessive lawn watering, redirected runoff onto landslides, unlined drainages atop cut slopes, and cuts and fills that locally reduce slope stability.