POSSIBLE CAUSES AND IMPLICATIONS OF ENIGMATIC DROUGHT-YEAR LANDSLIDES 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.