A HISTORY OF DEBRIS FLOWS IN UTAH

Historical records indicate debris flows in Utah are triggered by intense thunderstorm rainfall or rapid snowmelt and often cluster around specific events such as wildfires, overgrazing, and unusual climatic conditions. Utah has an extensive history of damaging debris flows since settlement in 1847, largely due to development on alluvial fans with little regard to debris-flow hazards. Debris-flow damage has shifted from agricultural land to residential subdivisions as agricultural areas become urbanized. The potential for damage continues to grow as development encroaches on alluvial fans, although increasingly more communities recognize and address the hazard.

Overgrazing and wildfires in drainage basins along the Wasatch Range in Davis County north of Salt Lake City resulted in 27 debris flows from 22 drainage basins during the 1920s and 1930s. These events prompted excavation of contour trenches in the upper drainage basins, and construction of deflection walls and debris basins on alluvial fans at canyon mouths. The unusually wet climate of 1983-84 led to widespread snowmelt-induced debris flows across northern Utah, mostly triggered by debris slides into main channels. The debris flows prompted many affected communities to enact risk-reduction measures that typically included constructing debris basins and concrete-lined channels. Since 1999, during a recent drought in northern Utah, 26 fire-related debris flows occurred in seven wildfire-burned areas. Again, risk-reduction measures following the debris flows included construction of debris basins, and deflection berms with sediment collection areas.