DEBRIS FLOWS FROM EXTREME CONVECTIVE PRECIPITATION IN A SEMI-ARID MOUNTAINOUS REGION

Depositional evidence on the alluvial fans of southeastern Arizona indicates debris flows have been an active geomorphic process in the geologic past; however, few debris flow have occurred historically. The debris flows that have been observed in southeastern Arizona before July 2006 were generally small and traveled less than a few 100 m, which led many to speculate southern Arizona debris flows were a predominantly Pleistocene process. On July 31, 2006, an unusual set of atmospheric conditions aligned to produce record floods and an unprecedented number of debris flows in this region. Heavy, multi-day rainfall resulted in at least 400 hillslope failures and subsequent debris flows in four mountain ranges, including more than 35 in the northern Chiracahua Mountains of southeastern Arizona and more than 65 at the southern end of the Huachuca Mountains. In the Santa Catalina Mountains, north of Tucson, Arizona, at least 250 hillslope failures spawned debris flows that impacted structures, roads, and, in some instances, homes on Tucson's urban boundary. The large number of failures and debris flows in an area with few historical events underscores the rare occurrence of this type of storm. Most failures appeared to be shallow-seated slides of colluvium in the steepest section of a hillslope. Progressive bulking of the mobilized debris flows during movement down the colluvial slope added significant mass to the flows, causing deep scour of chutes and substantial aggradation of channels downstream. The magnitude and duration of precipitation and slope characteristics appear to be the key variables governing failures, while antecedent moisture had a significant role in determining specific failure sites. Analysis of the geomorphic response of the hillslopes and channels to this extreme storm offers insight into processes shaping semi-arid mountainous regions.