DEBRIS FLOWS – AN UNDERAPPRECIATED FLOOD (?) HAZARD IN SOUTHERN ARIZONA

Geologists have recognized the potential for debris flows in steep mountain drainages in Arizona for decades. Although Arizona's climate is generally arid to semiarid, it is subject to incursions of very moist subtropical air masses that interact with higher-latitude systems to produce wet intervals with impressive amounts of precipitation. The most extreme storms have generated 20-30 cm of precipitation in 3-24 hours and caused multiple debris flows in mountain drainages. Links between forest fire, increased runoff, and debris flow occurrence in the western U.S. have heightened concerns about debris flow hazards in Arizona, as the state has been subject to numerous large fires and some subsequent debris flows in the past 15 years. Because nearly all historical debris flows have occurred in uninhabited or lightly inhabited areas, however, debris flow hazards have not been systematically considered in the context of floodplain management. An extreme precipitation sequence set up over Arizona in late July, 2006, when a persistent low-pressure system centered in eastern Arizona interacted with a moist monsoon flow from the south. Increasingly wet storms culminated on July 31, when debris flows occurred at numerous mountain locations throughout southeastern Arizona. Hundreds of slope failures occurred in the Santa Catalina Mountains on the northern fringe of the Tucson metropolitan area, and some of these coalesced into large, boulder-laden debris flows that continued several kilometers down canyons to the mountain front. Public infrastructure and roads sustained extensive damage, and runout from a debris flow damaged several homes near a canyon mouth. Only a few small debris flows had occurred in this area in the past several decades and none had caused significant damage. Relatively young-looking, coarse bouldery deposits near the mouths of all of the sizable canyons draining the south side of the Santa Catalina Mountains attest to the occurrence of past large debris flows, however. We are currently working with the local flood control district to map these deposits in detail and to provide a preliminary assessment of the extent of potential debris flow activity along the mountain front.