Identification of debris-flow hazards in warm deserts through analyzing past occurrences: Case study in South Mountain, Sonoran Desert, USA

Article history: Received 14 March 2016 Received in revised form 7 August 2016 Accepted 8 August 2016 Available online 16 August 2016 After recognition that debris flows co-occur with human activities, the next step in a hazards analysis involves estimating debris-flowprobability. Prior research published in this journal in 2010 used varnishmicrolamination (VML) dating to determine a minimum occurrence of 5 flows per century over the last 8100 years in a small mountain range of South Mountain adjacent to neighborhoods of Phoenix, Arizona. This analysis led to the conclusion that debrisflows originating in smallmountain ranges in arid regions like the SonoranDesert could pose a hazard. Twomajor precipitation events in the summer of 2014 generated 35 debris flows in the same study area of South Mountain—providing support for the importance of probability analysis as a key step in a hazards analysis inwarm desert settings. Two distinctmechanisms generated the 2014 debris flows: intense precipitation on steep slopes in the first storm; and a firehose effect whereby runoff from the second stormwas funneled rapidly by cleaned-out debris-flowchutes to remobilize Pleistocene debris-flowdeposits.When compared to a global database on debris flows, the 2014 stormswere among themost intense to generate desert debris flows— indicating that storms of lesser intensity are capable of generating debris flows in warm desert settings. The Sr/Sr analyses of fines and clasts in South Mountain debris flows of different ages reveal that desert dust supplies the fines. Thus, wetter climatic periods of intense rock decay are not needed to resupply desert slopes with fines; instead, a combination of dust deposition supplying fines and dirt cracking generating coarse clasts can re-arm chutes in a warm desert setting with abundant dust. © 2016 Elsevier B.V. All rights reserved.