ASSESSING DEBRIS FLOWS INITIATION PROCESSES FROM HISTORIC RAINFALL IN SEPTEMBER 2013 – GILA NATIONAL FOREST, NM (Invited Presentation)

In September 2013, a record-breaking series of rainstorms in the southwestern U.S. hit Glenwood, New Mexico and surrounding areas, including areas burned by the 2012 Whitewater-Baldy Complex wildfire, leading to unprecedented, and deadly flooding and debris flows. This study characterizes generation processes of debris flows that were initiated in response to a high intensity rainfall event occurring on a landscape that had been primed by two previous days of heavy rainfall. The specific storm event lasted approximately 10 hours starting on the night of 14 September with maximum 10 and 30 minute intensities of 117 and 108 mm/hr, respectively, and storm precipitation totals as high as 190 mm. A total of 688 debris flows were documented in a 939 km² area centered on the Whitewater Creek watershed in the Gila National Forest. This event offers an outstanding opportunity to characterize the local influences on debris flow initiation in both burned and unburned areas. Generally, debris flows initiated on south and southwest facing slopes. Based on aerial imagery and field observations, it was possible to infer the debris flow initiation mechanism (i.e. overland runoff generated or mobilized from a shallow landslide) for two thirds of the identified debris flows. Runoff generated debris-flows primarily occurred in burned areas. Fifty-three percent of the debris flows occurred on slopes with no prior evidence of debris flow scars or rills while the remaining flows, classified as reactivated debris flows, occurred in areas where there was evidence of debris flow activity prior to the historic rainfall event. Reactivated debris flows occurred on less steep slopes compared with new debris flows and had watersheds with higher mean burn severities. The large percentage of debris flows on previously unscarred slopes led to a substantial increase in the drainage density of this landscape. Field measurements indicate approximately 46,200 m³ of sediment was mobilized in the 139km² Whitewater Creek watershed alone. Geospatial analysis of debris flow initiation locations and relevant landscape characteristics, such as slope and burn severity, can provide important insights into the prevalence of particular debris flow initiation mechanisms in different environments that could help assess hazards associated with future events.