DEBRIS FLOWS, SLOPE FAILURES, AND FLOODS RELATED TO THE AUGUST 19, 2014 EXTREME PRECIPITATION EVENT, NEW RIVER MOUNTAINS, ARIZONA

An extreme storm event on August 19, 2014 delivered intense precipitation to the mountains and mesas north of metropolitan Phoenix near the town of New River, initiating over 200 slope failures and debris flows. Downstream along the New River, Skunk Creek, and Cave Creek drainages, numerous gages experienced record flows, resulting in intense lateral erosion, changes in channel morphology, and severe flooding that impacted northern metropolitan Phoenix. Perhaps most notably, Skunk Creek overflowed its channel and flooded the northbound lane of Interstate 17, then continued south and spilled into the Central Arizona Project canal, a major source of water for metropolitan Phoenix.

Use of the historical imagery feature of Google Earth Pro allowed for identification of slope failures and debris flows that resulted from this storm, delineated by the relatively light-colored, sparsely vegetated scars visible on hillslopes in imagery after the storm. The initiation point of the slope failure at the top of each scar was marked with a placemark, then converted to a point shapefile with ArcGIS. Currently, 225 slope failures have been identified, most of which were aligned along the axis of the New River Mountains, where orographic lift concentrated precipitation. At least 20 of the slope failures can be classified as debris flows based on the presence of boulder levee and/or snout deposits, even at the limited resolution of the Google Earth imagery.

Field investigations have allowed for confirmation of several of the debris flow deposits, although most of the area is rugged and difficult to access. The volumes of accessible debris flow boulder snout deposits were estimated using multiple methods. GPS tracks and thickness measurements allowed a rough estimation of volumes, which can be compared with measurements made using Structure from Motion photogrammetry and used as an input in debris flow runout models of the area. The August 19, 2014 storm delivered an unprecedented amount of precipitation, flooding, and slope failures to the New River area. Although such extreme events are rare by human standards, they are important in shaping the hillslopes and drainages of semiarid central Arizona and can have significant economic consequences for downstream infrastructure.