Road Repair to Deal with Accelerated Erosion of Argiboroll and Cryoborall Soils of the San Francisco Peaks, Arizona, after a 2010 Wildfire: Successes and Failures.

The Schultz Fire burned 6,100 ha on the eastern slopes of the San Francisco Peaks, a dormant Holocene to Middle Pliocene stratovolcano in northern Arizona. The fire burned between June 20^th and 30^th, 2010, across moderate to very steep ponderosa pine and mixed conifer watersheds. About 40% of the fire area was classified as high-severity, mostly on  mountain slopes greater than 30% and in places exceeding 100%. A series of flood events beginning in mid-July 2010 initiated erosion that transported soil and parent material from fire-impaired argiborall and cryoborall soils off of the upper slopes of the watersheds and into urban areas 1,000 m below. In the process, a road used to service a waterline for the City of Flagstaff, Arizona, was cut or damaged at 28 locations. This waterline provided 20% of the city’s water supply. In some locations, channels incised 5+ m into C horizons and cut the waterline. Repair involved construction of low-water crossings and installation of rock-filled gabion baskets and concrete-covered roadways. This paper examines successes and failures in restoring the roadway and reducing erosion. Successes included immediate armoring of drainage channels, cessation of channel erosion, and efficient conveyance of sediment through low-water crossings. Failures consisted of inadequate keying of concrete channel crossings, roadway design geometry errors, downslope gabion design errors, underestimation of runoff and sediment volumes, and insufficient correction of significant erosional head cuts below critical road sections.