Wildfire, Rain and Floods: A case study of the June 2010 Schultz Wildfire, Flagstaff, Arizona
In early August of 2010, AZGS geoscientists joined researchers from the U.S. Forest Service Rocky Mountain Research Station’s (RMRS) Southwest Watershed Team to document and quantify the effects of fire on geomorphic and watershed processes following the Schultz Fire. Our goals are to provide land managers and government agencies with the tools necessary for assessing and effectively mitigating post-fire geologic hazards, potential impacts to long-term water quality and supply, threats to infrastructure, human life and property, and to understand local landscape evolution processes.
Watersheds subject to moderate to high severity burns are prone to much greater runoff – both in volume and velocity - due to decreased interception and infiltration (Neary and Ffolliott, 2005). Excess runoff increases hillslope and channel erosion resulting in sediment-laden flood flows or debris flows. Newly formed rills and gullies on the upper hillslopes of the Schultz Fire burn area fed into existing drainages or swales, further concentrating runoff and scouring channels to bedrock. Precipitation that, prior to the fire, would not have produced much runoff, produced very large post-fire flood flows and debris flows.
The Schultz Fire burned several peaks on the east side of the San Francisco Peaks composite volcano. Drainages descend from the steep mountain slopes into confined channels within Pleistocene alluvial fan deposits. Near the forest boundary the channels emerge onto unincised, heavily modified Holocene alluvial fans where housing developments have sprung up over the past quarter century. The Schultz Fire was a high-impact fire with the majority of the fire area burned at moderate (27%) or high (40%) severity on moderate to very steep slopes that were covered by Ponderosa Pine and mixed conifer forests (Figure 4, U.S. Forest Service, 2010). Several watersheds burned almost completely (Figure 4). Removal of the forest floor litter, alteration of soil properties, development of fire-induced water repellency, and loss of tree canopy in the moderate and high severity burn areas dramatically impacted the hydrologic behavior of this landscape.
Figure 4. Schultz Fire BAER basins with burn severity and ALERT stations. BAER basins identified as Basins of Concern are in blue. Data from Coconino National Forest. Basin outlines generated from a 10m DEM.
Burned Area Emergency Response (BAER)
Several days before containment, a U.S. Forest Service Burned Area Emergency Response (BAER) team began to asses the fire impacts on the forest to identify potential resources at risk and to determine appropriate mitigation measures. Their mitigation goals were to reduce flooding potential and retain on-site soils (Robichaud and others, 2010). Of particular concern were the City of Flagstaff’s waterline road (FR146), which provided approximately 20% of Flagstaff’s summer water from the Inner Basin of the San Francisco Peaks, cultural resources, soil erosion, and flooding impacts to downstream developments (U.S. Forest Service 2010). The BAER team assessed 11 basins within the burned area (Figure 4) and identified five as basins of concern (Figure 4) due to the burn severity, total area burned, and the steepness of the slopes (US Forest Service, 2010).