This study evaluated methods to quantify the risk of debris-flow initiation and runout potential to impact alluvial fan flooding hazards in Maricopa County. Debris flows are unsteady, non-uniform, very poorly sorted sediment slurries that are generated when hillslope soils become saturated and fail. While there is some evidence that debris flows have occurred in Maricopa County on very steep slopes of mountainous watersheds, there are no documented cases of historic debris flows impacting flood hazards on mid-piedmont alluvial fans. Based on known general characteristics of debris-flow behavior, as well as on the specific climatic and geologic conditions in Maricopa County, the expected recurrence interval for debris flows in Maricopa County probably exceeds 1,000 years. Furthermore, because of the regional physiography and watershed characteristics, it is likely that future debris flows will have low volumes because of limited sediment supplies, will travel only short distances from their point of initiation due to their coarse sediment composition and low clay content, and that most will not reach the active areas of alluvial fans, particularly the fans that are located away from the mountain front. To assess potential debris flow impacts on alluvial fan flooding, a combined approach of geologic reconnaissance and mapping, with a two-phase application of the LAHARZ debris-flow runout hazard model is recommended. Geologic reconnaissance will confirm the presence or absence of relatively young debris-flow deposits, and provide details of the basin and piedmont conditions which will be useful for calculating and evaluating potential debris-flow volumes. Geologic mapping will provide data regarding minimum number of deposits, relative ages, and travel distances of past debris flows. Debris-flow runout models will provide hazard information regarding potential travel distances, and the volumes required to reach those distances.This report is part of a larger product, Refinement of Methodology: Alluvial Fan Flood Hazard Identification & Mitigation Methods, by J.E. Fuller on behalf of the Flood Control District of Maricopa County.Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact [email protected]
