Analyzing debris flows with the statistically calibrated empirical model LAHARZ in southeastern Arizona, USA

Hazard-zone delineation for extreme events is essential for floodplain management near mountain fronts in arid and semiarid regions. On 31 July 2006, unprecedented debris flows occurred in the Santa Catalina Mountains of southeastern Arizona following extreme multiday precipitation (recurrence interval > 1000 years for 4-day precipitation). Most mobilized sediment contributing to debris flows was derived from shallow-seated failures of colluvium on steep slopes. A total of 435 slope failures in the southern Santa Catalina Mountains released 1.34 million Mg of sediment into the channels of 10 drainage basins. Five drainages produced debris flows that moved to the apices of alluvial fans on the southern edge of the mountain front, damaging infrastructure and aggrading channels to reduce future flood conveyance. Using the statistically calibrated, empirical debris-flow model LAHARZ and modified model coefficients developed to better match conditions in southeastern Arizona, we predicted the approximate area of deposition and travel distance in comparison to observed depositional areas and travel distance for seven debris flows. Two of the modeled debris flows represented single slope failures that terminated downslope with no additive influence of other debris flows or streamflow flooding. Five of the simulated debris flows represented the aggregation of multiple slope failures and streamflow flooding into multiple debris-flow pulses. Because LAHARZ is a debris-flow hazard-zone delineation tool, the complexity of alternating transport and deposition zones in channels with abrupt expansions and contractions reduces the applicability of the model in some drainage basins.