DEBRIS-FLOW TRIGGERS, THRESHOLDS, AND INITIATION MECHANISMS IN THE INTERMOUNTAIN WEST

Debris flows in the Intermountain region of the western U.S. initiate from a variety of geologic terrains and are generally triggered by intense rainstorms or rapid snowmelt. The influence of each trigger is at least partially controlled by latitudinal variations in the timing of maximum monthly precipitation. In areas south of about N39° latitude, maximum precipitation occurs in the months of July and August and intense rainstorms are the dominant trigger. North of N39°, maximum precipitation occurs between March and June and it is unclear if either trigger dominates.

Rainfall and snowmelt thresholds for triggering debris flows have been previously defined for some environments in the region. A rainfall threshold for the alpine zone in the Front Range of Colorado (Menounos, 1996) is defined as I=13.0D^-0.65, where I = rainfall intensity (mm/hr) and D = rainfall duration (hrs). This threshold is poorly constrained at rainfall durations of less than 1 hr. A threshold for recently-burned areas in Colorado is well constrained for durations of less than one hour and takes the form I=7.0D^-0.60 (Cannon et al., 2003). A study of snowmelt-triggered debris flows in Colorado, Utah, and Wyoming (Chleborad, 1998) showed that most debris flows occurred within 2 weeks after the first yearly occurrence of a six-day moving-average air temperature of 58° F (14.4° C).

Common mechanisms of debris-flow initiation in the region include: 1) erosion and entrainment of colluvium or channel material by runoff and rilling, 2) a firehose effect where concentrated runoff from bedrock mobilizes downslope material, and/or 3) mobilization from infiltration-triggered landslides. Field observations and published reports indicate that runoff processes dominate in sparsely vegetated, semi-arid areas south of about N39°. To better understand initiation processes and short-duration rainfall triggers for runoff debris-flows, three debris-flow basins in central Colorado were instrumented with event-recording rain gages and soil-moisture sensors. Two debris flows occurred in one of the basins in 2004, the first year of monitoring. Triggering rainfall for these two debris flows was 5 mm in 15 minutes (I = 20 mm/hr) and 15 mm in 23 minutes (I = 39 mm/hr). Theses values are less than the alpine threshold and at or above the threshold for burned areas.