Past Research Highlights
November/December 2006
Debris flows and Record Floods from Extreme Convective Thunderstorms over the Santa Catalina Mountains, Arizona
Robert Webb, U. S. Geological Survey
Peter Griffiths, U. S. Geological Survey
Chris Magirl, U. S. Geological Survey
Diane Boyer, U. S. Geological Survey
Erik Pytlak, National Weather Service
Craig Shoemaker, National Weather Service
Michael Schaffner, National Weather Service
Phil Pearthree, Arizona Geological Survey
Ann Youberg, Arizona Geological Survey
On July 31, 2006, an unusual set of atmospheric conditions aligned to produce record floods and an unprecedented number of debris flows in the Front Range of the Santa Catalina Mountains. During the week prior to the event, an upper-level area of low pressure centered near Albuquerque, New Mexico generated widespread heavy rainfall in southern Arizona. After midnight on July 31, a strong and widespread complex of thunderstorms developed over the Mogollon Rim in central Arizona in a deformation zone that formed on the back side of the upper-level low. These thunderstorms interacted with a low- to mid-level zone of atmospheric instability to create an initial wave of rainfall across the Tucson metropolitan area in the early morning hours.
A second wave of thunderstorms and heavy rain developed over the Santa Catalina Mountain near dawn. This intense rain falling on already saturated soil triggered multiple debris flows in several canyons. The Sabino Canyon Recreation Area, a heavily used recreation area administered by the U.S. Forest Service, was the epicenter of mass wasting where at least 18 debris flows removed structures, destroyed the roadway in multiple locations, and closed public access for months.
Figure 1. Three-dimensional map of the south face of the Santa Catalina
Mountains showing 240 slope failures (red dots), 3 debris-flows that
exited the mountain front (purple line), and the boundary of the 2003
Aspen fire (orange line).
To the west in Rattlesnake Canyon, a similar number of debris flows coalesced to travel several kilometers down the channel washing out the roadway, debouching into Sabino Creek, and contributing to a net 5-7 m of deposition within Sabino Creek. To the east in Soldier Creek, several slope failures generated a debris flow that exited the mountain front, eventually stopping at a small bridge at the Mount Lemmon Short Road. This debris flow completely filled the channel above the bridge, forcing the following flood to find a new path to the east over the alluvial fan and through several residential properties.
Figure 2. Oblique aerial photograph of terminal debris-flow deposition
where Soldier Creek meets the Mt. Lemmon Short Road. (13 September 2006.
P. Griffiths, USGS)
These debris flows were followed by streamflow floods which eclipsed the record discharge in the 75-year gaging record of Sabino Creek. The floods reworked most of the debris-flow deposits in or adjacent to the channel or buried them beneath coarse, sandy deposits. Slopes in the affected watersheds are now destabilized and remain vulnerable to future failure and debris-flow activity from less intense precipitation. The large volume of sediment released into mountain canyons by these slope failures will gradually move out of the Santa Catalinas in waves of sediment, with the potential to compromise Pima County flood control system for some time to come.
A PowerPoint presentation about the debris flows in the Santa Catalina Mountains is available for download in Adobe pdf format (24 MB).
For more information contact:
Robert Webb (rhwebb@usgs.gov)
Peter Griffiths (pggriffi@usgs.gov)
