July 31, 2006 Flood and Debris Flow Event

Project Purpose and Need

A series of meteorological events at the end of July, 2006 triggered unprecedented flooding along the Santa Catalina Mountains. While flooding on some watercourses met or exceeded 100-year peak discharge estimates, the most extraordinary result of the events was the high number of debris flows on the southern front of the Santa Catalina Mountains.

In order to better understand the nature of this event and determine whether or not adjustments to the District's floodplain management practices were warranted, the District, in partnership with the U.S. Geological Survey the Arizona Geological Survey and the University of Arizona, studied the event in detail. The results of those studies are presented here.

Scope of Work:

• Data Collection: Develop a data base of all debris flows associated with the 2006 event and document evidence of past debris flow events.
• Hydrology and Hydraulics: Determine the characteristics of the event that triggered the unprecedented number of debris flows.
• Debris Flow Dynamics: Determine the mechanics and processes driving debris flows in the Santa Catalina Mountains.
• Probability Analysis: Determine the probability of the 2006 event and future events of this magnitude.

Event Summary:

July 31, 2006 Flooding and Debris Flows

The 2006 summer monsoon season turned out to be a record year for rainfall and stream flow in eastern Pima County.  The total rainfall received in June, July, and August was 8.6 inches at the Tucson International Airport.  This rainfall total is 2 inches more than the average rainfall for the same time period. 

Rainfall in mid-July created saturated soil conditions in the upper watersheds especially the Rillito-Tanque Verde-Pantano watershed.  In late July, moisture from Tropical Storm Emilia created a period of intense rainfall in eastern Pima County starting on July 27 and ending on July 31, 2006.  During this five-day period, rainfall totals ranged between 5 to 11 inches in the Catalina and Rincon Mountains and from 1 to 6 inches in the valley with many locations receiving over 50% of their average annual rainfall. 

Saturated conditions increased so that each successive day, an increasing amount of rainfall went to runoff so that by July 31, 2006 over 90% of the rainfall on the Catalinas resulted in runoff. Rainfall on the morning of July 31, 2006 was especially intense over the Tanque Verde Creek Watershed where 4 to 6 inches of rainfall occurred between midnight and 7:00 a.m.  The National Weather Service (NWS) estimates that the 4-day rainfall event was a 1,000-year event. 

The U.S. Geological Survey’s (USGS) estimate of the flood peak in the Rillito River is 38,700 cubic feet per second (cfs) and the flood peak on the Santa Cruz River at Continental is 42,000 cfs.  By comparison, the Federal Emergency Management Agency’s (FEMA) estimate for the 100-year flood on the Rillito River is 32,000 cfs, and the 50-year flood on the Santa Cruz River is 48,000 cfs. The USGS estimates that the flood on the Rillito exceeded the 500 year event.

Flows in many of the mountain washes exceeded the 100-year event.  The most intense flood damage occurred in the Tanque Verde Creek Watershed including Sabino Canyon and other mountain washes along the southern Catalina Mountains where heavy rains on the weekend of July 27, 2006 to July 31, 2006 deposited 6.97 to 10.28 inches of rain. For example, the 100-year estimate for Bear Canyon Wash is 1,940 cfs and the estimate for July 31st is 2,400 cfs, and the 15,700 cfs discharge at Sabino Canyon was estimated to be about a 200 yr flood.

The most unique feature of the storms and flooding are the debris flows along the southern Catalina Mountains.  The USGS has identified over 435 slope failures and debris flows which occurred in the Catalina Mountains between Esperero Canyon and Solider Canyon.  The Sabino Canyon Recreation Area was impacted by 36 debris flows.  The USGS characterized the storm and subsequent debris flows as an extreme event.  There has been only isolated individual debris flow activity in historic times and the USGS estimates that this magnitude of debris flow activity has not taken place in Catalina Mountains for at least 2,000 years. 

In order to better understand the possible risk of future debris flows, the District contracted with the USGS, Arizona Geological Survey and the University of Arizona to assess what happened on July 31, 2006, and to determine the risk of similar debris flow activity in the future.