Deadly debris flows in the SW U.S. fueled by drought, wildfire, and rain
On January 24, the LA Times reported that residents in the Montecito, California, area received conflicting information regarding evacuation zones prior to the worst post-fire floods and debris flows (often called mudflows) in recent California history. This is troubling news, as 21 people perished in these flows with two still missing, 65 homes were destroyed and another 462 were damaged. Also troubling is the report, in the same article, that Santa Barbara County officials estimate only 15% of people in the mandatory evacuation zone actually heeded the warnings and evacuated.
Southern California is no stranger to post-fire debris flows; authorities anticipated post-fire debris flows from the Thomas Fire burned area. What they didn’t anticipate was the size, velocity, and travel distance of the debris flows, and the magnitude of destruction, which was surprising.
The Thomas Fire began on December 4, 2017, and grew to 281,893 acres in the next month, becoming the largest wildfire in state history. The steep drainages in the Santa Ynez Mountains above Montecito and Carpinteria were the last areas burned by the Thomas Fire. By January 5, 2018, officials were issuing warnings to the residents of Montecito that a large storm was approaching. They went a step further and presented maps of likely hazard zones that ultimately were similar to the areas impacted by post-fire debris flows (Joseph Sarna, LA Times reporter).
Post-fire debris flows are generated by runoff when rainfall intensities (the rate of rainfall) surpass a threshold. Thus, it is not the amount of rainfall received but rather how fast the rain falls that is important. Rainfall-intensities that one might expect in any given year (1-year recurrence interval) can be sufficient to generate post-fire debris flows in burned, denuded watersheds.
The USGS Landslides Hazards group assessed the burned area prior to the January 9th storm and estimated that a 15-minute rainfall-intensity of 24 mm/h (~0.25” rainfall in 15 minutes) would likely generate debris-flows in the watersheds above Montecito. NOAA Atlas 14 indicates that this design storm rainfall intensity has a return interval of less than one year. Rainfall intensities in the January 9th storm were much greater than this.
Dr. Luke McGuire (Univ. of Arizona Geosciences) analyzed rainfall intensities for several gauges from the Thomas Fire burn area (Figure 1) and found that the three gauges closest to Montecito had peak 15-minute rainfall-intensity rates of 70-80 mm/h (0.7”-0.8” in 15 minutes) – far above the threshold values. Dr. Francis Rengers of the USGS reported that the Montecito gauge recorded 0.5” in 5 minutes – an extreme precipitation event with an estimated recurrence interval of 200 years! Given this combination of intense precipitation on steep, recently burned, denuded watersheds, it is little wonder the debris flows were so large and destructive.
Arizona is subject to similar post-fire hazards. In June 2010, a residential area northeast of Flagstaff experienced extensive post-fire flooding from debris flows following the 15,000-acre Schultz Fire (Figure 2). The first debris-flow producing storm on July 20 resulted in extensive downstream flooding and one death. The peak 15-minute intensity for this storm was between a 10- and 25-year recurrence interval, well over the minimum threshold but much less extreme than the storm that hit Montecito on January 9th.
The Ecological Restoration Institute at Northern Arizona University estimated the full economic cost of the Schultz Fire was between $133 and $147 million (Combrink et al., 2013). Coconino County realized that they needed to prepare for future events. To help them with this, JE Fuller Hydrology & Geomorphology and AZGS conducted a study assessing potential post-wildfire hazards if fires were to occur around the San Francisco Peaks and Bill Williams Mountain (AZGS OFR-17-06). Currently, Coconino County and the City of Williams are actively working with several partners to reduce and mitigate potential post-wildfire hazards.
Combrink, T., Cothran, C., Fox, W., Peterson, J., and Snider, G., 2013, A full cost accounting of the 2010 Schultz Fire: Ecological Restoration Institute, Northern Arizona University, Flagstaff, AZ, 44 p.
Posted by Ann Youberg, 29 Jan. 2018