"Arizona communities in the urban-wildland interface urgently need building and flood ordinances that anticipate wildfires and potential post-fire events." This is the conclusion of a report released today by AZGS following the Schultz fire near Flagstaff that led to damaging floods this past summer.
Residents living downslope of the June fire did not appreciate the nature of sheet flow and alluvial fan flooding. Neither did local government. Most assumed that because they are relatively high on the alluvial fan they were immune to flooding. But sheet floods sweep across slopes and a wash that may not have carried runoff for decades suddenly becomes the main channel.
The article is published in the Winter edition of our online news magazine Arizona Geology. Authors Ann Youberg (AZGS), Karen Koestner, and Dan Neary (both with the USFS Rocky Mountain Research Station in Flagstaff) also warn that:
More large floods may occur within the next few years. Future flooding depends on future rainfall amount and intensity and the recovery of vegetation on the mountain slopes, and is impossible to predict in the long-term. Until robust vegetative cover returns to hillslopes, floods over the next few years will likely carry a fair amount of sediment. With the natural re-growth of vegetation, and with the mitigation efforts of the Coconino National Forest and Coconino County, flooding should diminish over the next several years to near pre-fire levels.The article links to 50+ photos the flood and debris flow hazards and a video of the runoff that flooded homes downslope.
Our goal is to provide education to developers, home buyers, and local officials on the nature and scope of these geologic hazards. It is in everyone's best long-term interests to recognize where such hazards exist and what steps can be taken to prevent or mitigate them. Otherwise, more communities are destined to face the same calamities as those down slope from the Schultz fire did.