Dealing with alluvial fan flooding and debris flows in Flagstaff

There is more debris coming off the area burned by last summer's Schultz fire than expected, and "This debris threatens to fill in many channels that engineers could design to push floodwaters around the communities," according to one of several stories in today's [Flagstaff] Daily Sun newspaper by Cyndi Cole. [right, AZGS geologist Ann Youberg points out the thickness of a debris flow above the Timberline neighborhood, triggered by monsoon rains last year, following the Schultz fire. Officials fear the volume of debris on the hillsides could overwhelm proposed flood control structures and create additional flooding. My photo]

County and federal officials are struggling with ways to mitigate flooding and debris flows but there are no easy or cheap answers.

The problems of alluvial fan flooding are described in a companion story. The Flagstaff communities of Timberline and Doney Park were surprised last year when hundreds of homes on the slopes below the Schultz fire were hit by flood waters. Flooding is going to be a problem for perhaps another 3-5 years until the upper slopes are revegetated and the soils can absorb rainfall. Meanwhile, the nature of alluvial fans causes runoff to jump from channel to channel, from one wash to another, with any given storm. Small channels can become the main stream with no notice.

Meanwhile, prison inmates are filling tens of thousands of sandbags to assist homeowners.