Debris flow assessment above Flagstaff

AZGS geologist Ann Youberg (right) is in Flagstaff, mapping and assessing the debris flows that accompanied the July 20 flood that damage so many homes in the Timberline and Doney Park subdivisions on the east side of the city. The floods and debris flows were a result of the denuded slopes from the recent Schultz fire.

Ann is working with Dan Neary and his team from the US Forest Service research lab based at NAU. I spent the afternoon in the field with Ann and Karen (USFS) and was astounded at the size of the debris flows on the mid-slopes (below the steep slopes of the upper mountain and above the alluvial fan where people live). [right, Ann Youberg points out the deposition from one debris flow triggered by flooding on July 20]

It's expected that it will take 2-3 years for vegetation to be restored on the upper slopes and naturally mitigate flooding and debris flows. In the meantime, the amount of sediment in channels and swales has the potential to be remobilized into lower channels during heavy rains, affecting how runoff is channeled down the alluvial fan where the residences are.