Tuesday, July 23, 2013

Assessing Yarnell post-fire flood and debris flow hazards

AZGS geologists Ann Youberg and Phil Pearthree have been working with state and federal colleagues around the Yarnell wildfire area to assess the potential for flooding and debris flows.   Burned over areas are more susceptible because monsoon rains are not slowed by vegetation.

The teams are examining areas most at risk, so rain gauges and automatic warning systems can be installed to warn residents downslope of potential risks.    When rainfall exceeds certain rates, floods  are more likely and in areas with debris-laden channels, debris flows can be initiated. [Right, Yarnell Hill fire area.  Credit, Arizona State Forester]

In addition, they have been in frequent contact with Yavapai Co., Arizona Division of Emergency Management (ADEM), and Arizona Dept. of Water Resources (ADWR) staff.  Our debris flow expert Ann Youberg was in the field with a team from the USGS Water Science Center to try to collect some ground LiDAR data in at least one watershed upstream from Yarnell.

SR 89 climbs up the west side of the Weaver Mountain - the Yarnell grade. Several small steep washes that cross the older, west-bound lane of this divided highway have clear evidence of past debris flows, including medium to large boulders and scoured chutes above the highway crossings. These watersheds were partially burned, but the burned parts are the steepest upper slopes. Extra runoff is expected, and that could cause some issues for the West-bound lane. If the area is unfortunate enough to experience a slightly out-of-the-ordinary storm (something like a 5-yr storm), then there is a reasonable chance that these drainages could have debris flows that would impact the highway. 

We're keeping our counterparts at ADOT and other agencies apprised of our findings.

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