Sunday, December 9, 2007

hazard maps

by Guy McCarthy

REDLANDS, Nov. 14 -- Better safe than sorry.
Even though another dry winter is forecast, authorities are preparing for rains that could trigger post-fire debris flows or flash floods in communities left vulnerable in the wake of last month's intense wildfires.
If hard rains do fall, communities in and below areas burned in the Santiago Fire in Orange County are among the most at-risk in Southern California, federal scientists said today.
Sue Cannon, a U.S. Geological Survey landslide specialist, is "furiously working on post-fire maps for the Santiago Fire," said Jim Bowers, a USGS hydrologist based in San Bernardino County. "Santiago is our number one priority for our debris flow folks."
Four years ago, Cannon worked with FEMA and the Environmental Systems Research Institute of Redlands (ESRI) to produce preliminary post-fire debris flow hazard maps after the Old and Grand Prix fires in October 2003. Sixteen people died in boulder-laden flash floods near the mouths of canyons at the base of the San Bernardino mountains on Christmas Day 2003.
Cannon and other USGS scientists are leading efforts to produce 25 hazard assessment maps addressing concerns for all the burned areas in Southern California, said USGS spokeswoman Catherine Puckett. More than 750 square miles burned in the fires last month, forcing thousands of residents from their homes.


The Santiago Fire burned areas are of particular concern because hundreds of residents live in canyons that are partly denuded of vegetation.
"This is as bad as they've seen, as far as potential for erosion," Orange County Fire Chief Chip Prather said Wednesday of field reports from his staff, who are working closely with Burned Area Emergency Response team scientists. "Fire damage isn't unusual, but some of these canyons haven't burned in our recorded history. You could expect to see a lot of run-off."
Federal Emergency Management Agency staff are part of the current mapping project, Puckett said.
The maps Cannon, FEMA and ESRI produced in San Bernardino County in 2003 gave preliminary assessments of debris-flow probability, and peak discharge estimates that could be generated by debris flows in watersheds that burned in the Old and Grand Prix fires. She projected how watersheds might react to one-hour duration storms of 25-year, 10-year, and 2-year rain intensity.
Cannon could not be reached for comment Wednesday.
"She's working on maps that will come out in a few days, or next week," said Leslie Gordon, a USGS spokeswoman based in Menlo Park.


The forecast for this winter is dry, said Bill Patzert, a climatologist at NASA's Jet Propulsion Lab in Pasadena. But it is impossible to rule out unexpected, intense rains that could unleash debris-laden flash floods, Patzert said.
"The consensus here is for below average precipitation," Patzert said. "That's good news for the people in the burned areas, but bad news for people in places like Idyllwild. We could still have more fires. It's a double-edged sword. There might be another Santa Ana over Thanksgiving.''
Even if nothing else burns again for the next five years, it's going to take time for more than 750 square miles of watersheds to recover from the most recent wildfires. More than 2,000 homes burned and the death toll directly linked to the fires climbed to 10 yesterday.
And whether it rains this winter or not, canyon communities in the Cleveland National Forest still need protection that includes weather monitoring devices, Prather said. He was gratified Wednesday to hear about Cannon's mapping project, as well as plans to install at least one camera on Santiago Creek.
"We already have a creek monitor gauge on Santiago Creek," Bowers said. "We'll install an online web camera there, capable of taking a picture a second. We are working hard on this."
Bowers worked on a prototype post-fire debris flow warning system touted by USGS and the National Oceanic and Atmospheric Administration in September 2005. The prototype system focused in part on the Grand Prix and Old Fire burned areas.
Prather said he is seeking information from communities that have dealt with post-fire debris flows in recent years. He said he wants to learn more about planning and potential clean-up if debris flows do occur.



The following recommendations are verbatim from Cannon's 2003 report, "Emergency Assessment of Debris-Flow Hazards from Basins Burned by the Grand Prix and Old Fires of 2003, Southern California."
-- It is imperative to insure that people occupying businesses, homes, and recreational facilities downstream of the basins identified as the most hazardous are informed of the potential dangers from debris flows and flooding.
-- Warning must be given even for those  basins with mitigation structures at their mouths in the event that the structures are not  adequate to contain potential debris-flow events. We further recommend site-specific debris-flow hazard assessments be performed above structures and facilities identified as being at  risk and that could be impacted by flows from basins smaller than those evaluated here. In addition, this assessment is specific to post-fire debris-flow activity; further assessment of potential hazards posed by flash floods is necessary.
-- And last, we highly recommend the  establishment of an early-warning system for both flash floods and debris flows. Such a system should consist of an extensive reporting rain gage and stream gage network coupled with National Weather Service weather forecasts. Any early-warning system should be coordinated with existing county and flood district facilities.

Links to NOAA/USGS post-fire debris flow warning system information:

Link to Cannon's 2003 post-fire debris flow hazard report:

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