Jesusita Fire Adds Relevance to Student Study

July 1, 2009


Blaze enables testing of computer-based procedure for assessing post-fire flood risks

Santa Barbara, Calif. – The Jesusita fire destroyed nearly 80 homes and burned more than 8,000 acres of Santa Barbara hillside in may , but a team of UC Santa Barbara graduate students may be able to find some good  in the devastation. The fire has given the students, all of whom received their master’s degrees at the Bren School of Environmental Science & Management on June 12, a chance to test a new synthesis of computer models designed to predict the risks of flooding and debris flows during the rainy season following a wildfire.

Comparisons between a burned watershed (top right) and an unburned one (top left) in the mountains behind Santa Barbara were the basis for a study in which students assessed the potential impacts of water, debris, and sediment flows in areas downslope of fire-affected terrain. As part of their field work, students examined a debris barrier (bottom left) constructed after the 2008 Gap Fire and hiked up a stream (bottom right) to see a sediment catchment barrier in a creek bed.

In spring 2008, the five students—Leslie Abramson, Milli Chennell, Erica Eisch, Alicia Glassco, and Thomas Holley and their advisor, Bren professor of hydrology and geomorphology Thomas Dunne—began a yearlong study of two analogous watersheds. One had been affected by the Gap Fire, while the other, the Mission Creek watershed, had not burned for decades. The group received important support from many individuals and agencies, particularly the U.S. Forest Service’s Burned Area Emergency Response (BAER) team and former Santa Barbara County Flood Control District employee Candice Constantine.

“We consulted with a variety of helpful local experts to build upon the existing knowledge and post-fire practices of the Santa Barbara County Flood Control and the U.S. Forest Service's Burned

Area Emergency Response (BAER) Team,” said Alicia Glassco.

Runoff and erosion rates rise dramatically in the rainy seasons following wildfire, increasing the risk of destructive floods, sediment accumulation, and debris flows. Watershed assessments are typically performed after a fire, but it’s difficult to quantify the risk of flooding and to take mitigating action when a fire occurs just before the rain begins. “In highly vulnerable areas, such as the Mission Creek watershed, pre-fire analysis of post-fire risk is warranted,” the team wrote in the final report of their study, titled Post-fire Sedimentation and Flood Risk Potential in the Mission Creek Watershed of Santa Barbara.

Less than a month after the work was completed and the group presented its findings to the public, the Jesusita fire blackened approximately 50 percent of the study site, the Mission Creek watershed. As destructive as the fire was, it means that the group could collect real-world data to test the validity of predictions from their computer-model synthesis and then adjust it to be more useful to city and county planners responsible for mitigating the risk of post-fire flows.

“We can now test our calculation,” said project manager Alicia Glassco. To do so, she explains, “We would basically use runoff data from stream gauges after a storm event and compare them to our modeling output based on the size of the storm. Another simple check would be to compare Burned Area Emergency Response (BAER) Team predictions for Mission Creek with our results.”

The group presented its findings to Santa Barbara County Flood Control just after the Jesusita fire, and their final project report has been sent to Mayor Marty Blum, the Santa Barbara City Council, and County Supervisor Salud Carbajal by way of contributing to discussions in which local government officials are deciding how best to prepare the burn area for the rainy season.

“The group’s report was interesting,” said Andrew Raaf, a Bren School alumnus who is a resource biologist at the Santa Barbara County Flood Control and Water Conservation District and invited the group to present their findings. “It’s one of many resources the agency is using as it considers how to address post-fire flood risks. Having been through several fires, and two in the past ten months, the engineers already have some things they do after a fire of this kind.”

The Group Project, which serves as the master’s thesis for Bren School students, involved using data from the watershed burned in the Gap Fire to create a synthesis of computer models to predict the risk from flooding, sediment accumulation, and debris flows. The group used field observations of the burned watershed, spatially explicit data on watershed characteristics, historic rainfall and runoff measurements, and accepted modeling techniques to estimate post-fire changes in hydrologic and sedimentary processes in the Mission Creek watershed.

By comparing two similar watersheds, the team was able to accomplish three objectives: They refined predictions of how a fire in the upper Mission Creek watershed would affect hydrologic and sedimentary processes in the following rainy season, they identified the post-fire risk of flooding to downstream communities, and they provided information that local government agencies can and are using to assess current mitigation strategies and emergency response to potential flooding.

In the wake of the Jesusita fire, the group and public agencies making use of the study could use the 2009-2010 rainy season to get a sense of how well they did.