When Flood Follows Fire

By Harrison Tasoff, UC Santa Barbara

The deluge Santa Barbara county received on January 9 occurred exactly five years after devastating debris flows smothered Montecito in 2018. While downtown Santa Barbara saw flooding as Mission Creek surged forth, the storm didn’t trigger similarly destructive landslides.

Still, with more rain in the forecast this winter, residents are justifiably concerned that the hills may come rushing down to the sea yet again, especially in areas still recovering from recent fires.

It’s difficult to determine when a burned area recovers enough that it’s no longer at heightened risk of landslides. “I will say that substantial hydrologic recovery occurs within two to five years following fire,” said Naomi Tague, an ecohydrology professor at UC Santa Barbara.

“It’s worth noting, though, that recovery times can vary substantially with how severe the fire was and with post-fire climate,” she continued. “Drought following a fire tends to slow recovery.” And complete recovery of vegetation can take more than a decade.

“Recent fire scars — such as the Alisal Fire scar (Oct 2021) — represent very dangerous conditions because the vegetation burned less than a year ago, followed by a really dry winter and spring,” explained Professor Dar Roberts, in the Department of Geography. “However, it does help that we received enough rain to get a healthy growth of annual vegetation prior to this set of storms.”

The Cave Fire was farther back, in November 2019, but was followed by poor rainfall from 2020 through 2022, so conditions there are likely fairly dangerous too. “The Thomas Fire scar is probably less dangerous,” Roberts continued, “but Montecito did receive a huge amount of rain, so even if it had not burned, it would still be a concern.”

Soil stability in a burn scar hinges on many factors, but it all comes down to one thing: how much material can be mobilized on the hillsides. “The less erodible the hillside is, the lower the potential for a mudslide,” said Assistant Professor Vamsi Ganti, who studies river morphology in the Department of Geography. Mud can act as a lubricant for rocks and boulders, initiating a landslide. And just after a fire, a hydrophobic layer can form causing water to rush off unprotected slopes.

“When the Montecito Debris Flow occurred, the conditions were about as bad as possible,” said Roberts. “A very high rain rate falling on a landscape striped of vegetation on recently burned soils that were likely hydrophobic.”

Vegetation plays a huge role in stabilizing sediment. For instance, plants protect the soil and reduce runoff by intercepting rainfall and helping water soak into soils. “More extreme fires typically strip away this vegetation, exposing bare soil to erosion by the elements,” Roberts said. “And over longer timeframes, fire can remove many of the roots that help hold the soil in place.”

The extent of regeneration after a fire has a large impact on soil stability. As a burn scar recovers, grasses and herbaceous plants can dissipate some of the force from rain drops, and a shrub will do an even better job. Shrubs and trees also have more extensive root systems, especially those that re-sprout after fires.

“That said, decades of research suggests that up to 10 years post-fire there’s still elevated risk for landslides and debris flows below burn scars,” explained ecosystems scientist Marc Mayes, at UCSB’s Earth Research Institute. As climate change progresses, extreme storms in close sequence to one another may occur more often. Under this regime, debris flows can happen even in areas without recent burns once soils are saturated.

The intensity and duration of the rainfall matters. If more rain falls faster soil erosion increases. “Think of the difference between using a watering can for gardening, or dumping all the water out of a 5 gallon bucket,” said postdoctoral researcher Paul Alessio, who studies hydrology and slope stability.

Indeed, strong storms can create hazards on their own. “Given the rain rates we have seen, even an unburned landscape could still produce landslides, rock falls and mudflows,” Roberts said. “There were quite a few of these in grasslands during the 1998 El Niño.”

In 2018, graduate students at the Bren School of Environmental Science & Management penned a report on the flood risks of the Mission Creek watershed. The authors found that “flood discharge associated with the 100-year storm is four times more likely after fire, and even small storms will flood areas of downtown Santa Barbara.”

These floods indeed came to pass on January 9. And there were landslides and debris-choked waterways throughout the region. Fortunately, measures like debris basins and debris flow barriers prevented major destruction.

Our monitoring systems are also improving. “I think an important, optimistic part to the story of these 2023 winter rains is how much more available real-time hydrological data is, even compared to 2018,” Mayes said. This includes information on stream flows, flooding, and debris basins. The data enables scientists and public officials to evaluate risks and make informed decisions promptly to prevent loss of life and property.

“Even with great strides since 2018, more investment is needed in real-time hydrologic monitoring infrastructure across more front-country watersheds,” Mayes added.

Santa Barbara County Flood Control now has a real-time data dashboard. Anyone can log on and see photos updated every 5-10 minutes of what’s going on at the debris basins protecting their communities downstream during storms or through their post-storm cleanup. “During these recent storms, I spoke with at least three family and friend groups whose decisions to evacuate were solidified by prompt emergency communication from Santa Barbara County Office of Emergency Management, and by being able to log onto a data dashboard and see real-time debris basin conditions from their kitchen tables,” Mayes said.

“But regardless of what danger may or may not be apparent on dashboards, it is critically important to take emergency officials’ warnings seriously,” he added. More information about disaster preparedness is available on the county website and the campus emergency website.


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  1. So here’s a quandary – if we stop with fire suppression so as to allow the forest to begin returning to its natural state, how to we deal with the subsequent debris flows? Do we only suppress fires that would foreseeably cause debris flows that could damage homes or property? A fire that doesn’t pose an immediate threat of burning down homes (one that could be allowed to burn), might be nevertheless close enough to create conditions for a debris flow down canyon that *would* be a danger to homes, farms, ranches, etc. I ask because I know some here are very anti-suppression. Interested to hear thoughts on how that would play out in these situations.

    • SAC – I believe the southern states have some of the largest forests in the US. Why is it we’re always hearing about the fires in the west and not so much in other parts of the US. It’s because, as you state in your comment, “some here are very anti-suppression.” People ‘gonna have to sacrifice some of those pretty flowers, trees and fish to maintain the forest properly.

    • CHIP – interesting points. How feasible is it, though, for Cito and SB to expand/upgrade their basins? Honestly, I don’t know. But, it seems like a good idea to balance the need for extra protection from debris flows with the need to let distant fires (relative to populated areas) burn out on their own.

    • Doulie, in a more natural state, with regular small intensity fires, there would actually be more trees and flowers because entire landscapes wouldn’t be completely denuded and all life killed above and into the soil from the raging infernos we’ve experienced – the trees wouldn’t be burned down, just the brush and litter on the ground. It also wouldn’t cause the same mudslide risk as hillsides wouldn’t be stripped of all vegetation and root structures.

    • Sac, I think you raise a very good question about how to manage the forest lands above Santa Barbara and montecito. Fire suppression policies cause Thomas fire type events which are a danger in their own right in addition to creating hazardous conditions that lead to flooding and debris flows for years to come. I would suggest a three pronged approach. 1: Improve access to the area and add more fire roads, use mechanical thinning and clearing in strategic areas to create fire breaks. 2: conduct planned burns on a rotating schedule. The planned burns will make unplanned fires much less likely to occur and much easier to manage when they do. In addition, the planned burns will limit the extent of any particular watershed that is vulnerable to erosion at any given time by ensuring vegetation remains in the majority of each watershed at all times. Finally, upgrade debris basins and waterways to suitable capacities. Carpinteria got buried in mud in a post-fire debris flow back in the 60s. They built the Santa Monica debris basin after that, and as a result carpinteria was protected from what would otherwise have been devastating flooding and debris flows after the Thomas fire. Santa Barbara and montecito to this day have refused to upgrade their debris basins to an adequate capacity to protect their residents.

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