Fact-Checking the Forecast
The standing room-only crowd that filled Montecito Union School’s meeting hall on the evening of December 5 wanted one thing above all else: good news. And by and large, that’s what they got from the group of city, state, and federal officials who presented their latest findings on both the winter weather forecast and the state of the watershed in the canyons and debris corridors above town.
Kevin Cooper of the Montecito Fire Protection District estimated that 80 percent of the groundcover in the upper areas of the watershed and 90 percent of the downslope cover that was lost to the 2017 Thomas Fire, which preceded the lethal debris flows of January 9, 2018 has already grown back. “That’ll go a long way to stop another debris flow,” he said. “The soil has a few more years to recover from before the fire, but we’ve come a long way and are looking at a very different scenario.”
Eric Boldt, a warning coordination meteorologist with the National Weather Service, gave a similarly upbeat update about the weather. “The outlook is that there is a seventy percent chance of a neutral weather season, meaning it is often going to be a drier than usual season,” Boldt stated. “The three-month outlook is dry, drier than normal.”
Pointing to the fact that debris nets had been installed in key debris corridors and that the debris basins were unaffected by recent rains, Montecito’s fire chief Kevin Taylor added to the sense of measured reassurance. “The risk of a high-velocity debris flow is much less,” he declared. “So far, the watershed is absorbing material and the creeks are clear.”
Following the presentation – and a brief moment of silence for the 23 victims of the debris flow – the crowd dispersed after only a few questions. It was a scene reminiscent of the film Jaws, when the mayor tells the townspeople that it’s safe to go back in the water. Clearly, the crowd had heard both what they wanted to hear, and what our public officials wanted them to hear. With that in mind, the Montecito Journal figured it’d be worthwhile to talk to a few more experts and see how well the official story compares to a deeper dive into both the winter weather and the state of the watershed.
First off, we spoke to Tom Dunne, a geomorphologist and hydrologist who has been a professor at UCSB’s Bren School of Environmental Science & Management and Department of Earth Science since 1995. Dunne, who says he last hiked the area in August at the end of the summer growing season, took issue with the notion that there is really 80 to 90 percent ground cover anywhere in the mountains above Montecito. “Before the fire there wasn’t eighty percent ground cover,” he said. Because chaparral vegetation doesn’t have a very dense ground cover, Dunne reasoned that officials must be talking about canopy cover – the leaves in the bushes or trees that rain drops off on to the ground. “To wave around a number like ninety percent ground cover certainly defies my observation in August,” he said. “My observation is that is a wildly unrepresentative estimate of ground cover.”
According to Dunne, ground cover with actual roots growing at ground level, is “by far the most effective” form of erosion control. During the Thomas Fire, Dunne pointed out, the topsoil burned completely to ash at an average depth of three to four centimeters, as compared to the recent Camp Fire, where the soil only burned one centimeter deep. “I am not an alarmist, but there is more material up there where all the roots were burned out and below that there is fine soil where all the roots are intact. It will take time for the roots to grow to the surface. Meanwhile that loose topsoil can erode, water can flow over it and scour it from the surface, so even with gentle rain, it can be saturated and collapse.”
Dunne suggested we speak with Jonathan Schwartz, a geologist with the U.S. Forest Service, who along with Cooper, hiked the watershed this year and provided the ground cover estimates. “I wouldn’t want to be one who argues with Tom,” Schwartz said, chuckling. “But I would say that both are important – the root system and the canopy ground cover. The root systems are holding the soil particles together and preventing it from washing down. But the ground cover is important because it is providing protection from the direct impact of raindrops on the soil.”
Schwartz has been studying the Montecito debris flow event for two years and one of the most interesting discoveries he’s made involves not what occurred on the burn area, but rather what did not occur in the unburned patches. “In the middle of these watersheds, you had a bunch of islands of unburned chaparral,” he explained. “I uncovered an unburned patch of vegetation that experienced the exact same storm following the fire, but it looked as if not one drop of rain fell.” All the organic matter on top of the soil was completely intact. “You wouldn’t know a rainstorm had occurred there, much less a 200-year event,” he said. “Not one grain of sand moved.”
According to Schwartz, some areas impacted by the debris flow are now covered with thick vegetation. “To crawl into it, I had to crawl on my hands and knees,” he recalled. “It was like crawling into a jungle.” And Schwartz stands by the ground cover estimate that Cooper provided to the public on December 5. “Me and Kevin last went out a couple of months ago and we determined the ground cover was eighty percent. It might not be the same type of ground cover we had before the fire, but it’s substantial and a major element that reduces the potential for debris flows.”
It’s impossible to overstate the importance of a layer of loose fine particles at ground level to fuel fatal debris flows, Schwartz continued. “People usually think of the big boulders as the destructive power of debris flows,” he said. “But these boulders do not behave the way they do without an incredible number of particles to make these boulders float. Without the fine particles, you might have a fluid that might have boulders, but they will roll down the creek and won’t have the destructive energy a debris flow would have. It usually tends to behave, to stay in the creek bed.”
That said, Schwartz urges residents to continue to monitor the weather and take precautions. “We don’t want to create a false sense of security,” he said. “The public needs to pay attention to warnings from the National Weather Service (NWS) and so on. I’m just saying that, based on what I’ve seen in the top of the watersheds, we do have a very good ground cover and revegetation.”
Speaking of the weather forecast, Boldt’s prediction of a neutral weather system for winter was confirmed when the NWS made that news official in an agency press release a week later. Nonetheless, we contacted Jonathan J. Gourley, a research hydrologist with the National Oceanic and Atmospheric Administration (NOAA) and the National Severe Storms Laboratory in Norman, Oklahoma, to ask him about recent developments in Montecito’s weather readiness. Gourley uses computer modeling to estimate rainfall with a software program he developed with The Partnership for Resilient Communities (TPRC) in Montecito. The problem with current weather forecasting, he said, is that too much of it relies on local rain gauges, which means by the time you are able to calculate the rate of rainfall, the event is already underway.
“In collaboration with the Partnership and local community, we developed software to take rainfall rates and controlled measurements to predict flash floods with weather radar,” Gourley explained. “This software enables you to compute rainfall rates where storms are still over the ocean. Once we ran it in playback mode, we saw rainfall rates that were well above criteria for debris flows. So we can use that information to say if this holds and hits landfall, this rain will cause problems.”
According to Gourley, while there may be no way to predict if another rare storm approaching Montecito will have the power to unleash a lethal debris flow, the town will at least know a bit more ahead of time if one is coming. “I have to give credit to the folks who reached out to us,” he said of the TPRC. “It hadn’t occurred to us that there is some fairly low-hanging fruit to implement that was pretty straightforward to do. When it was brought to our attention that there was a problem and a need, we were actually able to do something, and it only took a few weeks.”