Just before it started, the geology meeting at the Santa Barbara Central Library on Thursday looked like this from the front of the room (where I also tweeted the same pano):

Our speakers were geology professor Ed Keller of UCSB and Engineering Geologist Larry Gurrola, who also works and studies with Ed. That’s Ed in the shot below.

As a geology freak, I know how easily terms like “debris flow,” “fanglomerate” and “alluvial fan” can clear a room. But this gig was SRO. That’s because around 3:15 in the morning of January 9th, debris flowed out of canyons and deposited fresh fanglomerate across the alluvial fan that comprises most of Montecito, destroying (by my count on the map below) 178 buildings, damaging more than twice that many, and killing 23 people. Two of those—a 2 year old girl and a 17 year old boy—are still interred in the fresh fanglomerate and sought by cadaver dogs.* The whole thing is beyond sad and awful.

The town was evacuated after the disaster so rescue and recovery work could proceed without interference, and infrastructure could be found and repaired: a job that required removing twenty thousand truckloads of mud and rocks. That work continues while evacuation orders are gradually lifted, allowing the town to repopulate itself to the very limited degree it can.

I talked today with a friend whose business is cleaning houses. Besides grieving the dead, some of whom were friends or customers, she reports that the cleaning work is some of the most difficult she has ever faced, even in homes that were spared the mud and rocks. Refrigerators and freezers, sitting closed and without electricity for weeks, reek of death and rot. Other customers won’t be back because their houses are gone.

Highway 101, one of just two freeways connecting Northern and Southern California, runs through town near the coast and more than two miles from the mountain front. Three debris flows converged on the highway and used it as a catch basin, filling its deep parts to the height of at least one bridge before spilling over its far side and continuing to the edge of the sea. It took two weeks of constant excavation and repair work before traffic could move again. Most exits remain closed. Coast Village Road, Montecito’s Main Street, is open for employees of stores there, but little is open for customers yet, since infrastructural graces such as water are not fully restored. (I saw the Honor Bar operating with its own water tank, and a water truck nearby.) Opening Upper Village will take longer. Some landmark institutions, such as San Ysidro Ranch and La Casa Santa Maria, will take years to restore. (From what I gather, San Ysidro Ranch, arguably the nicest hotel in the world, was nearly destroyed. Its website thank firefighters for salvation from the Thomas Fire. But nothing, I gather, could have save it from the huge debris flow wiped out nearly everything on the flanks of San Ysidro Creek. (All the top red dots along San Ysidro Creek in the map below mark lost buildings at the Ranch.)

Here is a map with final damage assessments. I’ve augmented it with labels for the canyons and creeks (with one exception: a parallel creek west of Toro Canyon Creek):

Click on the map for a closer view, or click here to view the original. On that one you can click on every dot and read details about it.

I should pause to note that Montecito is no ordinary town. Demographically, it’s Beverly Hills draped over a prettier landscape and attractive to people who would rather not live in Beverly Hills. (In fact the number of notable persons Wikipedia lists for Montecito outnumbers those it lists for Beverly Hills by a score of 77 to 71.) Culturally, it’s a village. Last Monday in The New Yorker, one of those notable villagers, T.Coraghessan Boyle, unpacked some other differences:

I moved here twenty-five years ago, attracted by the natural beauty and semirural ambience, the short walk to the beach and the Lower Village, and the enveloping views of the Santa Ynez Mountains, which rise abruptly from the coastal plain to hold the community in a stony embrace. We have no sidewalks here, if you except the business districts of the Upper and Lower Villages—if we want sidewalks, we can take the five-minute drive into Santa Barbara or, more ambitiously, fight traffic all the way down the coast to Los Angeles. But we don’t want sidewalks. We want nature, we want dirt, trees, flowers, the chaparral that did its best to green the slopes and declivities of the mountains until last month, when the biggest wildfire in California history reduced it all to ash.

Fire is a prerequisite for debris flows, our geologists explained. So is unusually heavy rain in a steep mountain watershed. There are five named canyons, each its own watershed, above Montecito, as we see on the map above. There are more to the east, above Summerland and Carpinteria, the next two towns down the coast. Those towns also took some damage, though less than Montecito.

Ed Keller put up this slide to explain conditions that trigger debris flows, and how they work:

Ed and Larry were emphatic about this: debris flows are not landslides, nor do many start that way (though one did in Rattlesnake Canyon 1100 years ago). They are also not mudslides, so we should stop calling them that. (Though we won’t.)

Debris flows require sloped soils left bare and hydrophobic—resistant to water—after a recent wildfire has burned off the chaparral that normally (as geologists say) “hairs over” the landscape. For a good look at what soil surfaces look like, and are likely to respond to rain, look at the smooth slopes on the uphill side of 101 east of La Conchita. Notice how the surface is not only a smooth brown or gray, but has a crust on it. In a way, the soil surface has turned to glass. That’s why water runs off of it so rapidly.

Wildfires are common, and chaparral is adapted to them, becoming fuel for the next fire as it regenerates and matures. But rainfalls as intense as this one are not common. In just five minutes alone, more than half an inch of rain fell in the steep and funnel-like watersheds above Montecito. This happens about once every few hundred years, or about as often as a tsunami.

It’s hard to generalize about the combination of factors required, but Ed has worked hard to do that, and this slide of his is one way of illustrating how debris flows happen eventually in places like Montecito and Santa Barbara:

From bottom to top, here’s what it says:

1. Fires happen almost regularly, spreading most widely where chaparral has matured to become abundant fuel, as the firefighters like to call it.
2. Flood events are more random, given the relative rarity of rain and even more rare rains of “biblical” volume. But they do happen.
3. Stream beds in the floors of canyons accumulate rocks and boulders that roll down the gradually eroding slopes over time. The depth of these is expressed as basin instablity. Debris flows clear out the rocks and boulders when a big flood event comes right after a fire and basin becomes stable (relatively rock-free) again.
4. The sediment yield in a flood (F) is maximum when a debris flow (DF) occurs.
5. Debris flows tend to happen once every few hundred years. And you’re not going to get the big ones if you don’t have the canyon stream bed full of rocks and boulders.

About this set of debris flows in particular:

1. Destruction down Oak Creek wasn’t as bad as on Montecito, San Ysidro, Buena Vista and Romero Creeks because the canyon feeding it is smaller.
2. When debris flows hit an obstruction, such as a bridge, they seek out a new bed to flow on. This is one of the actions that creates an alluvial fan. From the map it appears something like that happened—
   1. Where the flow widened when it hit Olive Mill Road, fanning east of Olive Mill to destroy all three blocks between Olive Mill and Santa Elena Lane before taking the Olive Mill bridge across 101 and down to the Biltmore while also helping other flows fill 101 as well. (See Mac’s comment below, and his link to a top map.)
   2. In the area between Buena Vista Creek and its East Fork, which come off different watersheds
   3. Where a debris flow forked south of Mountain Drive after destroying San Ysidro Ranch, continuing down both Randall and El Bosque Roads.

For those who caught (or are about to catch) Ellen’s Facetime with Oprah visiting neighbors, that happened among the red dots at the bottom end of the upper destruction area along San Ysidro Creek, just south of East Valley Road. Oprah’s own place is in the green area beside it on the left, looking a bit like Versailles. (Credit where due, though: Oprah’s was a good and compassionate report.)

Big question: did these debris flows clear out the canyon floors? We (meaning our geologists, sedimentologists, hydrologists and other specialists) won’t know until they trek back into the canyons to see how it all looks. Meanwhile, we do have clues. For example, here are after-and-before photos of Montecito, shot from space. And here is my close-up of the latter, shot one day after the event, when everything was still bare streambeds in the mountains and fresh muck in town:

See the white lines fanning back into the mountains through the canyons (Cold Spring, San Ysidro, Romero, Toro) above Montecito? Ed explained that these appear to be the washed out beds of creeks feeding into those canyons. Here is his slide showing Cold Spring Creek before and after the event:

Looking back at Ed’s basin threshold graphic above, one might say that there isn’t much sediment left for stream beds to yield, and that those in the floors of the canyons have returned to stability, meaning there’s little debris left to flow.

But that photo was of just one spot. There are many miles of creek beds to examine back in those canyons.

Still, one might hope that Montecito has now had its required 200-year event, and a couple more centuries will pass before we have another one.

Ed and Larry caution against such conclusions, emphasizing that most of Montecito’s and Santa Barbara’s inhabited parts gain their existence, beauty or both by grace of debris flows. If your property features boulders, Ed said, a debris flow put them there, and did that not long ago in geologic time.

For an example of boulders as landscape features, here are some we quarried out of our yard more than a decade ago, when we were building a house dug into a hillside:

This is deep in the heart of Santa Barbara.

The matrix mud we now call soil here is likely a mix of Juncal and Cozy Dell shale, Ed explained. Both are poorly lithified silt and erode easily. The boulders are a mix of Matilija and Coldwater sandstone, which comprise the hardest and most vertical parts of the Santa Ynez mountains. The two are so similar that only a trained eye can tell them apart.

All four of those geological formations were established long after dinosaurs vanished. All also accumulated originally as sediments, mostly on ocean floors, probably not far from the equator.

To illustrate one chapter in the story of how those rocks and sediments got here, UCSB has a terrific animation of how the transverse (east-west) Santa Ynez Mountains came to be where they are. Here are three frames in that movie:

What it shows is how, when the Pacific Plate was grinding its way northwest about eighteen million years ago, a hunk of that plate about a hundred miles long and the shape of a bread loaf broke off. At the top end was the future Malibu hills and at the bottom end was the future Point Conception, then situated south of what’s now Tijuana. The future Santa Barbara was west of the future Newport Beach. Then, when the Malibu end of this loaf got jammed at the future Los Angeles, the bottom end of the loaf swept out, clockwise and intact. At the start it was pointing at 5 o’clock and at the end (which isn’t), it pointed at 9:00. This was, and remains, a sideshow off the main event: the continuing crash of the Pacific Plate and the North American one.

Here is an image that helps, from that same link:

Find more geology, with lots of links, in Making sense of what happened to Montecito. I put that post up on the 15th and have been updating it since then. It’s the most popular post in the history of this blog, which I started in 2007. There are also 58 comments, so far.

I’ll be adding more to this post after I visit as much as I can of Montecito (exclusion zones permitting). Meanwhile, I hope this proves useful. Again, corrections and improvements are invited.

30 January

• USGS Geologists Join Efforts in Montecito to Assess Debris-Flow Aftermath.
• The Independent has a comprehensive and frequently updated Montecito Mudslides Disaster Assessment Map.
• Noozhawk reports Caltrans, County Need to Replace Multiple Montecito Bridges After Storm Damage
• Felicia S. blogs The Silence of Epiphany: An Unexpected Lesson in Geology.
• CNN Report: Her parents died in the mudslide. A stranger brought her mom’s earrings back to her. The Facebook page in the report is Montecito Lost and Found.

6 April, 2020
*I was told later, by a rescue worker who was on the case, that it was possible that both victims’ bodies had washed all the way to the ocean, and thus will never be found.

In this Edhat story, Ed Keller visits a recently found prior debris flow. An excerpt:

The mud and boulders from a prehistoric debris flow, the second-to-last major flow in Montecito, have been discovered by a UCSB geologist at the Bonnymede condominiums and Hammond’s Meadow, just east of the Coral Casino.

The flow may have occurred between 1,000 and 2,000 years ago, said Ed Keller, a professor of earth science at the university. He’s calling it the “penultimate event.” It came down a channel of Montecito Creek and was likely larger on that creek than during the disaster of Jan. 9, 2018, Keller said. Of 23 people who perished on Jan. 9, 17 died along Montecito Creek.

The long interval between the two events means that the probability of another catastrophic debris flow occurring in Montecito in the next 1,000 years is very low, Keller said.

“It’s reassuring,” he said, “They’re still pretty rare events, if you consider you need a wildfire first and then an intense rainfall. But smaller debris flows could occur, and you could still get a big flash flood. If people are given a warning to evacuate, they should heed it.”