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):
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 because around 3:15 in the morning of January 9th, simultaneous debris out of multiple canyons 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 3 year old girl and a 17 year old boy—are still interred in at places unknown in the fresh fanglomerate, sought
The town was evacuated after the disaster, and is gradually being repopulated. I’m guessing the audience was comprised almost entirely of evacuees and people who knew victims or had friends who knew victims. (I’m in that last group.)
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 flows converged on the highway and used it as a catch basin before overtopping it 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, the town’s Main Street, is open for employees of the stores there, but nothing is open for customers yet, since water service hasn’t been restored. (Or so I heard. Corrections welcome.) Opening Upper Village will take longer.
Here is a map with final damage assessments. I’ve augmented it here with labels for the canyons and creeks (with one exception for a parallel creek west of Toro Canyon Creek—a correction is welcome there too):
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 also took some damage, though less than Montecito.
Ed Keller put up this slide to explain what conditions 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. They are also not mudslides, so we should stop calling them that.
Debris flows tend to require sloped soils left bare and hydrophobic—resistant to rain—after a recent wildfire has burned off the chaparral that normally (as geologists say) “hairs over” the landscape. Though wildfires are common, and chaparral is adapted to them, rainfalls as intense as this one are not. It was, said a number of locals interviewed for TV, “biblical.”
It’s hard to generalize about the combination of factors required, but Ed has worked hard to do that, and this slide is one way of illustrating how debris flows happen eventually:
From bottom to top, here’s what it says:
- Fires happen almost regularly, spreading most widely where chaparral has matured to become abundant fuel, as the firefighters like to call it.
- Flood events are more random, given the relative rarity of rain and even more rare rains of “biblical” volume. But they do happen.
- 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.
- The sediment yield in a flood (F) is maximum when a debris flow (DF) occurs.
- 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.
So, did these flows clear out the canyon floors? We (meaning our geologists, hydrologists and other specialists) won’t know until they get 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 fresh muck:
See the white lines fanning back into the mountains through the canyons (Cold Spring, San Ysidro, Romero, Toro) above Montecito? Ed Keller explained that these appear to be the beds of creeks feeding into those canyons. Here is his slide showing Cold Spring Creek before and after the event:
By Ed’s graphic above, one might say that there isn’t much sediment left to yield there, and that the valley floor has returned to stability.
Does this mean Montecito has had its 200-year event, and a couple more centuries need to pass before we have another one of these.
Ed and Larry caution against conclusions beyond recognizing that nearly all of Montecito and Santa Barbara are their beautiful selves by grace of debris flows such as these. If your property is graced with boulders, Ed said, a debris flow put them there, and not long ago in geologic time.
For an example of that, here are some boulders we quarried out of our yard more than a decade ago, when we were building our house into a hillside:
This is deep in the heart of Santa Barbara.
These boulders were deposited by one or more debris flows that happened within the last few dozen thousand years.
The matrix mud we now call soil 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 hard parts of the Santa Ynez mountains and are so similar that only the trained can tell them apart. All four of those geological formations were established long after dinosaurs vanished, accumulated originally as sediments, mostly on ocean floors, probably not far from the equator.
UCSB has a terrific animation of how the transverse (east-west) Santa Ynez Mountains came to be where they are. Here are three frames:As 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 was the future Point Conception. The future Santa Barbara was west of the future Newport Beach. When the Malibu end got jammed at Los Angeles, the Point Conception end swept out, clockwise, starting at 5 o’clock and stopping (though it hasn’t) at 9:30. It was, and remains, quite a show.
I’ll be adding to this after I get a break later today, and again after I visit as much as I can of Montecito tomorrow. Meanwhile, I hope this proves useful. Again, corrections and improvements are invited.