I was just looking at the national radar for the rain storm which is pummeling those of us in greater Appalachia right now, and I noticed off in the far edge of the map that they also have a big system in NoCal today:
I know that they had horrible mudslides down in the Santa Barbara area when a rain storm moved in after last year’s fires, and I got to wondering whether Napa & Sonoma were at risk of anything similar to that
Here’s a pretty terrifying Youtube from Montecito [just south of Santa Barbara] after their fire-scorched hillsides were visited by a rain storm:
Landsides are fairly common during big winter storms up here. The ladscape between NoCal and SoCal is very dfferent though. Even though we had fires up here the land was and remains to be forested and the tree roots tend tp hold the hillsides together. I think the danger of slides is much higher in hilly residential areas since the land has been cleared and excessive drainage cannot be controlled. I’ll be driving through the Tubbs fire zone on my way to Calistoga within the hour. The front has passed so the roads should be fine. I’ll keep my eyes peeled for any signs of slides.
More worrisome in these parts are the rocks that loosen and “slide” into the roadway. Silverado Trail is where this happens between St. Helena and Calistoga: it is a very narrow, winding road carved into the hillside in many areas, and what is worse, the very narrow bike lane that is difficult on a good day is the repository for those rocks. Bikers beware.
Right after the Santa Barbara mudslides, I heard one of the major reasons why we didn’t have mudslides up north like they did down south, is because a lot of the grasses and other wild plants that help keep the hillsides stable had started to grow back already up north. The fires in Napa/Sonoma/Mendocino were the week of October 8. The fires in southern California were in December, less than a month before the big rainstorm in early January. Nothing had grown back down south. Plus what Brian was talking about, the differences between rural hillsides and residential ones.
I just looked at the rainfall totals from today’s storm. Some places in the Santa Lucia Mountains got 7-8 inches of rain in the past 24 hours. I hope all that water doesn’t run down to Big Sur like last year.
One of the other major difference, from what I’ve heard from a few sources, is that when the mudslides started down here, pipes that led to a few reservoirs were fractured, releasing a whole lot more water to add to the rainfall. This certainly changed the amount of run off substantially . . .
I am a geotechnical engineer and did some reconnaissance with GEER after the Montecito debris flow. One of the factors that made this debris flow so impactful was the soil’s water repellent (hydrophobic) nature post fire. A gas released during the combustion of vegetation penetrates the soil during the fire, and then post-fire condenses to form a waxy, water repellent coating on the surface.
This means that during the rainfall event, instead of rainfall penetrating the ground, there was a huge amount of overland flow due to this waxy coating. All of this water then started flowing downhill together at high velocity, allowing it to pick up a huge amount of material, including 6ft diameter boulders. So these weren’t deep-seated failtures where half a hillside crumbles away due to instability, but an overland flow of water that then rips up material from the ground surface as it flows.
There was a developed draining system in Montecito (including multiple pre-dug debris flow basins, meant to catch this material before it made it to the infrastructure). However the volume of the flow was so large that it overtopped these channels, and the velocity was so high that it wasn’t contained when the channels turned. There were also many small bridges across the channels that ended up serving as chokepoints for the flow, causing it to overtop the channel and impact the nearby homes.
I didn’t see any broken water lines, but by the time I was there the utility companies had already been doing a lot of recovery work.
