Our Rainy Season Isn’t Over Yet, And Coastal Bluffs Will Most Likely Keep Collapsing
It's a striking sight: Four multi-family homes sitting very, very close to where a hillside started collapsing down to the beach this Wednesday. Beyond the dramatic images, the collapse renewed longstanding concerns about coastal erosion.
Those buildings — no longer safe to occupy — have now been evacuated and red tagged.
Though the collapse is still being assessed by experts, landslides like it aren’t all that rare, especially along our coast. It’s just that this year, the problem is being exacerbated by one of the wettest winters we’ve had in some time.
With another atmospheric river slated to arrive next week, we thought we’d dive into the science of collapsing bluffs so that you can better understand the risk.
Why rain can make collapses more likely
There are countless factors behind why any individual bluff along the coast might collapse.
To determine why, experts have to consider things like the slope, drainage, the weight of the community that’s been built on top of it, as well as soil type, which can range from very hard to very soft, all within a short distance.
“The rocks in the southern half of the state tend to be a little more erodible than the northern part of the state, which is a lot more granite,” said Patrick Barnard, research geologist with the U.S. Geological Survey.
Stresses within the soil work to hold everything in place. When it’s dry, covered with vegetation and filled with roots, it can stick together quite well.
Saturate that same soil with water during an exceptionally rainy year like this one and that strength can be compromised.
“When you get tons and tons of rain, it’s more likely that these sorts of events are going to happen,” said Barnard.
When heavy rainfall stops, the risk of deep-seated landslides begins. These can happen weeks/months after heavy rains. CGS monitors this hazard by reviewing historical slide activity & rain data to identify susceptible areas. https://t.co/fbQZAfDJaO @calconservation @Cal_OES 1/4 pic.twitter.com/v9Hhmk2aUy— California Geological Survey (@CAGeoSurvey) March 17, 2023
When soil is wetted by intense rainfall and runoff, both the weight of the soil and how stress is distributed throughout it, changes. The soil itself becomes heavier and the water effectively lubricates individual grains, making it harder for them to stick together.
Enough water and a weak point can present itself, which is when chunks of a bluff can give away.
…And then there’s sea level rise
This year, heavy rain is no doubt a factor.
But there’s another sizable force that’s being exerted on cliffs all up and down our coast as well: the ocean.
A bluff’s structural integrity can be compromised over time as waves lap up against its base, slowly eroding it. A natural process that becomes a bit of a nuisance when you have homes that you’ve built along cliffs, that you’re hoping don’t fall into the ocean. It’s a problem that’s expected to get worse, and leave a whole lot of important infrastructure vulnerable, as sea levels rise.
“So maybe without the big rains, this slide might not have happened. But when you have this confluence of things happening all at once, this is where it gets really complicated and tricky,” said Juliette Finzi Hart with the Pathways Climate Institute.
We’ve talked about the issue before, including why many of our beaches will disappear if we decide to armor our coastlines against rising seas, instead of letting them migrate inland.
If you’d like to see how far the ocean is projected to creep, the USGS helped put together a map of California that includes just that.
Can you know if a cliff is going to collapse?
There are some signs that might indicate a landslide is imminent, according to the The California Department of Conservation:
- New cracks in your home
- The sound or sight of the earth shifting, including the movement of material downhill
- Bulges at the bottom of your hill
- Broken water pipes and gas lines that signal things are moving that shouldn’t be
But, even if you don’t see these warning signs, it doesn’t mean that a hill won’t fail catastrophically.
If you’ve got a home perched on a hillside, you should call an expert if you think you might have a reason to worry.
“It really requires a careful geotechnical examination,” said Brett Sanders, professor of civil and environmental engineering at UC Irvine.
“You would never be able to know just from looking at the ground from above it to know which homes are anchored well into good rock and which ones aren’t, because the materials in place along cliffs can be highly variable.”
Sanders recommends a subsurface investigation and examination of the geotechnical design to help you best determine the home’s safety.
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