Thread by @phaneritic

1/ The washout of #HWY1 at Rat Creek is really impressive, so I think I'd like to make a thread about things that I notice in pics & videos, then hypothesize on how the large rain event may have triggered this washout. TLDR I think the road was responsible for its own demise.

2/ Let's orient ourselves first. Here's Rat Creek (yellow pin) in comparison to the rest of central California. (This thread will develop slowly as I put info together, so please bear with me)
earth.google.com/earth/rpc/cc/d…

Rat Creek is part of the AMAZING Sant Lucia Range which has the steepest coastal slopes in the Lower 48. (important for later). The range is also home to the @LosPadresNF , the Ventana Wilderness and a multitude of breathtakingly beautiful state parks and like Pfieffer Big Sur.

Many factors within a complex system conspired to eliminate this section of #Hwy1. Who were the conspirators? Geology, topography, fire, epic rainfall and engineering hubris (IMO).

And before I get too much further along, many of the washout images I’ll be using are clipped from the amazing @bigsurkate blog. Original photographer credits are in the embiggened photos.

*Santa* Lucia, doh.

Let's start with a simplified version of how the geology contributed to #Hwy1's washout. @UsgsNgmdb's new interactive geologic map browser makes it easy to zoom into the area. Rat Creek is center of view. ngmdb.usgs.gov/mapview/?cente…)

Notice Rat Creek cuts through purple "fs" Franciscan Complex sandstone (called greywacke) at the road. These rocks are "fossilized" subduction zone sediments that were deposited btwn 120-90Ma in a trench & smooshed hard but only lightly cooked about 80Ma.

Over the next 62 million years sediments continued to erode off the N American continent & into the deep Pacific Ocean to est thicknesses of 20,000ft! Those sedimentary rocks are labeled Kss/Kcg on the map by Dibblee who also described area geology here--> fs.usda.gov/detail/lpnf/le…

Starting 38 million years ago, several periods of mountain building, erosion and subsidence/sea level rise followed in what are now the Coast Ranges. By 6 Ma the uplift of the modern coast ranges had begun 📽️ Dr. Tanya Atwater.

In essence, over the past 6Ma the flat layers of old marine sedimentary rock have been squeezed and folded between a complex set of faults created as the Pacific Plate drags past the North American plate to the northwest along the famous San Andreas Fault.

As a result of all the squeezing, folding, breaking and uplift, the layers of old marine sedimentary rocks (purple & green on map) are now titled 45 to the NW, very much like the edges of a deck of cards being shuffled. Generally speaking, steeply tilted sed rocks are not stable.

Imagine a ream of paper, with each page representing a layer of sedimentary rock. If you pick up one end of the stack and tilt it enough, the pages will slide along their surfaces when the force of gravity overcomes the friction. Rock layers can behave the same way.

What's interesting about the Rat Creek area is that the layers of greywacke (fs) and conglomerate (Kcg) actually tilt away from the road, which would generally be good news since the mountain would support the base of the "pages" and prevent them from sliding.

But one quick glance at the geologic map (Rat Creek at top left) shows an abundance of cream colored "Qls": geologically-recent LANDSLIDE DEBRIS. Why so many landslides if the dip of the sedimentary layers seem to be somewhat favorable? Well...

The short answer to explain the landslides is that the MOUNTAINS ARE STEEP. Google Earth lets us quickly calculate the average slope of the topographic profile (red line). 25%(!) slope. For hikers, that's about 2000' per mile. Trucks get warning signs at 6%.

Quick detour into physics. Generically, the force of gravity is the driver of landslides. But it's how gravity's component forces interact with the weight, strength and friction of rocks that determines if they are destined to slide...
📷Open Geology opengeology.org/textbook/10-ma…

When the slope of a mountain steepens, there is less friction (fn) to resist an increase in shear force "fs" (which you can think of the force that tries to push the rocks along the slope). If shear force is greater than friction, the rocks will slide.
📷opengeology.org/textbook/10-ma…

Applying our newfound love (really?) of physics (no not really) our slope at Rat Creek & all the coast ranges normally😉 has a frictional force that is a wee bit bigger than the pushing sheer force. But not by much.

Lets also remember that the small fragments of sediment that make the greywacke sandstone and conglomerate that are exposed at the surface are only staying put based on how well they are cemented together.

Quick recap... we know the mountains of sedimentary rocks around Rat Creek were smooshed up by folding/faulting. The smooshing forces have made the topography steep. Steep is bad for slope stability because friction is low.

When I pick this thread back up tomorrow we'll investigate how the topography at Rat Creek also influences the amount of rain in the region, the speed of runoff, AND HOW AND WHY THE RAT CREEK ROAD WASHED OUT.

I’m BACK! Before we leave the geology-topography relationships in the Rat Creek system it's worth looking at the slopes directly under and next to the road. It’s kinda odd, here.

Steeply-sloped creeks lead to high velocity runoff if rain is plentiful. If runoff flows over erosion-prone meta/sedimentary rocks punctuated erosion events will lead to V-shaped valleys that drain downhill. But(!) Rat Creek’s canyon abruptly runs uphill at HWY 1. Why?

It appears that engineers of the 1930's likely thought bridges were too expensive for little canyon crossings, so it seems they opted to fill the canyon with dirt and put the road on top of it. Following gradient and greening of canyon, I think everything I traced is fill.

Filling canyons with dirt is also known as building a dam in most circles. If there is enough water. Which brings us to our next interaction of the system: geology/topography and now WATER... and fire.

Oooh, I almost forgot... Here's the StreetView of the the road in October 2017. Oversteepened slopes, even engineered ones can show signs of weakness. The black tar fill and crescent-shaped patch demonstrate the road was not on the most solid of footing.

Now, back to the interaction of water! As @SoildocTony pointed out earlier today, the Rat Creek watershed (the land area that feeds water to Rat Creek) is VERY small. About 2.8km^2 (or 1 square mile). Modelmywatershed.org makes it really easy to see! modelmywatershed.org/project/33387/…

With such a small watershed, engineers likely thought in the 1930's that a small drain or culvert under the road would prevent erosion of the roadbed. There is drain somewhere in here, its just hard to see... and likely kicks out near sea level so as to not undercut the roadbed.

ModelMyWaterShed let's us see some really cool things to help us understand the old-timers thinking. In an average year, the watershed would likely see ~30" of rain.

B/C the mountains are so steep, I have to assume the soils are not very thick. Which leaves mostly (not all, but mostly) the rocks to absorb the rainfalls. The graph shows how well water absorbs into the rocks.
Franciscan greywacke =D
Younger conglomerates and sandstone = C & B.

Now lets put the model to use and run a few different rainfall scenarios to see how infiltration, evapotranspiration (evaporation from the soil and water vapor "exhaled" by plants photosynthesizing) and runoff are affected by different rain rates.

In a light rain of about 0.60" in 24hrs, all the rain is absorbed or put back into the atmosphere through evapotranspiration. Note there is NO runoff after a light storm. Which means, nothing to flow down Rat Creek.

Here's a comparison between 2, 4 and 6" rain rates per 24hrs. 2-4" would be fairly common during good storms, and 6" would be a very big storm (I'll show you why in a second). What do you notice?

Did you notice that with the larger storms there was A LOT more runoff?
2"= 560,000cf
4" = 2,668,000cf
6" = 5,448,000cf
The drain under the roadbed could probably handle that since 6" storms likely only happen a few times each decade.

I think this is also a great place to mention why the Coast Ranges get so much more rain than the Central Valley, just a few tens of miles to the east. Again, is has to do with the topography created by the geology. (Here's a recent storm's 24hr total)

I'm a big fan of @CartoonKahuna's skills, so I'll borrow one of his @latimes illustrations to, um, illustrate the concept of orographic precipitation. The Coast Range forces stormy air full of water vapor coming off the pacific from where it evaporated up and over the mountains.

The gist is that rising air cools, condenses and makes lots of cloud and rain. As the air descends into the Central Valley it warms a bit, & coupled with the fact that there's less moisture in the air after raining in the Coast Ranges, the air has a lower relative humidity.

The main idea here is that the topography of the Rat Creek drainage squeezes out rain. But on Jan 26-30th, the drainage was subjected to a stalled atmospheric river.

Between Jan 26th & Jan 29th at 7am (when the photo of the washout was taken) 9"(!) of rain had fallen! That's a lot. But not unprecedented. So how come the road washed out THIS TIME?
wrh.noaa.gov/map/?&zoom=8&s…

We've already established that the roadbed that filled Rat Creek Canyon had a drainage system that had handled runoff like this before without catastrophic failure. So what gives?

To find that answer out we need to look at the pics. First I note a large "reservoir" of stained water on the upslope side of the roadbed. This is a big clue that the drain, well, didn't drain. So what plugged it up?

These pics by @JoshEdelson of the aftermath help clue us in a bit. The roadbed "dam" appears to have filled with lots of mud and large logs, likely plugging the drainage system that could normally handle water with *some* sediment. Not sediment with *some* water.

In order to figure out why logs and debris filled the canyon, we have to veer a bit into biology, wildfire and climate change. I'll leave you tonight with a streetview looking up Rat Creek Canyon in 2017. What do you notice and wonder?goo.gl/maps/qLy7qxFj5…

Time to buckle-in for the exciting conclusion!

Zooming-in on the StreetView of Rat Cr Canyon allows us to see a diverse mix of plants- from scrubby chaparral mixed with invasive weeds, to taller conifer forests. But one also notices the ghost forest of white standing snags... evidence of fire.

The Coast Range's varied biomes are due in part to the microclimates influenced by the topography, geology and meteorology of the hills. Cooler, wetter areas (like canyons or upper elevations) have conifers, while the fast-draining, sun-exposed slopes hold chaparral.

The Coast Range is adapted to fire. The USFS says that the Chaparral typically burned 3x's a century prior to colonization. (pic of a cool fire model USFS uses)
fs.fed.us/psw/topics/fir….

The area near Rat Cr has burned quite frequently since 2000. The Rat Cr watershed burned in the mid-80's & again during the large and intense 2008 Basin Complex started by massive summer lightning storm which created the ghost trees. But then 2020 decided to 2020 Rat Creek.

2019-20 was a paltry rain-year accompanied by a very hot summer which dried fuels out and made them quite explosive. I just spent a ton of time looking for 2020 Fuel Energy Release Graphs, but all I can get are 2021. Trust me, by August the area was a tinderbox.

Then on August 18th, 2020 an as yet undetermined ignition source sparked the #DolanFire which would burn through the Rat Creek watershed early and spread to 120,000+ acres over the next several month. It was officially contained on 12/31/2020(!) inciweb.nwcg.gov/incident/7018/

Thanks to @sentinel_hub we can see what the vegetation looked like in the months and days prior to the fire. Near-Infrared wavelengths reflect off chlorophyll from healthy plants and are captured from space 768km(!) in orbit. The data is false-colored red. RED=HEALTHY VEG!

Here's the same area 10 days and a few months after the fire. I notice some veg in the canyons that survived the fire appear to have died out by November. This all means that when last week rolled around there was much less veg on the slopes than before the fire.

Argh... ink layer misaligned.

Trees and veg both blunt the force of falling rain & reduce velocity of water moving down slopes. Imagine plinko as a model for the path the water drop has to take downhill. Its not as direct.

Leaf litter and roots also hold soils in place and slow water's roll, especially on steep slopes like those surrounding Rat Cr. But when they burn, not only do they disappear, but they leave behind a hydrophobic layer of soil.

As the @NWSLosAngeles put it in this great tweet, "heavy rains runoff as it would on pavement". That's not good.

Let's recap:
-9 inches of rain in 48hrs fell onto
-very steep slopes
-fresh burn scar
-runoff rates/velocity greater than normal due to a lack of infiltration & ground cover.
-burned topsoil and ash carried in fast runnoff
-headed to a roadbed "dam" with a small drain.

The resulting muddy, rocky and woody debris flows plugged the drain and filled the canyon behind the roadbed. Note in the📸 by @JoshEdelson that mud and former conifer snags are at road level. But this didn't cause an immediate failure of the road. Nope.

Referring back to @bigsurkate's blog and photos from January 29th, it obvious most of the road is still intact that morning. It's also obvious that the canyon has been filled with debris (short event?) and mud/water from relentless rain is flowing over the top of the road.

This pic was the impetus of this gargantuan thread. I saw it and immediately thought "Oooh! Waterfall undercutting and retreating". All of the runoff that was blocked by the clogged drain had to overflow onto the hard asphalt and poured onto the dirt fill of the roadbed.

Waterfall pic was from @Wikipedia
en.wikipedia.org/wiki/Waterfall

Remember how we already established the roadbed was showing signs of slumping? Well, guess what happens when a lot of its supporting fill is washed away? Adios road bed!

The rains and runoff apparently continued to cause more headward through at least January 31st when these pics by @JoshEdelson were taken. Don't you love the cute delta of larger debris at the base, surrounded by muddy waters? Wonder how that affects near-shore life?

Here's the Sentinel Image from Feb 2nd showing the muddy runoff from Rat Creek (among others) still flowing into the Pacific.

Because the #DolanFire was so widespread, so too were the debris flows along many of its drainages.

IN CONCLUSION (for the road, anyway), It appears water overtopping the asphalt eroded the roadbed because its drain was plugged by a debris flow caused by steep slopes in burn scar after very heavy rains. Should there have been a bridge there to begin with? You decide.

P.S. Teacher-me wants to draw attention to the fact that what seemed like a simple engineering problem was in fact a hugely complex system of interacting components that all need to be understood in order to have discussions about how to solve (or walk away from) the problem.

For my k12 T's and geosci ed professionals I'd also like to point out that this Rat Creek conundrum is a great example of how local place-based geoscience phenomena can drive entire units of interdisciplinary science study that most MS & HS schools in CA aspire to achieve.

I used geology, geography, physics, chem, biology, ecology, hydrology, meteorology, remote sensing and mentioned engineering in order to tell a rich story about as silly road at Rat Creek. That's the power of geoscience: it can give all of the other sciences meaningful context.

That was a fun few nights of pretending to sound like I know what I’m doing. Thanks for enduring the @SethAbramson-esque length. Now I shall join my cats. Zzzzzzzzzzz.....