Mudflows Devastate Parts of Southern California

By Erik Klemetti | January 12, 2018 9:14 am
Sediment from January mudflows burying cars and homes in Montecito, California, seen on January 10, 2018. California National Guard/Flickr.

Sediment from January mudflows burying cars and homes in Montecito, California, seen on January 10, 2018. California National Guard/Flickr.

This week has been a tragic one for parts of Southern California. Heavy rains have triggered landslides and mudflows that have killed over 17 people with dozens more missing. Now, this tragedy is a sequel to an earlier disaster: Wildfires ravaged the coastal mountains near Santa Barbara. However, they are two events that tend to go together because the effects of one prompt the other.

The wildfires that burned forests and homes in 2017 have the net impact of destabilizing rugged terrain like is found in towns like Montecito, where numerous mudflows have struck. As vegetation is burned – both trees and ground plants – the soil is exposed to rain water (or snow melt, in other places). The soil absorbs the water, but instead of having plant roots to keep much of that soil in place, it becomes a slurry that’s vulnerable to the force of gravity.

The material can move downhill into stream and river channels. It can also cause whole hillsides to become unstable and create landslides that move debris into valleys and, again, into rivers and streams. They can occur with little-to-no warning after rainstorms, so getting people out of their path is challenging (more on that below).

All this soil, rock, burned trees and whatever else is caught up in the water-logged flow then moves downhill at speeds that might reach tens of kilometers per hour. This isn’t like a muddy river, however. Much like volcanic mudflows (lahars), these mudflows carry so much material that they have the strength to destroy homes and cars in their path. If you have any question about that, check out the images of what happens to cars during these mudflows (above and below) — they end up barely recognizable as vehicles.

They have the consistency and density  of wet cement, so if you can imagine trying to escape from a river full of such material, you can see what mudflows can be so deadly. Once the mudflow has come to a rest, it tends to separate and lose its water, both trapping victims and creating floods in low areas where the flow stops, usually due to the slopes flattening out.

Landsat 8 image of the aftermath of the Montecito mudflows, seen on January 10, 2018. NASA Earth Observatory.

Landsat 8 image of the aftermath of the Montecito mudflows, seen on January 10, 2018. NASA Earth Observatory.

The NASA Earth Observatory posted telling images for the Montecito mudflows (see above). You can clearly see the hills above the town where wildfires destroyed vegetation (light brown, relative to the greener hills to the left). Landslides in the hills fed debris into streams that then rushed into the town below and the brown debris can be seen in and around those river channels. If you happen to live near these streams or were near them when the mudflows came barreling down the channel, things were not good for you.

For California, the question now becomes one of both rebuilding after these mudflows and protecting people from further mudflows.

Volcanic mudflows are some of the deadliest hazards associated with volcanoes, so mitigating against those types of debris flows has been a focus for decades. Some cities near volcanoes have built special channels to help keep lahars confined to channels, but that comes at great expense. Other places such as the towns downhill from Mt. Rainier in Washington have employed early warning systems, where mudflows far uphill trigger alerts to citizens below.

There is concern that the people of Montecito did not get or heed these warnings when the mudflows roared through their town in the early morning hours. Officials estimate that only 10-15 percent of the population evacuated after a voluntary order was issued for concern of mudflows — and even after being warned for days beforehand of the risk. This is yet another tragic example of how to communicate risk to the general population. It is possible that people just didn’t understand the high level of danger regarding the potential for mudflows or maybe they suffered from “disaster fatigue” after the summer and fall of wildfires.

Rescue crews are still looking for survivors from the mudflows, but even if they are found, extracting them from the mud can be very difficult (as we learned during the 1985 eruption of Nevado del Ruiz in Colombia). With the high likelihood of more mudflows in Montecito and in other towns near areas damaged by the 2017 wildfires, this question becomes of great concern to prevent more deaths.

  • OWilson

    Erosion is a natural process here on Earth, and this is just a snaphot in time of how it works over eons.

    Eathquakes, river delta floods, landlsides, mudslides, volcanoes, rains and frosts, and plate techtonics, can wash out entire valleys, reduce Everest scale mountains to mere bumps, and sink entire continents.

    Humans do best when they stay out of it’s way!

    • Uncle Al

      You engineer, you abandon, or you get stomped. So has it ever been. You can buy paradise with dollars, but you cannot keep it without IQ points.

  • Uncle Al

    Montecito is a substantial parcel of deeply wooded extreme wealth in what would otherwise be chaparral desert. Nobody had shutters on their windows and vents, or copper-piped sprinklers for under eaves and atop roofs. Roofs were not ceramic pantiles or even Shepherd arctic color IR-reflective. Burn, baby., burn.

    Given full ten-day warning of a major rain event, residents went looking for sandbags that morning. The rain-fed landslides sheared water mains from total 12 million gallon impoundments.

    Physical reality is not subject to popular vote. Ask šithole Haiti about major earthquake zone building codes that ignore rebar.

  • Mark

    Relative to fire effects to soils in the recent California debris flows…. It is my understanding (based on my long-ago geomorphology classes) that chaparral and creosote bush-type fires burn very, very hot and consume much of the A-Horizon organic materials which hold soils in place.

    This means that there’s little to nothing to keep these remaining mineral soils stationary in the presence of precipitation.When rains come after the fire, that the soils are basically water-proofed against infiltration/absorption.

    This causes there to be less infiltration and more immediate surface and subsurface run-off. Combine that with steep slopes and mineral soil angles of repose and/or mineral soil grain frictional characteristics…. and you get more of a sheet flow runoff effect which is turbulent enough to entrain tiny mineral soil particles. That leads to larger particles being entrained as density and gravity do their thing…

    Next thing you know its pebbles, cobbles and then boulders being entrained….. And those will pick up and move everything else in their path at a certain density and continued gravity flow downslope (cars, homes, trees, various household and community objects. These flows have the consistency of wet concrete…just like lahars…..they will devastate nearly everything in their path….

    What needs to be drilled into the heads of people and government bureaucracies, is that herd/collective memory disappears after the fires are gone. Once the debris flow basins are mined for their water; then emptied of rocks and sands to feed the aggregate industry; the prime location properties still remain. In many of these areas, the property values are higher, the higher up the canyon they are located

    So, the irony is that after the property parcels are cleared off and vegetation returns, many property owners who experienced a total loss , or near-total loss, will either eventually sell their properties or dump them at a steep discount to realtors.

    Those who experienced minor damage and/or were left relatively unscathed, because they were very lucky…. they are also affluent enough to then hire engineers who will have constructed (hopefully robust) armored and reinforced diversion walls to direct flows away from their homes; sometimes to the detriment of their unwitting eventual new neighbors or their fellow survivors down or cross- slope….

    So, lack of herd/collective memory entices new affluent property buyers; who come in and rebuild on very desirable-appearing, exclusive properties. But they don’t armor their homes against debris flow potentials….and it used to be there were no requirements for architects and engineers to design for debris-flow mitigation, in contrast to earthquake survivability design requirements for many new construction and retro-fit requirements for existing structures…..

    In the end, there need to be design requirements for debris flows for canyon construction in areas which have historically experienced these events. The city fathers/local municipalities have often already placed catch basins at the base of these canyons to mine the water and catch the aggregate resulting from these flows…


Rocky Planet

Rocky Planet covers all the geologic events that made and will continue to shape our planet. From volcanoes to earthquakes to gold to oceans to other solar systems, I discuss what is intriguing and illuminating about the rocks beneath our feet and above our heads. Ever wonder what volcanoes are erupting? How tsunamis form and where? What rocks can tell us about ancient environments? How the Earth might change in the future? You'll find these answers and more on Rocky Planet.

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