Landslides and Slope Mitigation in California's Great Valley...Wait...What?

 

I used to talk to my students about the geological hazards that we face as inhabitants of California's Great Valley (or Central Valley, for those who don't appreciate its actual greatness). I would go down the list of things to worry about: earthquakes, droughts, wildfires, volcanic eruptions, flooding, and so on. But then I somewhat jokingly described things we didn't have to worry about such as hurricanes (Florida's problem), tsunamis (a problem for coastal cities), tornadoes (Oklahoma's problem), and mass wasting (also known by the generic term 'landsliding').

Unfortunately, over the years I've become aware that some of those unlikely hazards actually can be a factor in living in the valley. We've had a fair number of tornadoes in recent years, including two that came within a few miles of my house (they weren't anything like the monsters of Tornado Alley in the Midwest, but still a bit scary). A powerful tropical storm hit Southern California last summer that came up just short of being a hurricane, and the heavy downpours were statewide. And then there is mass wasting (slope failures and landslides). I know of at least two fatalities caused by mass wasting in the last few years. One was a homeless person who had dug a tunnel into a river embankment that later collapsed, and another was a person who was driving along a freeway in heavy rains when the freeway embankment collapsed as a mudflow and spread across the lanes causing a fatal accident.

The Great Valley is famously flat so mass wasting doesn't seem to be much of a danger to those who live here, since landslides and other slope failures require, well, a slope to happen. But the valley is not quite so flat as people may think. The valley is 400 miles long, and most of it is close to sea level. Much of it is low-lying river floodplains, but other sections sit at slightly higher elevations because of complex history of climate change and glacial ice ages over the last 1,000,000 years. These bluffs and terraces protect my city and others nearby because even the worst floods are contained within the floodplains and do not spill over onto the terrace surfaces where cities like Modesto and Turlock have been built near the Tuolumne River.

During the ice ages glaciers never reached the valley floor, but meltwater from the Sierra Nevada glaciers swelled the rivers to several times their average flow, and they carried tremendous amounts of muddy sediment that spread widely across the valley floor building up alluvial fans. When the glaciers receded, the rivers flowed less, but carried clear water that was more capable of eroding the soft sediments of the alluvial fans, forming channels and floodplains several tens of feet deep. Once these channels developed, floods never covered the terraces again. It's the bluffs that form the boundaries of these terraces that provide the conditions that can result in slope failure.

The heavy rains of 2022-23 led to widespread flooding across many parts of California including some real problems on the Tuolumne River Parkway Trail when I regularly go birdwatching. I wrote about these in January of 2023 in the aftermath of one of the biggest storms. The most serious problem was the access road to our town's water treatment plant. It's on the river floodplain about 60 feet below the river terrace. Slumping had caused major cracks to form in and near the pavement.

Eventually the rains subsided and the soil dried up. The slide seemed to stabilize, but the threat to the roadway remained and would eventually have to be dealt with. That is what was new this week: the cranes and were in place to start the slope mitigation process.

The main problem is that the access road traverses unstable debris and soil that slumped in the 2023 event. They would need to re-engineer the slope by rebuilding it from scratch. Their strategy was complicated by the fact that all the equipment and materials had to traverse the very road they were trying to repair. Truckloads of heavy boulders were going down the road every few minutes. Meanwhile a huge long-reach excavator was digging away at the slope below the road!

After digging away and smoothing off the slope they covered it with felt matting and then started piling many tons of boulders on the slope. The boulders are intended to buttress the slope and hopefully keep it stable during future weather events.

It's a lot of work being done to keep a single paved road open, but it's a pretty important road since it provides the only access to the water-treatment plant for the city of Waterford. And thus we are dealing with slope mitigation in what is supposedly the flattest place in the country!

A Short Primer on Mass Wasting, Courtesy of California's Atmospheric River Storms

I live in California's Great Valley, known to some as the plain old "Central Valley", and most know it as a very flat place. A VERY flat place. Over the four-hundred-mile length of the valley elevations barely rise above 300 feet above sea level, and much of the valley is floodplain. As we emerge from the unrelenting series of atmospheric river storms that dropped near-record (and some record) amounts of precipitation all over the state, one might assume the greatest problem here is flooding. Some areas have indeed been hit very hard, and lives were lost.

One might be surprised to hear that even though the rivers rose, some areas were less affected by the flooding. In the case of my home county, Stanislaus, there were (and continue to be) problems along the lower reaches of the Tuolumne and San Joaquin Rivers, but on the east side of the valley there were few ill effects. Along my usual walkway, the Tuolumne River Parkway Trail in Waterford, the damage was of a type not often associated with a flat valley floor: mass wasting, or mass movement.

The reason has to do with a quirk of the geological history of our region. During the Pleistocene ice ages over the last two million years, glaciers covered perhaps 30% of the Sierra Nevada on repeated occasions. The ice never reached the Great Valley, but the streams of ice ground up vast amounts of rock to sand and mud, and the rivers were swollen with muddy meltwater. Rivers like the Tuolumne and Merced built up vast alluvial fans that resulted in higher elevations near the mountain's edge, on the order of a few tens of feet. That doesn't sound like much, but when the glaciers ebbed, the muddy rivers turned clear, and the rivers began to erode into those old alluvial fans, forming terraces and bluffs.

On the one hand, these bluffs and terraces have protected towns like Modesto and Turlock from river flooding because even the worst of floods cannot overtop the bluffs where most of the region's cities are located. On the other hand, the bluffs are steep and are composed of loosely consolidated sediments. That's the ideal recipe for mass wasting, the downhill movement of loose debris and rock under the influence of gravity. I got an excellent introduction to a variety of mass wasting events after the final storm last week. It was a mess along the trail.

Mass wasting happens because of gravity, but an overaccumulation of water can substantially add to the intensity and degree of movement. The movement takes three forms: falls, flows, and slides. I saw examples of all three this week.

In the picture above, there was so much water built up in the soil that the slope failed rapidly and the fluid mix of silt and water flowed and covered part of the trail below. This is called a mudflow. In different circumstances, especially involving glaciers and erupting volcanoes or desert cloudbursts, mudflows are one of the most dangerous forms of mass wasting. A single volcanic mudflow in Colombia in 1985 killed some 25,000 people. 

A short distance away, the slope was more coherent, but water had added a great deal of weight to an already steepened slope (from the carving of the trail itself), and the slope failed as a single mass that slid downhill as a slump (above). Slumps are usually much slower-moving than a mudflow and thus rarely kill anyone. But they can do considerable damage to homes, roads and other developments. The slump shown above is inconsequential, but I saw a much more serious problem a short ways down the trail... 

The town's water treatment plant has been built on a lower terrace next to the Tuolumne River at the base of the steep bluff. A paved access road was necessary, and they carved it into the slope, oversteepening the upper slopes, and putting additional weight on the slope below the road. A slump has begun forming right next to the road, and is ominously slipping an inch or two a day so far. I don't know if it will stabilize now that we've had some dry weather, but they are going to have to do some mitigation work in coming weeks.

The over-steepened slope above the access road has always been a problem, as rockfalls have been a constant, if minor, problem even in dry weather. The rains made the problem far, far worse, and after the final storm, the road was a real mess. There had been some wild tobacco shrubs whose roots helped hold back the rock, but they could do little to stabilize things in the face of intense rain.

Mass wasting consists of flows, falls, and slides, but one of the most pervasive and efficient forms of mass-wasting is almost mundane in the face of all the drama seen above. Over time all exposed surface weather and develop into a loose ground cover called regolith. If the regolith can support plant life, it is referred to as soil. If any slope exists at all, the soil and regolith will move move downhill imperceptibly over many months or years. Soil creep is not dramatic, but in the big picture it probably moves more material than any other form of mass wasting. It never kills anyone, but it will deform and bulldoze structures built into the slope over time. It's why old barbwire fences on hilly country roads always seem to be tilting over. It can even tilt telephone poles.

Soil creep was not much in evidence as a result of the storms, but it is clear that the trail builders knew it would be a problem over time. That's why many sections of the trail have walls built on the uphill side of the trail, to hold back the process for awhile (see below).

In one week, my modest hiking trail showed off nearly all the major forms of mass wasting, with the only exception (thankfully) of a debris avalanche that is capable of wreaking serious havoc, and solifluction, a form of creep known from artic environments. How did things play out where you live? I've heard a lot of stories of serious damage coming from around the state. I hope you've avoided the worst of it.

What's Wrong With This Picture? Part 3: The Mass Wasting of Del Puerto Canyon

It's no secret that I am against the construction of a useless wasteful dam in Del Puerto Canyon. I have had a lot to say about the project in public testimony and in recent blogs, and my opposition is deeply rooted in my appreciation of the canyon as a unique and irreplaceable outdoor laboratory for understanding the geology and natural history of the Diablo Range. It ought to be a national park or monument, not a stagnant pool of mud. But my opposition to this project also follows from my understanding of the hazards of constructing a reservoir in unsuitable and unstable rocks with a proximity to earthquake fault zones of unknown potential activity.

The words "mass wasting" in the title might be interpreted in several ways. It would certainly be a huge waste of money, and a squandering of a treasure of national significance. But to a geologist, mass wasting has a specific meaning: it is the downhill movement of rock and debris under the influence of gravity. Most people call this landsliding, but "land" is a non-specific term, and "sliding" is too specific. Mass wasting can involve sliding debris, but also flowing and falling material.  This post is not about the future potential hazard of mass wasting in Del Puerto Canyon. It is about the active slides that are already there.

Let's see what the Geotechnical Memorandum of the Environment Impact Report has to say about the mass wasting hazard in Del Puerto Canyon:

A significant number of landslides are found within and in the immediate vicinity of the reservoir inundation area, the majority of these landslides are located within units of the Cretaceous Moreno formation, upstream from the proposed main dam. At least seven landslides are mapped within the inundation area of the proposed reservoir – six are in the Moreno formation and one landslide occurs in the Panoche formation (Figure 5). It is expected that additional small landslides and movement of existing landslides would occur as a result of reservoir infilling and operations. These landslides would be expected to experience continuous deformation without some form of stabilization/mitigation. The rate of movement of these landslides would likely be slow. Stability of the reservoir rim, including potential for seismically triggered landslides would be required for design of the Project.

But wait, as they say, there's more...

The proposed reservoir would inundate areas underlain by the Cretaceous Moreno and Panoche Formations. Landslides are found within and in the immediate vicinity of the Project Area, the majority of which are located within units of the Moreno formation, upstream from the proposed main dam. Movement of these landslides is expected as a result of infilling and seasonal operations of the reservoir. It is expected that additional landslides would form as well. Movement of existing and any newly developed landslides resulting from reservoir operation is expected, any deformation of the landslide would be relatively slow and at scale that would not form seiche waves of significant magnitude that would overtop the proposed dam. An assessment of landslide potential and impacts to the Project would be needed for final design of the reservoir and dam.

That's pretty much it. Seven landslides within the reservoir inundation zone, with no specifics about the age of the failures, or the volume and length, and nothing about the current activity. These are precious few words for what could be one of the most hazardous aspects of the proposed reservoir.

Here's figure 5 from the EIR (above). It is a geologic map that shows the different rock formations found at the dam site and the location of the landslides mentioned in the quote above. The proposed reservoir is outlined in blue, and the landslides are the white areas are in white, with black arrows showing the direction of movement. The scale of this map is very roughly 1 inch = 1 mile. These mapped landslides are not small. The smallest mapped slides are about 1/5 of a mile in length, while the largest is about a mile. A mile. These are not minor earth movements.

The three pictures in this post provide a perspective for understanding the size of these unstable masses. The images show the same slide from three different angles, and the trees and farm buildings provide scale. It's huge. It is probably several thousand years old, as it filled the canyon it occupies, pushing the stream all the way to the right against the mountain slope. But it is not done...as Del Puerto Creek has undercut the base (the toe) of the slide, the lower parts have been reactivated, forming the sharp terraces (scarps) above the creek.

Whatever stability exists with this slide lies in the friction between the mass and the underlying rock. But the slide is slated to be at least halfway inundated by the waters of the reservoir, and water is a hugely destabilizing force in mass wasting. It gets between rock surfaces and in essence breaks down the frictional resistance. Geologists other than myself will have to assess the possibilities of inundation, but one of the worst-case scenarios would involve a rapid flow or slide of a huge volume of rock and debris into the reservoir, displacing vast amounts of the water over the top of the dam (the seiching mentioned in the EIR). A less catastrophic outcome, but no less significant, would be the slow flow of debris into the lake. Every cubic yard of debris going into the lake is a cubic yard less of water storage. The slide in these pictures is the smallest of the mapped slope failures. Imagine what happens when all six or seven of the slides are rejuvenated by the waters of the proposed reservoirs.

These are serious concerns, and although the draft EIR addresses some of them, they are buried within something like a thousand pages of the EIR itself, and the background memorandums. This dam is being proposed for construction in an area of unstable and unsuitable rocks and sediments within an area where the earthquake risk is poorly known. I've been learning about the region for thirty years, and I was surprised to find that there was a 6.1 magnitude earthquake in 1881 in the immediate vicinity of the dam site (six miles), but that "the location is highly uncertain given its pre-instrumental age and is based on intensity estimates documented in the public record". That is the kind of uncertainty that should give everyone pause about the magnitude and risks of this dam proposal.

There will be many reasons offered as to why this reservoir is SO necessary, and how there will be SO many benefits. But the questions need to be asked, what is this taking away from all of us, and what is the full magnitude of the threats we will face if it constructed?

If you are on Facebook, check out https://www.facebook.com/groups/463664377903706/

Read the Environmental Impact Report at this link. If Del Puerto Canyon has significance to you, please respond and be active in the opposition! If you have expertise in any of the areas that will affected, you need to be heard from.

There are several important meetings and deadlines coming up very soon:

1/27 5:00pm Public Comments DUE. ahansen@delpuertowd.org OR Anthea Hansen PO Box 1596 Patterson CA 95363 (use the forms outlined in the EIR documents)
1/28 9:00am Board of Supervisors Meeting 1010 10th St Modesto CA - voice concerns, they have final decision