Saturday, January 21, 2023

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.

Sunday, January 1, 2023

Dry Creek: Anatomy of a Flood (One of Many Across California Today)


After days of gloomy and wet weather, New Year's Day dawned bright and sunny, and we couldn't resist driving out into the California prairie to have a look at the beautiful landscape. The streams across the prairies east of Modesto were full and flowing in a way we haven't seen for a number of years. And that's the problem of course. 

This creek, normally dry, was just one of many dozens of tributaries to Dry Creek, which is itself an unregulated, undammed tributary to the Tuolumne River. This entire area received upwards of five inches of precipitation in the last day or two, and all the water had to go somewhere.

I included a picture of Dry Creek in my post yesterday, when it was flowing at about 600 cubic feet per second (cfs). I am including another picture below, taken at the same time, but from an angle that shows the pasture to the left. I knew that more water would be coming downstream, maybe as much as 1,500 cfs, an amount that would actually be more, by a wide margin, than the main drainage in the area, the Tuolumne River.

That's not quite what happened...

When we crossed the Dry Creek Bridge north of Waterford today, the creek was running at 6,000 cubic feet per second, more than ten times the flow of the previous day. Take a look below at what happened to the pasture (not to mention all the shrubs and brambles at the base of the oak trees).

By the time we arrived in the prairies in the afternoon today, most of the floodwaters had subsided in the upper watershed, but we could see evidence everywhere that a significant flood event had taken place. Rocks were strewn across the roadways, and every watercourse showed evidence of having been feet deeper the previous day. One bridge we crossed would have been four feet underwater during the height of the storm. 

The flood hydrograph below tells the story. The data is taken from a stream gage downstream in Modesto. The bar graph at the top shows the pattern of the rainfall in the storm up in the watershed, and the subsequent rise of Dry Creek. Notice how the rise of the creek lagged behind the precipitation. This so-called lagtime makes sense because it takes time for the water to gather into the tributaries and then to flow the twenty miles or so downstream. Lagtime represents the critical hours that residents downstream can prepare for the oncoming flood.

Wouldn't it be nice if there were a government entity that could monitor all rivers and all flood events so that when such events unfold, there could be timely warnings? Perhaps even keeping records of storms over the course of a century or more, so that specific warnings could be made about the timing and the expected intensity of the oncoming flood? Unlike earthquakes, floods can be predicted, and there are in fact government institutions that are tasked with this job, mainly the United States Geological Survey (across the entire US), and the Department of Water Resources specifically in California.

Which brings us to the handy-dandy bottom portion of the hydrograph. The blue line on the graph is what happened already. The pink line is the prediction. We have another intense storm coming on Wednesday, and after dropping to around 300 cfs, Dry Creek is going to rise again to at least 6,000 cfs and maybe more. Isn't it nice that we have several days warning? That's just one huge example of the value of science in our society.

Of course, no one is perfect, and all models and predictions can be affected by unknown and unexpected factors. The storm this week offers one tragic example. Although most streams and rivers behaved more or less as predicted, the Cosumnes River defied the predictions and produced record flooding, well beyond the predicted levels. 

What went wrong? Those who do science fully understand that errors happen, and it their goal is to understand the reason for such errors. The factors in the Cosumnes River flooding are being analyzed and may include an unexpected slowing of the storm front causing increased precipitation, two or three broken levees, and the Caldor Fire of 2021 that ravaged much of the watershed upstream. If you want to follow the analysis, check out the Weather West blog by Daniel Swain (@Weather_West on Twitter).

So, there is my science homily for the day. But we were out to explore some nature, and in any case, we need to appreciate the gifts we have been given. The day, a respite from a long series of expected storms, was beautiful. 

Mountain Bluebirds are not common on the valley floor, but we saw a small flock along the road.

An American Kestrel is a sharp-looking small member of the falcon family. This one remained perched near our car for a few moments.

Bald Eagles are not especially abundant in the region, but we found one. So had an 'unkindness' of Common Ravens, and they were making their displeasure known to the eagle.

And finally, an old horse seemed to appreciate the sunshine. The horses were brought to the continent by the Spaniards in the 1500s, but they actually have a long heritage here. They evolved in North America tens of millions of years ago! They migrated across the Bering Land Strait and spread throughout the world, but for some reason went extinct along with many other large mammal species in North America about 12,000 years ago.