Los Angeles: Negative Effects of Natural Disasters

A question proposed by Professor Diggins asked, “what do you get when you combine: mountains, tectonics, eroded soil, forest fires, and infrequent but cataclysmic rainfall events?” you get an average day in California, specifically Los Angeles. The City and County of Los Angeles has been fighting the elements for years and it has only been getting worse. They are experiencing a large-scale positive feedback loop where the geology is slowly being weakened with every geological event that occurs, which makes more natural disasters even more likely to transpire.

As the rock units erode to soil and the soil becomes weaker, it is easier for more erosion and disasters to happen.

Los Angeles has a geological environment that is composed of mountain ranges formed primarily by thrust faults along transform plate boundaries and they have a series of fault lines spread throughout California. The LA Basin is “coastal lowland area located at the edge of the Pacific Plate” (Diggins). “A basin is a depression, or dip, in the Earth’s surface.

Basins are shaped like bowls, with sides higher than the bottom” (NatGeo). Basins can be formed by the natural process of weathering and erosion or due to the shifting of tectonic plates; and since California is tectonically active, basins are spread out through the state. The Los Angeles area is located in between the San Gabriel Mountain Range and the Pacific Ocean. Due to their proximity along the San Andreas Fault, California experiences a multitude of earthquakes. “The range is bounded on the north by the San Andreas Fault zone, on the south and southwest by thrust and reverse faults of the Cucamonga-Sierra Madre fault complex, and on the east by faults of the San Jacinto zone” (Geomaps).

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Due to the proximity to these faults, geological hazards happen often in the surrounding area of Los Angeles.

The geological hazards include a multitude of mass wasting events such as rock falls, landslides, mudslides, and liquefaction. With these events, flooding can also occur due to the basins, levees, and reservoirs not performing the way they were designed to and failing because they are overburdened with debris. Other natural disasters that occur are brush fires and earthquakes; both cause the preventative engineering measures that are put in place to possibly fail in disastrous ways. These types of rock falls are controlled by discontinuities. There are different types of discontinuities, which include joints, bedding planes, foliation, shear zones, faults, and blast fractures. Rock falls also have their own types, such as toppling, planar, wedge and block failure. Each can be caused by different factors, for example, joints, raveling, and differential weathering.

Since Los Angeles is an arid environment, erosion of rock units is even greater than in a cold environment. Some other factors that make California a haven for disastrous geological events can be attributed to the fault lines running through and along the state. Infrequent rainfall causing flooding due to the unusually high volumes of water. “Since July 1, downtown Los Angeles has received 11.33 inches of rain as of Friday, which is 178% of average at this point in winter. Santa Barbara has received 12.03 inches, which is 149% of average” (Serna). The endless accidental forest fires and even some set purposefully also add to their long list of complications that plague the area. Mostly these massive brush fires are caused by the Santa Anna winds. The National Weather Service defines them as ‘a weather condition in which strong, hot, dust-bearing winds descend to the Pacific Coast around Los Angeles from inland desert regions” – “The result is the strong, hot Santa Ana winds, which increase the potential for wildfires by drying out vegetation. The winds also accelerate the speed of wildfires once they start” (Wotus).

Continents form due to converging, diverging and transform plate boundaries causing a variety of formations. Mostly mountain ranges are formed due to subducting plates where In regard to Los Angeles, the San Gabriel Mountain Range is considered to be a key player in the battle of elemental destruction. They formed due to lower and upper plate rocks along the Vincent Thrust. “The Vincent Thrust brought the upper-plate and lower-plate rock masses together in Late Cretaceous or earliest Tertiary time” (Geomaps). The composition of the mountain is primarily high-grade metamorphic rock, metamorphosed sedimentary rock, plutonic and volcanic rock. All of which can date back centuries, even to the Mesozoic Period.

Weathering is a major factor that has been affecting the area such that forest fires are constantly ravaging the area and decreasing the structural stability of the soil. The charred earth causes surface instability making erosion more occurring. Another factor is the San Andreas Fault that runs through California. It is the cause of a multitude of earthquakes throughout the state. In recent news, forest fires have been tearing through thousands of acres of land forcing the evacuation of residents from their homes. The destruction that pairs with these fires are considered to be the main problem to the average individual, but a closer look at the geology of the area after these fires occur reveal a larger problem that can put even more people at risk. The reason that fires contribute is that “in a burned area, a wildfire can make the soil repellent to water, creating a flood like flow on the ground that picks up rock and debris” – “In an area that has not burned, soil can become saturated. Pressure builds up underground, and soil starts moving and begins picking up mud and debris as it starts flowing downhill” (Serna).

Debris basins and different forms of tieback anchors and fencing have been put in place to help protect the surrounding areas, mostly around communities and roadways. They help control the flow of debris when storms occur, they help limit rock falls, and they help protect property and people. Other methods such as reservoirs help divert the direction of water or debris flows away from populated areas. Methods that have been implemented to prevent rock falls are scaling, which is the removal of unstable blocks. The decrease of slope angles and benching can help with steep slopes along roads and buildings. Other systems to help stabilize the ground are retaining walls, rock anchors and bolted wire mesh. And to help with water flow, drainage systems constructed behind retaining walls can divert the pooling of water. Without the installation of drainage systems “storms without intervening dry periods of at least eight hours are considered to indicate an increased potential for the occurrence of large debris flows” (Cannon).

The current engineering controls are effective to a certain degree but cannot account for the exponentially increasing debris after each new natural disaster. There is the constant fear that these current preventative measures will fail to cause a loss of life and the cleanup will be at a high cost to its people. The debris basins are easy to fill up, but at the same time they are difficult to empty for the next event. Not only is clearing the basins difficult and time-consuming, but it is also expensive for the government and population to pay for!

Urban expansion has been the most destructive reason for the change in the environment. The smartest option that will help prevent damage is to no longer allow development of structures in the areas considered to be in the most danger of catastrophic events occurring. The government could even go as far as to relocate its citizens to safer areas. Another approach would be to carve out parts of the mountain closest to these developed areas at a low slope ratio. The most stable slope would be covered in shrubbery and have a gradient ratio of one per one and a half feet. Even though this option would make it much safer, the expense to perform such a task is extremely high. Many other options are present but they each have their downsides such as continuing to construct basins to protect communities; which has the downsides of pricing and an endless loop of cleanups and construction.

The cost of building on this already weak, eroded land are that it causes a more unstable structural integrity of the rock underneath. The engineering to prevent damage has also changed the environment at points causing a loss of stabilization in the rock structure beneath these structures. A few benefits to these engineering structures, besides their goal of protection, is that dams can collect water for consumption and some can produce hydroelectricity to power certain parts of the area.

Los Angeles is an arid environment with frequent brush fires that enhances weathering and erosion of rocks, making rockfalls and debris flows happen more often. Positive feedback loops of erosion and weathering create a dangerous environment to live on and around. The environment also acts as a haven for brush fires that weaken the structural stability of the soil. This causes mass wasting events to occur more often. Mass wasting events can wreak havoc in a multitude of ways and when they do, they make the ground even weaker as well. Different engineering designs have helped divert these disasters, but over time they destroy their surrounding environment and they are not always a permanent solution. Basins and trenches can overflow, damns can fail, slope gradients can still be too steep. They aren’t perfect defensive measures. Each design has its flaws, some more than others, but they will not always work out in the end. Potential alternatives can be the government creating laws to protect the safety of the city and the state’s population through the controlled regional development of houses. By preventing the construction of homes and roads near places that have a high risk of disaster there would be less destruction when geologic events occur.

In regard to humans attempting to control nature, it is most definitely a futile effort. The costs of trying to control these natural occurrences have continuously proved that they outweigh the benefits. Even though they do help in keeping cities and their people safe, they slowly destroy the ground that they are developed on. Many times, these protective measures are done improperly due to the developers trying to take the easy way out of a project.