Landslides in the Mountains of the West

July 13, 2023
3-5 minutes


While Landslides may not first come to mind when thinking about meteorological phenomena, these events are deeply tied to weather. Wind, rain, snow, and heat all play a crucial role in creating the conditions under which a landslide may occur. Subsequently, as our world attempts to address an increasing number of extreme weather events, we must continue to be on the lookout for landslides and the chaos they may bring. In the United States, recent years have seen uncharacteristically inclement weather hit regions along the west coast, spurring damage, proving that landslides are a force to be reckoned with, and, most importantly, sparking discussion and education about landslides as a whole.

What is a Landslide?

Landslides can be defined as any movement of debris or earth down a slope. Categorically, landslides account for five types of slope movements — falls, topples, slides, spreads, and flows. Falls and topples occur when large heavy materials, like boulders or blocks of a cliff, fall down a slope. Slides take place when smaller surface materials loosen and separate from the under-layer of a slope. Lastly, spreads and flows ensue when the earth is moved laterally — oftentimes because a powerful force like water or earthquakes is pushing material.

Landslides can be triggered in many ways, with gravity, geology, precipitation, land structure, and human activity all playing a part in instigating these events. For example, a loss of vegetation from wildfire, dehydration of land from drought, or heavy rains can all wear away at the root systems or plants that hold the earth intact, making the ground more susceptible to landslides. Similarly, human actions like agriculture, construction, deforestation, and irrigation can also weaken or destabilize a slope.

Where in the United States do Landslides Occur?

Landslides can take place anywhere in the world but generally occur in mountainous spaces with steep slopes, heavy precipitation, and weakened morphology. In the United States, the Rocky Mountains and the Pacific Coastal Ranges of the West are some of the mountain ranges that best encapsulate these conditions and subsequently serve as locations for frequent landslides.

Landslides occur most frequently in western mountain ranges because of the geological youth and the weather patterns of the region. Compared to other parts of the country, the states of Washington, Oregon, California, Colorado, and Idaho have young, and subsequently weak, rocks and mountains. Similarly, the youth of these mountains means they have been exposed to less erosion than their eastern counterparts, leading to much steeper peaks. In fact, many of the mountains on the west coast are still tectonically active today — meaning that new steep cliff faces are continuing to appear.

The weather patterns in the western mountains of the United States also create landslide conditions. The region often experiences short windows of heavy precipitation. This rapid influx of rain does not allow for the water to soak into the ground, causing the sediment to be easily dislodged and washed away.

Finally, the Western United States also experiences more earthquakes than other parts of the nation, which can break, displace, or crumble rock material and lead to landslides.

How Is Climate Change Making Landslides Worse?

Climate change is further exacerbating many of the conditions that create landslides. For example, higher global temperatures lead to larger storms, higher sea levels, and more water-dense clouds. These weather phenomena all play a role in increasing the likelihood of rain and flooding — known instigators of landslides. Similarly, higher temperatures mean that more precipitation is falling in the form of rain instead of snow. This transition further feeds landslides as rain better weakens and dislodges mountainsides as compared to its more stationary counterpart.

On the west coast in particular, climate change is increasing the likelihood of wildfires, which pose a landslide-inducing threat. When wildfires move through a region, the fires heat up and create an almost plastic-wrap-like layer of chemicals in the soil, that prevent soils from absorbing rain leading to landslides.

What kind of Damage can Landslides Inflict on Human Populations?

Landslides are a serious hazard that causes around one billion dollars in damages each year in the United States. These damages come in the form of disrupted electric, gas, and sewage lines, destroyed roads and railways, and inhibited routes of communication and transport. Each year these landslides and the subsequent damages that ensue lead also to between 25 and 50 deaths in the United States.

Case Study: Montecito Landslide

On January 9th of 2018, the small town of Montecito was struck by almost an inch of rain in the span of only 30 minutes. Because the area had experienced intense wildfires the preceding fall, there was a plethora of loose debris and weakened earth in the region — leading to a catastrophic landslide. The landslide brought plant debris, boulders, and mud reaching 15 feet deep down from the mountains and into the town. In total, the landslides killed 23 people, destroyed 130 homes, required 7,000 evacuations, and caused billions of dollars in damages.

Case Study: Southern California 2023

A rainy winter in the state of California has arisen conversations about the continued possibility and danger of landslides. In the winter spanning 2022 - 2023, Southern California saw rainfall levels that were 400 to 600 percent higher than average — with some areas receiving up to 30 inches of rain. Accordingly, in the month of January 2023 alone, California saw nearly 700 individual landslides. These storms and landslides resulted in the deaths of at least 20 individuals, as well as the flooding of many roadways and rivers around the state. In total, the landslides caused hundreds of millions of dollars of damage to the state and its industries.

Landslides Mitigation Solutions

Thankfully, much of the potential damage that the 2023 California rainy season brought was avoided thanks to several lessons learned from prior catastrophic rainy seasons — such as the 2018 Montecito Landslide. Some notable solutions include the creation of debris basins, to catch falling debris, and the construction of nets, to prevent large materials from causing damage. Recent years have also seen technological advancements in forecasting data — which may be the key to the future of landslide safety and prevention.

In the present, weather forecasting technology, such as HD Rain sensors, play a crucial role in mapping where rains will occur with the most intensity. By understanding where exactly rain accumulates, public safety officials can delegate when evacuations need to happen, where nets and basins need to be built, and what actions should be taken. Exact data on where rain will fall can be the difference between catastrophe and calm. As landslide reports are continually linked to weather data, analysis can be done to produce increasingly precise predictions on where landslides will occur.

HD Rain WebApp, Vallée de la Roya, France,  28th, June 2023.