Landslides

The generic term “landslide” refers to a wide range of mass wasting events during which rock material moves downhill. The rugged topography of the Western US makes landslides a common problem. Landslides are usually triggered by high rainfall, but they can also be triggered by earthquakes, erosion, deforestation, groundwater pumping, and volcanic eruptions. Not all mass wasting events are rapid—slow land movement, known as soil creep (Figure 10.11), usually does not cause loss of life, but it can still destroy roads and buildings. Mud and debris flows are very fast landslides that are likely to kill anyone unfortunate enough to be caught in their path, as they can reach speeds that exceed 32 kph (20 mph).

The slowest kind of landslide is known as creep. When clay in the soil on a hillside absorbs water, it will expand, causing the soil to swell. As the clay dries and contracts, the particles settle slightly in the downhill direction. The process causes fences and telephone poles to lean downhill, while trees adjust by bending uphill (Figure 10.11).

Figure 10.10: The steps involved in a megathrust earthquake and tsunami.

The Arctic tundra of Alaska is experiencing an unusual form of creep called solifluction. During the summer, rain passes through the thawed upper layer of the soil but cannot penetrate the permafrost below. Instead, the water builds up against the impermeable permafrost. As a result, the water and soil move slowly downhill like thick porridge.

The Pacific Northwest is particularly prone to landslides because of its high levels of rainfall. The most damaging landslide in recent history was the March 22, 2014 Hazel mudslide in Oso, Washington. The Hazel landslide has been active for over 60 years, causing numerous smaller slides—the March 22 slide came from a high terrace made of a poorly cemented layer of glacial sand and gravel. After a period of extremely high rainfall, a section of the terrace collapsed into a fast-moving mudflow that engulfed 49 homes and took 41 lives. Incredibly, most of the homes were located across a river from the terrace, on a bluff nine meters (30 feet) above the river.

Figure 10.11: The effects of soil creep.

The coastal erosion of hillsides often results in landslides. Portuguese Bend on California’s Palos Verdes Peninsula contains layers of rock that slope towards the ocean. Among these layers is bentonite, a very slippery clay layer. In the 1950s, houses were built on an area that soon started to slide once the city, in the process of road construction, had added material to the top of the slope. Although homeowners successfully sued the city for initiating the slide that destroyed their homes, they were also partially responsible since they added water to the slide by irrigating their lawns and using septic tanks. The land is still moving, and no houses can be built there.

Debris flows are a dangerous mixture of water, mud, rocks, trees, and other debris that move quickly down valleys (Figure 10.12). The flows can result from sudden rainstorms or snowmelt that creates flash floods. Areas that have experienced a recent wildfire are particularly vulnerable to debris flows, since there is no vegetation to hold the soil. El Niño climate conditions, which bring high precipitation, also increase the danger of debris flows. In response to these dangerous slides, many communities have built debris basins across their canyons to protect houses downstream (Figure 10.13). The USGS and NOAA are in the process of instituting a flash flood and debris flow early-warning system.

Figure 10.12: This debris flow occurred on February 6, 2010 in La Canada-Flintridge, California as a result of a flash flood generated in Mullally Canyon.

Figure 10.13: This debris basin in Los Angeles county is designed to capture the sediment, boulders, and other debris washed from the canyon during a storm.

One of the most dangerous kinds of debris flow is a lahar, which is composed of water and volcanic debris such as ash. An eruption is not necessary to trigger a lahar. A melting glacier, a debris dam being breached, or a crater lake’s walls collapsing can all send water and ash down the volcano at speeds up to 100 kph (60 mph). Mt. Rainier in Washington is considered the most dangerous volcano in the United States because of its proximity to high populations and its potential for lahars (Figure 10.14). The Puyallup River valley, directly downstream of the mountain, is built upon 500-year-old lahar deposits. The town of Orting is at the junction of two river valleys that descend from Mt. Rainer, making them capable of delivering a lahar. Since lahars are predicted to flow through the valley every 500 to 1000 years, the USGS has set up a lahar warning system to give people enough time to evacuate in the event of a lahar racing down the flanks of Mt. Rainier.

Figure 10.14: Lahar paths of Mt. Rainier. This map shows three major events that occurred in the last 10,000 years. Note how the lahars follow the river valleys.