Landslide Occurrences in Logged Areas of Washington State

Jonathan H. Friend | 05.02.11 | GEOG 465



landslide_2008.jpgLandslides are a type of mass wasting event that occurs in nature where there is a combination of factors. Slope angle, climate, geology, soil type, and vegetation cover all play a factor in where landslides may occur. One factor that is linked to landslide occurrences is areas that have been cleared of vegetation – more specifically areas that have been stripped of trees by clear-cut logging practices. Trees and their root structures strengthen slopes and help keep soil in place. After an area that has been clear cut of trees, roots deteriorate and lose the ability to hold soil in place on mountain or hilly slopes. The trigger for landslides is typically from rainfall that saturates the deforested slope or in some circumstances earthquakes can bring down masses of earth.

In Washington State landslides are common, especially from the Cascade Mountains westward to the Pacific Coastline. One of the most plentiful natural resources in the state is its vast forests that cover large parts of the state. Logging is prevalent in Washington as it is in neighboring Oregon. The most common of logging practices is clear cutting, where virtually all the trees in a given tract of land are cut down leaving behind little to no vegetation to anchor the soil and rock material in place. The climate of the western part of the state is a significant factor that promotes landslides events. Most areas west of the Cascade Mountains experience high precipitation amounts on an annual basis, especially at higher elevations and near the coast. In addition to rainfall being a trigger for landslides on logged slopes, the geology of the state is also important to note. The geology of Washington State is complex with many different rock types with some rock bodies being more stable than others. Many fault lines run throughout the state with the most important tectonic feature being the subduction zone 75-100 miles off the western coastline. Given this type of geologic setting, earthquakes and weaker rock layers can trigger off landslides.

The goal of this project is to see the spatial distribution of landslides in the State of Washington. Logging practices may not be the sole cause for landslide in the state, so the distribution analysis was expanded to cover possible natural causes for these mass wasting events. This study also looks at other possible triggers of landslides on a geographical scope. The distribution of landslides is correlated to not only deforested areas, but to areas of seismic activity, high precipitation areas, and to the type of underlying bedrock.


What deforestation does to the land

Over the years, many case studies have proven that clear cutting of large trees and other vegetation has had a drastic impact on the stability of the land. The frequency of landslides is increasing and the probability of them occurring in logged areas is high. These studies have laid out several mechanisms of clear cutting that have contributed to the increase:

1.                Large trees provide strong root structures that penetrate fragile cracks in the underlying bedrock and anchor the soil.

2.                Old growth trees natural have large leaf surface areas which cause much water transfer. The water is drawn up from the soil and eventually transpired back to the atmosphere.

3.                Forests create a canopy of foliage high up in the air to help dissipate rain fall over large areas.

4.                Heavy logging machinery damages precious topsoil and decreases its ability to absorb water.

5.                Logging slash (leftover tree limbs) or debris blocks natural drainage basins.

6.                Anytime large amounts of vegetation are removed from an area the root systems eventually die off, which leaves the soil vulnerable to over saturation.




Natural vs. Human Caused Landslides

A common natural cause of landslides is earthquakes. Seismic waves shaking the ground have always been a main cause of landslides throughout the world’s history. When earthquakes occur in areas with steep slopes, many times the soil & loose rock slips - causing landslides. Also ashen debris flows (lahars) caused by earthquakes or volcanic eruptions can also trigger mass movement of soil and rock. Excessive rain that falls and saturates steep sloped areas can destabilize soil layers causing landslides. Without the help from trees and other vegetation root support systems the soil simply runs off when it is oversaturated with water.


Landslides occur every year in Washington State. The combination of climate, topography, and geologic setting is a perfect setting for landslides. In Western Washington, the majority of landslides are triggered during fall and winter. Losses from landslides are difficult to track, but conservative estimates of average losses over the last 30 years are more than $20 million per year. Across the U.S., landslides account for more than $2 billion in losses annually and result in an estimated 25 to 50 deaths a year. Landslides occur most frequently during high precipitation storm events that commonly move through the Pacific Northwest from November to March. Areas that are absent of vegetation or urbanized are the most vulnerable to these mass wasting events. This hazard increases as slope of the land increases and with the presence of unconsolidated sediments/loose rock. The clear cutting method of timber harvesting; which completely removes all old growth timber from the area, increases the chances of landslides. This logging method is hazardous because it destroys the existing mechanical root structure in the area. Another cause of landslides is mining activities where the ground is disrupted and destabilized. Surface mining especially increases the risk of landslides due to vast areas of ground that is cleared of vegetation. Mining activities completely destabilized the land and create a high risk environment for landslides.

Lands.jpgMapping Analysis

The map to the left shows areas where active logging has taken place since 1989 to early April 2011. Most logging takes place in locations west of the Cascade Mountain divide and the northeast corner of Washington. Logging takes place on National Forest lands, National Scenic lands, Native American owned lands, and other privately owned lands. The logging layer here contains 85,508 objects, which was overlain to show the distribution of logged areas with respect to different types of lands in the State of Washington.




slidelog.jpgThe map on the far left shows the six different logging regions in the state. The map to the right shows the distribution of landslides that have occurred in logged areas by region. All landslide records are indicated on this map. The distribution of landslides in logged areas statewide are concentrated in the western slopes of the Cascade Mountains, the Willapa Hills of the southwest, and the northwestern corner of the Olympic Peninsula. Some 22,140 separate landslides in logged areas were found using overlay analysis.  




The pie chart to the left shows how common landslides have been in the different logged areas according to the different regions of Washington. Almost 70% of landslides occur within the Pacific-Cascade and South Puget Sound logging areas. Very few landslides occur in the logged areas of the northeastern corner of Washington with respect to the rest of the state.










Geology and rock type is important to consider when studying landslides and the likelihood of them occurring in a given area. This map depicts what geology rock layers underlie all the landslide areas in Washington. Rock types of different ages may be significant in determining areas that are at higher risk from landslides. For example loose, poorly compacted or cemented rocks – sediments would be more unstable then hard, weather resistant rocks such as granite or metamorphic rocks. The diagram below shows, which rock layers are most commonly involved in landslide areas.




This diagram shows the frequency of the different geologic rock layers and how often each layer is associated with landslides in the state. The analysis conducted involved 65,475 objects created by intersecting a statewide geology rock layer map and all landslides that have been mapped out in Washington. Tertiary rocks are the dominant rock type that is involved with landslides in the state. Most Tertiary rocks were formed from sedimentary marine sediments, which are not held together very well. Active tectonic zones are the next major geologic feature in correlation with landslides, which is not surprising.



faults.jpgThis map depicts the fault-lines throughout the state of Washington. A three mile buffer is shown to represent areas that would have the highest potential of seeing landslide in an earthquake event. The small distance used for the buffer is due to the fact that most of these faults are fairly minor and don’t produce far reaching damaging effects. A significant feature that is not depicted here is the Cascadia subduction zone off the coast some 75-100 miles. This seismic zone has a history of producing large earthquakes with far reaching effects. This seismic zone has the potential to cause landslides and ground movement on a larger scale across the Olympic Peninsula and maybe as far east as the Puget Sound in a major seismic event. 


precip.jpgA major natural factor to take into consideration when analyzing landslides is the climate factor. The coastal and generally the western region of Washington receives large amounts of precipitation on an annual basis. The interior of the state receives far less precipitation due to the rain shadow effect of the Cascade Mountains. It is easy to see there is some geographical relationship to the higher precipitation areas of the west to the landslide areas, which are also concentrated over those same areas. It is a fact that most landslide in Washington do occur in the wetter months of the year when days of rain can weaken sediment and rocks. Oversaturation of the land occurs and gravity takes control in bringing down slopes that aren’t stabile. In some cases forested slopes can be vulnerable to landslides if enough moisture is present in the ground or in some cases seismic activity can be the trigger. This would explain the areas of reported landslides inside of Olympic National Park where there has never been logging activities.



Preventative Measures

preventive.bmpThe image to the left shows one way the Washington DOT tries to prevent loose sediment and rocks from falling down onto roadways. Chain link fencing type material is draped over slopes that have a history of landslides and rock-falls. This preventative measure helps slow down and stop rock from falling down onto roadways. Large scales slide would not be stopped by this kind of setup, but would involve more elaborate measures. In the state of Washington and other areas in the region, entire hillsides along roadways and near urban areas have drainage systems. Engineers drill metal pipes into the bases of hillsides to help drain water out in order to lessen the weight and increase the stability of slopes.

Throughout the Pacific Northwest and southward into California landslides are common both from natural causes and manmade causes. With the expansion of urban areas and poor logging practices landslide are on the rise. Clear cutting techniques create a whole host of environmental issues with landslides being the most destructive and violent consequence. Less harmful logging practices in conjunction with measures to stabilize existing slopes is needed in order to help prevent these costly mass wasting disasters.


References & Data Sources:

Washington State Geospatial Data Archive |

Washington State Department of Natural Resources GIS Data Center |

Washington State Department of Transportation GeoData Distribution Catalog |

Washington State DNR – Geologic Hazards & Mapping |

Washington State DNR – GIS Data |