Science behind the choices: Protecting unstable slopes on forested state trust lands and private forests

DNR Earth Sciences Program
Scientist from DNR’s Earth Science Program gathers information about sediments in this stream on state trust lands. Photo: Slaughter/DNR

Shallow landslides can be a significant source of sediment that gets into streams on managed forestlands in the Pacific Northwest. But science is helping us make changes to help prevent sediment runoff into streams from landslides. This is especially important considering the heavy rainfall that comes with fall and winter storms in Washington State.

Landslides do occur naturally. However, our scientists help us prevent landslides that could occur due to human activities such as timber harvests and road building. To help prevent these landslides and sediment delivery into streams in forests on state trust lands—especially where there is significant rainfall on the 1.2 million trust acres in western Washington—DNR’s Earth Sciences Program is hard at work. Among other efforts, Earth Science geologists have worked with trust land managers to develop a computer-based screening tool to help identify areas of potential shallow landslides. They concentrate their study on areas where there are proposed timber harvests and road construction activities.

DNR has all kinds of data about the lands that we manage. These are captured on many layers of various geographic information system (GIS) programs. Using the data, DNR is working with some effective tools to improve detection—2-meter resolution Light Detection and Ranging (LiDAR) digital elevation models on the GIS.  Staff successfully use slope angle and slope convergence to identify landforms commonly recognized as sources of shallow landslides (for you technical folks it’s the colluvial hollows and low-order inner gorges). Here is a large technical presentation on how DNR uses LiDAR.

The use of slope angle and convergence shows great promise as a way to remotely identify potentially high hazard landforms. The goal of the project is to produce a new, LiDAR-based shallow landslide screening tool that allows natural resource managers to identify hazardous landforms during the early stages of the forest management planning process and, as a result, decrease the rate of shallow landsliding associated with forest management activities.

So how do our foresters use the information for designing a timber sale? We leave trees and vegetation on the unstable areas, to hold the soils together and slow the rainfall and runoff.

Trees and vegetation protect slopes stability
DNR’s Forest Practices Division uses maps and data from geologists and other scientists to evaluate  ‘forest management prescriptions’ for these unstable areas on 9 million acres of private and state forestlands. Forest Practices foresters are examining harvest applications that involve areas with unstable slopes, and closely tracking those on the ground, to follow up and make sure they are protected.

Forest Practices rules restrict harvest on potentially unstable slopes because trees and vegetation help protect slopes by locking together the soils through inter-connected root systems; the leaves and branches of the canopy intercept the pounding rain, protecting the ground from direct impact of water. There also is evaporation of water off of the leaf surfaces. Some have speculated that big trees are heavy and may cause a slope to slide in a rain event, but the “weight of trees” argument is negligible compared with the other factors.

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