Roads-Unstable Hillslopes

Step Wise Guide

Transportation/Energy

· What is the landslide potential associated with roads?

Roads, particularly logging roads, can lead to heighted slope instability potential because roads can cause drainage diversion (e.g., routing ditch flow over the road and onto unstable slopes), road side cast failure (particularly relevant to older roads), road cut slopes that can lead to instability, and removal of hillslope vegetation. NetMap classifies road segments based on the predicted slope stability potential of the hillslope (underlying the road). First, NetMap breaks roads at pixel cell borders, creating road segment lengths of meters to 10 meters. Next, each pixel scale road segment is given a value of the underlying predicted hillslope stability, either Generic Erosion Potential (GEP), or shallow landslide potential. Road - stream crossings can also be classified by headwater debris flow potential, providing an index of the potential of a road to trigger a debris flow or the potential for a debris flow to impact or destroy a road.

Refer to the "Warning" button in the tool interface. Follow up any remote sensing work, including using NetMap, with field work to verify environmental conditions and landslide and debris flow risk. There is never zero risk, there is only degrees of risk, which often is best considered on a relative basis.

Step 1: Go to NetMap Road Modue > Road Stability. Follow the instructions.

Step 2: Go to NetMap Road Modue > Road Stability > Results to Streams. This tool function allows a user to summarize the road stability predictions per slice of hillslope (e.g., drainage wing) and then report the results to stream channels. Predictions are made for both the left and right side of the stream channel, as well as the total. The user can then go to the Overlap tool (Basic Tool Module > Risk Analysis > Stream Basin Overlap) and locate those hillsides that have the highest predicted road instability potential, and where it intersects valuable aquatic habitats. For example, the Overlap Tool is used to calculate where the highest 1% (or 5% or 10%) of road instability potential is located with respect to the best (highest 10%) of fish habitat.