Debris Flow Potential

Step Wise Guide

Erosion

· Which headwater streams are most susceptible to debris flows or gullying?

Debris flows represent one of the most hazardous erosion processes in mountainous watersheds. Debris flows are often triggered by shallow landslides and they can travel great distances along steep, headwater streams. Debris flows can impact fish habitats, roads and other engineered structures.

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 Erosion Tools > Channelized Mass Wasting > Debris Flows. This tool provides five types of predictions pertinent to debris flow potential. The first two in the drop down list “Debris Flow Susceptibility-Reaches” and “Debris Flow Susceptibility-Junctions” provide site specific predictions about the likelihood of debris flows in headwater streams. The first one displays a map involving all stream channels in a watershed, focusing particularly on headwater locations.

The second one in the drop down list (Debris Flow Susceptibility-Junctions) focuses on tributary confluences only, since that is where debris flows commonly impact important resources such as fish habitats, roads and other engineered structures.

The values (probability) originate from a specific dataset from the Oregon Coast Range. However, the controlling variables of debris flow initiation and movement (number of landslide source areas, steepness of headwater streams, tributary junction angles) are common across many landscapes. Hence, the debris flow parameter can be used as a screening tool in many landscapes, although the range of values should be considered in a relativistic sense rather than an absolute set of probability values, including within the Oregon Coast Range.

Steps 2 through 4 (optional): There are three other types of information (grids) that pertain to debris flows or the combination of shallow landslides and debris flows (available in the drop down list). “Debris flow traversal” provides a display of those hillslopes and headwater streams that have a debris flow potential that will intersect river channels of a specific gradient threshold. In other words, if the gradient threshold is 5%, very few headwater streams will be predicted to have debris flows that will reach channels of 5% or less. Conversely, setting the gradient threshold at 20% will allow many more hillside areas and headwater streams to route debris flows to those channels.

The next two attributes “traversal proportion grid” and “source-traversal proportion grid” displays debris flow and landslide-debris flow maps but in terms of proportion of risk, starting with the highest 20%, the next highest 20% and so on. Refer to Tech Help.