From each pixel with a nonzero landslide density, we trace the downslope flow path defined by elevation points in the DEM. We treat this as a relative susceptibility for debris-flow initiation PI. Generic erosion potential or landslide density translates to a probability of finding a landslide within any DEM pixel area. The landslide density and delivery models are used to calculate the relative susceptibility for direct debris-flow impacts to stream channels in a parameter called Debris Flow Susceptibility - Reaches (Figure 2). Other names given to debris flows include debris torrents, sluice outs and mud flows. Debris flows typically stop at valley floors. Entrainment of additional sediment and organic debris in first- and second-order channels can increase the volume of the original landslide by 1000 percent or more, enabling debris flows to become more destructive as their volume increases with distance traveled (Benda and Cundy, 1990). Debris flows contain 70 to 80 percent solids and only 20 to 30 percent water by volume. Debris flows are initiated by liquefaction of landslide material concurrently with failure or immediately thereafter as the soil mass and reinforcing roots break up. Model Description (1/4): Debris Flow Risk in StreamsĪ debris flow is defined as a highly mobile slurry of soil, rock, vegetation and water that can travel many hundreds of meters from its point of initiation through steep (more than 5 degrees gradient) and confined mountain channels (Figure 1). NetMap Module/Tool: Erosion Processes/Debris Flow Although the model is calibrated using a population of stream segments following a major (1996) landslide triggering storm in the Oregon Coast Range, it is assumed that the relative likelihood of debris flows found in that case can be applied to other landscapes where debris flows are driven by similar topographic and climatic controls. Hence, the model output provides a relative likelihood of debris flow potential in any stream reach, relative to all other similar stream reaches in a landscape. We expect, therefore, out of 1000 reaches with a P_DF_AVE value of 0.02, we should find twice as many with evidence of debris-flow deposition or scour than in 1000 reaches with a P_DF_AVE value of 0.01.

The P_DF_AVE value reflects the relative proportion of similar reaches mapped with debris-flow effects. For example, all reaches with the same modeled P_DF_AVE value have similar physical characteristics found empirically related to debris-flow occurrence (number of landslide sources, the length, steepness, and confinement of the potential debris-flow tracks leading from those landslide source areas to the reach). Units: The modeled relative spatial frequency of debris flow scour and deposition (and traversal, neither scour or deposition). Grid name: proportions_basinID Common Name: Source Traversal proportion grid Grid name: travprop_basinID Common Name: Traversal proportion grid Grid name: trav_ID Common Name: Debris flow traversal (the probability of debris flows traversing channel grid cells) Also see Warning about using Slope Stability Models.įield Name: P_DF_AVE Common Name: Debris Flow Susceptibilityįield Name: DF_Junct Common Name: Debris Flow Susceptibility – Junctions A calibration tool is planned for NetMap in later 2014. Although the model employs universal topographic indicators of debris flow initiation and runout potential increased accuracy and resolution can be obtained when debris flow model is calibrated using local data (georeferenced locations of debris flow initiation and runout). Thus when used outside of that landscape, the model should be considered as a screening tool that provides a relative ranking of instability potential(e.g., low - high). Important Notice: This erosion model is calibrated for the Oregon Coast Range. There are four component models that address debris flow rish in streams, debris flow risk at tributary confluences (fish eye view), proportion of debris flow risk (0-100%) based on predicted delivery to streams of a specified channel gradient threshold (see Delivery Tool ), and combined shallow landslide-debris flow risk proportion (also using a user specified delivery channel gradient threshold).

Predictions include debris flow susceptibility across entire river networks, focusing on headwater streams, and debris flow susceptibilities in mainstem (fish bearing) channels at headwater tributary junctions (e.g., likely areas of debris flows impacts from steeper headwater channels). Parameter Description: The susceptibility of channels to debris flow movement, scour and deposition, in terms of probability an empirical model.

Netmap valley width