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Step Wise Guide
· Which hillsides in my watershed are most prone to landsliding?
In many landscapes, landsliding is an important erosion process that can pose a risk to resources such as fish habitats, roads or other engineered structures. Landsliding can also be an important ecological process, supply sediment and organic material to channels and valley floors. Either from a hazard prevention point of view or from an ecological perspective, it is useful to map the locations in a watershed that are most likely prone to landsliding. Also see the “debris flow” attribute in NetMap, as well as “deep seated landslides/earthflows”.
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 >Generic Erosion Potential. On the right hand side of the interface you will find a drop down list of 6 attributes, starting with the “Generic Erosion Potential Grid”. Grid refers to “grid cells” (or a raster in GIS parlance) or the erosion prediction on hillsides (at the scale of underlying digital elevation model [DEM], commonly 10 x 10 meters).
GEP is a topographic index of slope steepness and convergence. The Generic Erosion Potential Grid does not account for sediment delivery to streams (but the fourth attribute in the list does, see below). GEP is based on the work of Miller and Burnett (2007).
Step 2: The next two attributes in the drop down list provide a channel- or fish-eye view of erosion potential; “Generic Erosion Potential-Segment Scale” is the erosion potential summarized into the stream at the scale of stream adjacent “drainage wings” (see Tech Help). Drainage wings are stream segments local contributing drainage areas on both sides of the channel (e.g., from channel to ridgetop). Channel segments that are approximately 100 m long create drainage wings of approximately 0.1 km2 in area. “Generic Erosion Potential-summed downstream” is the routed version of segment scale GEP. This GEP value is summed and area weighted (GEP/kms), providing information on watershed scale spatial patterns of erosion potential at any scale defined by the channel network.
Step 3 (optional): Channel representations of GEP can be overlaid with other watershed attributes such as fish habitat quality or channel sensitivity, to identify spatial intersections of habitats and stressors (see “Habitat-Stressor” Step Wise Guide). Refer to the Overlap Tool.
The fourth through sixth attribute in the drop down list provides attributes that consider delivery of material to stream channels. Note that the prediction of delivery in GEP, and in other erosion attributes, is based on a user defined channel gradient threshold (See “Delivery” tool under NetMap’s Erosion Tool). The default prediction, loaded in NetMap digital watersheds, is a gradient threshold of 200%, e.g., delivery to all channels.
Step 4 (optional): Alternatively, Go to NetMap Erosion Tools > Shallow Landslides. The “shallow landslide” attribute is a calibrated version of Generic Erosion Potential (GEP), in terms of number of landslides per km2 (using empirical data from the Oregon Coast Range, Miller and Burnett 2007).
A new tool in NetMap (fall, 2013) will allow users to import point and line data on landslide locations and create their own calibrated version of landslide potential (using GEP).