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This feature class contains landslides compiled from a variety of sources, spanning the past few decades. The Landslide Compilation contains landslides from the following sources: (1) Landslides that are mapped within 1:24,000-scale geologic mapping (not statewide) and our statewide 1:100,000-scale geologic map compilation; (2) a miscellaneous compilation of pre-existing landslide data from several sources, including the Washington Geological Survey, the Department of Natural Resources’ Forest Practices Division, and other federal and private entities; (3) landslides mapped as part of Watershed Analysis efforts for the Forest Practices Board; (4) reconnaissance-scale mapping of landslides associated with significant landslide events; and (5) a study of near-shore landforms along the Salish Sea.
1:24,000-scale and 1:100,000-scale Landslides from Geologic Mappings how landslides that were extracted from 1:24,000- and 1:100,000-scale geologic maps. Landslide mapping was not the primary purpose of these projects, and the absence of a mapped landslide in a particular location does not suggest that no landslide exists or that there is no landslide risk. Also, due to the scale of the final mapping products, landslides with deposits too small to be identified in any included scale of geologic mapping (for example, debris flows, debris avalanches, rock topples, etc.) are not typically included. Only partial coverage exists for landslides mapped at 1:24,000 scale. 1:100,000-scale mapping covers the entire state.
Miscellaneous Landslide Mapping is compiled landslide data from many sources focused on landslide mapping. Each project has its own protocol and methods, so it is up to the user to determine how best to interpret the landslide data. To avoid the risk of misinterpretation, invalid results, and erroneous conclusions, users should consider original map scale, collection methodology, original mapping purpose, currency of data, and any other conditions specific to every data element and each dataset as a whole.
Watershed Analysis Landslide Mapping polygons were created to support forest practices rules in identification of unstable slopes in various watersheds throughout the state. Rule-identified landform mapping was conducted using aerial photographs, maps (including geology, soil, and topographic), field observations, and limited lidar. Landforms were identified and mapped using a Watershed Analysis or the Landslide Hazard Zonation Protocol. All rule-identified landforms are not shown; only areas identified as landslides in the original watershed analyses are included in this data.
Reconnaissance-Scale Landslide Mapping is compiled landslides mapped from surveys immediately following widespread rain or rain-on-snow events in Washington. Mapping techniques for these large-scale surveys vary between aerial surveys from small aircraft, aerial photography, or lidar identification, with minimal field verification.
The Salish Sea Landforms identifies landforms along 2,200 miles of Salish Sea shoreline that have characteristics of deep-seated landslides, but lack the thorough investigation necessary to classify these landforms as landslides. All landforms in this layer were identified using historic aerial imagery and 3- to 6-foot LiDAR digital elevation models (DEM). Based on interpretation of LiDAR derivatives (such as hill shades, contour lines, and slope gradient) and aerial imagery, observations of geomorphic features representative of a deep-seated landslide were delineated in a GIS. Due to the method of data collection and the certainty of the data, this dataset should not be confused with a landslide inventory, which typically undergoes a thorough, historic aerial imagery analysis, field validation, and peer review.
Landslides from various projects may or may not have used lidar for landslide identification. Landslides in this compilation are mapped at various scales and with varied purpose. Many of these compilation layers are not statewide in extent – the absence of a mapped landslide in a particular location does not suggest that no landslide exists or that there is no landslide risk.
To avoid the risk of misinterpretation, invalid results, and erroneous conclusions, users must consider original map scale, collection methodology, currency of data, and any other conditions specific to every data element and each dataset as a whole. This dataset is not intended as a substitute for a detailed investigation of potential slope instability by a qualified practitioner. Site-specific analysis may give results that differ from those displayed on the map.
In areas where the Landslide Compilation is overlapped by the lidar-based Landslide Inventory, the overlapping zones have been attributed as ‘superseded.’ These layers are not displayed on the portal but are available in the GIS download of the Landslide Compilation.