Identification_Information: Citation: Citation_Information: Originator: Nicolazzo, J.A. Publication_Date: 2026 Title: Landslide mapping and susceptibility modeling in Wrangell Island, Alaska Geospatial_Data_Presentation_Form: data Series_Information: Series_Name: Report of Investigation Issue_Identification: RI 2026-2 Publication_Information: Publication_Place: Fairbanks, Alaska, United States Publisher: Alaska Division of Geological & Geophysical Surveys Other_Citation_Details: 27 p., 3 sheets, scale 1:25,000 Online_Linkage: https://doi.org/10.14509/32057 Description: Abstract: Landslide mapping and susceptibility modeling in Wrangell Island, Alaska, Report of Investigation 2026-2, provides a regional assessment of landslide hazards on Wrangell Island, Alaska, following the fatal 2023 Zimovia Highway landslide, which underscored the safety and financial risks that slope failures pose to people and infrastructure. To support the City and Borough of Wrangell resilience to landslide hazards, the Alaska Division of Geological and Geophysical Surveys (DGGS) expanded an existing landslide inventory and modeled shallow landslide susceptibility and potential debris flow inundation. DGGS integrated previously mapped Tongass National Forest landslides with newly identified slope failures; more than 90 percent of all inventoried features are classified as debris flows. Shallow landslide susceptibility was evaluated using frequency ratio analysis that compared inventoried landslides with terrain factors influencing slope failure. The highest susceptibility is associated with slopes steeper than 31 degrees, southwest- to west-facing aspects, proximity to drainages, and colluvial soils. Model performance was strong, with an area-under-curve value of 0.88. DGGS also modeled the initiation and downslope movement of channelized debris flows and their inundation extents for relatively common events occurring every few years to decades, as well as rare events with recurrence intervals of roughly 1,000 years. Collectively, these analyses provide a regional framework for understanding landslide hazards on Wrangell Island and can inform future land-use planning, development, and scientific investigations. All files can be downloaded from the Alaska Division of Geological & Geophysical Surveys website http://doi.org/10.14509/32057. Purpose: The purpose of this report is to assess landslide hazards on Wrangell Island by expanding the landslide inventory, modeling shallow landslide susceptibility, and estimating debris-flow inundation extents. The work supports community resilience by identifying terrain conditions associated with high susceptibility and by providing regional hazard information to guide land-use planning and future investigations. Supplemental_Information: DGGS completed this landslide hazard assessment for the City and Borough of Wrangell by developing a landslide inventory and database, a map of modeled shallow landslide susceptibility, and a map of modeled channelized debris flow initiation and runout. Together these products provide a regional assessment of slope failure hazards that can support community planning, emergency preparedness, and the identification of areas where more detailed investigations may be warranted. The maps identify areas that are relatively more susceptible to slope failure but do not predict the timing, probability, or recurrence intervals of future landslides. Site‑specific geologic and geotechnical investigations should be completed prior to development in potentially hazard prone areas. The results presented herein are for informational purposes only and are not suitable for legal, engineering, or surveying uses. DGGS is not a regulatory agency; therefore, this report and maps are non‑regulatory. >wrangell_landslide_susceptibility_db: Landslide inventory geodatabase and accompanying data dictionary, includes raster data file depicting slopes susceptible to shallow landslides. Time_Period_of_Content: Time_Period_Information: Single_Date/Time: Calendar_Date: 2026 Currentness_Reference: publication date Status: Progress: Complete Maintenance_and_Update_Frequency: None planned Spatial_Domain: Bounding_Coordinates: West_Bounding_Coordinate: -132.395763 East_Bounding_Coordinate: -131.920505 North_Bounding_Coordinate: 56.489379 South_Bounding_Coordinate: 56.116687 Keywords: Theme: Theme_Keyword_Thesaurus: ISO 19115 Topic Category Theme_Keyword: geoscientificInformation Theme: Theme_Keyword_Thesaurus: Alaska Division of Geological & Geophysical Surveys Theme_Keyword: Bedrock Theme_Keyword: Colluvial Deposits Theme_Keyword: Colluvium Theme_Keyword: Database Theme_Keyword: Debris Flow Theme_Keyword: Debris Flow Deposits Theme_Keyword: DGGS Theme_Keyword: Emergency Preparedness Theme_Keyword: Engineering Theme_Keyword: Environmental Theme_Keyword: Geodatabase Theme_Keyword: Geologic Theme_Keyword: Geologic Hazards Theme_Keyword: Geological Process Theme_Keyword: Geology Theme_Keyword: Geomorphology Theme_Keyword: Geotechnical Theme_Keyword: GIS Theme_Keyword: Hazards Theme_Keyword: Image Interpretation Theme_Keyword: LAHARZ Theme_Keyword: Landslide Theme_Keyword: Landslide Deposits Theme_Keyword: Landslide Susceptibility Theme_Keyword: Liquefaction Theme_Keyword: Mass Flow Deposits Theme_Keyword: Mass-Movement Theme_Keyword: Methods Theme_Keyword: Modeling Theme_Keyword: Physical Properties Theme_Keyword: Rock Avalanche Theme_Keyword: Rockfall Theme_Keyword: Slope Theme_Keyword: Slope Instability Theme_Keyword: Soils Theme_Keyword: Statistics Theme_Keyword: Surface Theme_Keyword: Surface Water Theme_Keyword: Surficial Theme_Keyword: Surficial Geology Theme_Keyword: Tonalite Theme_Keyword: Topography Theme_Keyword: Water Place: Place_Keyword_Thesaurus: Alaska Division of Geological & Geophysical Surveys Place_Keyword: Capel Point Place_Keyword: Cemetery Point Place_Keyword: Chichagof Peak Place_Keyword: City and Borough of Wrangell Place_Keyword: Dewey Hill Place_Keyword: Earl West Creek Place_Keyword: Earl West Marsh Place_Keyword: Highbush Lake Place_Keyword: Institute Creek Place_Keyword: Long Lake Place_Keyword: Nemo Point Place_Keyword: Pat Creek Place_Keyword: Pats Lake Place_Keyword: Petersburg Mining District Place_Keyword: Point Highfield Place_Keyword: Point Shekesti Place_Keyword: Rainbow Falls Place_Keyword: Thoms Creek Place_Keyword: Thoms Lake Place_Keyword: Thoms Place Place_Keyword: Thoms Point Place_Keyword: Trout Lake Place_Keyword: Turn Point Place_Keyword: Wrangell Place_Keyword: Wrangell Island Access_Constraints: This report, map, and/or dataset is available directly from the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (see contact information below). Use_Constraints: Any hard copies or published datasets utilizing these datasets shall clearly indicate their source. If the user has modified the data in any way, the user is obligated to describe the types of modifications the user has made. The user specifically agrees not to misrepresent these datasets, nor to imply that changes made by the user were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys. The State of Alaska makes no express or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product. Point_of_Contact: Contact_Information: Contact_Organization_Primary: Contact_Organization: Alaska Division of Geological & Geophysical Surveys Contact_Position: GIS Manager Contact_Address: Address_Type: mailing and physical Address: 3354 College Rd City: Fairbanks State_or_Province: AK Postal_Code: 99709-3707 Country: USA Contact_Voice_Telephone: 907-451-5020 Contact_Electronic_Mail_Address: dggsgis@alaska.gov Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays Data_Set_Credit: This work was funded by the Federal Emergency Management Agency (FEMA) Cooperative Technical Partnership (CTP) Program grant number EMS-2024-CA-05006. We thank Katreen Wikstrom Jones and our other reviewers for their expertise and thoughtful comments that helped improve the quality of this product. Cross_Reference: Citation_Information: Originator: Hubbard, T.D. Originator: Daanen, R.P. Originator: Stevens, D.S.P. Publication_Date: 2025 Title: Debris flow hazard evaluations for multi-hazard risk mapping in Sitka, Alaska Series_Information: Series_Name: Report of Investigation Issue_Identification: RI 2024-2 Publication_Information: Publication_Place: Fairbanks, Alaska, United States Publisher: Alaska Division of Geological & Geophysical Surveys Other_Citation_Details: 14 p., 6 sheets, scale 1:20,000 Online_Linkage: https://doi.org/10.14509/30187 Cross_Reference: Citation_Information: Originator: Nicolazzo, J.A. Originator: Larsen, M.C. Publication_Date: 2025 Title: Alaska landslide inventory database Series_Information: Series_Name: Digital Data Series Issue_Identification: DDS 23 Publication_Information: Publication_Place: Fairbanks, Alaska, United States Publisher: Alaska Division of Geological & Geophysical Surveys Other_Citation_Details: 14 p Online_Linkage: https://doi.org/10.14509/31697 Cross_Reference: Citation_Information: Originator: Nicolazzo, J.A. Originator: Wikstrom Jones, K.M. Originator: Salisbury, J.B. Originator: Horen, K.C. Publication_Date: 2024 Title: Post-landslide elevation changes detected from multi-temporal lidar surveys of the November 2023 Wrangell, Alaska, landslides Series_Information: Series_Name: Preliminary Interpretive Report Issue_Identification: PIR 2024-2 Publication_Information: Publication_Place: Fairbanks, Alaska, United States Publisher: Alaska Division of Geological & Geophysical Surveys Other_Citation_Details: 22 p Online_Linkage: https://doi.org/10.14509/31124 Cross_Reference: Citation_Information: Originator: Zechmann, J.M. Originator: Wikstrom Jones, K.M. Originator: Wolken, G.J. Publication_Date: 2023 Title: Lidar-derived elevation data for Wrangell Island, Southeast Alaska, collected July 2023 Series_Information: Series_Name: Raw Data File Issue_Identification: RDF 2023-28 v. 1.1 Publication_Information: Publication_Place: Fairbanks, Alaska, United States Publisher: Alaska Division of Geological & Geophysical Surveys Other_Citation_Details: 14 p Online_Linkage: https://doi.org/10.14509/31098 Cross_Reference: Citation_Information: Originator: Zechmann, J.M. Originator: Wikstrom Jones, K.M. Originator: Wolken, G.J. Publication_Date: 2024 Title: Lidar-derived elevation data for Wrangell Island, Southeast Alaska, collected November 28-29, 2023 Series_Information: Series_Name: Raw Data File Issue_Identification: RDF 2024-1 Publication_Information: Publication_Place: Fairbanks, Alaska, United States Publisher: Alaska Division of Geological & Geophysical Surveys Other_Citation_Details: 9 p Online_Linkage: https://doi.org/10.14509/31106 Data_Quality_Information: Attribute_Accuracy: Attribute_Accuracy_Report: Mapping for this project is desktop-based and relies on available datasets without field verification, and lidar provides only a snapshot of landscape conditions at the time of acquisition, so map accuracy may decline as new landslides occur, technology improves, or mapping techniques evolve. Some slope failures were likely missed or misinterpreted due to lidar limits, imagery quality, vegetation cover, and the absence of field checks. The shallow-landslide susceptibility model uses post-failure lidar, meaning mapped initiation points may not represent pre-failure conditions, and the frequency ratio method identifies associations rather than causation, assumes conditioning factors are independent, and produces relative likelihood values rather than absolute probabilities. Channelized debris flow modeling is based on estimated growth factors and volumes, so actual flow behavior may differ, and infrastructure, topographic changes, and mitigation measures can alter flow paths. Material differences, such as woody debris, can affect flow behavior and runout, and lidar analysis cannot always distinguish natural elevation changes from man-made structures. For additional discussion of these limitations, see the accompanying report. Logical_Consistency_Report: The relationships among the observations are generally consistent at a regional scale, but they are constrained by data quality, lidar interpretations, statistical assumptions, and modeling uncertainties. We used frequency ratio analysis to quantify statistical relationships between landslide occurrence and terrain attributes. Some observed relationships may be incomplete, may not represent pre-failure conditions, or may vary as new data, improved techniques, or additional landslides occur. For additional discussion of these limitations and tests used to determine data quality, see the accompanying report. Completeness_Report: The dataset is broadly complete where lidar coverage, imagery, and terrain derivatives adequately capture landscape conditions at the time of acquisition, providing a consistent regional basis for desktop mapping and susceptibility modeling. However, completeness is limited by vegetation cover, imagery quality, lidar resolution, and the absence of field verification, which may result in some slope failures being missed or misinterpreted. Susceptibility relationships derived from frequency ratio analysis reflect statistical associations rather than causation, leaving some interactions among terrain factors unresolved. Debris-flow modeling relies on estimated parameters and does not fully represent real-world variability, infrastructure effects, or material differences, and lidar may not distinguish natural elevation changes from man-made structures. For more detail on data quality and completeness, see the accompanying report. Positional_Accuracy: Horizontal_Positional_Accuracy: Horizontal_Positional_Accuracy_Report: Horizontal accuracy is controlled primarily by the quality of the lidar point cloud and the georeferencing of source datasets. Lidar provides a consistent regional framework, but its snapshot nature means mapped feature positions may not fully represent pre-failure conditions or areas affected by vegetation, shadowing, or limited point density. Desktop interpretation of lidar and imagery introduces additional uncertainty, and some mapped features may be offset or generalized relative to their actual ground locations. For more detail on horizontal accuracy and related limitations, see the accompanying report. Vertical_Positional_Accuracy: Vertical_Positional_Accuracy_Report: Lineage: Process_Step: Process_Description: Geodatabase deployment - DGGS compiled geologic and surficial soils information by incorporating 1:63,360-scale geologic maps of the Petersburg B-1 and B-2 quadrangles, a 1:150,000-scale regional compilation, a 1:7,200-scale surficial soils map of the Wrangell area, USDA STATGO2 soils data, and U.S. Fish & Wildlife Service wetlands data. DGGS incorporated the U.S. Forest Service Tongass National Forest landslide inventory, excluding snow avalanches, and used it to calibrate identification of additional slope failures. DGGS integrated slope failures identified by others and compared two lidar datasets collected in July and November 2023. Following published landslide mapping protocols, DGGS used ArcGIS Pro to create feature classes for deposit polygons, flank polygons, scarp polylines, and landslide initiation points. DGGS interpreted hillshade and slope rasters derived from lidar-based digital terrain models, supplemented by aerial imagery from Maxar, Esri Wayback, and Google Earth, to identify additional slope failures throughout the project period. DGGS mapped landslide components where visible, inferred initiation points where necessary, and included scarp polylines for unstable bedrock areas. These steps produced the landslide inventory geodatabase used in subsequent susceptibility modeling and debris-flow analysis. Process_Date: 2025 Process_Step: Process_Description: Shallow-landslide susceptibility model - DGGS modeled shallow-landslide susceptibility by compiling terrain and environmental variables and evaluating their relationships to inventoried landslides using frequency ratio methods. DGGS derived slope angle, slope aspect, curvature, and topographic wetness index rasters from lidar-based digital terrain models and calculated distances to drainages and roads. DGGS incorporated geologic mapping to represent material conditions. DGGS binned each conditioning factor, computed frequency ratio values for each bin, and combined the results to produce a susceptibility surface. DGGS evaluated model performance using area-under-curve analysis. Process_Date: 2025 Process_Step: Process_Description: Channelized debris flow inundation model - DGGS modeled channelized debris flow initiation, transport, and inundation using the Laharz-based workflow described by Burns and others (2022). DGGS applied the modified Laharz tools to simulate debris flow initiation from shallow landslides or channel-adjacent erosion, routed modeled flows downslope through drainage channels, and allowed modeled volumes to expand during transport. DGGS delineated watershed susceptibility classes by evaluating initiation and transport potential for each watershed. DGGS generated inundation and deposition extents for multiple modeled event severities to produce debris-flow susceptibility and inundation mapping. Process_Date: 2025 Spatial_Data_Organization_Information: Direct_Spatial_Reference_Method: vector Spatial_Reference_Information: Horizontal_Coordinate_System_Definition: Planar: Grid_Coordinate_System: Grid_Coordinate_System_Name: Universal Transverse Mercator Universal_Transverse_Mercator: UTM_Zone_Number: 8 Transverse_Mercator: Scale_Factor_at_Central_Meridian: 0.999600 Longitude_of_Central_Meridian: -135 Latitude_of_Projection_Origin: 0 False_Easting: 500000.000000 False_Northing: 0 Planar_Coordinate_Information: Planar_Coordinate_Encoding_Method: coordinate pair Coordinate_Representation: Abscissa_Resolution: .00000001 Ordinate_Resolution: .00000001 Planar_Distance_Units: Meters Geodetic_Model: Horizontal_Datum_Name: NAD83 (2011) Ellipsoid_Name: GRS 80 Semi-major_Axis: 6378137 Denominator_of_Flattening_Ratio: 298.257222101 Entity_and_Attribute_Information: Detailed_Description: Entity_Type: Entity_Type_Label: ri2026-2_wrangell_landslide_susceptibility_db.gdb, ri2026-2_wrangell_landslide_susceptibility_db_data_dictionary.xlsx Entity_Type_Definition: Landslide inventory geodatabase and accompanying data dictionary, includes raster data file depicting slopes susceptible to shallow landslides. Entity_Type_Definition_Source: DGGS Entity_Attribute_Layer_Name: wrangell_landslide_susceptibility_db Attribute: Attribute_Label: landslide_points Attribute_Definition: Points match polygons Attribute_Definition_Source: this report, DGGS staff Attribute_Domain_Values: Unrepresentable_Domain: File Geodatabase Feature Class Attribute: Attribute_Label: deposit_polygons Attribute_Definition: Polygons tracing deposit extents, or the scar extents of a paleolandslide Attribute_Definition_Source: this report, DGGS staff Attribute_Domain_Values: Unrepresentable_Domain: File Geodatabase Feature Class Attribute: Attribute_Label: flank_polygons Attribute_Definition: Polygons tracing landslide flanks Attribute_Definition_Source: this report, DGGS staff Attribute_Domain_Values: Unrepresentable_Domain: File Geodatabase Feature Class Attribute: Attribute_Label: scarp_polylines Attribute_Definition: Polylines tracing scarps Attribute_Definition_Source: this report, DGGS staff Attribute_Domain_Values: Unrepresentable_Domain: File Geodatabase Feature Class Attribute: Attribute_Label: transport Attribute_Definition: Polylines representing the transport path for a debris flow Attribute_Definition_Source: this report, DGGS staff Attribute_Domain_Values: Unrepresentable_Domain: File Geodatabase Feature Class Attribute: Attribute_Label: watershed Attribute_Definition: Polygons representing the watershed contributing to each potential debris flow Attribute_Definition_Source: this report, DGGS staff Attribute_Domain_Values: Unrepresentable_Domain: File Geodatabase Feature Class Attribute: Attribute_Label: inundation_typical Attribute_Definition: Polygons representing debris flow inundation/deposition extents modeled for a "typical" event Attribute_Definition_Source: this report, DGGS staff Attribute_Domain_Values: Unrepresentable_Domain: File Geodatabase Feature Class Attribute: Attribute_Label: inundation_extreme Attribute_Definition: Polygons representing debris flow inundation/deposition extents, modeled for an "extreme" event Attribute_Definition_Source: this report, DGGS staff Attribute_Domain_Values: Unrepresentable_Domain: File Geodatabase Feature Class Attribute: Attribute_Label: shallow_susceptibility Attribute_Definition: Raster image depicting slopes susceptible to shallow landslides; pixel values indicate risk classification: 1 = very low/none, 2 = low, 3 = moderate, 4 = high, 5 = very high Attribute_Definition_Source: this report, DGGS staff Attribute_Domain_Values: Unrepresentable_Domain: File Geodatabase Raster Dataset Distribution_Information: Distributor: Contact_Information: Contact_Organization_Primary: Contact_Organization: State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys Contact_Address: Address_Type: mailing and physical Address: 3354 College Road City: Fairbanks State_or_Province: AK Postal_Code: 99709-3707 Country: USA Contact_Voice_Telephone: 907-451-5020 Contact_Facsimile_Telephone: 907-451-5050 Contact_Electronic_Mail_Address: dggspubs@alaska.gov Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays Contact_Instructions: Please view our website (http://www.dggs.alaska.gov) for the latest information on available data. 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User specifically agrees not to misrepresent this metadata file, nor to imply that changes made by the user were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys. Metadata_Extensions: Online_Linkage: http://www.dggs.alaska.gov/metadata/dggs.ext Profile_Name: dggs metadata extensions