Landslide hazard susceptibility mapping in Homer, Alaska

Salisbury, J.B.2024Landslide hazard susceptibility mapping in Homer, Alaskageodatabase, rasterReport of InvestigationRI 2024-3Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys21 p., 3 sheetshttps://doi.org/10.14509/31155Landslide hazard susceptibility mapping in Homer, Alaska, Report of Investigation 2024-3, provides a map and database of historical and prehistoric slope failures, maps of shallow and deep-seated landslide susceptibility, and a map of simulated debris flow runouts for the City of Homer, Alaska and nearby populated areas including Kachemak City and Millers Landing. The landslide inventory map integrates existing maps of landslides caused by the 1964 Great Alaska Earthquake and newly mapped slope failures identified in sequences of aerial photos since 1950 and high-resolution light detection and ranging (lidar) data collected for this project. The Alaska Division of Geological & Geophysical Surveys (DGGS) staff created a shallow landslide susceptibility map following protocols like those developed by the Oregon Department of Geology and Mineral Industries, which includes incorporating landslide inventory data, geotechnical soil properties, and lidar-derived topographic slope to calculate the Factor of Safety (FOS), which serves as a proxy for landslide susceptibility. Debris flow runout extents were generated using the model Laharz, which simulates runout extents based on catchment-specific physical parameters (e.g., hypothetical sediment volumes). Data from these analyses are collectively intended to depict locations where landslides are relatively more likely to occur or are relatively more likely to travel. The results provide important hazard information that can help guide planning and future risk investigations. The maps are not intended to predict slope failures and are site-specific; detailed investigations should be conducted before development in vulnerable areas. Results are for informational purposes and are not intended for legal, engineering, or surveying uses. These data and the interpretive maps and report are available from the DGGS website: http://doi.org/10.14509/31155.This study provides a regional evaluation of landslide hazards in and around the City of Homer. The study aims to help the community better understand landslide hazards, inform mitigation efforts, guide future development activities, and protect public safety.>scarps_lidar: Feature class containing landslide head scarp lines identified and digitized based on geomorphological characteristics in the high-resolution 2019 lidar data. >deposits_lidar: Feature class containing polygons outlining the extent of landslide deposits identified in the high-resolution 2019 lidar data. >deposits_photo: Slope failures identified in georeferenced aerial images. >runouts: Feature dataset collection of feature classes that provide categorical volume and dimensions of modeled hypothetical debris flow runouts from steep catchments in the Homer area. Runout zones are computed using LaharZ (Iverson and others, 1998; Schilling, 1998), and debris flow material characteristics are from Griswold and Iverson (2008). Volumes for runout zone modeling were estimated per catchment based on percentages (5, 10, 25 percent) of the total volume of topsoil in the catchment, as well as the percentage of catchment identified as having sustained slope failure in the historical period. Estimated points beyond which sediments are deposited are based on the geomorphological characteristics of each catchment. >FOS-5m: Raster image data model of the relationship between shear forces acting to move material downslope and forces acting to resist downslope movement. This model was used as a proxy for shallow landslide susceptibility.19642023ground conditionCompleteNone planned-151.732814-151.37294359.70940159.598718ISO 19115 Topic CategorygeoscientificInformationAlaska Division of Geological & Geophysical SurveysCoastalCoastal and RiverColluviumDGGSDebris AvalancheDebris FlowDebris Flow DepositEarthquake Related Slope FailureEngineeringEngineering GeologyEnvironmentEnvironmentalGeologicGeologic HazardsGeological ProcessGeologyGeomorphologyGeotechnicalHazardsHydrology and Surficial GeologyLandslideLiDARModelingRaster ImageSlopeSlope InstabilitySurfaceSurface WaterSurficialSurficial GeologyAlaska Division of Geological & Geophysical SurveysBear CanyonBeluga LakeBidarki CreekBluff PointBridge CreekBridge Creek ReservoirCity of HomerCity of KachemakCoal PointCook InletCrossman RidgeDiamond CreekDiamond GulchDiamond RidgeHomerHomer SpitKachemak BayKachemak CityLampert LakeMud BayNeilson CanyonPalmer CreekSterling HighwayTwitter CreekWaterman CanyonWoodard CanyonThis 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).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.Alaska Division of Geological & Geophysical SurveysMetadata Managermailing and physical

3354 College Road

FairbanksAK99709-3707USA(907)451-5020(907)451-5050dggspubs@alaska.gov8 am to 4:30 pm, Monday through Friday, except State holidaysPlease view our website (https://www.dggs.alaska.gov) for the latest information on available data. Please contact us using the e-mail address provided above when possible.DGGS collected and processed lidar for use in this landslide hazard resiliency project for the City of Homer, funded by the Federal Emergency Management Agency (FEMA) through Cooperating Technical Partnership (CTP) with the City of Homer and DGGS under federal grant number CTP EMS-2018- CA-00016-S01. DGGS thanks the Homer Planning Commission for guidance throughout the multi-year project and Jonathan Godt, Rich Buzard, and Bretwood Higman for constructive reviews that significantly improved this manuscript. We also thank Amy Macpherson for GIS support and Kristen Janssen for designing the report layout.Buzard, R.M.2021Photogrammetry-derived historical orthoimagery for Homer, Alaska from 1951, 1952, 1964, and 1985Raw Data FileRDF 2021-21Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys10 phttps://doi.org/10.14509/30824Buzard, R.M.Overbeck, J.R.2022Coastal bluff stability assessment for Homer, AlaskaReport of InvestigationRI 2022-5Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys22 p., 2 sheets, scale 1:50,000https://doi.org/10.14509/30908Reger, R.D.1978Bluff Point landslide, a massive ancient rock failure near Homer, AlaskaGeologic ReportGR 61BFairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveysp. 5-9https://doi.org/10.14509/409Salisbury, J.B.Daanen, R.P.Herbst, A.M.2021Lidar-derived elevation models for Homer, AlaskaRaw Data FileRDF 2021-2Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys6 phttps://doi.org/10.14509/30591Suleimani, E.N.Nicolsky, D.J.Salisbury, J.B.2019Updated tsunami inundation maps for Homer and Seldovia, AlaskaReport of InvestigationRI 2018-5 v. 2Fairbanks, Alaska, United StatesAlaska Division of Geological & Geophysical Surveys97 p., 11 sheetshttps://doi.org/10.14509/30095The slope failure inventory, shallow landslide susceptibility, modeled debris flow runout map, and integrated results maps were developed using the best available data; however, there are inherent limitations. The intended use of these data products is to help identify the relative slope failure risk in and around Homer, provide a basis for regional planning and increased resiliency, and help identify localities where more detailed slope failure hazard mapping is warranted. Maps are not intended for use at scales other than the published map data scale. The accompanying report provides a complete list of specific limitations and potential sources of error.Not applicableThis data release is complete.DGGS used lidar point cloud data to produce a high-resolution (1.6 ft [0.5 m]) digital terrain model (DTM) and a digital surface model (DSM) for Homer (Salisbury and others, 2021). The DTM, also known as a bare-earth elevation model, was essential for identifying landslide geomorphology beneath dense vegetation, confirming evidence of landslide activity identified in aerial photograph sequences, making FOS calculations, and modeling potential debris flow runouts. The positional accuracy the slope failure inventory, factor of safety, debris flow runout models were developed using the best available data; however, there are inherent limitations to the calculated and interpreted feature extents. The accompanying report provides a complete discussion of specific limitations and potential sources of error.Lidar Acquisition and Processing - DGGS used lidar point cloud data to produce a high-resolution (1.6 ft [0.5 m]) digital terrain model (DTM) and a digital surface model (DSM) for Homer (Salisbury and others, 2021). The DTM, also known as a bare-earth elevation model, was essential for identifying landslide geomorphology beneath dense vegetation, confirming evidence of landslide activity identified in aerial photograph sequences, making FOS calculations, and modeling potential debris flow runouts.2019Landslide Inventory - The comprehensive landslide inventory presented here spans 1952-2019 and was generated by (1) collecting and organizing existing information about previously identified landslides; (2) obtaining, georeferencing, and analyzing sets of aerial photographs since 1952; (3) acquiring, processing, and analyzing high-resolution lidar elevation data; (4) compiling all landslide information into a geodatabase; and (5) generating a landslide inventory map.2021Shallow Landslide Susceptibility - We estimated the FOS for shallow landslides, or earthflows, that are approximately the thickness of the mapped soil column (~5 ft [1.25 m] or less, USDA NRCS, 2005). Data were processed using methods described in the accompanying report.2021Deep-Seated Landslide Susceptibility - Deep-seated landslides involve the failure of materials, as the name implies, several tens of feet below layers of active soil and the uppermost weathered bedrock in an area. We estimated the FOS for Deep-seated landslides using methods described in the accompanying report.2021Debris Flow Runout Modeling - We used LaharZ, a computer model developed by Schilling (1998) for the U.S. Geological Survey, to simulate the behavior and forecast areas likely to be inundated by hypothetical future debris flow events. For all hypothetical runouts, we used ArcGIS Pro focal statistics and conditional tools to smooth the results.2021rasterUniversal Transverse Mercator50.999600-1530500000.0000000coordinate pair.00000001.00000001MetersNAD83 (2011)GRS 806378137298.257222101scarps_lidarFeature class containing landslide head scarp lines identified and digitized based on geomorphological characteristics in the high-resolution 2019 lidar data.DGGSscarps_lidarAgeEstimated year of occurrence of mapped landslides, if applicable. Known 1964 landslide scarps are those that are identified in the geomorphology that are coincident with post-1964 landslides mapped by Waller, 1966.this reporttextNameMostly N/A, except for the deep-seated Bluff Point landslide.this reporttextShape_LengthGeodesic length of digitized headscarp line in meters.this reportnumberScarp_TypeDenotes whether a feature is a landslide head scarp or an intermediate scarp (a scarp within a landslide body.this reporttextdeposits_lidarFeature class containing polygons outlining the extent of landslide deposits identified in the high-resolution 2019 lidar data.DGGSdeposits_lidarAgeEstimated year of occurrence of mapped landslide deposits, if applicable. Known 1964 landslide deposits are those that are identified in the geomorphology that are coincident with post-1964 landslides mapped by Waller, 1966.this reporttextShape_LengthGeodesic length of landslide deposit perimeter in metersthis reportnumberShape_AreaGeodesic area of landslide deposit polygon in square metersthis reportnumberdeposits_photoSlope failures identified in georeferenced aerial images.DGGSdeposits_photoAgeDate range during which the slope failure occurred, based on collection dates of various sets of aerial images.this reporttextShape_LengthGeodesic length of landslide deposit perimeter in meters.this reportnumberShape_AreaGeodesic area of landslide deposit polygon in square meters.this reportnumberrunoutsFeature dataset collection of feature classes that provide categorical volume and dimensions of modeled hypothetical debris flow runouts from steep catchments in the Homer area. Runout zones are computed using LaharZ (Iverson and others, 1998; Schilling, 1998), and debris flow material characteristics are from Griswold and Iverson (2008). Volumes for runout zone modeling were estimated per catchment based on percentages (5, 10, 25 percent) of the total volume of topsoil in the catchment, as well as the percentage of catchment identified as having sustained slope failure in the historical period. Estimated points beyond which sediments are deposited are based on the geomorphological characteristics of each catchment.DGGSrunoutsValueCalculated volumes for runout zone modeling. The values are estimated per catchment and based on percentages (5, 10, 25% or "map" which is the volume of soil mapped in shallow landslides since 1952) of the total volume of topsoil in the catchment, as well as the percentage of catchment identified as having sustained slope failure in the historical period.this reportnumberShape_LengthGeodesic length of modeled hypothetical debris flow runout perimeter in meters.this reportnumberShape_AreaGeodesic area of modeled hypothetical debris flow runout polygon in square meters.this reportnumberri2024-3-homer-ls-hz-FOS-5m.tifRaster image data model of the relationship between shear forces acting to move material downslope and forces acting to resist downslope movement. This model was used as a proxy for shallow landslide susceptibility.DGGSFOS-5mAlaska Division of Geological & Geophysical SurveysMetadata Managermailing and physical

3354 College Road

FairbanksAK99709-3707USA(907)451-5020(907)451-5050dggspubs@alaska.gov8 am to 4:30 pm, Monday through Friday, except State holidaysFGDC Content Standard for Digital Geospatial MetadataFGDC-STD-001-1998If the user has modified the data in any way they are obligated to describe the types of modifications they have performed in the supporting metadata file. User specifically agrees not to imply that changes they made were approved by the Alaska Department of Natural Resources or Division of Geological & Geophysical Surveys.https://dggs.alaska.gov/metadata/dggs.extdggs metadata extensions