Inventory map of submarine and subaerial-to-submarine landslide features in Barry Arm Fjord, Prince William Sound, Alaska

Documenting and assessing submarine or subaerial-to-submarine landslides is critical for understanding the history of slope failures and related tsunami impacts in rapidly deglaciating fjord environments. The discovery of the ~500-million-cubic-meter slow-moving subaerial Barry Arm Landslide in northwest Prince William Sound, Alaska (Dai and others, 2020) highlights the need to better understand locations, frequencies, volumes, and mobilities of landslides in fjords. This improved understanding could lead to more accurate models of potential landslide-generated tsunamis.

Here, we present an inventory of submarine and subaerial-to-submarine landslide features at Barry Arm Fjord, Alaska. Data include geographic information system (GIS) polygons of landslide source areas and deposits, polylines of scarps, and bathymetry-derivative rasters used to identify and map the features. The bathymetric rasters include a slope map and a topographic openness map created from a National Oceanic and Atmospheric Administration (NOAA) multi-beam sonar bathymetric digital elevation model (DEM). The bathymetric DEM was collected in August of 2020, covers 52.94 square kilometers, and is available at https://www.ngdc.noaa.gov/nos/H12001-H14000/H13396.html. To minimize resolution bias while maintaining visual fidelity and mapping consistency, we resampled the bathymetry from its original range of resolutions between 1 and 16 meters, to 4 meters, which represents the minimum resolution for mapped areas containing most landslide scarps. Lower resolution areas generally cover deeper and flatter portions of the fjord where fewer landslides are present. For mapping, we used the topographic openness map (Yokoyama and others, 2002) in combination with the slope map (see Red Relief Image Map in Chiba and others, 2008), which allows for good discernment of subtle concavities and convexities in the bathymetry and is well-suited for identifying landslide scars and deposits. To assure that landslide mapping corresponded with the scale and resolution of the bathymetry data, we mapped landslide scarps where the distance between lateral flanks was approximately 25 meters or more and mapped all features to be accurate at a scale of approximately 1:2,500. For subaerial portions of the landslides, we used 5-m Interferometric Synthetic Aperture Radar (IFSAR) and 1-m lidar elevation data. IFSAR and lidar data are available from the Alaska Division of Geological and Geophysical Surveys (DGGS) Elevation Portal (https://elevation.alaska.gov). Neither subaerial DEM data nor any subaerial derivative raster products are included in this data release.

We mapped submarine scarps and submarine landslide deposits but were not able to associate source and deposit areas. Additionally, we mapped two subaerial-to-submarine landslides for which we could associate the source and deposit areas.
 
We mapped a total of 1,108 submarine landslide scarps, irrespective of their deposits. Scarps ranged in total length from 33 to 401 m. Though we did our best to map only arcuate-shaped scarps typically formed by landslides (that is, single-mass failures), as opposed to geomorphic features formed by glacial erosion or submarine currents (for example, sediment waves), we acknowledge the possibility that some mapped scarps may have been formed by processes other than landsliding.

We mapped 101 individual submarine landslide deposits, for which unique source areas could not confidently be identified. Mapped deposits ranged in area from 1,569 to 649,446 square meters. These features were mapped and differentiated based on their morphology (for example, lobate shape or hummocky terrain) and may represent material from both single failures and composites of several failures (for example, submarine fans). In some cases, where nested deposit boundaries were clear, we mapped smaller deposits within the bounds of larger deposits. Because the grain size and origin of sediment could not be confidently distinguished at the scale of bathymetry, we did not differentiate between distal deposits that formed from density currents (for example, turbidite flows) and deposits that formed from high-mobility submarine landslides (for example, subaqueous debris flows).

Subaerial-to-submarine data include two landslides with subaerial source areas and submarine deposits. We observed other near-coastal subaerial landslides during our work, but we only mapped polygons if we identified an associated submarine deposit. For example, we did not map the Barry Arm Landslide because we could not identify any unambiguous submarine expression of the landslide. Additionally, we did not map subaerial and submarine deposits that appeared to be deposited by subaerially initiated debris flows and/or water-dominated flows (for example, debris-flow fans and alluvial fan deltas). The mapped subaerial-to-submarine landslides had deposits with areas of 64,950 and 99,875 square meters.

For purposes of submarine landslide susceptibility mapping, scarp and deposit data are intended to be used in conjunction with other data, such as slope angle, geologic substrate, temporal constraints, or geomorphic units. Ultimately, the full dataset is meant to serve as a component to inform future submarine and subaerial landslide susceptibility assessments in Barry Arm and Prince William Sound at large.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

References Cited:
Chiba, T., Kaneta, S., and Suzuki, Y., 2008, Red relief image map: new visualization method for three dimensional data: The international archives of the photogrammetry, remote sensing and spatial information sciences, v. 37, no. B2, p. 1071–1076. 

Dai, C., Higman, B., Lynett, P.J., Jacquemart, M., Howat, I.M., Liljedahl, A.K., Dufresne, A., Freymueller,J.T., Geertsema, M., Ward Jones, M. and Haeussler, P.J., 2020, Detection and assessment of a large and potentially tsunamigenic periglacial landslide in Barry Arm, Alaska. Geophysical research letters, v. 47, no. 22, p. e2020GL089800.

National Oceanic and Atmospheric Administration [NOAA], 2020, Report for H13396: National Oceanic and Atmospheric Administration [NOAA] web page, accessed April 5, 2021, at https://www.ngdc.noaa.gov/nos/H12001-H14000/H13396.html.

Yokoyama, R., Shirasawa, M., and Pike, R.J., 2002, Visualizing topography by openness: a new application of image processing to digital elevation models: Photogrammetric engineering and remote sensing, v. 68, no. 3, p. 257–266.