Field Observations of Past Debris Flows, February 2024, Pohnpei State, Federated States of Micronesia

Landslide hazards pose a significant threat to communities and infrastructure across the Federated States of Micronesia (FSM). To support hazard assessment and mitigation efforts, the USGS Landslide Disaster Assistance Team (LDAT) produced landslide susceptibility and debris-flow runout models and map products for Pohnpei State, FSM. For more information on the LDAT FSM project, refer to the project landing page: https://www.sciencebase.gov/catalog/item/6619520ad34e7eb9eb7dcacb. Field observations help validate and calibrate landslide susceptibility and debris-flow runout model output. This data report includes our reconnaissance field observations, deposit descriptions, and photographs, collected in February 2024 at select accessible sites on the main island of Pohnpei State, FSM. The data table “FSM_Pohnpei_FieldObservationsOfPastDebrisFlows_Cyr.csv” includes our determinations of the presence or absence of previous debris-flow activity based on those observations. These determinations include 1) yes, there was sufficient field evidence of one or more debris flows at a location; 2) no, there was not sufficient field evidence of debris flows at a location; 3) maybe, because field observations were conflicting or inconclusive at a location; 4) undetermined, usually because site conditions precluded making enough field observations to make a more definitive determination; and 5) other, which were observations we thought important to include, but were not related to debris-flow activity. Additional details, including the definition of the degree of confidence in each determination, are provided in the Entity and Attribute section of this metadata. Observations were primarily made where stream drainages crossed roads. At sites where our determination was “yes” for previous debris-flow activity, we observed deposits typically characterized by massive, cohesive, clay and fine-grained sediment matrix, commonly with an orangish hue and minimal evidence of pedogenesis, that supported subangular to subrounded, poorly sorted, pebble- to boulder-sized clasts that were disorganized (without imbrication). The “maybe” sites often exhibited some morphological aspects of debris-flow deposition, but we could not precisely determine their origin during our reconnaissance. For example, we frequently observed a deposit which we referred to colloquially as being a “puddingstone” or having a “puddingstone” appearance. These are unlithified deposits of cohesive mud with sparse, moderately- to well-sorted, pebble- to cobble-sized clasts evenly distributed throughout, in other words, uniformly sized clasts “floating” in a deposit of mostly massive, cohesive mud. Additional details can be found in the individual field descriptions of sites where this type of deposit occurred. No field observation can be exhaustive, and the findings presented here are limited to what observations were possible at the time.

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