Field Observations of Past Debris Flows, February 2024, Kosrae 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 Kosrae 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 Kosrae State, FSM. The data table “FSM_Kosrae_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.

滑坡灾害对密克罗尼西亚联邦（Federated States of Micronesia, FSM）境内的社区与基础设施构成严重威胁。为支撑灾害评估与减灾工作，美国地质调查局（United States Geological Survey, USGS）滑坡灾害援助小组（Landslide Disaster Assistance Team, LDAT）为密克罗尼西亚联邦科斯雷州（Kosrae State）研发了滑坡敏感性（landslide susceptibility）与泥石流运动范围（debris-flow runout）模型及制图产品。如需了解该LDAT-FSM项目的更多详情，请访问项目主页：https://www.sciencebase.gov/catalog/item/6619520ad34e7eb9eb7dcacb。

野外实地观测可用于验证与校准滑坡敏感性及泥石流运动范围模型的输出结果。本数据报告收录了2024年2月于密克罗尼西亚联邦科斯雷州主岛选定的可抵达点位开展的野外踏勘观测、沉积物描述及影像资料。数据表"FSM_Kosrae_FieldObservationsOfPastDebrisFlows_Cyr.csv"收录了我们基于上述观测得出的既往泥石流活动存在性判定结果。此类判定共分为五类：1）是：某点位存在足够野外证据表明曾发生过一次或多次泥石流；2）否：某点位无足够野外证据证明存在泥石流活动；3）待定：某点位野外观测结果存在矛盾或不明确；4）未确定：通常因点位条件限制，无法开展充分野外观测以得出明确结论；5）其他：虽认为该观测内容有收录价值，但与泥石流活动无关。本元数据的实体与属性章节将提供更多细节，包括各类判定的置信度定义。

本次观测主要聚焦于溪流穿越道路的点位。在判定为“是”的既往泥石流活动点位，我们观测到的沉积物通常具有以下特征：以块状、黏结性黏土与细粒沉积物为基质，整体呈橙黄色调，成土作用（pedogenesis）证据极少；基质中混杂棱角次圆状、分选性差的细砾至巨砾级岩屑，排列杂乱无章（无叠瓦构造（imbrication））。判定为“待定”的点位通常呈现部分泥石流沉积的地貌特征，但在本次踏勘中无法精准确定其成因。例如，我们常观测到一种被我们俗称为“布丁岩”或具有“布丁岩”外观的沉积物。此类沉积物为未固结的黏性泥质沉积物，其中均匀散布少量分选中等至良好的细砾至圆砾级岩屑，简言之即粒径均一的岩屑“漂浮”于以块状黏性泥为主的沉积物中。该类沉积物的详细信息可参见对应点位的单独野外描述。任何野外观测均无法做到面面俱到，本次呈现的研究结果仅涵盖当时可开展的观测内容。