March Meeting: Landslides, Erosion, and Uplift in the Olympic Peninsula
Event Details
Landslides, Erosion, and Uplift on the Olympic Peninsula, Washington: A Tale of Two Applied Geologic Projects
Presentation by Kathy Goetz Troost, PhD, LG
Associate Teaching Professor, Earth and Space Sciences Department, University of Washington, Seattle
Senior Consultant, Shannon & Wilson, Inc., Seattle, Washington
Zoom Link: https://us06web.zoom.us/j/89300156145
This tale of two sites is really a tale of many enigmas in part, because so little geologic work has been focused there. We, a team of applied geology students, faculty, and volunteers, have been working on geologic questions on the Pacific coast at Rialto Beach and inland at a large landslide on Undi Road, on the Bogachiel River.
Our objectives at Rialto Beach focus on geologic hazards and their impacts to coastal communities and the environment: 1) did uplift and landslides occur at Rialto Beach because of the last major Cascadia Subduction Zone earthquake, and 2) what is the cause of an apparent increase in erosion rates at Rialto Beach and does that increase relate to uplift? To address these questions, we have been slowly gathering data and mapping a low-elevation terrace to determine the age of onlapping landslides, interpret sedimentary facies changes, measure erosion, and monitor trees on the terrace. We found that landslides have been occurring there for at least hundreds of years, including one about 300 years ago. The low-elevation terrace, which stands about 1-2 meters above the beach, hosts a wave-cut bedrock surface that stands 1-2 meters above the modern wave cut bedrock platform suggesting uplift. Uplift and an interruption of sediment supply to the beach may have triggered renewed erosion of the terrace. In addition, changes in climate and sea level could be contributing to an increase in erosion rates. Our best estimate is an increase of 0.5 to 3-5 m/year over the last 50 years. Ongoing studies will help to refine our answers.
Our objectives at the Undi Road landslide focus on geologic hazards and their impacts to recreation, fisheries, riverine/hillslope ecosystems, and residents. Movement in the central portion of the landslide finally destroyed part of the road causing Jefferson County to build a bypass road in 2016 to maintain access for people living upstream and for recreation. We have been mapping and monitoring the landslide over the past two years to evaluate rate of movement, possible triggers, possible mechanics, and factor of safety of the slope. Over the winter of 2021-2022, movement occurred throughout parts of the larger landslide complex, including at the uppermost headscarp, 65 meters above the most active portion of the landslide. Three benchmarks were destroyed, and two others are nearly gone. Movement exceeds tens of meters in the vertical and meters in the horizontal directions. Low shear strength in a basal alpine glaciolacustrine unit and continued encroachment by the Bogachiel River play roles in the failures. Careful interpretation of the geologic history suggests that buried failure planes and unconformities set the stage for continued failure and sediment loading of the Bogachiel River. Ongoing studies are focused to help us answer our questions.
Kathy Troost, PhD, LG, has over 42 years of experience as a Quaternary geologist providing expertise on geologic hazards and engineering geology for the public and private sectors. She began her career with Shannon & Wilson, Inc. (S&W), where she worked for 19 years in three of their offices. She left S&W to get her graduate degrees at the University of Washington. While there, she co-directed (then later directed) the Pacific Northwest Center for Geologic Mapping Studies (GeoMapNW) where she designed and managed the subsurface database that now resides at the Washington Geological Survey. Over her 12 years with GeoMapNW, she completed numerous geologic maps and derivative maps of urban areas. After her time with GeoMapNW, and after a few years consulting as Troost Geosciences, Inc., she rejoined the UW in 2012, this time as a faculty member. Kathy now teaches and co-directs the MESSAGe program (Masters in Earth and Space Sciences applied geosciences) at UW. In addition to teaching courses such as Engineering Geology, Advanced Geology Investigation Methods, Technical Communications, and Professional Practice, she mentors and advises many graduate students each year. She has been and is very active in outreach, networking, and professional organizations. She is currently a member of the Washington Geology Licensing Board and the State Mapping Advisory Board. Most recently she rejoined S&W as a part-time senior consultant.