LANDSLIDE HAZARD STUDY OF MERCER ISLAND, WA

Mercer Island, located in the Puget Lowland of northwest Washington State, has high landslide hazard for many reasons. The Puget Lowland was repeatedly glaciated in the Quaternary, and the advances and retreats of the ice left a complex, spatially variable glacial and interglacial stratigraphy behind. The final retreat of the ice also left over-steepened hillsides throughout the Puget Lowland, and Mercer Island is almost entirely surrounded by such hillsides situated in the middle of a deep, subglacially scoured trough. Finally, the Puget Lowland receives abundant annual rainfall, especially in the winter. The combination of poorly consolidated glacial sediments, steep hillsides, and wet conditions for much of the year de-stabilize hillslopes and cause landslides. Recent LiDAR data show that Mercer Island has several historic (likely older than 150 years old), as well as abundant recent landslides. In 2009, Troost and Wisher used field mapping, geomorphic analysis, a geotechnical database, and GIS to create a landslide inventory and hazard map of Mercer Island. Our new study uses protocols from the Oregon Department of Geology and Mineral Industries (DOGAMI) and the Washington State Department of Natural Resources to map landslide hazards on Mercer Island using GIS, geotechnical parameters, and field mapping. We found good correlation between areas that are modeled as “susceptible” to landslides and the locations of existing landslides on Mercer Island, as mapped by Troost and Wisher (2009). As expected, the areas that are most susceptible to landslides on Mercer Island are those around the edges of the island, especially in areas of steep slopes, deep ravines, and/or former landslides. The DOGAMI protocols overestimate the area of high risk to landslides due to the abundance of steep, but short landscaping elements and road cuts. This study also identified landslide hazards on Mercer Island in accordance with Washington State’s Growth Management Act.