Landslide mapping in Seattle, Washington

Abstract Seattle, Washington, has one of the most progressive landslide ordinances and mitigation programs in the United States. The necessity for this is driven by concentrated winter precipitation, steep slopes, and glacial soils that are susceptible to instability. Early in the European development, engineers recognized the vulnerability of Seattle hillsides to landslides, particularly when the hillsides were disturbed. In this paper, we trace the history of mapping of landslide-prone ground in Seattle, started by Miller (1973), detailed by Tubbs (1974, 1975), and most recently catalogued by Shannon & Wilson (2000, 2003). Owing to the relative homogeneity of geology (glacial and other Pleistocene nonglacial soils), landsliding consists mainly of debris landslides, debris avalanches, and slumps. Debris avalanches that engage the shallow colluvial soils are by far the most common. Deep-seated slumps are not as common but can encompass a large area and affect many property owners. Several studies agree that ~80% of the landslides include one or more human influences. To limit the amount of damage that landslides cause to private properties and City of Seattle infrastructure, rules were promulgated by the city in 1984. These rules have been revised three times, and they now serve as an example for the rest of the state of Washington. Following destructive, widespread slope instability in 1997 and the Shannon & Wilson studies in 2000 and 2003, Seattle Public Utilities instituted a state-of-the-industry landslide mitigation program that has already paid dividends for the city.