MITIGATION OF LANDSLIDE HAZARDS ALONG PUGET SOUND SHORELINES

Thousands of single-family, multi-family and/or vacation residences are located along the shorelines of Puget Sound in western Washington. Many of these residences are situated at the base of bluffs that are prone to landsliding as a result of the geologic history and geomorphic evolution of the Puget Lowland. Numerous such houses have been destroyed and in some cases occupants killed or injured. The risks of damage, injury and death are likely to increase as houses which were originally built as summer cabins are increasingly occupied year-round. The risks also increase as the density of land use increases, and can further increase due to upland land use changes. Economic impact increases as the value of the properties at risk increases due to escalating real estate prices.

Simple avoidance of the landslide-prone shoreline areas is not practicable as the sole mitigation, particularly for existing developed sites. In most locations, one or more of four general approaches can be used to mitigate the landslide hazards: (1) The steep bluff source area can be modified to reduce the potential of landslide occurrence or the potential for damage from landslides. (2) Appropriate structures or other provisions can be made to detain or deflect landslide debris. (3) The at-risk buildings can be replaced, re-constructed, or retrofitted, with specific design measures employed to minimize damage to the structures and/or risks to the occupants. (4) Use of the houses or other buildings can be restricted. All of these mitigation approaches have been used in the Puget Sound area and are illustrated through examples.