We assess possible threats to homes and property on the Reservation due to shoreline erosion. Shoreline erosion occurs when water erodes the coastal bluff causing instability to the surrounding property. The threat of sea level rise and extreme weather resulting from climate change may increase the rate of shoreline erosion in the future.
The shoreline is an important tribal resource, which provides not only water access and views, but also important ecological functions. Beaches exist where erosion by tides and waves is balanced by a renewal of beach material, like sand and gravel, which in Port Gamble Bay is supplied by eroded bluff material. This type of erosion is a natural process and is necessary to maintain intertidal habitat for important species of forage fish, clams, oysters, crab and juvenile salmon.
The coastal bluff will continue to erode and retreat. The coastal bluff is eroding in response to different mechanisms, such as waves and rainfall. The rate of coastal bluff retreat is controlled by the properties of the material in the bluff and the forces acting upon them. In Port Gamble Bay, the water runoff and drainage from the uplands are the dominant forces.
The Tribe has completed many studies of its shoreline over the past decades for site-specific analysis and in response to sudden geological events.
- In 1975, the Army Corps of Engineers was engaged to investigate bluff erosion and found that “the slope is not retreating by wave erosion, but that slope failure is the result of springs accelerated by the addition of septic tank effluents.” The Tribe has since installed a sewer system so septic effluent no longer drains to the bluff. The Corps also concluded that “planting vegetation may slow the rate of slope sloughing, but will not eliminate the cause.”
- In 1987, Applied Geotechnology, Inc. performed geotechnical investigations of a single slope failure at a residence south of Point Julia, and they found that the slide was caused by seepage flow through a sand layer overlying a silt and clay layer. They recommended managing drainage and surface water controls.
- In 1997 a study by Shannon & Wilson, Inc. investigated reports of sliding at five residences in December 1996 and January 1997. They suggested it may be most cost-effective to relocate residences away from the slope at a distance two times the height of the bank away from the toe of the slope.
- In 2016, a team of hydrologists, engineers and ecologists conducted a study to consider both the bluff erosion issue and the ecological function of the shoreline of the entire reservation. Their recommendations included forming a shoreline workgroup and initiating an erosion monitoring program.
As a part of the shoreline management and monitoring project, Environmental Program staff inspected each bluff property on the reservation to look for signs of erosion. They continue to consult with property residents and are working on a comprehensive plan to manage shoreline erosion on the reservation.