Author(s): Jim Johannessen and Andrea MacLennan
The shores of the Puget Sound region provide valuable recreational and economic benefits for the burgeoning population. Beaches and bluffs of the Puget Sound region provide critical nearshore habitat functions and values for the region’s fish and wildlife. Coastal bluffs are the primary source of beach sediment along the Puget Sound shore, and their natural erosion is essential for maintaining beaches and associated nearshore habitats. Critical habitats dependent on functioning coastal systems include coastal forests, spawning beaches for forage fish (such as surf smelt), eelgrass beds, and salt marshes, all of which shape the health of salmon populations.
Puget Sound and the Northern Straits encompass the central feature in the Puget Lowland, consisting of a complex series of deep, generally north-south-trending basins. Puget Sound was created by the repeated advance and retreat of glacial ice sheets, the most recent of which advanced between 15,000 and 13,000 years ago (Booth 1994). The area’s glacial legacy has resulted in abundant steep bluffs (sometimes referred to as sea cliffs, although locally termed bluffs) of up to 400 feet in elevation, which are both dramatic and dynamic features.
The shores of Puget Sound are highly convoluted, encompassing approximately 2,380 miles of shore length (Washington State Department of Natural Resources (WDNR) 2001). Extensive sand and gravel beaches provide a wide variety of coastal configurations that in turn serve as habitat links between rivers and the marine environment for salmon and the many interdependent species of the Puget Sound region.
The general shore types found throughout the region include rocky coasts, coastal bluffs, bluff-backed beaches, depositional beaches, deltaic shores, and spits associated with protected lagoons and salt marshes. The most prevalent of these shore types are bluff-backed beaches: coastal bluffs fronted by narrow mixed sand and gravel beaches. However, excluding spits and other types of depositional beaches, most Puget Sound area beaches are only a thin veneer of sediment atop a relatively flat erosional platform (Shipman 1995), and are therefore subject to erosion when conditions are altered due to human-induced change.
Bluff erosion is caused by marine, subaerial and human-induced erosion, often in concert. Waves caused by windstorms typically drive bluff erosion over the long term. Wave attack interacts with locally variable bluff geology and aspect, toe protection and other factors, including management practices. Precipitation frequently triggers landslides at over-steepened bluffs (Tubbs 1974). Coastal bluffs are the primary source of sediment for most Puget Sound beaches (Keuler 1988), such that bulkheads and other shore modifications that limit bluff erosion and coastal sediment transport have led to major changes in sediment supply and associated changes in beach and habitat stability.
Cumulatively, more than 805 miles, or 34 percent, of the Puget Sound and Northern Straits shore has been modified (WDNR 2001). Bulkheads and other shore-parallel structures along coastal bluffs impound potential beach sediment, commonly bury upper beach spawning habitat and fundamentally alter the beach and backshore, resulting in a decrease in the amount of drift sediment available for maintenance of down-drift beaches. Burial of the backshore results in reduced beach width (Griggs 2005) and loss of habitat area. The remaining upper beach typically suffers from changed hydraulic conditions. Although research has been very limited locally, bulkheads are thought to cause some degree of localized beach erosion (MacDonald et al. 1994). As a result of these changes, beaches become more coarse-grained and gravel-dominant, which does not provide the same quality of habitat as a finer grained beach (Thom et al. 1994, MacDonald et al. 1994). Cross-shore structures, such as groins and jetties, also impact coastal processes by impeding sediment transport alongshore. This often results in sediment accumulating up-drift of the structure and erosion along the down-drift shore. Other common shore modifications in the Puget Sound nearshore include causeways, fill and dredge areas, mine and quarry areas, and overwater structures.
Sea level rise (SLR) and the impacts of global climate change are currently bringing accelerated change to the region’s beaches and bluffs, with increased erosion rates and coastal flooding, heightening the need for new long-range planning efforts. Spatial variability in Puget Sound tectonics processes produces different rates of sea level rise across the region, referred to as relative sea level rise. Relative sea level rise in north Puget Sound is close to the global average, and is up to double the average in south Puget Sound (Snover et al. 2005).
Sea level rise will result in the landward migration of the shoreline, a response to the changes in the elevation of breaking waves on the beach profile. At unprotected shores, the response is generally a self-regulating process, as additional (eroded) sediment from the backshore or bluffs allows for down-drift shores to become higher and move landward, thereby maintaining the beach profile (Bray and Hooke 1997). Existing shore protection at beaches and bluffs will likely be less effective at preventing erosion with sea level rise, and the structures will likely incur damage due to increased water depths and greater wave energy and runup. This would result in increased probability of structural damage, necessitating retreat or some form of repair (Bray and Hooke 1997). As a result, soft-shore protection is likely to be more effective at managing erosion control in the context of rising sea levels.
Coastal restoration that involves removing impairments to physical processes along Puget Sound area beaches and bluffs has begun to benefit nearshore habitats. This involves structure removal, repairing changes in estuarine function, and beach enhancement. Conservation of functioning areas and public education are also seen as key components of ecosystem recovery. A conceptual model for beaches and bluff restoration developed for this paper presents proposed restoration actions (management measures) and links these to restored natural processes, structural changes and anticipated functional responses. The model focuses on restoration of physical processes in order to improve the functioning of natural processes and habitats.
A number of significant data gaps exist in our understanding of Puget Sound beaches and bluffs, as we have not invested adequately in understanding fundamental coastal and bluff processes. Gaps in our knowledge and information include:
- Net shore-drift rates in the Puget Sound and Northern Straits;
- Historic shore changes, including using oral histories on undocumented changes such as beach mining;
- Quantitative sediment budgets;
- Long-term beach and bluff monitoring;
- Biological response to beach nourishment (response of beaches and surrounding habitats);
- The effects of climate change and sea level rise on the Puget Sound and Northern Straits shores.
This paper discusses beaches and beach processes, bluffs and bluff processes, and factors affecting these systems, including development, sea level rise and coastal restoration. The study area described is all of Puget Sound and the Northern Straits, from the mouth of the Elwha River on the northern Olympic Peninsula to the Whatcom County border with British Columbia, Canada. This paper is not intended to be a comprehensive review of these topics; instead it is a synthesis of existing knowledge and research that highlights the connections among issues and gaps in our knowledge. Existing sources with more depth include technical beach papers by Finlayson (2006) and Kirk (1980), a bluff paper by Shipman (2004), and a less technical book on local coasts by Downing (1983).
Johannessen, J. and A. MacLennan. 2007. Beaches and Bluffs of Puget Sound. Puget Sound Nearshore Partnership Report No. 2007-04. Published by Seattle District, U.S. Army Corps of Engineers, Seattle, Washington.