Puget Sound Feeder Bluffs: Coastal Erosion as a Sediment Source & Its Implications for Shoreline Management
Beaches make up about 1400 miles of Puget Sound’s 2500-mile shoreline. They are an important component of the region’s coastal environment and support a broad range of ecological functions, from spawning habitat for forage fish to the formation of estuaries and salt marshes. These beaches are complex geological systems that respond to changes in the availability of sediment and its transport along the coast. On Puget Sound, some of the sand and gravel on the beaches may come from streams and rivers, but much of it is derived from erosion of coastal bluffs. These bluffs are called feeder bluffs, for which we offer the following definition:
Feeder Bluff: An eroding coastal bluff that delivers a significant amount of sediment to the beach over an extended period of time and contributes to the local littoral sediment budget.
The potential significance of feeder bluffs on Puget Sound has been recognized for decades, but their role in beaches and nearshore ecosystems has not been well understood and there has been no Puget Sound-wide mapping of their distribution. This project emerged from the need to understand these landforms better, to describe their location and extent, and to use this information to improve shoreline management in the region.
In this report, we investigate the geologic characteristics that influence the formation and evolution of Puget Sound beaches. This coastline is strongly influenced by the legacy of the last glaciation, which left a steep, bluff-dominated coast, a complex wave environment that leads to transport of sediment along the shoreline, and an abundance of coarse sand and gravel that erodes rapidly and builds beaches. The coast is divided into hundreds of relatively small littoral cells (also called drift cells), each with its own sources of beach sediment. In some locations these sources may includes rivers or streams, but the primary source of beach-size sediment in many areas is the erosion of the coastal bluffs themselves.
The fact that eroding bluffs are important sources of beach sediment has several ramifications for Puget Sound’s coast:
- Beaches and nearshore ecosystems. Bluff erosion delivers sediment to beaches and is important for the long-term maintenance of beach ecosystems within the local littoral cell. The ongoing delivery of sediment to the beach system is a good example of an ecosystem process that supports a broad range of ecological functions.
- Shoreline armoring. Measures to prevent erosion of coastal bluffs, even if successful at protecting upland structures and property, diminish the supply of sediment to adjacent beaches. Concerns about the environmental impacts of shoreline armoring are not unique to Puget Sound and are not limited to their effect on sediment supply, but this is a particularly relevant management issue in this region where bluffs are common.
- Sea-level rise. Rising sea levels will be an increasing concern on all coasts during the coming decades. On a bluff-dominated coast such as Puget Sound, the ability of bluffs to erode naturally will allow beaches to migrate landward with less impact on beach character and coastal ecosystems and will also provide critical sediment that will benefit other beaches in the vicinity and allow them to adjust to the higher water levels.
Protecting feeder bluffs and their role in providing sediment to Puget Sound beaches is challenging since it requires balancing the need to maintain natural erosion with landowners’ concerns about safety and the loss of shoreline property. There are a number of different approaches to managing and protecting these bluffs, ranging from improved development practices to the implementation of conservation strategies. The report looks at the following issues:
- Building on are above feeder bluffs poses two problems. First, these are often hazardous areas specifically because of the potential for erosion or landslides. Second, efforts to prevent erosion diminish the ability of the bluffs to continue to provide sediment to nearby beaches. Large setbacks can reduce risks and delay the need for protective measures. When structures do become threatened, emphasis should be placed on removing or relocating buildings and improvements rather than armoring the shoreline.
- Shoreline Management Programs (SMPs) provide numerous opportunities to better manage feeder bluffs. Local governments are required to establish policies that maintain ecological functions. Shoreline inventories can identify important bluffs and help prioritize sensitive shorelines. Environment designations can be used to target appropriate policies for important coastal bluffs. Setbacks can be established that reduce development in the most hazardous locations while also reducing the need to armor the shoreline in the long-term. Shoreline stabilization policies can include strict standards for new armoring on feeder bluffs and can encourage alternative approaches such as relocation of at-risk structures and mitigation of potential impacts on sediment supply.
- Conservation and acquisition may be the most effective ways of assuring permanent and effective protection of feeder bluffs, but can be expensive and may be best used for particularly high value shoreline areas. Several important feeder bluffs on Puget Sound have been acquired recently, protecting the long-term function of the bluffs, assuring that development is not built in hazardous locations, and providing additional environmental and recreational benefits.
- Restoration of feeder bluffs may be possible in some locations. Many historical feeder bluffs on Puget Sound have been armored and in many cases, concerns about property ownership and renewed erosion make restoration unlikely. Opportunities remain in some locations, however, for removal of shoreline armor and the restoration of natural sediment supplies. On some highly developed shorelines, artificial beach nourishment may have a place in restoring sediment to historically degraded beaches.
This project is intended to increase understanding of the role and distribution of feeder bluffs on Puget Sound. This should lead to improved policies for managing coastal bluffs and nearshore ecosystems, more generally. It may help funding agencies and local groups target key shorelines for protection and conservation efforts. The higher resolution maps should help refine earlier work by the Puget Sound Nearshore Ecosystem Restoration Project (PSNERP) that examined the degradation of sediment supply to different drift cells and may improve efforts by the Puget Sound Partnership (PSP) to monitor progress on the shoreline armoring vital sign.
There are also significant opportunities to improve our current knowledge. Mapping could be refined to better characterize the distribution of shoreline armoring and its relationship to historical feeder bluffs. Geological studies could help quantify erosion rates and sediment budgets, and to better characterize the sensitivity of beaches to changes in sediment. Ecological studies should look more carefully at how beach ecology relates to changes in the physical conditions of beaches. Together, these types of studies would help us better predict long-term trends in the condition of Puget Sound beaches and to consider scenarios of increased development, accelerated sea level rise, or changes in coastal management.