What's a feeder bluff?
A feeder bluff is an eroding coastal bluff that delivers sand and gravel to a beach over time. Feeder bluffs contribute to the amount of sediment on the beach below. Most bluffs erode to some extent and are likely to provide sediment to the coastal environment.
The rates at which bluffs erode, the availability of appropriately sized sediment, and the influence on local beaches differ from place to place.
Mapping Puget Sound feeder bluffs
In 2013, we completed mapping different categories of feeder bluffs and made this material available on our Washington State Coastal Atlas. In addition to mapping the extent and location of feeder bluffs, we mapped other types of coastal land forms, including estuaries, rocky shorelines, and barrier beaches.
The term feeder bluff has been widely used on Puget Sound to describe actively eroding bluffs that provide sediment to nearby beaches. Knowing the role feeder bluffs have in the ecosystem, the location of feeder bluffs, and their relationship with adjacent beaches and drift cells informs shoreline management, including efforts to prioritize protection and restoration.
The amount of sediment delivered by coastal bluffs depends on several factors including:
- Height of the bluff
- Proportion of beach-size sediment in the bluff
- Rate of bluff retreat or erosion
Beaches and bluffs are a legacy of the region’s glacial history. The glaciers left three important legacies for beaches:
- Steep coastlines and coastal bluffs
- Complex shorelines and wave environment
- Mixed sand and gravel beaches
Beaches can form wherever sand and gravel are abundant and there is sufficient wave energy to move them around. Puget Sound's irregular coastline influences sediment transport and the development of local shoreline features such as feeder bluffs and barrier beaches.
Eroding bluffs deliver sediment to the beach. The sediment is transported by waves along the shoreline in drift, or littoral, cells. Beach cells are sections of the shoreline that contain their own sources and sinks of beach sediment that are largely isolated from adjacent cells. This sediment supply is a critical geologic process that supports a wide range of ecological functions. The geological processes related to beaches include:
- Bluff erosion
- Sediment transport
- Formation of spits
- Regular cycles of beach erosion and growth
Bluff processes support healthy habitats
These processes influence the formation of spawning habitat, the accumulation of large wood and organic debris, and maintenance of riparian forest and other coastal habitats. The beach itself supports a variety of habitats, ranging from substrate for forage fish spawning to tidal wetlands behind spits.
Shoreline armoring impacts
We generally discourage any armoring, also called shoreline hardening, along coastlines. Shoreline armoring in Puget Sound poses a serious threat to the function of feeder bluffs. Seawalls and retaining walls cut off the natural supply of sediment to the local drift cell, which affects the supply of sediment to nearby beaches and can have long-term impacts on beach conditions.
We don’t want to diminish the potentially serious ramifications that erosion and landslides can have on coastal development. However, there are trade-offs associated with decisions to harden the shoreline. Many cities and counties have rules about armoring in their Shoreline Master Programs.
Sea level rise
In general, sea level rise will make bluffs erode faster by increasing the wave energy at the foot of the slope. This may increase pressure to build more shoreline armoring. As sea levels rise along armored shorelines, the beach will gradually become narrower or even disappear. Water levels at the seawall will increase, requiring more robust armoring structures.
Washington beaches will be greatly diminished or lost unless shorelines are allowed to erode landward. The increased availability of sediment from eroding bluffs may be critical to retain the resilience of low-lying beaches from rising water levels and to sustain important nearshore ecosystems.
See how we're helping Washington adapt to coastal hazards and sea level rise.
Building on feeder bluffs
Any bluff is a potentially hazardous location but feeder bluffs tend to have higher erosion rates and more serious slope stability issues. Building on feeder bluffs presents challenges because it requires safely developing property while simultaneously protecting the natural functions of the bluff. At the same time, any effort to slow erosion or otherwise stabilize the bluff impairs the bluff’s important role as a source of beach sediment.
The simplest way to avoid property damage and allow the natural erosion of the bluff is to avoid putting development in harm’s way in the first place. However, many structures already exist on bluffs. Also, constraints on how the site can be developed lead to structures being built closer to the edge than would otherwise be appropriate.