III. A. 1. Feeder Bluffs |
The geology of Bainbridge Island is the product of a series of glacial advances and retreats, and its morphology is a result of sculpting, sea level rise, and erosion since the retreat of the last major ice sheet approximately 13,000 years ago. The primary surface material on the island consists largely of the Vashon-Lodgemont Till, a poorly sorted, very compact, non-stratified mixture of gravel, sand, silt and clay with occasional boulders, and Vashon advance outwash sand and gravel and lacustrine silts and clays. The beach sediment reflects this predominantly glacial origin.
The erosion of the bluff may be initiated from the upland side by hydrologic and hydrogeological processes that lubricate and weaken the soil, or from the water side by waves and fluctuating high water levels that undercut the bluff and cause collapse. In either case, water and gravity work together as the major forces of bluff erosion. Once the sediment is on the beach, wave action may work against gravity to temporarily move the sediment up the beach face, but gravity always prevails. Eventually the sediment is transported into deep water where it is unavailable to the coastal system.
Few large rivers deposit beach sediment into Puget Sound (Downing 1983). Only relatively small streams provide fine-grained sediment to Bainbridge Island shores, and most is retained in the heads of embayments or in tidal marshes. Beaches in Puget Sound are typically supplied with sediment by bluff erosion. The contribution of the predominantly glacial bluffs results in beach materials that range in size from cobbles to silts/clays (see Figure III-1 for the general process). Grain-size descriptors often used to classify sediment are summarized in Table III-1. The Wentworth (1922) system is shown mainly for historical purposes, because it is most frequently associated with the grain-size distribution scale. The system proposed by Dethier (1990) can be used for habitat classification and for general beach characterization. The system of Komar (1998a) is best for detailed studies of grain-size distribution, for instance, in designing a beach nourishment project.
Until recently, engineering guidance for beach nourishment projects depended on a comparison of the grain-size distribution of the native beach material relative to the borrow material. An overfill factor, RA, and a renourishment factor, RJ, were calculated based on the mis-match between the native and borrow grain-size distributions. More recent guidance treats sediment characteristics using a single grain-size parameter, the median grain diameter, D50, which is why a relatively narrow gradation such as Komar’s is required. Additional guidance is based on equilibrium beach profile concepts, an assessment of storm-induced erosion, and an assessment of wave-driven longshore transport losses (Dean 2002; National Research Council 1995; U.S. Army Corps of Engineers 2001a).
Figure III-1. Bluff erosion and littoral transport alongshore. (Source: King County Dept of Natural Resources).
Table III-1. Grain-Size Classifications
Grain Diameter, mm |
Size Description | ||
| Wentworth (1922) | Dethier (1990) | Komar (1998) | |
| > 256 | Boulder | Boulder | Boulder |
| 64 to 256 | Cobble | Cobble | Cobble |
| 4 to 64 | Pebble | Gravel | Pebble |
| 2 to 4 | Granule | Sand | Granule |
| 1 to 2 | Coarse sand | Very coarse sand | |
| 0.5 to 1 | Coarse sand | ||
| 0.25 to 0.5 | Medium sand | Medium sand | |
| 0.125 to 0.25 | Fine sand | Fine sand | |
| 0.0625 to 0.125 | Very fine sand | ||
| 0.0039 to 0.626 | Silt | Mud | Silt |
| < 0.0039 | Clay | ||
Groundwater and Surface-Water Drainage
The coastal bluffs of Bainbridge Island are typical of those in other areas of Puget Sound in their predominantly glacial origin. Several sources are available that describe the processes contributing to bluff erosion and steps that property owners can take to reduce landslide activity (Macdonald and Witek 1994; Zelo and Shipman 2000). Coastal bluffs are normally stable at slopes of 30 to 40 degrees. Many of the Bainbridge Island coastal bluffs, however, are steeper than 40 degrees and are susceptible to downslope movement. Heavy rains that saturate and weaken soils provide lubrication between layers and contribute to slope failure. The process is exacerbated by removal of vegetation and by the increased runoff that usually accompany development. When slopes are nearly vertical, waves also can undercut the toe of the bluff or remove sediment, which also contributes to the erosion process. These factors are illustrated in Figure III-2.
Figure III-2: Factors related to erosion of the nearshore upland (Source: Manashe 1993).
Development, Vegetation, and Bluff Stabilization
More than 82% of the Bainbridge Island shoreline is classified as developed (P. Best, COBI unpublished data; personal communication, 2002). With the exception of Eagle Harbor, single-family residential is the primary land use in nearshore uplands. Development of an area typically involves land clearing, excavating and backfilling of soils, the compacting of soils, installation of septic drain fields, and the building of roads. All of these activities can have a profound influence on the stability of the nearshore uplands and bluffs. They can affect the groundwater and surface water flows (as discussed in the previous section) and may cause erosion of the nearshore uplands (Manashe 1993). Figure III-3 illustrates these factors with additional details provided by Manashe.
Figure III-3. Effects of vegetation in minimizing erosion (Source: < Manashe 1993) .
Mapped Eroding Bluff and Feeder Bluff Locations
The Coastal Zone Atlas (Washington State Department of Ecology 1980) shows locations of feeder bluffs and erosion scars from past slope failures. The term “eroding bluff” is a more general category than a “feeder bluff.” The primary difference between an eroding bluff and feeder bluff is the type of sediment delivered to the beach, although specific criteria for Puget Sound feeder bluffs have not been developed (H. Shipman, WDOE, personal communication, 2002). Eroding bluffs contribute sediment to the beach irrespective of its size or gradation (e.g., the full range of fine materials, sands, silts, and clays to coarse materials, such as gravel, with no distinction between proportions of each). Feeder bluffs, on the other hand, are typically comprised of highly erodable coarser sediment, and as a result, contribute higher proportions of coarser materials, particularly sand and gravel.
The Geologic Stability Map in Appendix A illustrates known locations of unstable bluffs and landslide activity. Areas of eroding bluff activity were identified by Anchor Environmental and Applied Environmental Services during the fall of 2001 along the shorelines south of Agate Pass, at Battle Point, north of Fletcher Bay, near Blakely Harbor, near Yeomalt Point and Ferncliff, around Skiff Point northward nearly to Faye Bainbridge State Park, and west of Port Madison Bay near Agate Point. More investigation is needed to discern which eroding bluffs are contributing coarse sediment (gravels) in proportions substantial enough to be considered “feeder bluffs” versus eroding bluffs. A combination of historical aerial photography and on-site investigation is needed to make these distinctions.
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