VI. A. 2. Regional Focus - Bainbridge Island |
“Hard” shoreline stabilization or “armoring” is one of the more prevalent forms of nearshore modification in Puget Sound resulting from rapid population growth that has caused a surge in commercial and private development. By the mid-1990s, over 29% of Puget Sound’s shoreline had been armored, with 1.7 miles of Puget Sound shoreline being newly armored each year (Canning and Shipman 1995b). In King County, armoring comprises 75% to 87% of the coastline (Washington State Department of Natural Resources 2001; Williams et al. 2001; Bumgartner 1990). Of 2262 shoreline parcels on Bainbridge Island, over 82% have been developed, with single-family residential use representing the vast majority of these cases (P. Best, COBI, personal communication, 2002). Approximately 52% of the Bainbridge Island shoreline has some type of armoring or modification according to the WDNR ShoreZone Inventory (2000). However, more recent surveys by the City of Bainbridge Island suggest that this number may be higher.
Vertically oriented bulkheads are the most common shoreline stabilization structures encountered on Bainbridge Island (P. Best, COBI, personal communication, 2002) (Figure VI-3; Figure VI-4). Bulkheads are vertical shoreline structures designed to prevent sliding or erosion of the land behind it, primarily by protecting it against waves and currents (Williams and Thom 2001). Functionally, bulkheads provide a vertical separation of land from water and are built to protect adjacent uplands from erosion and create shoreline real estate. Predominant bulkhead designs used in Puget Sound are vertically oriented concrete, rock, or wood structures that are in direct contact with water action (Downing 1983; Cox et al. 1994; Canning and Shipman 1995a).
Figure VI-3. Vertical bulkheading along Rolling Bay (© WA Dept of Ecology, 2000).
Figure VI-4. Vertical bulkheading along Port Orchard Bay (Source: Applied Environmental Services, Inc.).
Similar structures that are used to prevent landslides and to protect uplands from wave action include seawalls, which are self-supporting, and revetments, which are placed on or against an existing sloping embankment. Seawalls are large freestanding structures designed to protect bluff and bank habitats exposed to moderate to very severe waves; they are not a common structural feature along the shores of Bainbridge Island. A revetment is an armored slope built to protect existing shorelines or embankments against the erosive forces of current, wave action, or storms (Anchor Environmental 2000). Revetments generally parallel the contours of the shoreline and are commonly composed of riprap (randomly placed rock rubble), gabions (rectangular steel wire baskets filled with stones), interlaced concrete forms, or grout- (concrete) filled bags (Downing 1983; Cox et al. 1994). Revetments are generally used for protecting bank and bluff habitats on residential parcels, are relatively easy to construct and maintain, and usually do not extend beyond the mean low water (MLW) mark (Cox et al. 1994). In general, there are very few revetments on Bainbridge Island, none of which reach MLW (P. Best, COBI, personal communication, 2002).
Groins are rigid, self-supporting structures built out at an angle from the shore (usually perpendicular) to protect it from erosion or to trap sand. They commonly function to provide or maintain a beach by trapping littoral drift sediment and reducing the rate of sediment loss. Groins on Bainbridge Island generally are narrow, made of concrete or quarried rock, of varying lengths, and may be spaced at intervals along the shoreline in what are referred to as “groin fields” (See Figure VI-5 and VI-6).
Figure VI-5. Groin (Source: Applied Environmental Services, Inc.).
Figure VI-6. Groin field (© WA Dept of Ecology 2001)
Breakwaters and jetties are constructed to dissipate wave energy, channel tidal action, and/or to protect and stabilize navigation channels and harbor areas. Neither type of structure is typically found along Bainbridge Island shorelines.
Soft armoring approaches strive to reduce impacts to nearshore habitats associated with hard armoring methods and are considered a preferred approach to rebuilding beaches or protecting shorelines from erosion. Beach nourishment serves as a “soft,” sacrificial barrier that functions to prevent beach recession, provide protection from storms and flooding damage, and enhance recreational opportunities (Williams and Thom 2001). Additional advantages include the preservation of beach aesthetic values and an increased supply of sediment to downdrift beaches. Beach nourishment in Puget Sound typically uses gravel-sized material placed by truck or barge along the upper beach, and spread to the design contour by bulldozer (Shipman 1998). A number of shoreline restoration projects around Bainbridge Island have used small-scale beach nourishment (less than 1000 cubic yards of placed material) to control erosion (Zelo and Shipman 2000).
Other soft approaches use natural vegetation and large woody debris to reduce soil erosion, increase slope stability, trap sediment, and absorb wave energy along shorelines (Myers 1993; Macdonald et al. 1994; Manashe 1993; Macdonald et al. 1994). Along sloping shorelines and backshore areas, this approach involves planting vegetation to harness the hydrological and mechanical benefits of plant foliage and root systems to stabilize soil. Stumps, drift logs, and tree root masses are a natural component of Pacific Northwest shorelines and can form semi-permanent stockpiles, which trap beach sediment and promote the establishment of vegetation (Downing 1983) (Figure VI-7). However, soft techniques to stabilize shorelines are less common in practice, with fewer than 20 known sites located on Bainbridge Island (P. Best, COBI, personal communication, 2002) (Zelo and Shipman 2000). Most of these projects consist of cabled logs and discretely placed boulders as alternatives to bulkheads.
Figure VI-7. Naturally occuring large woody debris stabilizing shoreline (Source: Applied Environmental Sciences, Inc.)
| << vi.a.1. types of structures |