III. A. 2. Sediment Supply and Availability |
The focus of the guidance provided by Manashe (1993) is to assist coastal property owners in stabilizing the coastal bluff. Slope failure is a natural process that supplies sand and gravel to the Island’s beaches. Artificial stabilization (i.e., bulkheads, retaining walls) of the bluff deprives or slows this contribution and can exacerbate beach erosion. Sediment around Bainbridge Island is derived from predominantly glacial deposits that comprise the bluffs and nearshore upland areas. River deposits are not a significant source of material for Puget Sound beaches (Downing 1983; Shipman and Canning 1993). Stream discharge into several of the inlets on Bainbridge Island may contribute a small amount of additional beach material. These areas include Manzanita Bay, Fletcher Bay, Pleasant Beach, Blakely Harbor, Eagle Harbor, Point Monroe Lagoon, and Murden Cove.
Both substrate type and sediment abundance information for Bainbridge Island are available from the Washington State ShoreZone Inventory (Washington State Department of Natural Resources 2001). The substrate type is illustrated on the Substrate Type Map in Appendix A. The map illustrates that the dominant nearshore substrate types are gravel and sand (mixed coarse). Also shown are sand and muds and fines in the embayments of Port Madison Bay, Eagle Harbor, Blakely Harbor and Fletcher Bay. Rocks, gravels, and sand are shown around Restoration Point.
Sediment abundance (see Sediment Abundance Map in Appendix A) is a qualitative estimate of sediment abundance within the shore-unit as quantified by ShoreZone (Washington State Department of Natural Resources 2001). ‘Abundant’ indicates areas with accretional landforms and highly mobile sediment; ‘Moderate’ means some mobile sediment but not likely to rapidly move; and ‘Scarce’ signifies areas of bare rock or rock with cobble/boulder veneer. Most of Bainbridge Island is in the ‘Moderate’ rating of sediment abundance. Tips of points, such as Battle Point and Yeomalt Point are noted to contain sediment in abundance (Figures III-4a and III-4b). Restoration Point, an area of hardened mudstone at the southeast tip of the Island, has little sediment abundance (Figure III-5).
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