III. Nearshore Physical Processes
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Bainbridge Island is essentially an eroding coastal system in which sediment along its shoreline is derived from the island itself. The sediment found in beaches, mud flats, and tidal marshes is the result of upland erosion. The several small streams that flow into the headwaters of the bays on Bainbridge Island contribute fine material that may eventually make its way to the open coast, but by far the largest volume of coastal and beach sediment is derived from the recession of the bluffs, which are comprised mainly of glacial deposits.
Once the unconsolidated sedimentary material reaches the beach, it is redistributed and sorted by waves and currents, and is formed into gravel or sand beaches, cuspate forelands, and spits. Under normal wave and tide conditions, the forces of water and gravity sort the beach into zones, leaving a pavement of coarse gravel material high on the beach and winnowing the fine sands and mud, which are deposited in a low-tide terrace. Finer material (silt and clay-sized particles) that remains in suspension is removed from the shoreline by the tidal currents and is deposited in sedimentary basins, usually in deep water, though some may be retained on tidal flats. During storm conditions, even relatively coarse material may be removed from the beach and be permanently deposited out of the reach of waves. Storms, which typically occur from late fall into spring, may produce water levels that are higher than normal, as well as large waves that remove material from the toe of the bluff. The rain may also saturate the soil and further weaken bluffs, which slide down slope and add sediment to the beach. Though bluffs may remain stable for a number of years, when they do slide, the effect may be significant and catastrophic. The mudslide that occured on January 19, 1997, at Rolling Bay destroyed several homes and took the lives of an entire family. The type and extent of bluff erosion depends on a number of geologic factors and environmental conditions, and not all slides are as extensive as that of January 1997 (Macdonald and Witek 1994). Because beach material on the Island shoreline is primarily derived from erosion, the permanent stabilization of a bluff through the use of structures or other methods deprives the beach of its natural source of replenishment.
A great deal of engineering knowledge about general coastal processes has been developed. The Army Corps of Engineers (USACE), Coastal and Hydraulics Laboratory (CHL) publishes engineering guidance and calculation tools for wave prediction, sediment transport, beach protection methods, structure design, and coastal construction. The Coastal Engineering Manual (CEM) is replacing the Shore Protection Manual, which for many years was an international standard for coastal engineering technology. As chapters are developed and reviewed, they are placed on the (CHL) web site (http://www.wes.army.mil/). The guidance in the CEM is the latest available, has been reviewed by international experts, and is respected as a worldwide authority. The guidance has been developed based on theory, tested in laboratory experiments, and confirmed by field measurements. In spite of this rigor, few of the conditions examined correspond with those encountered in Puget Sound (e.g., large tidal range with strong currents, coarse gravel beaches). Specific applications of the CEM technologies must consider the conditions under which they were developed, and adjustments using engineering judgement must often be made for the site of interest.
The presentation below will consider the Bainbridge Island coastal system from landward to seaward, proceeding from the supply of sediments to the beach, to the oceanographic and hydrodynamic processes, and finally to the transport of the beach material itself. A summary of the key findings and conclusions for general applications is given at the end of the chapter.
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