III. C. 1. Drift Cells |
The shoreline of Puget Sound has been characterized in a series of reports beginning in the 1970s by Schwartz et al. (1991) as consisting of a number of drift cells. These cells are compartmentalized zones along the shoreline that act as discrete systems with respect to transport of beach sediment. A drift cell consists of segments of shoreline that include the source of sediment, the area where they accumulate or deposit (a sink), and the connecting path or driftway between the two (Downing 1983). The concept was developed from observations on California beaches, where it was noted that beach sands could be traced from a source (river) along shore (transport) and eventually to a sink (submarine canyon), where the sand was removed from transport in the coastal system. It is a particularly useful concept on open coastlines where cells can be more easily identified than in Puget Sound. Drift cells around Bainbridge Island have been identified (Schwartz et al. 1991; Terich 1987) and are illustrated on the Drift Cells Map in Appendix A.
The drift cells and direction of net drift were inferred from observation of geomorphic and small-scale features along the shoreline. Though the general directional trends are often obvious from observation of large-scale features such as spits and cuspate forelands, the details of the drift direction are sometimes in error and should be used with caution. In other cases, the drift cells do not represent closed systems, because sediment may bypass spits and embayments in deeper water and enter an adjacent drift cell. The selection of the cells does not consider processes that may take place in the active transport zone in deeper water, nor does it consider long-term transport trends.
The maps prepared by Schwartz indicate direction of inferred net transport. Estimates of potential transport rate are available in the Washington Coastal Zone Atlas (1980). These estimates are based on seasonal observations of wind direction and estimates of the resulting wave transport calculated from empirical formulas available in the early editions (1977) of the Shore Protection Manual (U.S. Army Corps of Engineers 1984). These should be considered as very rough indicators of potential transport and should not be used for most engineering purposes.
Spits and tidal flats are the most common types of shoreline features encountered around Bainbridge Island. Several spits are obvious around Bainbridge Island; e.g., Point Monroe (Figure III-11), Battle Point, Fletcher Bay, and Wing Point.
Tidal flats and flood tidal deltas were also observed in Murden Cove, Rolling Bay, and the inner harbors and bays of Manzanita Bay, Port Madison Bay, Fletcher Bay (Figure III-12), Blakely Harbor, and Eagle Harbor. A low-tide tombolo formation was observed behind Treasure Island in Port Madison Bay, and cuspate forelands were observed near Rolling Bay and Skiff Point, and near Yeomalt Point and Wing Point. Taggart (1984) inventoried and detailed the different drift cells in Kitsap County. He pointed out that artificial modifications of the shoreline could affect net shore drift by forming an artificial drift-cell terminus, a region of no apparent net shore drift, or could result in erosion.
Figure III-11 Spit, lagoon, and tidal flat at Point Monroe ( © WA Dept. of Ecology, 1992).
| <<iii.c. sediment transport processes |