2.2.3 Geomorphic Classification |
Understanding a shoreline’s geomorphic setting provides not only the basis for deriving consistent comparisons between nearshore structure and function, but also a context for comparing existing conditions with pristine or historical conditions and setting restoration goals. With this in mind, each reach of Bainbridge Island shoreline was classified into one of five major geomorphic categories, following the shore types outlined by Terich (1987):
- Low Bank
- High Bluff
- Spit/Barrier/Backshore
- Marsh/Lagoon
- Rocky Shore
Large river deltas were perhaps the only category of Puget Sound coastal geomorphology not exhibited on Bainbridge Island, and were not included in the classification scheme.
Geomorphic classes were assigned using data from the ShoreZone inventory (WDNR 2001), which included information on sediment source and substrate class, visual interpretation of aerial photographs (WDOE 1977, 1992, 2000), LiDAR-based slope maps (15+ and 40+ %) (Harding and Berghoff 2000; Puget Sound LiDAR Consortium 2000), slope stability maps (Small 2001), and expert knowledge (Table 3). Historical topographic sheets from the U.S. Coast and Geodetic Survey (Figure 4; Puget Sound River History Project 2003) were used in cases in which shorelines had been modified so extensively that classification into one of the above classes was uncertain (e.g., major fill). The following are some key attributes that were used to assign each class:
| Low Bank – (photo example at left) slope often greater than 40% (though not very wide); usually greater than 15%; height less than 5 meters; usually narrow foreshore (beach) with high water line at or on the bank; trees at waterline often indicate low bank rather than beach or wide backshore class; raised bedrock terraces assigned low bank if characterized by a sand and gravel beach; backed by low scarp |  |
High Bluff – (photo example at right) slope greater than 40%; height greater than 5 meters (estimated from aerial photo, GIS verified); often unstable or with visible face (little or no vegetation); sediment source often from backshore; high stairs and setback houses also indicate bluff |
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Spit/Barrier/Backshore – (photo example at left) wide beach face; slope <15%; wide backshore is key to distinguishing between bank and beach; spits and barrier beaches are generally self-evident |
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Marsh/Lagoon – (photo example at right) protected embayments, often with fluvial sediment sources; substrate is composed of fines; diagnostic salt marsh vegetation; lagoons may empty completely at low tide (extensive tide flats) and may have a residual basin that holds water at low tide |
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Rocky Shore – (photo example at left) backshore rocky; foreshore often bedrock with veneer of other substrata on top; raised terraces with bedrock classified as rocky if shoreline characterized by little sediment movement. |
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The distribution of geomorphic classes over Bainbridge Island is shown in Figure 4. It should be noted that this classification scheme inherently seeks to simplify the habitat continuum, and some reaches may share several geomorphic characteristics because of the scale at which the ShoreZone database distinguished individual reaches. The predominant geomorphic landform class was assigned to those reaches that exhibited a combination of geomorphic traits. For example, if a 1000-ft long reach of shoreline was composed predominantly of high bluffs (75% of linear shoreline), with some smaller proportion composed of low banks, this reach was considered “high bluff.”
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