Kitsap County

Shoreline Stewardship

3.2.1 Agate Passage Management Area
(Reaches 3217-3233; 3487-3491)

MA CF Qualitative Rating = Moderate Impact
Mean Reach CF Score (Normalized) = -0.47 (Reach score range: -0.69 to 0.00)
Median Reach CF Score (Normalized) = -0.54
Number of Reaches = 12

Mean Reach EF Score = 28.4 (Range: 22 to 32)
Median Reach EF Score = 29

Description
Agate Passage (MA-1) comprises 19,495 ft of shoreline along Agate Passage and around Agate Point (Table A-1).  It comprises 12 reaches, most of which are classified as high bluff (9) and the remainder classified as low bank (3) (Figure B-17).  MA-1 comprises two drift cells with northerly alongshore drift that converge at Agate Point.  The southern boundary of the first drift cell begins at a divergence zone north of Manzanita Bay and moves along the western shoreline along Agate Passage, encompassing a long stretch of shoreline with eroding feeder bluffs (Figure 3).  The smaller second cell begins at a divergence zone located just southeast of Agate Point on the western shore of Port Madison, and also includes a small area with unstable feeder bluff activity.  Relative to wave exposure, shorelines are considered “semi-protected” along Agate Passage to “protected” along Port Madison (Figure B-18).  There are no major watersheds in MA-1, with all runoff considered coastal sheetflow.

Overhanging riparian vegetation covers approximately 36% of the MA-1 shoreline, the highest value observed of any MA (Table A-1, Figure B-23).  Similarly, within the 200-ft riparian zone, naturally vegetated surfaces (coniferous and deciduous trees, shrubs, and wetlands) compose 71% of land cover (Figure B-22).  Impervious surfaces (e.g., roads, roofs) represent 17% of the riparian zone land cover.  As such, riparian zone land-cover class values in Agate Passage represent one of the best ratios of percentage of natural vegetation to %TIA on Bainbridge Island.

Shoreline development in MA-1 is primarily residential in nature.  Approximately 57% of shoreline is modified by armoring and 21% of the shoreline has armoring that encroaches into the intertidal zone (Table A-1; Figure B-19).  A total of 235 point modifications were recorded along MA-1 shorelines, at an average of 12.1 modifications per 1000 ft, among the highest densities on Bainbridge Island (Figure B‑20).  The most common structures were stairs (64), a common feature for shoreline access along high bluffs, followed by overwater structures, such as docks or piers (32).

Limiting Factors
The average normalized CF score within MA-1 was -0.47, well within the middle range of MA scores on Bainbridge Island (Table A-2; Figure B-10, Figure B-13).  Consequently, the Agate Passage MA was rated “moderate” in terms of relative impact to shoreline controlling factors on Bainbridge Island.

Relative to other areas, MA-1 scored particularly low (poor) in the following controlling factor metrics: loss of natural shade, sediment supply, pollution, and hydrology (Table A-3; Figure B-16).  The fact that MA-1 had the lowest (worst) average natural shade score (-2.75) while also having the highest total percentage (36%) of shoreline with overhanging riparian vegetation was puzzling.  Closer examination of the data revealed that most short reaches in the MA exhibited relatively low vegetation scores, whereas some longer reaches had riparian cover exceeding 90%.  While still useful, this suggests that a simple, additive index alone may prove misleading when average assessment scores are scaled up within a larger landscape.  Alternative measures, such as using median and upper/lower quartiles may characterize the cumulative landscape scores better than simply using the average.  Sediment supply scores were influenced primarily by the high percentage of armoring in front of important feeder bluffs and backshore sediment sources.  Pollution scores were low in MA-1 because of recreational shellfish closures and warnings in five of twelve reaches associated with proximity to a sewer treatment plant outfall.  High %TIA in the marine riparian zone and outfall densities along the shoreline in some reaches further influenced low pollution and hydrology metric scores.

On a positive note, MA-1 exhibited high (good) scores in the physical disturbance metric relative to other MAs (Table A-3, Figure B-16).  This value is primarily attributed to the residential nature of the shoreline, high forested cover, and relatively moderate number of docks, mooring buoys, and ramps due to poor shoreline access afforded by the high bluff geomorphology of most reaches.

Opportunities
The most obvious opportunities for improving scoring of controlling factor metrics include removal of armoring structures in front of feeder bluffs to allow natural sediment processes.  This approach will yield obvious results at down-drift reaches dependent upon alongshore sediment sources.  Some high-bluff reaches in this MA were in particularly good condition, including Reach 3488, one of the least-impacted, high-bluff reaches on Bainbridge Island.  This reach exhibits natural feeder bluff activity, large woody debris (LWD) recruitment, downed trees and overhanging riparian vegetation at the base of the bluffs, high forest cover in the landward riparian zone, and no linear or point modifications along the shoreline.

The EF scores are particularly high in MA-1 relative to the rest of Bainbridge Island because of the prevalence of forage-fish (herring, sandlance, and surf smelt) spawning areas documented along most reaches (Table A-4; Figure B-11, Figures B-24, B-25, B-26, Appendix D).  Eelgrass beds and geoduck resources are also abundant along most shorelines.

Individual Reaches Within MA-1 (Figure B-27, Appendix C, Appendix D)

Reach 3217

Reach 3218

Reach 3219

 Reach 3220

 Reach 3221

 Reach 3222

 Reach 3223

 Reach 3487

 Reach 3488

 Reach 3489

 Reach 3490

 Reach 3491

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