3.2.9 Manzanita Bay Management Area (MA9) (Reaches 3492-3501) |
Qualitative Rating = Moderate Impact
Mean CF Score (Normalized) = -0.49 (Range: -0.71 to -0.13)
Median CF Score (Normalized) = -0.54
Number of Reaches = 10
Mean EF Score = 23.9 (Range: 20 to 33)
Median EF Score = 22.5
Description
Manzanita Bay (MA-9) is the smallest management area on Bainbridge Island, comprising 18,879 linear feet of shoreline that encompass all of Manzanita Bay and Arrow Point (Table A-1, Figure 2). MA-9 comprises only 10 reaches composed of the following geomorphic classes: marsh/lagoon (3), high bluff (3), spit/backshore (2), and low bluff (2) (Figure B-61). MA-9 is basically defined by four drift cells (Figure 3). The first drift cell begins at a divergence zone located outside the northern margin of Manzanita Bay and moves south into Little Manzanita Bay. The second drift cell is quite small and moves eastward into Little Manzanita Bay along its south shore. The third and fourth drift cells move south from divergence zones near Arrow Point and south of Little Manzanita Bay and converge at the head of Big Manzanita Bay. Relative to wave exposure, shorelines of MA-9 are considered “very-protected” in upper embayment reaches, or “protected” (Figure B-62). No feeder bluffs have been documented in MA-9. Manzanita Bay receives upland flows from two watersheds with low to moderate levels of land use: Manzanita Creek (WRIA 15.0344; TIA 5%) and WRIA unnumbered 29 (TIA <1%).
Overhanging riparian vegetation covers approximately 35% of the MA-9 shoreline, ranking this MA among the best on Bainbridge Island in this category (Table A-1, Figure B-67). Within the 200-ft riparian zone, naturally vegetated surfaces (coniferous and deciduous trees, shrubs, and wetlands) compose 70% of land cover, whereas impervious surfaces (e.g., roads, roofs) represent only 12% of land cover (Figure B-66). As such, riparian zone land-cover in Manzanita Bay is among the best of all Bainbridge Island MAs.
Shoreline development in MA-9 is primarily residential. Approximately 57% of the shoreline is modified by armoring, and 29% of the shoreline has armoring that encroaches into the intertidal zone, among the highest rates for any MA on Bainbridge Island (Table A-1, Figure B-63). A total of 218 point modifications were recorded along MA-9 shorelines, at an average of 11.5 modifications per 1000 ft (Figure B-64). Most of the modifications along this protected embayment were represented by stairs (63), docks (42), overwater structures (12), groins (7), and piers (9). A total of 9 outfalls were also recorded along MA-9 shorelines.
Limiting Factors
The average normalized controlling factor (CF) score within MA-9 was -0.49, within the middle range of MA scores on Bainbridge Island (Table A-2, Figure B-10, Figure B-13). However, Manzanita Bay (MA-9) had among the lowest (worst) scores of all MAs in five of the nine metrics: natural shade, artificial shade, sediment supply, substrate type, and depth/slope (Table A-3, Figure B-16). As was seen in MA-1, the Manzanita Bay MA scored low (bad) on the natural shade metric despite having a relatively high total percentage (35%) of shoreline with overhanging riparian vegetation. Closer examination of the data revealed that the shoreline along most reaches in MA-9 displayed relatively low coverage of overhanging vegetation (0% to 18%), whereas a few long marsh/lagoon reaches had cover exceeding 50%. Artificial shade scores were influenced primarily by the high density of shade-causing structures such as docks, piers, boats, buoys, and other overwater structures found in this protected anchorage. Sediment supply scores were affected primarily by the high percentage of armoring (57%), especially in front of areas with documented backshore sediment sources, combined with multiple groins (7) and drift-intercepting ramps in areas with alongshore sediment sources. The substrate type metric was similarly influenced by heavily armored shorelines combined with relatively high densities of point modifications. Low (poor) depth and slope scores also reflected the relatively high rate of armoring, combined with encroachment rates (29%), along Manzanita Bay shorelines.
Manzanita Bay (MA-9) had one of the best (highest) scores of all MAs in the pollution metric (Table A-3, Figure B-16). This is partly because of the absence of recreational shellfish closures and warnings, though it should be noted that Manzanita Bay has not been classified by the WA Department of Health. Low pollution metric scores were further aided by few marinas, low outfall densities, and a relatively low %TIA (12%) in the marine riparian zone.
Opportunities
One of the most obvious opportunities for improving scores in a number of MA-9 metrics would be to minimize and remove shoreline armoring, especially since this is not a high wave-energy area. Scores would also improve by removing or maximizing light penetration under existing shade-causing structures, or reducing the number of docks and overwater structures.
The EF scores are particularly high in MA-9 (23.9) relative to other MAs on Bainbridge Island (Table A-4, Figure B-11, Appendix D). This was likely due to the prevalence of documented spawning by all three forage-fish species (herring, sandlance, and surf smelt) (Figures B-69). Cutthroat trout, chum, and coho salmon are also documented to spawn in the Manzanita Creek (WRIA 15.0344), and efforts should be made to maximize adult passage and juvenile rearing functions, both in-stream and in nearby shoreline habitats. Eelgrass is fairly sparse along most Manzanita Bay shorelines, although geoduck and clam resources are abundant (Figure B-68, Figure B-70).
Individual Reaches within MA-9 (Figure B-71, Appendix C, Appendix D)
Reach 3492
CF Rating = Moderate Impact; Normalized score = -0.525
Most impacted CF metric: substrate type
Least impacted CF metric: natural shade, hydrology
Geomorphic class: Spit/Barrier/Backshore; Shoreline length: 2034 ft
Sediment source: Alongshore; Wave exposure: protected
Overhanging vegetation: 2.6%; Riparian zone land use: 22.6% TIA, 61.1% forested
Total shoreline armoring: 95.6%; Total encroaching: 5%
Total point modifications: 39; Density: 19.2/1000 ft; most common is stairs, with 5.9/1000 ft
Other:
EF Score: 22
Reach 3493
CF Rating = Moderate Impact ; Normalized score = -0.425
Most impacted CF metrics: artificial shade, substrate type, depth/slope
Least impacted CF metric: wave energy
Geomorphic class: Marsh/Lagoon; Shoreline length: 1209 ft
Sediment source: Alongshore; Wave exposure: protected
Overhanging vegetation: 57.3%; Riparian zone land use: 13% TIA, 72.2% forested
Total shoreline armoring: 38.7%; Total encroaching: 33.8%
Total point modifications: 11; Density: 9.1/1000 ft; most common is buoys, with 3.3/1000 ft
Other: small embayment, some fronting road
EF Score: 23
Reach 3494
CF Rating = Low Impact, Normalized score = -0.125
Most impacted CF metric: substrate type
Least impacted CF metric: wave energy, natural shade, pollution, hydrology, physical disturbance
Geomorphic class: Marsh/Lagoon; Shoreline length: 4804 ft
Sediment source: Foreshore; Wave exposure: very protected
Overhanging vegetation: 81.3%; Riparian zone land use: 7.5% TIA, 75.9% forested
Total shoreline armoring: 18%; Total encroaching: 4.5%
Total point modifications: 25; Density: 5.2/1000 ft; most common is stairs, with 2.5/1000 ft
Other:
EF Score: 26
Reach 3495
CF Rating = Moderate/High Impact ; Normalized score = -0.622
Most impacted CF metrics: natural shade, depth/slope
Least impacted CF metric: pollution, physical disturbance
Geomorphic class: High Bluff; Shoreline length: 1470 ft
Sediment source: Backshore; Wave exposure: protected
Overhanging vegetation: 0%; Riparian zone land use: 11.6% TIA, 81.3% forested
Total shoreline armoring: 99.9%; Total encroaching: 76.9%
Total point modifications: 16; Density: 10.9/1000 ft; most common is stairs, with 4.1/1000 ft
Other:
EF Score: 33
Reach 3496
CF Rating = Moderate Impact ; Normalized score = -0.556
Most impacted CF metric: substrate type
Least impacted CF metric: pollution, hydrology, physical disturbance
Geomorphic class: High Bluff; Shoreline length: 3076 ft
Sediment source: Backshore; Wave exposure: protected
Overhanging vegetation: 29.9%; Riparian zone land use: 14.4% TIA, 58.8% forested
Total shoreline armoring: 52.8%; Total encroaching: 36.6%
Total point modifications: 50; Density: 16.3/1000 ft; most common is floating docks, with 4.9/1000 ft
Other:
EF Score: 30
Reach 3497
CF Rating = Low Impact, Normalized score = -0.2
Most impacted CF metrics: natural shade, artificial shade, sediment supply
Least impacted CF metric: wave energy, pollution, hydrology, physical disturbance
Geomorphic class: Marsh/Lagoon; Shoreline length: 1698 ft
Sediment source: Foreshore; Wave exposure: very protected
Overhanging vegetation: 50.1%; Riparian zone land use: 8.2% TIA, 64.9% forested
Total shoreline armoring: 6.3%; Total encroaching: 6.3%
Total point modifications: 8; Density: 4.7/1000 ft; most common is stairs, with 1.2/1000 ft
Other:
EF Score: 23
Reach 3498
CF Rating = Moderate/High Impact, Normalized score = -0.711
Most impacted CF metrics: natural shade, substrate type, depth/slope
Least impacted CF metric: pollution
Geomorphic class: Low Bank; Shoreline length: 1414 ft
Sediment source: Alongshore; Wave exposure: protected
Overhanging vegetation: 0%; Riparian zone land use: 8.3% TIA, 73.9% forested
Total shoreline armoring: 90.1%; Total encroaching: 74.8%
Total point modifications: 24; Density: 17/1000 ft; most common is stairs, with 4.2/1000 ft
Other:
EF Score: 20
Reach 3499
CF Rating = Moderate Impact ; Normalized score = -0.6
Most impacted CF metrics: natural shade, substrate type
Least impacted CF metric: hydrology
Geomorphic class: Low Bank; Shoreline length: 2080 ft
Sediment source: Alongshore; Wave exposure: protected
Overhanging vegetation: 0%; Riparian zone land use: 12.6% TIA, 73.2% forested
Total shoreline armoring: 91.8%; Total encroaching: 16.8%
Total point modifications: 32; Density: 15.4/1000 ft; most common is stairs, with 4.8/1000 ft
Other: 1 marina(s) present
EF Score: 20
Reach 3500
CF Rating = Moderate Impact ; Normalized score = -0.514
Most impacted CF metric: substrate type
Least impacted CF metric: wave energy, natural shade
Geomorphic class: Spit/Barrier/Backshore; Shoreline length: 427 ft
Sediment source: Alongshore; Wave exposure: very protected
Overhanging vegetation: 0%; Riparian zone land use: 20.8% TIA, 65.4% forested
Total shoreline armoring: 99.5%; Total encroaching: 53%
Total point modifications: 6; Density: 14.1/1000 ft; most common is upland structures at waterline, with 4.7/1000 ft
Other:
EF Score: 22
Reach 3501
CF Rating = Moderate Impact ; Normalized score = -0.578
Most impacted CF metric: depth/slope
Least impacted CF metric: physical disturbance
Geomorphic class: High Bluff; Shoreline length: 668 ft
Sediment source: Backshore; Wave exposure: protected
Overhanging vegetation: 17.7%; Riparian zone land use: 14% TIA, 79% forested
Total shoreline armoring: 100%; Total encroaching: 100%
Total point modifications: 7; Density: 10.5/1000 ft; most common is stairs, with 4.5/1000 ft
Other:
EF Score: 20
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