Projections of uture Transitions in Tidal Wetlands under ... Wetland Response to SLR_Final...

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Projections of Future Transitions in Tidal Wetlands under Sea Level Rise within the Port Gamble S’Klallam Traditional Use Areas

Mary F. Ramirez and Charles A. Simenstad

School of Aquatic and Fishery Sciences, University of Washington

January, 2018

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Table of Contents Background ................................................................................................................................................... 1

Approach ....................................................................................................................................................... 2

Projected Sea Level Rise .......................................................................................................................... 4

Methods ........................................................................................................................................................ 4

Elevation ................................................................................................................................................... 5

Wetland ..................................................................................................................................................... 6

Land Cover................................................................................................................................................ 7

Levee ......................................................................................................................................................... 7

Analysis .................................................................................................................................................... 7

Results ........................................................................................................................................................... 9

Point Gamble S’Klallam Tribe Primary Traditional Use Area ................................................................. 9

Major River Deltas .................................................................................................................................. 12

Kilisut Harbor and Port Gamble Bay ...................................................................................................... 22

Discussion ................................................................................................................................................... 28

Conclusions ................................................................................................................................................. 30

Acknowledgements ..................................................................................................................................... 30

References ................................................................................................................................................... 31

Figures Figure 1. Generalized schematic of estuarine wetland types along the tidal inundation gradient (top).

Transgressive migration is the process by which tidal wetlands migrate landward in response

to new tide range elevations (middle). Where natural or artificial barriers prevent

transgressive migration, the acceleration of sea level rise puts tidal wetlands at risk of

submergence (bottom). .............................................................................................................. 3

Figure 2. Port Gamble S’Klallam Tribe Traditional Use Area. ................................................................ 5

Figure 3. Representative Puget Sound estuarine wetland types................................................................ 7

Figure 4. Proportional composition of tidal and non-tidal wetland categories within the Tribe traditional

use area (PGSK) and major river deltas under the initial condition and by year 2100 relative

to three scenarios of sea level rise. .......................................................................................... 10

Figure 5. Area (hectares) of wetland categories and corresponding wetland classes at time steps

between 2004 (initial condition) and 2100 under three scenarios of sea level rise (SLR). ..... 10

Figure 6. Scaled bubble plot of the area (hectares) of transgressive migration by PSNERP process unit

(PU) and proportional loss of tidal wetlands relative to the initial area by PU under 1.4 m

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SLR. Increase in bubble size corresponds with more migration of tidal wetlands landward

(green) and a greater proportion of tidal wetland loss (blue). ................................................. 16

Figure 7. Area (hectares) of tidal and non-tidal wetlands within major river deltas under initial

condition (year 2004) and projected under three scenarios of sea level rise by 2100. ............ 17

Figure 8. Projected tidal wetland outcomes in the five major river deltas within the Tribe’s interest area

by year 2100 under 1.4 meters sea level rise. .......................................................................... 18

Figure 9. Initial condition (top row) and SLAMM modeled change by 2100 under three sea level rise

scenarios around the Dungeness River delta. Red inset boxes show zoomed areas (right

column). ................................................................................................................................... 19

Figure 10. Initial (2004) wetland and land cover in the Kilisut Harbor (left) and Port Gamble Bay (right)

areas………………………………………………………………………………………… 23

Figure 11. Example of a barrier lagoon with an adjacent barrier estuary at the mouth of Mystery Bay in

the Kilisut Harbor area (left) and Point Julia barrier lagoon at the entrance to Port Gamble

Bay (right). Washington State Department of Ecology Shoreline Photos (2006). .................. 24

Figure 12. Projected tidal wetland outcomes in Kilisut Harbor by year 2100 under 1.4 meters sea level

rise. Inset maps detail projections in two areas along Marrowstone Island. ........................... 26

Figure 13. Projected tidal wetland outcomes in Port Gamble Bay by year 2100 under 1.4 meters sea

level rise. Inset map detail projections around Point Julia barrier lagoon. .............................. 27

Tables Table 1. Input parameters to SLAMM. ....................................................................................................... 6

Table 2. Source wetland and land cover data for SLAMM classification. ................................................. 8

Table 3. Initial (2004) area (hectares) of individual SLAMM land cover classes and projected area and

percent change in area from initial condition by 2100 under three sea level rise scenarios. ...... 11

Table 4. Transition matrices comparing initial area (hectares; rows) to year 2100 area (columns) under

A) 0.6, B) 1.4, and C) 2.0 meters sea level rise. Highlighted cells show the area that did not

change land cover class (persistence). Open water classes were compiled into a single category

as were low tidal classes other than tidal flat.............................................................................. 13

Table 5. Area and percent change in land cover classes under three scenarios of sea level rise within each

of the five major river deltas (A-E) located in the study area. .................................................... 20

Table 6. Area and percent change in land cover classes under three scenarios of sea level rise within A.

Kilisut Harbor, and B. Port Gamble Bay. ................................................................................... 25

Table 7. Summary of sea level rise (SLR) outcome probabilities under 2 emissions scenarios by 2100

and 2150 (based on updated projections (unpublished) from CIG for the Washington Coastal

Resilience Project). ..................................................................................................................... 28

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Background Although the global projections of relative sea level (RSL) rise suggest a net trend of 0.3-0.8 m of eustatic

RSL by 2100 (Church et al. 2013), regional variability in sea level changes deviate substantially from that

global mean (Carson et al. 2016). Sources for this variability range from large-scale, dynamic ocean and

atmospheric processes as well as finer scale isostatic and tectonic adjustments. Planning for sea level rise,

and other related climate changes, at the regional scale often embodies equally uncertain variability,

suggesting that coastal zone management would benefit from more strategic analysis of the scope, scale

and direction of RSL. This is particularly the case for tidal wetlands, which through biophysical feedback

processes have the potential to build vertically commensurate with sea level rise, even under accelerating

RSL conditions (Kirwan et al. 2016). The Pacific Northwest coast of North America is one of those

regions with variable sea level rise (NRC 2012), and is identified to have less sea level rise, as much as

0.1 m lower than the global average (both Intergovernmental Panel on Climate Change (IPCC) RCP 4.5

and RCP 8.5 scenario models; Carson et al. 2016). Furthermore, factors affecting both projected RSL and

climate change-associated sediment delivery, sedimentation and accommodation area for transgressive

development of tidal wetlands in newly inundated shorelines are spatially variable even within the Salish

Sea/Puget Sound (Mauger et al. 2015).

It is well established that global warming is happening and impacts are already being observed in the

Pacific Northwest (Glick et al. 2007, Mauger et al. 2015). Among other changes due to climate change,

the region is expected to face higher air and water temperatures, decreased snowpack, changing seawater

chemistry due to ocean acidification, and rising sea level along most of the Puget Sound coastline

(Mauger et al. 2015). Coastal shorelines, and particularly tidal wetlands, throughout Puget Sound are

already under pressure from consistent and continued growth of the human population and the structural

changes and development that accompany such growth (Simenstad et al. 2011). Remaining tidal wetlands

are therefore critical in their provision of a wide variety of ecosystem functions for fish and wildlife

(Fresh et al. 2011), as well as ecosystem goods and services for humans, such as mediation of storm

surges, water quality improvement, and carbon sequestration (Simenstad et al. 2011). While wetlands

have often been able to persist under past gradual sea level rise with adequate sediment supply, accretion,

and surface elevation increase, the acceleration of sea level rise and human alterations to the nearshore

now puts them at risk of submergence (Figure 1).

Kirwan and Megonigal (2013) describe the biological and physical feedbacks that have enabled wetlands

to actively engineer and stabilize their position in the intertidal zone. Above-ground plant growth slows

water velocities on the marsh surface, lowers wave height, and reduces erosion, together enhancing

sediment accretion (Kirwan and Megonigal 2013). Below-ground root growth and decay contribute

organic matter to the soil, resulting in elevation gain through sub-surface expansion (Kirwan and

Megonigal 2013). These feedbacks are shown to vary with the depth and duration of flooding, with peak

productivity occurring at intermediate elevations in the intertidal zone. This would suggest that flooding

from sea level rise should be accompanied by enhanced sediment accretion and therefore wetland

stabilization. However, impacts to sediment delivery, such as shoreline barriers and dam construction,

will limit availability and reduce the threshold at which wetlands can sustain rising sea levels (Kirwan and

Megonigal 2013). Loss of tidal marsh may be offset by transgressive migration, the process by which

tidal wetlands migrate landward in response to new tide range elevations (Brinson et al. 1995; Glick et al.

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2007). Tidal barriers, such as levees, may prevent this migration as efforts are made to protect

developments. Transgressive migration of tidal wetlands can also be naturally blocked by shoreline

geologic formations, such as bluffs and cliffs.

Puget Sound tribes are rooted to the nearshore, having spent centuries living along the shores of the

Straits of Juan de Fuca, Admiralty Inlet, and the greater Salish Sea. Traditionally, tribes adapted their

lives to the natural resources of the sea, rivers, and land (Gorsline 2017). The Port Gamble S’Klallam

Tribe (Tribe) is committed to the sustainable management of natural and cultural resources on the Tribe’s

reservation, located on the northern portion of the Kitsap Peninsula in Kitsap County, Washington, as

well as throughout their greater primary traditional use area (Barrett et al. 2014). This includes the

protection, restoration, and management of Puget Sound tidal wetlands, which the Tribe places great

value on as a cultural, subsistence, and economic resource (Barrett et al. 2014). The varied potential for

tidal wetland submergence, resilience and transgressive growth due to accelerated sea level rise should be

considered in the Tribe’s conservation and restoration efforts. Thus, a more precise understanding of

variability in climate change effects on tidal wetlands and shorelines along the estuaries and beaches of

the Salish Sea/Puget Sound would better inform strategies for spatially-explicit adaptation and

management for the future.

Approach We conducted a spatially-explicit analysis of potential wetland responses to future sea level rise over the

Tribe’s primary traditional use area around northern Hood Canal and the eastern Strait of Juan de Fuca

(Figure 2). The Tribe’s interest area is located in portions of three Puget Sound sub-basins (as defined by

the Puget Sound Nearshore Ecosystem Restoration Project (PSNERP)): Hood Canal, Strait of Juan de

Fuca, and North Central Puget Sound. The area includes significant wetlands associated with the

Dungeness, Quilcene, Dosewallips, Duckabush, and Hamma Hamma rivers deltas. Working with the

Tribe to identify specific areas of particular restoration, conservation or other management interests, we

focus our analyses on these large river deltas and also Kilisut Harbor and Port Gamble Bay. This provides

a more targeted understanding of the variation in the expected changes and adaptive capacity of tidal

wetlands in response to sea level rise and shoreline exposure within the context of the broader landscape.

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Figure 1 Generalized schematic of estuarine wetland types along the tidal inundation gradient (top).

Transgressive migration is the process by which tidal wetlands migrate landward in response to

new tide range elevations (middle). Where natural or artificial barriers prevent transgressive

migration, the acceleration of sea level rise puts tidal wetlands at risk of submergence (bottom).

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As an element of her 2015 M.S. thesis research (University of Washington, School of Aquatic and

Fishery Sciences) Brittany Jones used the Sea Level Affecting Marshes Model (SLAMM 6.2, open-

source; Warren Pinnacle Consulting 2017) to simulate potential changes to tidal wetlands in the U.S.

portion of the Salish Sea under the influence of accelerated sea level rise and other projected changes in

climate (Jones 2015). An important advancement of her approach was to incorporate the spatial

variability among the various factors that influence tidal wetland adaptability and development, such as

tidal range, vertical land movement, sediment delivery and wind-wave exposure. Although Jones’ (2015)

research considerably advanced our understanding about variation in the potential adaptability of different

tidal wetlands around the Salish Sea, the current study improves upon those findings by applying more

definitive spatial and temporal resolution and targeted analyses conducted on wetlands and shoreline of

specific interest to the Tribe. More contemporary data inputs are now available, including the most recent

national land cover product (NLDC; Homer et al. 2015) and higher resolution elevation data. The current

study also improved the wetland coverage dataset by refining the Mean Higher High Water (MHHW)

NOAA datum boundary that delineates regularly from irregularly flooded marsh and correcting errors in

the current wetland distribution. Select parameter inputs were also adjusted including the SLAMM

conceptual model elevation ranges and the elevation datum correction.

Projected Sea Level Rise

Sea levels are rising, but the predicted extent of sea level change varies regionally. Globally, sea level is

projected to increase by 27.9 to 96.5 cm (11 to 38 in) by 2100, where the rate of rise depends on the

amount of 21st century greenhouse gas emissions (Mauger et al. 2015). In the Puget Sound region, sea

level is projected to increase 35.6 to 137.2 cm (14 to 54 in) by 2100, though local rates of rise could

deviate from this range depending on the relative rate and direction of vertical land movement. The

historic sea level trend measured at the Port Townsend NOAA station since 1972 is approximately 1.8 cm

(0.7 in) per decade. This is slightly greater than the historic global pace of sea level rise at 1.7 cm per

decade, signifying minimal land subsidence in the area at roughly 0.1 cm per decade (NOAA 2017).

Within the Tribe’s traditional use area, sea level was modeled to rise by 61.0 cm (24 in) under a moderate

scenario, 1.4 m (56 in) under a high scenario, and 2.0 m (79 in) under an accelerated scenario. SLAMM

scales the rate of SLR to the A1B maximum scenario described by the IPCC.

Methods The Sea Level Affecting Marshes Model (SLAMM) was used to simulate changes in wetland type, area,

and distribution under future sea level rise. SLAMM simulates the dominant processes involved in

wetland transitions and shoreline modifications during long-term sea level rise, including inundation,

accretion, erosion, and soil saturation. Distributions of wetlands are predicted based on elevation ranges

defined for each wetland type, salinity, and water saturation of dry land. The model uses a decision tree to

assess at each time step how a wetland type will transition when it falls below its lower elevation

boundary. For example, when transitional salt marsh (e.g. scrub-shrub marsh) falls below its lower

elevation boundary by rising sea levels, this category generally converts to regularly flooded salt marsh

(e.g. emergent marsh). SLAMM assumes all areas currently designated as developed will remain

protected and therefore, does not project inundation of existing developed areas.

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Figure 2 Port Gamble S’Klallam Tribe Traditional Use Area.

Model inputs to SLAMM include: elevation, slope, current distribution of wetland and other land cover

types, historical sea level trend, tidal range, accretion rates, and erosion rates. This study will follow the

approach taken by Jones (2015) to use NOAA tide gauges throughout the study area for input tidal

parameters and SLAMM default values for marsh and swamp erosion rates. We will draw on Jones’

(2015) incorporation of accretion rates from local or (lacking that) regional peer-reviewed data applied to

specific wetland types (Table 1). Spatial data layers were clipped to a 2 km shoreline buffer within the

Tribe’s traditional use area.

Elevation

Elevation is arguably the most important data input to the model because habitat distributions are based

on the elevation that drives how frequently wetlands are inundated. We reviewed two regional

bathymetric-topographic digital elevation models (DEM): NOAA’s National Geophysical Data Center

(NGDC) integrated 1/3 arc-second (approximately 10-m) resolution DEM of Puget Sound (Carignan et al.

2014), and the University of Washington Puget lowland 9-m resolution DEM (Finlayson 2005). The

Finlayson combined swath bathymetry and LiDAR topography was determined to provide the most

adequate region-wide elevation resolution for the Tribe’s interest area, primarily due to inconsistencies

identified in the NGDC bathymetry. Slope is also derived from the integrated dataset.

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Table 1 Input parameters to SLAMM.

Parameter Input Source

Historic SLR Trend (mm/yr) 1.82 Port Townsend NOAA Station (1972-2015)

MTL-NAVD88 Correction (m) 1.2-1.3 NOAA Tides and Currents

GT Great Diurnal Tide Range (m) 3.3741 2.5972

1Bangor and 2Port Townsend NOAA Station

Salt Elevation (m above MTL) 1.5 Port Townsend NOAA Station*

Marsh Erosion (horz. m /yr) 2 SLAMM default

Swamp Erosion (horz. m /yr) 1 SLAMM default

T.Flat Erosion (horz. m /yr) 0.15 Derived from Keuler 1988 and Shipman 2004*

Reg.-Flood Marsh Accretion (mm/yr)

3.16 Derived from Thom 1992*

Irreg.-Flood Marsh Accretion (mm/yr)

3.6 Derived from Craft et al. 1993*

Tidal-Fresh Marsh Accretion (mm/yr)

8.4 Derived from Neubauer et al. 2002*

Inland-Fresh Marsh Accretion (mm/yr)

5.35 Derived from Hansen and Nestlerode 2014*

Tidal Swamp Accretion (mm/yr) 2.26 Derived from Craft 2012; Kroes and Hupp 2010; Noe and Hupp 2009;

Rybczyk et al. 2002*

Swamp Accretion (mm/yr) 3.65 Derived from Hansen and Nestlerode 2014; Conner and Day 1991*

Beach Sed. Rate (mm/yr) 2.1167 Derived from Ball 2004*

* Acquired from Jones (2015)

Bare-earth, 1-m resolution, LiDAR data on the Big Quilcene, Dosewallips, and Duckabush river deltas,

collected in 2008, provides higher resolution elevation data for these three major river deltas within the

study area (Watershed Sciences 2008). While coverage of these LiDAR datasets does not extend to the

deeper edge of the low tidal mud and sand flats, their use in the SLAMM does produce more detailed

outputs of wetland change, relative to 9-m Finlayson DEM. SLAMM results derived from the 1-m

LiDAR data were compared to changes identified based on the Finlayson DEM as a way to validate

results of the coarser, region-wide elevation data.

Wetland

Current wetland distribution is based on the PSNERP delineation of four main tidal wetland classes: (1)

euryhaline unvegetated, such as mudflats; (2) estuarine mixing, characterized by emergent marsh

vegetation; (3) oligohaline transition, characterized by scrub-shrub woody vegetation; and, (4) tidal

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freshwater, predominantly forested swamps (Figure 3; Simenstad et al. 2011). Estuarine mixing wetlands

were further classified into low and high elevation marsh based on MHHW with reference to the National

Agriculture Imagery Program (NAIP) aerial imagery. Additional categories of wetlands as defined by the

National Wetland Inventory (NWI) supplement the PSNERP wetland classification. All wetlands in the

study area were broadly reviewed and corrected where inaccurate. PSNERP and NWI wetland

classifications were converted to SLAMM categories using the NWI classes to SLAMM six categories

table provided in the SLAMM Technical Documentation (Warren Pinnacle Consulting 2012; Table 2).

Figure 3 Representative Puget Sound estuarine wetland types.

Land Cover

The 2011 National Land Cover Database (NLCD 2011) is the most recent national land cover product and

is used to identify developed land that is assumed to be protected and excluded from inundation.

Developed land is defined as having greater than 20 percent impervious surface, and includes the NLCD

categories low, medium, and high intensity developed. Unprotected dry land provides opportunity for

transgressive migration, an important process by which new tidal wetlands form as sea level rises.

Levee

The PSNERP delineation of tidal barriers (line dataset of dikes, levees, roads, and other manmade

structures that impede tidal hydrology; Simenstad et al. 2011) was reviewed for use in the SLAMM.

Lands identified as protected by dikes are excluded from inundated in the model. In addition to the

PSNERP dataset, we used aerial imagery and local knowledge of the area to interpret any lands that

would be protected from inundation. The result was minimal dike coverage in the Quilcene delta and at

Point Whitney.

Analysis

SLAMM produces GIS and tabular data files of land cover at each time step modeled (25-year intervals

for this study). Summaries of land cover area and percent change in area from initial condition are

calculated from these outputs.

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Table 2 Source wetland and land cover data for SLAMM classification.

SLAMM Land Cover Category

SLAMM Land Cover Class

Source Classification Source

Open Water

15 – Inland Open Water

See SLAMM Technical Documentation

NWI

16 – Riverine Tidal Open Water NWI

17 – Estuarine Open Water NWI

19 – Open Ocean NWI

Low Tidal

11 – Tidal Flat Euryhaline Unvegetated PSNERP

10 – Estuarine Beach NWI

12 – Ocean Beach See SLAMM Technical Documentation NWI

14 – Rocky Tidal NWI

Salt Marsh 8 – Reg. Flooded Marsh Low Estuarine Mixing PSNERP

Transitional Marsh

20 – Irreg. Flooded Marsh High Estuarine Mixing PSNERP

7 – Transitional Salt Marsh (Scrub-Shrub Marsh)

Oligohaline Transition PSNERP

Freshwater Tidal 6 – Tidal Fresh Marsh See SLAMM Technical Documentation NWI

23 – Tidal Swamp Tidal Freshwater PSNERP

Freshwater Non-tidal

3 – Non-tidal Swamp NWI

5 – Inland Fresh Marsh See SLAMM Technical Documentation NWI

22 – Inland Shore NWI

Aggregated Dry Land 1 – Developed Low, Medium, High Intensity Developed NLCD

2 – Undeveloped Dry Undeveloped Land NLCD

To determine how individual raster grid cells transition over time, output raster grids were converted to

polygon (Raster to Polygon Conversion tool) and a spatial union of projected and initial land cover was

performed. This provides, for any given place in the landscape, what the initial wetland or other land

cover type was and what was projected under sea level rise at each time step. The fate of tidal wetlands is

described according to the following four outcomes:

1. Persistence: tidal wetland category (low tidal, salt marsh, transitional marsh, or freshwater

tidal) does not change

2. Loss: tidal wetland category converts to open water

3. Conversion: tidal wetland category converts to a different tidal wetland category (i.e.

transitional marsh converts to salt marsh)

4. Transgressive migration: landward movement of wetlands such that a non-tidal wetland

category or undeveloped dry land converts to a tidal wetland category

Spatial outputs were also intersected with PSNERP process units to describe variation in model

projections across the interest area. The process units are based on littoral sediment drift along the

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shoreline and tidal hydrology in the large river deltas (Simenstad et al. 2011). Process units extend water-

ward to 10 m depth and encompass the adjacent nearshore and upland drainage area, therefore capturing

within its boundary any wetlands associated with each unique drift cell or river delta.

Results

Point Gamble S’Klallam Tribe Primary Traditional Use Area

Under the modeled initial condition, as delineated in 2004 mapping, the Tribe’s interest area included

6,176 hectares of tidal wetlands and 986 hectares of non-tidal wetlands (Figure 4, Table 3). Tidal flats

(within the Low Tidal land cover category) dominate this coverage, comprising 76 percent of the total

wetland area and nearly 88 percent of the tidal wetland area. Freshwater non-tidal wetlands comprise the

second greatest coverage of total wetland area (14 percent), followed by salt marsh (6 percent),

transitional marsh (3 percent), and freshwater tidal wetlands (1 percent). The five major river deltas

together account for approximately one-quarter of the total initial tidal and non-tidal wetland area.

All three SLR scenarios predicted significant declines by 2100 in the aggregate area of tidal wetlands:

44.7 percent decline under 0.6 m, 50.3 percent decline under 1.4 m, and 52.7 percent decline under 2.0 m.

Tidal flats experience the largest loss in area as well as percent change by 2100 (60.2 to 78.8 percent

decline from initial condition) making this wetland class the most vulnerable to accelerated sea level rise

(Table 3). SLAMM projected roughly half of the tidal flat area will be lost within the first time step (2004

to 2025) under all three SLR scenarios (Figure 5).

Salt marsh (regularly flooded emergent marsh) is predicted to increase in area under all three scenarios

(Table 3). The area of regularly flooded marsh is stable through 2025, then increases sharply in 2050

(Figure 5). Change in salt marsh over subsequent time steps varies by SLR scenario. The transitional

marsh category includes irregularly flooded emergent marsh and scrub-shrub marsh. Irregularly flooded

marsh largely persists under 0.6 m SLR, but declines significantly under the higher scenarios,

demonstrating how this wetland class is sensitive to sea level rise estimates. There is little coverage of

scrub-shrub marsh in the initial mapping (19.3 ha), but this wetland class is projected to increase

substantially by 2100 under all three scenarios (Table 3). The SLAMM decision tree initially converts

non-tidal wetland and dry land to scrub-shrub marsh where transgressive migration occurs (Warren

Pinnacle Consulting 2012). This expansion in area is shown in Figure 5 where scrub-shrub marsh

increases over 3,000 percent in the first time step, corresponding to an initial wetland migration landward.

The area of scrub-shrub marsh then declines from the 2025 high point as it is projected to convert to

regularly flooded marsh with increasing sea levels.

By 2100 the modeled composition of tidal and non-tidal wetlands in the Tribe’s interest area has changed

substantially (Figure 4). Low tidal flats now comprise roughly 30 to 50 percent of the total wetland area.

Salt and transitional marsh together comprise 27 to 46 percent; a much larger contribution to the whole

than under initial conditions.

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Figure 4 Proportional composition of tidal and non-tidal wetland categories within the Tribe traditional use

area (PGSK) and major river deltas under the initial condition and by year 2100 relative to three

scenarios of sea level rise. Total area of tidal and non-tidal wetlands shown on the secondary axis.

DUN = Dungeness, QUL = Quilcene, DOS = Dosewallips, DUC = Duckabush, HAM = Hamma

Hamma.

Figure 5 Area (hectares) of wetland categories and corresponding wetland classes at time steps between

2004 (initial condition) and 2100 under three scenarios of sea level rise (SLR).

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Tidal Flat RegularlyFlooded Marsh

IrregularlyFlooded Marsh

Scrub-ShrubWetland

Tidal Swamp Inland FreshMarsh

NontidalSwamp

Low Tidal Salt Marsh Transitional Marsh FreshwaterTidal

Freshwater Non-tidal

Are

a (h

a)

0.6 m SLR 1.4 m SLR 2.0 m SLR

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Table 3 Initial (2004) area (hectares) of individual SLAMM land cover classes and projected area and

percent change in area from initial condition by 2100 under three sea level rise scenarios.

Area by Year 2100 (ha) Percent Change by Year 2100

Land Cover Initial Area 0.6 SLR 1.4 SLR 2.0 SLR 0.6 SLR 1.4 SLR 2.0 SLR

Open Water

Inland Open Water 215.18 172.39 167.74 164.91 -19.88 -22.05 -23.36

Riverine Tidal 14.85 9.93 7.93 5.98 -33.11 -46.57 -59.74

Estuarine/Ocean Open Water 75,649.87 79,247.45 79,984.79 80,385.80 4.76 5.73 6.26

Low Tidal

Tidal Flat 5,427.87 2,158.30 1,496.21 1,151.16 -60.24 -72.43 -78.79

Estuarine/Ocean Beach 0.48 19.98 34.41 41.91 4,091.23 7,119.30 8,692.98

Salt Marsh

Reg. Flooded Marsh 418.27 650.15 1,007.68 1,307.98 55.44 140.92 212.71

Transitional Marsh

Irreg. Flooded Marsh 228.14 212.98 100.20 61.22 -6.64 -56.08 -73.16

Scrub-Shrub Marsh 19.33 323.22 399.18 347.24 1,572.02 1,964.92 1,696.24

Freshwater Tidal

Tidal-Fresh Marsh 11.05 11.05 11.05 10.99 0 0 -0.61

Tidal Swamp 61.86 61.06 56.18 43.39 -1.28 -9.18 -29.86

Freshwater Non-Tidal

Non-Tidal Swamp 459.00 429.84 400.05 382.09 -6.35 -12.84 -16.76

Inland-Fresh Marsh 526.77 447.05 415.45 394.34 -15.13 -21.13 -25.14

Inland Shore 9.32 9.09 9.09 9.09 -2.51 -2.51 -2.51

Aggregated Dry Land

Developed Dry Land 5,201.62 5,201.62 5,201.62 5,201.62 0 0 0

Undeveloped Dry Land 60,162.17 59,451.66 59,114.21 58,898.08 -1.18 -1.74 -2.10

The transition tables (Table 4a-c) indicate both consistency and variation for each SLR scenario in the

persistence of land cover classes as well as change from one class to another by 2100. For example, tidal

flats initially cover 5,427.87 ha. Under 0.6 m SLR, 3,271.69 ha convert to open water and 2,156.18 ha

persist as tidal flat. Under 1.4 and 2.0 m SLR, the area of tidal flats lost to open water increases to

3,935.06 and 4,284.56 ha, respectively. Very little conversion from other land cover classes to tidal flats

is projected to occur.

Regularly flooded marsh is a tidal wetland class that is projected to both maintain much of its initial area

and expand in area through the conversion of other land cover classes. Conversion of irregularly flooded

marsh into regularly flooded marsh is minimal under 0.6 m SLR (8.59 ha), but considerable under 1.4 m

(123.4 ha) and 2.0 m (170.75 ha) SLR.

The extent of transgressive migration of tidal wetlands landward can also be derived from the transition

tables. Initial areas of inland-fresh marsh, non-tidal swamp, and undeveloped dry land covert to both

regularly flooded marsh and scrub-shrub marsh under all three scenarios. Under 0.6 m SLR (Table 4a),

12

approximately 17 ha of non-tidal wetland convert to regularly flooded marsh and nearly 90 ha convert to

scrub-shrub. Additionally, 464 ha of undeveloped dry land are projected to convert to tidal wetland, while

244 ha would be inundated by open water. Under 1.4 m SLR (Table 4b), approximately 110 ha of non-

tidal wetland convert to regularly flooded marsh and approximately 60 ha convert to scrub-shrub. An

additional 740 ha of undeveloped dry land are projected to convert to tidal wetland, while 308 ha would

be inundated by open water.

Transitions in tidal wetlands from accelerated sea level rise are not expected to occur evenly over the

Tribe’s interest area. Figure 7 summarizes projected change by process unit under 1.4 m SLR from tidal

wetland area gained through transgressive migration and proportional tidal wetland loss to open water.

Proportional to the initial tidal wetland area, the area of tidal wetland loss was relatively low around the

Kitsap Peninsula. Process units along Hood Canal were projected to have moderate to high tidal wetland

loss, typically reaching or exceeding half of the initial tidal wetland area. In the northern part of the study

area, along Admiralty Inlet and the Strait of Juan de Fuca, tidal wetland loss was predicted to be high,

exceeding 86 percent of the initial area in many of the PSNERP process units. This area was also

projected to have greater expansion overall in wetland area through transgressive migration, which may

offset to a certain degree some of the wetland loss projected to occur. Area of expansion was greatest in

the Dungeness River delta process unit. Transgressive migration was generally limited throughout Hood

Canal and the Kitsap Peninsula, though tidal wetlands were projected to expand between Point No Point

County Park and Hansville Greenway Alder Wetland, with potential impacts to shoreline housing.

Major River Deltas

Predicted change to wetlands in the five major river deltas within the Tribe’s interest area varied among

the PSNERP delta process unit boundaries. The composition of tidal and non-tidal wetlands associated

with the river deltas is similar to that described for the study area as a whole, although some variability

does occur between river systems (Figure 4). The initial areal extent is greatest in the Dungeness delta

with nearly 900 ha of wetlands. Of this, low tidal flats comprise nearly 68 percent and freshwater non-

tidal wetlands comprise another 23 percent. The Quilcene delta has the second largest initial expanse with

approximately 400 ha of wetlands. This area is primarily divided between low tidal flats (54 percent), salt

marsh (15 percent) and freshwater tidal wetlands (20 percent). The three remaining deltas (Dosewallips,

Duckabush, Hamma-Hamma) have between 165 and 210 ha of initial wetlands with a similar composition

dominated by low tidal flats and salt marsh (Figure 4).

All five of the river deltas were projected to lose tidal flat habitat under each of the SLR scenarios that

were modeled (Figure 7, Table 5). The Dungeness delta was projected to lose nearly 80 percent of its

expansive 600 hectares tidal flat, under the 0.6 m SLR scenario modeled, and up to nearly 90 percent

under 2.0 m SLR. The Quilcene delta, in comparison, was projected to lose less than one-quarter of the

initial tidal flat area under 0.6 m SLR, and maintains over half of the tidal flat area even under the high

2.0 m SLR scenario. Regularly flooded marsh and scrub-shrub marsh were predicted to expand in area

within all deltas under all three scenarios. This expansion occurred through either transgressive growth, or

conversion of irregularly flooded marsh. Figure 8 delineates where tidal wetland change, persistence, or

growth was projected in each of the major river deltas under 1.4 m SLR by 2100. While the Dungeness is

shown to incur extensive change through both tidal wetland loss and transgressive growth, the other

deltas all maintain substantial portions of their initial wetlands, including those waterward of the current

shoreline (Figure 8).

13

Table 4 Transition matrices comparing initial area (hectares; rows) to year 2100 area (columns) under A) 0.6, B) 1.4, and C) 2.0 meters sea level rise. Highlighted cells

show the area that did not change land cover class (persistence). Open water classes were compiled into a single category as were low tidal classes other than

tidal flat. Omitted from the tables is the developed dry land class (5,200 ha) that occurs within the study area, but does not change under projected sea level

rise.

A. 0.6 m SLR

2100 Land Cover Class

Area (ha) Open Water

Tidal Flat

Other Low Tidal

Reg.-Flooded Marsh

Irreg.-Flooded Marsh

Scrub-Shrub Marsh

Tidal Swamp

Tidal-Fresh Marsh

Inland Shore

Inland-Fresh Marsh

Non-Tidal

Swamp

Undevel-oped Dry

Land

Total Area Year

2004

Init

ial (

Year

20

04

) La

nd

Co

ver

Cla

ss

Open Water 75,879.90 75,879.90

Tidal Flat 3,271.69 2,156.18 5,427.87

Other Low Tidal 0.26 0.22 0.48

Reg. Flooded Marsh 25.44 0.21 392.61 418.26

Irreg. Flooded Marsh 6.4 0.24 8.59 212.91 228.14

Scrub-Shrub Marsh 0.32 0.87 18.14 19.33

Tidal Swamp 0.73 0.07 61.06 61.86

Tidal-Fresh Marsh 11.05 11.05

Inland Shore 0.23 9.09 9.32

Inland-Fresh Marsh 1.01 0.16 13.44 65.11 447.05 526.77

Non-Tidal Swamp 0.28 3.39 27.5 427.82 458.99

Undeveloped Dry Land 244.24 1.51 19.76 230.51 212.47 2.02 59,451.66 60,162.17

Total Area Year 2100 79,429.77 2,158.30 19.98 650.14 212.98 323.22 61.06 11.05 9.09 447.05 429.84 59,451.66

14

B. 1.4 m SLR



Tidal Flat

Other Low Tidal

Reg.-Flooded Marsh


Scrub-Shrub Marsh

Tidal Swamp

Tidal-Fresh Marsh

Inland Shore

Inland-Fresh Marsh

Non-Tidal

Swamp

Undevel-oped Dry

Land

Total Area Year

2004

Init

ial (

Year

20

04

) La

nd

Co

ver

Cla

ss

Open Water 75,879.90 75,879.90

Tidal Flat 3,935.06 1,492.81 5,427.87




Scrub-Shrub Marsh 0.32 4.82 14.19 19.33

Tidal Swamp 2.94 2.73 56.18 61.85

Tidal-Fresh Marsh 11.05 11.05



Non-Tidal Swamp 0.37 32.96 26.66 399.01 459.00


Total Area Year 2100 80,160.47 1,496.21 34.40 1,007.67 100.20 399.18 56.18 11.05 9.09 415.45 400.05 59,114.21

15

C. 2.0 m SLR



Tidal Flat

Other Low Tidal

Reg.-Flooded Marsh


Scrub-Shrub Marsh

Tidal Swamp

Tidal-Fresh Marsh

Inland Shore

Inland-Fresh Marsh

Non-Tidal

Swamp

Undevel-oped Dry

Land

Total Area Year

2004

Init

ial (

Year

20

04

) La

nd

Co

ver

Cla

ss

Open Water 75,879.90 75,879.90

Tidal Flat 4,284.56 1,143.31 5,427.87




Scrub-Shrub Marsh 0.33 0.01 7.57 11.42 19.33

Tidal Swamp 0.01 0.01 7.03 11.42 43.39 61.86

Tidal-Fresh Marsh 0.07 10.99 11.06



Non-Tidal Swamp 0.54 0.05 49.1 27.81 381.5 459.00


Total Area Year 2100 80,556.67 1,151.16 41.90 1,307.98 61.22 347.24 43.39 10.99 9.09 394.34 382.09 58,898.08

16

Figure 6 Scaled bubble plot of the area (hectares) of transgressive migration by PSNERP process unit (PU)

and proportional loss of tidal wetlands relative to the initial area by PU under 1.4 m SLR. Increase

in bubble size corresponds with more migration of tidal wetlands landward (green) and a greater

proportion of tidal wetland loss (blue).

17

The resulting composition of tidal and non-tidal wetlands also reflects the variability of these changes

among the deltas (Figure 4). As opposed to the Dungeness, the composition in the four deltas within Hood

Canal did not change significantly between initial condition and projections under 0.6 m SLR. Under the

two higher SLR scenarios, a shift in the proportional contribution of low tidal (decline) and salt marsh

(increase) did occur in the Hood Canal deltas. The Dungeness delta, by contrast, showed significant

changes in wetland composition under all three scenarios. Low tidal wetlands decreased to less than 25

percent of the composition, while salt marsh and transitional marsh increased from less than 10 percent of

the initial wetland area to 50 to 70 percent by 2100.

The Dungeness River delta is one of the areas expected to be most susceptible to wetland changes within

the Tribe’s interest area under the three SLR scenarios (Figure 9). In addition to the significant loss of tidal

flats, these maps demonstrate the sensitivity to SLR estimates. For example, under 0.6 m SLR irregularly

flooded marsh and tidal swamp largely persist, and scrub-shrub marsh accounts for much of the migration

into dry land. Under 1.4 m SLR, much of the irregularly flooded marsh transitions to regularly flooded

salt marsh and a combination of salt marsh and scrub-shrub migrates inland. This trend of expanding salt

marsh continues under 2.0 m SLR, and tidal swamp converts to more saline wetland types. Additionally,

tidal wetland growth in all three scenarios is projected to occur directly adjacent to, or surrounding,

housing development and roads in the Dungeness delta area (Figure 9).

Figure 7 Area (hectares) of tidal and non-tidal wetlands within major river deltas under initial condition

(year 2004) and projected under three scenarios of sea level rise by 2100. DUN = Dungeness, QUL

= Quilcene, DOS = Dosewallips, DUC = Duckabush, HAM = Hamma Hamma.

0

100

200

300

400

500

600

700

Low

Tid

al

Salt

Mar

sh

Tran

siti

on

al M

arsh

Fres

hw

ate

r Ti

dal

Fres

hw

ate

r N

on

-tid

al

Low

Tid

al

Salt

Mar

sh

Tran

siti

on

al M

arsh

Fres

hw

ate

r Ti

dal

Fres

hw

ate

r N

on

-tid

al

Low

Tid

al

Salt

Mar

sh

Tran

siti

on

al M

arsh

Fres

hw

ate

r Ti

dal

Fres

hw

ate

r N

on

-tid

al

Low

Tid

al

Salt

Mar

sh

Tran

siti

on

al M

arsh

Fres

hw

ate

r Ti

dal

Fres

hw

ate

r N

on

-tid

al

Low

Tid

al

Salt

Mar

sh

Tran

siti

on

al M

arsh

Fres

hw

ate

r Ti

dal

Fres

hw

ate

r N

on

-tid

al

Delta DUN Delta QUL Delta DOS Delta DUC Delta HAM

Are

a (h

a)

Initial Condition 0.6 SLR 1.4 SLR 2.0 SLR

18

Figure 8 Projected tidal wetland outcomes in the five major river deltas within the Tribe’s interest area by

year 2100 under 1.4 meters sea level rise.

19

Figure 9 Initial condition (top row) and SLAMM modeled change by 2100 under three sea level rise

scenarios around the Dungeness River delta. Red inset boxes show zoomed areas (right column).

20

Table 5 Area and percent change in land cover classes under three scenarios of sea level rise within each of

the five major river deltas (A-E) located in the study area.

A. Dungeness River Delta Area by Year 2100 (ha) Percent Change by Year 2100


Open Water


Riverine Tidal 4.97 4.30 3.62 2.87 -13.47 -27.10 -42.26

Estuarine Open Water 813.85 1,294.87 1,333.07 1,361.33 59.11 63.80 67.27

Low Tidal

Tidal Flat 605.42 132.09 94.99 67.93 -78.18 -84.31 -88.78

Estuarine Beach 0.07 3.21 9.05 14.48 4,699.99 13,424.98 21,549.97

Salt Marsh

Reg. Flooded Marsh 23.29 44.46 201.29 303.16 90.93 764.34 1,201.79

Transitional Marsh


Scrub-Shrub Marsh 0 162.38 144.44 105.22 100.00 100.00 100.00

Freshwater Tidal

Tidal Swamp 11.35 11.29 9.82 5.28 -0.52 -13.41 -53.43


Non-Tidal Swamp 110.27 85.04 66.47 60.78 -22.88 -39.72 -44.88


Aggregated Dry Land

Developed Dry Land 297.26 297.26 297.26 297.26 0.00 0.00 0.00

Undeveloped Dry Land 1,176.00 1,066.89 987.20 944.03 -9.28 -16.05 -19.73

B. Quilcene River Delta Area by Year 2100 (ha) Percent Change by Year 2100


Open Water

Inland Open Water 0.74 0.74 0.74 0.74 0.00 0.00 0.00

Estuarine Open Water 39.53 91.12 126.92 146.37 130.52 221.10 270.28

Low Tidal

Tidal Flat 217.34 166.17 130.48 111.15 -23.55 -39.97 -48.86

Salt Marsh

Reg. Flooded Marsh 60.70 61.58 75.75 91.23 1.45 24.80 50.28

Transitional Marsh


Scrub-Shrub Marsh 0.91 5.75 17.74 26.01 531.19 1846.83 2754.13

Freshwater Tidal

Tidal Fresh Marsh 9.17 9.17 9.17 9.11 0.00 0.00 -0.73

Tidal Swamp 16.75 16.73 16.35 13.37 -0.10 -2.35 -20.17


Non-Tidal Swamp 36.63 36.63 35.25 31.65 0.00 -3.77 -13.60


Aggregated Dry Land



21

Table 5 cont.

C. Dosewallips River Delta Area by Year 2100 (ha) Percent Change by Year 2100


Open Water

Inland Open Water 10.19 10.19 10.18 10.17 0.00 -0.08 -0.25

Riverine Tidal 2.34 2.25 1.81 1.31 -3.93 -22.50 -43.93


Low Tidal

Tidal Flat 140.97 100.28 40.41 27.12 -28.87 -71.33 -80.76

Salt Marsh


Transitional Marsh



Freshwater Tidal

Tidal Swamp 4.10 4.10 4.06 3.62 0.00 -0.82 -11.63


Non-Tidal Swamp 7.97 7.97 7.96 7.90 0.00 -0.10 -0.84

Inland-Fresh Marsh 1.64 1.64 1.64 1.64 0.00 0.00 0.00

Aggregated Dry Land


Undeveloped Dry Land 813.47 812.17 806.27 798.63 -0.16 -0.89 -1.82

D. Duckabush River Delta Area by Year 2100 (ha) Percent Change by Year 2100


Open Water


Low Tidal

Tidal Flat 118.60 71.24 44.05 31.30 -39.93 -62.86 -73.60

Salt Marsh


Transitional Marsh

Irreg. Flooded Marsh 5.02 5.03 2.48 2.96 0.17 -50.50 -41.00


Freshwater Tidal

Tidal Swamp 6.21 6.17 6.01 4.74 -0.67 -3.23 -23.69


Non-Tidal Swamp 10.33 10.33 10.33 10.32 0.00 0.00 -0.16


Aggregated Dry Land



22

Table 5 cont.

E. Hamma Hamma River Delta Area by Year 2100 (ha) Percent Change by Year 2100


Open Water


Riverine Tidal 7.54 3.39 2.50 1.80 -55.10 -66.85 -76.16


Low Tidal

Tidal Flat 131.04 65.95 36.05 19.42 -49.67 -72.49 -85.18

Salt Marsh


Transitional Marsh



Freshwater Tidal

Tidal Swamp 8.95 8.95 8.95 8.84 0.00 0.00 -1.21


Non-Tidal Swamp 0.74 0.74 0.70 0.61 0.00 -4.55 -17.05

Inland-Fresh Marsh 1.91 1.91 1.91 1.86 0.00 0.00 -3.06

Aggregated Dry Land



Kilisut Harbor and Port Gamble Bay

Kilisut Harbor and Port Gamble Bay both provide shallow intertidal habitat, characterized primarily by

low tidal flats, with some fringing salt marsh (Figure 10). The Kilisut Harbor study area (defined by the

area of PSNERP process units that intersect the bay) includes one barrier estuary and four barrier lagoons

(one of which is located outside the bay, on the south side of Indian Island at Indian Island County Park).

Port Gamble Bay includes one barrier estuary at the southern end of the bay, and one barrier lagoon

located at the eastern edge of the bay entrance along the study area boundary (Figure 10, Figure 11).

Kilisut Harbor was projected to lose 83 to 92 percent of the area’s tidal flats through the three SLR

scenarios (Table 6). The loss of tidal flats to open water would occur along the entire shoreline, with few

exceptions (Figure 12). Two areas where tidal flats may persist are at the barrier lagoons located along the

western shoreline of Kilisut Harbor at Bishops Point and at Indian Island County Park (Figure 12). The

remaining three small embayments were projected to lose all or most of their fringing tidal flats, while the

associated regularly and irregularly flooded marsh may persist or expand. Within the Kilisut Harbor study

area, regularly flooded marsh was estimated to expand by approximately 10 to 20 ha, and up to 10 ha of

dry land may convert to scrub-shrub wetland (Table 6). Some low elevation dry land (including developed

shoreline) may also be susceptible to conversion to open water if left unprotected. For example, sections

of SR 116 that run parallel to the shoreline along Mystery Bay were predicted to be inundated under all

three SLR scenarios (Figure 12).

Over half of the tidal flat area in Port Gamble Bay was projected to persist even under the high 2.0 m SLR

scenario (Table 6). Under 1.4 m SLR, tidal flats were projected to decline by 40 percent, with losses

concentrated on the eastern side of the bay (Figure 13). Much of the tidal wetlands at Point Julia were

projected to persist under 1.4 m SLR, while areas that were initially delineated as irregularly flooded

marsh will convert to regularly flooded salt marsh. Salt marsh was also projected to expand into

previously undeveloped dry land around Point Julia (Figure 13 inset). Little change was projected for

23

wetlands on the western side of the bay, as well as much of the barrier estuary at the southern end, under

all three SLR scenarios.

Figure 10 Initial (2004) wetland and land cover in the Kilisut Harbor (left) and Port Gamble Bay (right)

areas.

24

Figure 11 Example of a barrier lagoon with an adjacent barrier estuary at the mouth of Mystery Bay in the

Kilisut Harbor area (left) and Point Julia barrier lagoon at the entrance to Port Gamble Bay

(right). Washington State Department of Ecology Shoreline Photos (2006).

25

Table 6 Area and percent change in land cover classes under three scenarios of sea level rise within A.

Kilisut Harbor, and B. Port Gamble Bay.

A. Kilisut Harbor Area by Year 2100 (ha) Percent Change by Year 2100


Open Water

Estuarine Open Water 837.13 1,077.31 1,098.53 1,108.56 28.69 31.23 32.42


Low Tidal

Tidal Flat 236.34 39.26 24.99 18.80 -83.39 -89.42 -92.05

Salt Marsh


Transitional Marsh


Scrub-Shrub Marsh 0.80 6.27 10.69 10.19 680.25 1,230.00 1,166.94

Freshwater Tidal

Tidal Swamp 1.91 1.91 1.89 1.89 0.00 -1.31 -1.31


Non-Tidal Swamp 6.33 6.31 6.31 6.30 -0.26 -0.26 -0.40


Aggregated Dry Land



B. Port Gamble Bay Area by Year 2100 (ha) Percent Change by Year 2100


Open Water



Low Tidal

Tidal Flat 80.03 57.54 48.51 43.49 -28.10 -39.38 -45.65

Salt Marsh


Transitional Marsh

Irreg. Flooded Marsh 0.74 0.74 0.20 0.02 0.00 -73.39 -97.11

Scrub-Shrub Marsh 0 1.02 2.84 5.00 100 100 100


Non-Tidal Swamp 22.02 22.02 22.02 22.02 0.00 0.00 0.00


Aggregated Dry Land



26

Figure 12 Projected tidal wetland outcomes in Kilisut Harbor by year 2100 under 1.4 meters sea level rise.

Inset maps detail projections in two areas along Marrowstone Island.

27

Figure 13 Projected tidal wetland outcomes in Port Gamble Bay by year 2100 under 1.4 meters sea level rise.

Inset map detail projections around Point Julia barrier lagoon.

28

Discussion This report describes projected changes to tidal wetlands under three scenarios of sea level rise. As part of

the Washington Coastal Resilience Project, the Climate Impacts Group (UW) has recently developed an

updated assessment of absolute sea level rise. Under RCP 8.5 (high greenhouse gas emissions scenario)

sea level rise in Washington State will most likely reach 51.8 cm (20.4 in) to 70.1 cm (27.6 in) by 2100.

Under RCP 4.5 (low greenhouse gas emissions scenario) sea level rise in Washington State will most

likely reach 30.5 cm (12 in) to 67.1 cm (26.4 in) by 2100. Given both of these emissions scenarios, our

wetland change projections under the 0.6 m (61.0 cm) SLR scenario are identified as the most likely to

occur. The 1.4 m SLR scenario, and associated wetland projections, are described as very unlikely by

2100 (less than five percent probability) given estimates of both high or moderate greenhouse gas

emissions. Wetland projections under the 2.0 m SLR scenario have less than one percent probability of

occurring by 2100.

However, the Port Gamble S’Klallam Tribe is committed to the sustainable management of their natural

resources for at least seven generations to come. Thus, it is helpful to also consider these projections in

terms of their probability of occurrence by 2150. Under that time frame, SLR is very likely to exceed 0.6

m under RCP 8.5, and is likely to meet or exceed 0.6 m under RCP 4.5. A 2.0 m SLR scenario is still very

unlikely by 2150, but projections of 1.4 m SLR range from likely under RCP 8.5 to very unlikely under

RCP 4.5. Table 7 highlights the outcome probabilities for the three SLR scenarios used in this study.

Table 7 Summary of sea level rise (SLR) outcome probabilities under 2 emissions scenarios by 2100 and

2150 (based on updated projections (unpublished) from CIG for the Washington Coastal

Resilience Project).

Emissions Scenario 0.6 m SLR 1.4 m SLR 2.0 m SLR

2100 RCP 4.5 Likely Exceptionally unlikely Exceptionally unlikely

RCP 8.5 Best estimate Very unlikely Exceptionally unlikely

2150 RCP 4.5 Likely Very unlikely Very unlikely

RCP 8.5 Very likely Likely Very unlikely

Projections of wetland change in the Tribe’s interest area identified tidal flats as the most vulnerable

wetland class to loss under sea level rise. The SLAMM projected a 60 percent decline in tidal flats under

0.6 m SLR by 2100, and over 70 percent decline under the other two scenarios. In particular, the

extensive flats associated with the Dungeness River delta are projected to decline by nearly 80 percent by

2100 under 0.6 m SLR. Mud and sand flats support many invertebrate species such as clams, oysters, and

crabs that are of economic and cultural importance, as well as shrimp, amphipods, insect larvae, and

worms that fish and other organisms feed on. Tidal flats also provide critical habitat for migrating

shorebirds to rest and find food. The projections would undoubtedly result in shifts in species

distributions and potential declines in populations as the composition of the nearshore dramatically

changes.

Intermittently exposed shallow-water also provides a broad buffer between shorelines and open water by

dissipating wave energy as it reaches the shoreline. Substantial losses of tidal flats would place coastal

communities more at risk of storm surges and flooding. This enhanced risk may lead to communities

constructing additional barriers along the shoreline such as levees, sea walls, or other armoring. While

these structures may help protect nearshore properties, their presence may also exacerbate tidal wetland

29

loss by preventing transgressive migration of wetlands inland as well as trapping natural deposition of

sediment, which helps wetlands keep pace with sea level rise (Glick et al. 2007).

The SLAMM did project increases in regularly flooded marsh and scrub-shrub marsh, primarily through

the conversion of non-tidal wetlands and undeveloped dry land. This transgressive migration of tidal

wetlands inland assumes no new barriers and, of course, would be limited if structures were built. For

example, projections around the Dungeness delta show significant expansion of tidal wetlands

surrounding houses along 3 Crabs Road that runs parallel to the shoreline. Much of the area southwest of

3 Crabs Road is currently marshy grassland that will likely become more saline and frequently inundated

with accelerated sea level rise. Similarly, tidal wetland expansion was projected under all three scenarios

near Point No Point County Park on the Kitsap Peninsula. Under the higher SLR scenarios, loss of low

tidal wetlands and expansion of salt marsh into dry land appear to threaten houses built between Point No

Point Road and the shoreline. Relocating houses in these areas would provide opportunities for restoration

actions that could enhance transgressive development.

Glick et al. (2007) conducted SLAMM simulations for the Pacific Northwest, including at 11 sites in

Washington and Oregon in 2007. Included in these 11 sites was Dungeness Spit and Sequim Bay, where

tidal flats were identified as extremely vulnerable. Glick et al. (2007) projected over 80 percent loss of

tidal flats in this area as well as salt marsh and transitional scrub-shrub marsh expansion under 0.71 m

relative SLR by 2100; results that are very similar to the findings in our study. Contrastingly, Jones

(2015) found tidal flats in Puget Sound to have high local persistence. For example, under 0.6 m SLR,

Jones projected nearly total persistence of tidal flats (at or above 97 percent) by 2100 in each of the five

river deltas described in our study. Under 1.4 m SLR, tidal flat persistence remained above 95 percent for

the Hood Canal deltas, and declined to 72 percent persistence in the Dungeness delta. Overall, Jones

reported relatively small changes to the total tidal wetland area in the Hood Canal sub-basin (5 percent

decline under 0.6 m SLR, and 4 percent decline under 1.4 m SLR). These results differed from those

found by our analysis. Discrepancies between these two studies are likely due to a model parameter that

was corrected in our study.

While this study improves our understanding of projected changes to tidal and adjacent non-tidal wetlands

under accelerated sea level rise around the Tribe’s traditional use area, there remain limitations in our

ability to adequately predict responses. All model results presented here are subject to known biases

associated with elevation data and are limited by the coarse resolution of the regional DEM. Comparisons

between the SLAMM results derived from the 1-m LiDAR data and the Finlayson (2005) DEM were

generally consistent, however, the coarser dataset will not be able to detect or delineate small scale

changes inherent in these processes. Emphasis should be placed on the interpretation of overall patterns in

wetland transitions, rather than small-scale projections at a given location. Our ability to make site-

specific, higher resolution projections of how tidal wetlands will resist or adapt to future sea level rise will

require improved DEM resolution, especially for low tidal elevations. Beaches and tidal flats are expected

to experience significant impacts with a changing climate, but there are very few sources of adequate

elevation data for these low tidal habitats.

We performed a sensitivity analysis in SLAMM that found, in addition to SLR scenario, change in

wetland classes was most sensitive to great diurnal tide range and salt elevation settings. A low number of

tide stations in the region provide these model sensitive parameters and consequently may be responsible

for some variation in results. Additionally, there are gaps in our knowledge of sediment accretion rates

throughout Puget Sound, and the delivery of sediment to tidal wetlands is likely to vary under sea level

30

rise and a changing climate (Czuba et al. 2011). Projections for increased extreme precipitation events

could affect flooding and stream flow, potentially resulting in high suspended sediment concentration in

rivers and increased sediment deposition in deltas. Alternatively, human modifications to the environment

have the potential to alter ecological processes like sediment delivery. Damming of rivers to ensure a

stable water supply, for example, has been shown to trap large amounts of sediment and dramatically

decrease downstream delta accretion rates (Yang et al. 2003). Additionally, this study used a simple

erosion model where marsh and beach erosion was triggered only when the average fetch of a cell

exceeds 9 km. Results from modeling future changes in storm events and resulting maximum wave

height projections could better qualify areas most vulnerable to erosion and improve our assessment of

changes to the nearshore.

Finally, this study did not address intertidal eelgrass (Zostera marina) distribution and response to sea

level rise. Seagrass distribution is regulated by complex interactions between water-level variation, wave

energies, temperature and light, which complicates the ability to predict the effects of climate change

(Short and Neckles 1999). Rigorous research directed toward eelgrass in Puget Sound is needed to better

understand the systematic and geographic variability in how these systems respond to the changing

climate.

Conclusions This study found tidal flats to be the most vulnerable wetland class to losses associated with sea level rise.

Regularly flooded marsh, on the other hand, is projected to have relatively high persistence. Estimates of

vertical accretion rates in regularly flooded marsh wetlands are higher than those for tidal flats, and

regularly flooded marsh is also able to tolerate a wider elevation range. These factors help protect this

wetland class against extensive losses to sea level rise. As scenarios of future sea level rise increase,

persistence of tidal wetlands decreases, particularly the tidal flat and irregularly flooded marsh wetland

classes. At the same time, higher rates of sea level rise provide for increased opportunity for transgressive

migration of tidal wetlands into adjacent dry land, particularly the regularly flooded and scrub-shrub

marsh wetland classes.

While wetlands are projected to be affected by sea level rise throughout the Tribe’s primary traditional

use area, tidal wetland loss was predicted to be greatest along Admiralty Inlet and the Strait of Juan de

Fuca. Hood Canal was projected to have moderate to high tidal wetland loss, while the Kitsap Peninsula

exhibited the greatest rates of local persistence. Transgressive migration was generally limited throughout

Hood Canal and the Kitsap Peninsula where the adjacent dry land appears to be of sufficient elevation

gradient to limit conversion to tidal wetland (though the area around Point No Point County Park on the

Kitsap Peninsula serves as an exception). The greatest area of tidal wetland expansion was projected to

occur along the Strait of Juan de Fuca around the Dungeness delta. Identifying large areas of wetland

persistence or transgressive migration is useful for setting restoration and conservation priorities. It is

inherent that we consider climate change associated impacts in order to successfully conserve for future

generations existing tidal wetlands and the ecosystem services they provide.

Acknowledgements We are grateful for the funding support provided by the Port Gamble S’Klallam Tribe and collaboration

of the University of Washington’s Climate Impact Group. We would also like to thank Brittany Jones for

sharing her model parameters and data inputs for the Hood Canal sub-basin.

31

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Supplemental Data

ArcGIS file geodatabase of SLAMM input rasters

ArcGIS file geodatabase of SLAMM output rasters

Tabular data of SLAMM output

http://tidesandcurrents.noaa.gov/map/

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