HomeMy WebLinkAboutCOW 2009-11-23 Item 4I.1 - Shoreline Master Program - Attachment A.05: Shoreline Restoration Plan Summary5. SHORELINE RESTORATION PLAN SUMMARY
5.1 Backeround
The state guidelines require that local governments develop SMP policies that promote
"restoration" of impaired shoreline ecological functions and a "real and meaningful"
strategy to implement restoration objectives. The City's shoreline inventory and
characterization report identifies which shoreline ecological functions and ecosystem
processes have been impaired. Local governments are further encouraged to contribute
to restoration by planning for and supporting restoration through the SMP and other
regulatory and non regulatory programs. As part of the SMP update process, the City
developed a Draft Shoreline Restoration Plan in February 2007. The draft plan was
finalized in May, 2008 following technical review by King County and Ecology and has
since been updated to include additional potential projects, address Ecology coininents
and refocus priorities to projects within the Transition Zone. It is included as Appendix B
to the SMP.
The restoration plan builds on the Inventory and Characterization Report and provides a
framework to:
Identify primary goals for ecological restoration of the Green/Duwamish
ecosystem;
Identify how restoration of ecological function can be accomplished;
Suggest how the SMP update process may accomplish the restoration of
impaired shoreline functions associated with the Green/Duwamish ecosystem;
and
Prioritize restoration projects so that the highest value restoration actions may
be accomplished first.
5.2 Assessment of Shoreline Functions
As summarized in the previous section, the Inventory and Characterization analysis
examined riverine and estuarine ecosystem processes that maintain shoreline ecological
functions, and identified impaired ecological functions. The inventory report identified
key ecosystem processes, and provided a qualitative assessment of their levels of
functioning at both a watershed and city reach scale. Key ecosystem functions identified
in the inventory, their level of alteration, and potential restoration actions are summarized
in Table 1.
As noted in the Inventory and Characterization Report and summarized in the Shoreline
Inventory and Characterization Summary Section, many of the alterations to shoreline
functions and ecosystem processes in the Green/Duwamish River are due to watershed
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scale issues within the upper watershed which cannot be fully restored or addressed in the
lower river section through Tukwila. However, hydrologic, water quality, and habitat
restoration measures in the City do have the potential to improve the overall functioning
of this important section of the Green/Duwamish River ecosystem that includes the
transition zone from fresh to salt water.
Function
Category
Hydrologic
Hydrologic
Water
Quality
Water
Quality
Large Woody
Debris
(LWD) and
Organics
LWD and
Organics:
Table 1. General Restoration Potential within the Shorelines of Tukwila
Function
Channel Floodplain
Interaction
Upland sediment
generation
Retention of
particulates and
contaminants
Nutrient cGycling
Maintain
cGharacteristic
R1 cGommunity
Source of LWD
Alterations to natural functioning
Presence of flood protection structures
(e.g., levees, river bank revetments,
flood gates) and significant fill and
development along the shoreline limit
channel floodplain interactions in
Tukwila.
Fine sediment contribution to the river
is increased due to build -up and wash
off from surrounding urban land uses.
Levees and revetments are virtually
continuous along the riverbanks,
limiting the potential to retain
particulates or contaminants contained
in stormwater sheet flows in the
fluvially dominated reaches.
Particulates, including sediment, are
retained in the tidally dominated
reaches, as evidenced by the need to
dredge the estuary turning basin.
As channel floodplain interaction was
reduced, the channel became a conduit
for nutrients, offering little opportunity
for contact time with soils.
The majority of the shoreline within the
City of Tukwila is currently dominated
by non native invasive weed species
(Himalayan blackberry, reed canary-
grass, and Japanese knotweed). Some
higher quality areas of cottonwood,
alder, and willow exist in riparian areas
bordering open space, parkland, and
residential zones.
Despite the lack of many sources for
LWD, there are some large
cottonwoods and big leaf maples occur
along the levees and revetment system.
'Potential,Resto ration Action within the
City,
1. Modify current levees and revetments to
increase channel and floodplain interaction;
2. Excavate back or side channels;
-1— Implement enhanced stormwater BMPs
for fine sediment removal in stormwater
runoff.
1. Modify current levees and revetments to
increase channel and floodplain area;
2. Install native riparian species to increase
bank roughness.
1 Increase riverine wetland area;
2. Install native riparian plant species.
3. Set back banks (revetments and levees).
1. Remove invasive plants and install native
riparian species;
2. Incorporate LWD into bank stabilization
and restoration projects;
3. Institute programmatic weed control
activities along shoreline.
4. Promote bioengineering techniques for
shoreline stabilization projects.
1. Install native riparian species;
2. Incorporate LWD into bank stabilization
and restoration projects.
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5.3 Plans, Programs. and Completed Projects
The importance of the Green/Duwamish ecosystem within the Puget Sound has resulted
in significant focus on this area in terms of restoration potential. With the federal listing
of Chinook and bull trout as endangered species, watershed planning in the region (e.g.,
WRIA 9) has focused on developing a Salmon Habitat Plan (WRIA 9, 2005), to which
the City of Tukwila is a party. The plan establishes goals, objectives, and programmatic
and site specific actions to address restoration of habitat critical to salmon species in the
Green/Duwamish watershed.
Tukwila has already engaged in the greater regional restoration effort for the
Green/Duwamish River. The City Council has ratified the WRIA 9 Plan and contributes
resources to maintain operating staff Tukwila has worked within the larger
Green/Duwamish River Ecosystem restoration project to acquire or donate properties that
are either currently functioning (Cecil B. Moses Park, Codiga Farm), or have the
potential for restoration (North Winds Weir. Duwamish Gardens). WRIA 9 and other
regional partners are currently working together to monitor baseline conditions. Several
projects from the WRIA 9 Plan are included on the City's Capital Improvement Program
list; other projects will be added as CIP projects are completed and funds are identified
for new projects.
The restoration plan identifies several projects that have already been completed in the
Green/Duwamish River. These projects provide an excellent opportunity to learn about
what river restoration measures are the most effective. For example, it appears that the
back channel that was excavated at Codiga Farm provides important habitat for migrating
juvenile fish.
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5.4 Restoration Onnortunities
Based on the key ecosystem functions that are currently altered, there appear to be two
five specific types of restoration actions that will most benefit the Green/Duwamish
ecosystem in Tukwila. These actions are intended to boost the levels of ecosystem
functioning as part of a self sustaining ecosystem that will limit the need for future
manipulation. While these projects are intended to restore many ecosystem functions, the
restoration activities will occur in the highly urban valley bottom, and as a result, cannot
fully achieve pre- disturbance channel conditions. In addition, some restoration actions
must occur at the watershed scale, which will restore ecosystem functions that cannot be
addressed solely within Tukwila or as part of the SMP.
Enlarging channel cross sectional area. This action could include setting back
levees and re- sloping banks to reduce steepnessrevctments, and the excavation of
historic fill or floodplain materials to create back channels. Th }ese actions will
increase flood storage, allow for more stable levees, restore some floodplain area,
provide a larger intertidal zone in this important transitional area, and provide a
more natural transition from aquatic to upland habitats. The Transition Zone is
identified in leMap 2.
Enhancinge existing habitats. These actions could include the removal of non-
native invasive vegetation, installation of native riparian vegetation, and
installation of LWD below Ordinary High Water. This action will improve the
functioning of the aquatic, riverine wetland, and riparian habitats that currently
exist along the Green/Duwamish River.
Creating off channel habitat areas. This action would create off channel areas
through the excavation of historic fill or floodplain materials to create back
channels as fish foraging and refugia areas.
Reconnecting wetland habitat to the river. This action would reconnect an old
oxbow wetland to the river. allowing for off channel habitat (Nelson Side
Channel).
Removing fish barriers where tributary streams discharge to the river. This
action would remove flap gates and install fish friendlv flap gates at the mouths
of Tukwila's three manor streams (Gilliam, Southgate and Riverton) and possibly
restore habitat area at these locations in the shoreline iurisdiction.
5.5 Potential Proiects and Priorities
The restoration plan summarizes 26 potential projects as specific restoration projects
within the shorelines of Tukwila. Most of the restoration projects are part of ongoing
restoration planning through the WRIA 9 watershed planning process. Additionally,
opportunities exist to enhance riparian vegetation along the majority of the
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Green/Duwamish River.
The restoration plan provides a preliminary qualitative (high, medium, low) project
ranking system. Within this ranking system. the highest priority location for restoration
nroiects is within the transition zone. The Transition Zone is manned in Map 2.
High priority projects will typically:
Address both hydrologic and habitat ecosystem functions;
Have opportunity for multiple funding sources;
Include freshwater tributary channels; and/or
Not require additional property acquisition.
Medium priority projects will typically:
Address limited ecosystem functions; and
Be eligible for multiple funding sources, and/or require property acquisition.
Low priority projects will typically:
Only focus on habitat enhancement;
Will be used as mitigation to offset impacts elsewhere; or
Not be eligible for multiple funding sources.
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