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HomeMy WebLinkAboutCOW 2009-11-23 Item 4I.1 - Shoreline Master Program - Attachment A.04: Shoreline Inventory and Characterization Summary4. SHORELINE INVENTORY AND CHARACTERIZATION SUMMARY Local jurisdictions updating their Shoreline Master Program (SMP) are required to prepare an inventory and characterization of the shoreline resources within their boundaries. As part of the City's SMP update, a Draft Inventory and Characterization Report and Map Folio was prepared in December 2006, and finalized in the spring of 2007 following technical review by Ecology and King County. The final report and map folio are included as Appendix A to this SMP. The purpose of the inventory and characterization report was to conduct a baseline inventory of conditions for water bodies regulated as "shorelines of the state" located in the City of Tukwila. The area regulated under Tukwila's SMP is approximately 12.5 linear miles along the banks of the Green/Duwamish River. For the baseline inventory, the river shoreline was divided into four reaches: 1) Reach G 1 -PAA (southern Potential Annexation Area); 2) Reach G1 (from the southern City boundary downstream to the Black River /Green River confluence); 3) Reach G2 (from the Black River /Green River confluence downstream to the northern City limits); and 4) Reach G2 -PAA (the northern Potential Annexation Area). The reaches are depicted on Map 3. The shoreline characterization identifies existing conditions, identifies current uses and public access, evaluates functions and values of resources in the shoreline jurisdiction, and explores opportunities for conservation and restoration of ecological functions. The findings are intended to provide a framework for updates to the City's shoreline management goals, policies, and development regulations. Key findings of the inventory and characterization are summarized below. 4.1 Watershed Context and Shoreline Modifications The City of Tukwila includes approximately 12.5 miles of the Green/Duwamish River and is situated in the Puget Sound Lowlands at the transition from the fresh water Green River to the tidally influenced Duwamish estuary ecosystem. The Green River basin is part of the Green/Duwamish Water Resource Inventory Area (WRIA 9). Historically, the Green/Duwamish River drained a significantly larger area than it does today. The Green/Duwamish River has undergone extensive modifications in the past to reduce channel migration and limit the extent and duration of valley flooding. The modifications include both natural river course changes and major engineering projects in the early part of the 20 century that diverted the White, Black and Cedar Rivers to neighboring basins. As a result, the overall freshwater discharge in the Green/Duwamish CL 19 11/19/2009 12:5500 PM W \Shoreline \Council Review \\Document \Council SMP 159 160 River has been reduced to around a third of the pre- diversion era. Seven pump stations also modify flows into the Green and Duwamish Rivers. Three of the pump stations, Black River, P -17, and Segale, are operated by the Green River Flood Control District, and four stations, Lift Stations 15, 17, 18, and 19 are operated by the City of Tukwila. The Black River pump station is the largest station discharging flows to the Duwamish River. This station is approximately 1,000 feet upstream of the Green Black River confluence, and is intended to both block floodwaters from the Green from inundating the Black River and Springbrook Creek in the City of Renton, and also regulates flows from Springbrook Creek into the Duwamish River. The P -17 pump station drains the P -17 Pond that collects surface water from a majority of the Urban Center. The Segale pump station was installed to regulate soil saturation and piping during high river events but does not add new flows to the river. The remaining City pump stations only operate when gravity discharge to the river is prevented by high river events. Levees and/or revetments were constructed along much of the Green/Duwamish River through the City of Tukwila to increase bank strength and reduce flooding. In addition, flows within the Green/Duwamish River were greatly modified by the construction of the Howard A. Hansen Dam and installation of water diversions. These modifications significantly reduced the severity of floods that historically covered much of the valley bottom. The condition of the current system of levees and revetments is a growing source of concern for King County and the cities involved, as many of the levees are aging and would not meet current standards for either flood conveyance or stability. 4.2 Biological Resources and Shoreline Functions The Green/Duwamish River within the City of Tukwila provides important habitat for several fish and some wildlife species, such as osprey. The aquatic environment within the channel is an important corridor located at the transition from the freshwater riverine environment to tidal estuarine environment of Elliott Bay. Almost every species of anadromous fish migrates through this transition zone. The entire length of the Green/Duwamish River within the City of Tukwila has been declared "critical habitat" for Chinook salmon and bull trout. Both species are listed as threatened under the Federal Endangered Species Act. One particularly important feature of Tukwila's shorelines is the habitat functions provided by the transition zone between fresh and salt water associated with the Duwamish estuary. In Tukwila, this area generally extends from the East Marginal Way bridge to the city's northern limits The transition zone between fresh and salt water has effectively been pushed upstream from its historic location due to: (1) a significant reduction (70 of fresh water flowing into the Duwamish estuary (owing to the diversion of the White and Cedar/Black Rivers), (2) channel dredging, and (3) reduction of flows as a result of the construction of the Howard A. Hanson Dam. The CL 20 11/19/2009 12:55:00 PM W \Shoreline \Council Review\ \Document \Council SMP establishment of heavy industrial uses in the transition zone has replaced wetlands with impervious surfaces, and the stream banks have been replaced by levees and other armoring, eliminating edge habitat which slows flows and creating unrestrained rapid downstream flows. Spatial structure, residence time, and the habitat available for fish refugia and rearing functions in the Duwamish estuary have therefore been reduced and constrained. High densities of fish have been observed utilizing what is left of this specific habitat. At the watershed scale, overall increases in salmonid survival rates are dependent on the availability of sufficient transition zone habitat to accommodate fish while they adjust from fresh to salt water (WRIA 9 Steering Committee, 2005). Modifications to the river system have resulted over time in reduced levels of ecosystem functioning, including hydrology, water quality, riparian habitat, and in- stream habitat. Changes to hydrology are the result of modified flow regime due to dam construction, diversion, and urban development. River management and levees have reduced the connection between the rivers and their floodplains, changing the spatial extent of habitats, and increasing the potential for negative water quality impacts. Disturbances to the channel banks have resulted in areas that are dominated by non native invasive species. Wood, in the form of riparian trees and in- channel wood, is generally lacking throughout the system, which negatively impacts riparian and aquatic habitats. 4.3 Land Use A. A History of the Green/Duwamish River and Tukwila's Shoreline: Origins of Land Development Patterns The Green River drains 492 square miles extending from the western Cascade Mountains to Elliott Bay. The City of Tukwila lies at the lower 1 /4 of the overall watershed. As the Green River flows into the southern boundary of the City of Tukwila, it has drained approximately 440 square miles, or about 78 percent of its total drainage basin. Approximately 12.5 river miles of the Green/Duwamish River are included within the City of Tukwila, from about River Mile (RM) 16 to RM 3.7. The Green/Duwamish River channel has been highly modified during the last 150 years. Modifications range from the installation of levees and revetments to straightening and dredging for navigation purposes. In general, the level of physical modification to the system increases with distance downstream, culminating at the artificial Harbor Island that supports industrial activities at the Port of Seattle. Several turning basins are maintained by periodic dredging throughout the straightened reach. The highly modified portion of the Green/Duwamish has also been the location of significant discharge of pollutants, resulting in portions of the river being designated as Federal Superfund sites. Remediation, source control and disposal activities are ongoing throughout the area. Prior to European settlement of the Lower Green River Valley, the floodplain likely consisted of a highly interspersed pattern of active and temporarily abandoned meandering CL 21 11/19/2009 12:55:00 PM W \Shoreline \Council Review \\Document \Council SMP 161 162 channels, secondary channels, logjams, riparian forest, and scrub -shrub wetlands. The proportion of open channel to forest in the floodplain appears to have varied depending on the severity and timing of floods. High flows resulted in wider channels and the creation of new channels across the floodplain. Accounts of the channel systems indicate that major floods resulted in channel avulsion (abrupt change in the course of a river), rerouting around logjams, and the formation of new logjams. The area presently occupied by the City of Tukwila appeared historically to contain oxbow channels, secondary and backwater channels, and extensive floodplain wetlands. As part of regional flood control and river management efforts, significant watershed -scale changes occurred to the major river drainages south of Elliott Bay, including changes to the alignments and discharge points of the Cedar, Black, Green and White Rivers. In general, these changes have reduced the amount of water flowing through the Green/Duwamish River to about one third of historic conditions. Land use changes between European settlement and the current day have occurred in two general phases. From the mid 1800s to World War II, agriculture and timber harvesting dominated the Lower Green River Valley. Population densities in the Lower Green River Valley remained low until the Howard A. Hanson Dam project was completed in 1962, providing flood protection for the valley. Levees have also been constructed along the banks of the Green/Duwamish River, ranging from federally- certified levees to non engineered agricultural berms. Since the dam and levee systems have significantly decreased the extent of flooding within the Lower Green River Valley, land development and urbanization have occurred. For more discussion on the character of the GreenlDuwamish River and an inventory of river conditions, see the Shoreline Inventory and Characterization Report, prepared by ESA/Adolfson, May, 2007 found in Appendix A. Historically, the GreenlDuwamish River valley was known for its farmland. Farming was established in the early 1900's after forested areas were cleared and transportation to the area was improved. In 1906, construction of the Lake Washington ship canal eliminated flows of the Black River into the valley, reducing valley flooding. As a result, the river valley developed into highly productive farmland for the region. In the early 1950's, the Port of Seattle proposed to convert much of the Green/Duwamish River valley to intensive industrial uses. These plans included converting the river into a shipping canal, possibly reaching as far south as the City of Auburn. Valley landowners countered this proposal by annexing large tracts of land into Tukwila to retain more control over future land use decisions. With the construction of Howard Hanson Dam in 1962 on the upper Green River, flooding in the valley was further reduced. Much of the river is now contained within levees and surrounded by commercial and industrial development. The Port's actions in the northern part of the River and drastic reduction in river flooding have had a major influence on the development of the river valley. Today, Tukwila's portion of the Green/Duwamish River is known as a center for retail, commercial and industrial uses. The river remains inaccessible to shipping activity south of the Turning CL 22 11/19/2009 12:55:00 PM W \Shoreline \Council Review \\Document \Council SMP Basin, where it can be accessed primarily by small water craft, kayaks and canoes only. Land uses along the river are mostly commercial and industrial activities, with a few residential areas. With the designation of the Southcenter area as an Urban Center and the Duwamish Corridor as a Manufacturing Industrial Center (MIC), this development pattern is expected to continue, and to intensify as redevelopment occurs. B. Riverbank Vegetation The natural environment along the river has been significantly altered from its original riparian corridor by intense urban development and river bank modification due to the construction of levees, revetments or other shoreline armoring. Most native stands of trees are gone, but have been replaced by new trees and plants in some areas. Landscaping with native and non- native plantings have also been completed in conjunction with new development along the corridor. Birds and small mammals are supported in both habitats. While more natural habitat is found up stream, re- development of the shoreline has the potential to provide appropriate landscaping and restoration of habitat that are more attractive to wildlife, people and a more environmentally sensitive form of development C. Public Access The regional Green River Trail provides public access to existing shoreline amenities and plans anticipate future linkages to Seattle's system. As redevelopment occurs, there will be opportunities to provide other types of public access, including viewing platforms, boat ramps and fishing areas_ 4.4 Restoration Opportunities and Potential Use Conflicts Past restoration work focused on the Green \Duwamish River (in Water Resource Inventory Area 9) has resulted in good data collection and identification of potential restoration opportunities. Significant restoration activities along the Green \Duwamish River are already underway in the form of the multi- agency Green River Ecosystem Restoration Project. Several opportunities have been identified on the river as part of the recently adopted King County Flood Hazard Management Plan. Restoration opportunities focus on several key elements: Removing non native, invasive plant species and re- vegetating with native riparian forest species; Removing artificial debris and walls that harden channel banks; Integrating the reconnection of floodplains, levee setbacks, and other ecosystem restoration techniques with future flood and river management efforts; and Property acquisition to allow for levee setbacks, side channel reconnection, and channel migration. CL 23 11/19/2009 12:55:00 PM W \Shoreline\Council Review \\Document \Council SMP 163 164 Two key issues illustrate constraints to implementing restoration and potential use conflicts in Tukwila: 1) levee maintenance and management; and 2) existing development patterns and anticipated redevelopment. Discussion of shoreline planning for the Green River in Tukwila must acknowledge the fact that, in light of the existing system of levees (including the federally certified "205" levees) and revetments, the City cannot act alone. There are a variety of regulatory jurisdictions outside of the City with different responsibilities for maintenance and management of the levee system, including the U.S. Army Corps of Engineers (the Corps), the Federal Emergency Management Agency (FEMA), King County River and Floodplain Management Unit (acting as part of the Green River Flood Control Zone District), and private property owners. The City of Tukwila Public Works Department has overall responsibility for maintenance of all levees, including the federally certified levee, which extends from about the I -405 crossing to the south City limits. The actual maintenance work on this levee is contracted by the City to King County. The restoration of native tree and shrub species along the levees would increase riparian habitat ecological functioning of this reach of the Green/Duwamish River, benefiting salmonids as well as other species. However, the Corps of Engineers (responsible for certifying the federal levee) believes that the root system of these trees could destabilize levees, resulting in water piping (e.g., water infiltrating into and through levees along root pathways at higher rates than it could through root free soil) at high flows, and potential levee failure if trees fall. For the Vegetation Free Zone of the levee, current Corps guidance only allows grass as vegetative cover on the levees (USACOE, Engineering Manual 1110 -2 -301). Current guidance also specifies a root -free zone where plantings can occur, but roots will generally not penetrate this structural zone. Therefore, under current regulations, to meet the requirements for federal levee certification, some vegetation was recently removed and ongoing vegetation management will be required to maintain the levee certification. Under the SMA, removing trees and vegetation from the riparian zone of shoreline of the state is in conflict with policies for vegetation conservation and enhancement. A possible solution is to step back and re -slope the levees to create mid -slope benches where vegetation can be planted that will not interfere with the levee prism as the levee system is reconstructed to improve its stability. This would require additional easement area beyond the existing maintenance easements that have been acquired along the length of the system. The existing development pattern also represents constraints to implementing restoration projects, including levee setbacks, off channel habitat restoration, wetland and stream restoration, and riparian zone enhancements. Most of Tukwila is fully developed, with portions having a dense, urbanized land use pattern. The City's SMP, in place since 1974, establishes a 40 -foot setback from the mean high water line. In many places, there is little more than this 40 -foot zone that is not intensely developed. Some places have somewhat more open space and less development and thus have greater flexibility to accommodate potential habitat restoration actions. The City's vision for future land use, CL 24 11/19/2009 12:55:00 PM W \Shoreline \Council Review\ \Document \Council SMP based on its comprehensive plan, includes maintenance and further development of its urban character, particularly its identity as a regionally significant center for manufacturing, industrial, and commercial development. A challenge lies ahead in determining how best to accommodate new and redevelopment near the shoreline in a manner consistent with both the Comprehensive Plan and the Shoreline Master Program in order to achieve "no net loss" of shoreline function. 4.5 Conclusions Like many rivers in the Puget Sound region, the course and dynamics of the Green/Duwamish River has changed significantly as a result of development and alteration of its watershed over the past century or so. Characteristic of many cities in the region, Tukwila has grown and become highly urbanized. Continued growth is anticipated and the City is planning for that growth. To a significant degree, the City has envisioned and maintained a development pattern that preserved public access to the Green River and assured setbacks of new buildings from the shoreline. Issues of concern today are focused on uncertainties about the ability of existing levees and revetments to protect existing development from flood hazards. There are many opportunities for conservation and restoration actions in the City to restore or replace habitat while managing natural hazard areas. CL 25 11/19/2009 12:55:00 PM W \Shoreline \Council Review \\Document \Council SMP 165