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Inventory and Assessment of Current and Historic Beach Feeding Sources/Erosion and Accretion Areas for the Marine Shorelines of Water Resource Inventory Areas 8 and 9

About this report

This report was initiated by King County Department of Natural Resources and Parks in order to provide much needed data and analysis of the marine shorelines within Water Resource Inventory Areas (WRIA) 8 and WRIA 9 . While drift cells were delineated many years ago, more spatially explicit characterization of the physical processes occurring within each drift cell has not been attempted within King County or southern Snohomish County previously. This study entailed field mapping to document the current geomorphic conditions within the study area, followed by research into the historic condition of all currently modified shores within this largely urban marine environment. Detailed mapping of Feeder Bluff and Accretion Shoreforms was carried out for both current and historic conditions at 1:24,000 scale throughout the approximately 120 lineal miles of the King County and southern Snohomish County study area.

This effort was originated through salmon conservation planning work, but is applicable to many different planning efforts. The primary objectives of this report were:

  • To understand the current conditions related to sediment input and transport processes within each drift cell and where modifications to those processes have occurred.
  • To be able to quantitatively prioritize conservation actions to protect currently functioning sediment sources.
  • To gain an understanding of the historic conditions of each drift cell, specifically in relation to sediment sources which have been disconnected and which sediment sources were likely providing the largest benefit to the shoreline so that restoration actions can be prioritized.
  • Provide coastal geomorphic context for salmonid restoration actions being undertaken within WRIAs 8 and 9.
  • To provide basic data for a characterization of ecosystem processes, which is required for Shoreline Master Plan updates under the Shoreline Management Act.

Comparison of current conditions to historic conditions mapping revealed that widespread and far reaching changes have occurred to coastal processes and the nearshore area throughout much of the study area. Historic analysis (combined with current conditions mapping) revealed that the most common shoretype mapped in pre-development conditions was Historic Feeder Bluff, which occurred along 35.3% of the 120-mile study area shore. Historic Feeder Bluff Exceptional was mapped along 15% of the shore, bringing the total Historic sediment sources to 50.3% of the shore, as compared to 18.4% in current conditions mapping. An additional 8.6% of shore was Potential Historic Feeder Bluff, but was not counted as Feeder Bluff/sediment source areas due to the ambiguity of data for these units. When comparing current to historic sediment sources there was a 63.4% loss for the entire study area, leaving only 36.6% of the historic sediment sources currently intact (not including Potential Feeder Bluffs).

Historic Accretion Shoreform mapping was performed independently from the sediment source mapping. The entire shore of the study area was examined for mapping of Historic Accretion Shoreforms using the best historic sources. Historic Accretion Shoreforms were mapped along 33.2% of the shore-equivalent length. Due to the different methods, some overlap of the Historic Accretion shoreforms and other units (mapped in current conditions work) occurred. Almost 40 miles of the shore was mapped as Historic Accretion Shoreform, which represented pre-development conditions; far more than the approximately 22 miles mapped during current conditions fieldwork.

Following the completion of current and historic conditions mapping a study area wide prioritization of all potential restoration and conservation sites was performed at landscape and drift cell scales. This prioritization was solely based on the geomorphology and geology of the study area. This report did not evaluate biological or habitat values, such that the data produced in this report may be of higher value when incorporated with biological data. In each case, historic (modified) and current Feeder Bluff and Feeder Bluff Exceptional unit Historic Sediment Source Index (HSSI) scores were used to determine the relative value of each segment as a source of beach sediment.

The first method of prioritization compared all individual modified (restoration) or intact (conservation) Feeder Bluff units against each other throughout the study area. The second prioritization approach compared HSSI unit scores within individual drift cells and listed the top three highest scoring units. The third prioritization approach summarized and scored data for entire drift cells and compared the scores across the study area.

The report is available for download below. No hard copies of the report were produced for distribution. The original files were broken up into pieces in order to ease downloading. The map files available for download have had their quality reduced in order to speed up download time, though the quality is still fairly high. Original map files are available on request to the project manager. The GIS data from this project is not included on the standard set of CDs available from the KC GIS Center; however it is available on request to the project manager.

View the report

    Summary, Introduction, Methods, and Results (4.84 MB)

    Results (con't) and References (773.89 KB)

    Appendix 1 (2.00 MB)

    Appendix 1 (con't) (1.80 MB)

    Appendix 1 (con't 2) (1.40 MB)

    Appendix 1 (con't 3) (1.70 MB)

    Appendix 1 (con't 4) (877.71 KB)

    Appendix 2 - List of Maps (50.00 KB)

    Appendix 2 (con't) - Maps 1-6 (3.40 MB)

    Appendix 2 (con't) - Maps 7-12 (3.30 MB)

    Appendix 2 (con't) - Maps 13-19 (2.81 MB)

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