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.
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