Tidal_PAD_V1_Sec4.pdf - Snohomish County PUD

Snohomish County PUD –Pre-Application Document
4 – Description of Existing Environment
the southwest. Admiralty Inlet is somewhat shallow relative to the rest of Puget Sound; a
shallow sill, approximately 60 m deep at its deepest, separates Puget Sound from the Strait of
Juan de Fuca. To the southeast of the sill, the channel deepens to over 100 m.
The geology around the site is dominated by Fraser-era glacial till, with smaller areas of Fraser-
glacial drift and outwash on Whidbey Island. Soils around the Admiralty Inlet site are
era
closely related to the geologic depositions, and consist mostly of loamy-skeletal, fine-mixed,
sandy-mixed, and mixed-mesic soils from the glacial till material.
The South Whidbey Island fault runs northwest to southeast within the potential project area, but
is understood to terminate south of the shallow-sill portion of the inlet. The land around the inlet
is a source of some mineral resources, primarily sand and gravel.
The channel substrate is estimated to be gravel near the channel center, a mixture of sand and
gravel toward the shores, and a mixture of more sand than gravel to the west between Point
Wilson and Marrowstone Point. Toward the southern portion of the Inlet, near Bush Point, the
channel bottom has small sand dunes, which migrate northward with the current (Polagye et al.
2007). A survey of the inlet’s seabed in the 1970s indicated exposed bedrock or bedrock
covered by thin layer of sediments, although USGS publications show more than a hundred
meters of sediment overlying bedrock (Polagye et al. 2007).
The potential for marine landslides at the site is not well studied, although the inlet has a very
steep portion just east of its deepest point. Terrestrial mass-wasting, however, has been better
documented. Figures 4-2 through 4-4, from the Washington Department of Ecology Coastal
Zone Atlas of slope stability maps (WSDE 2007), include the three areas of potential electricalgrid
interconnection for the Admiralty Inlet site. Figure 4-2 shows the Fort Casey area, the
northern of the two potential grid connection sites on Whidbey Island; Figure 4-3 shows the
Admiralty Head area on Whidbey Island; and Figure 4-4 shows the Port Townsend site. Each of
the above sites exhibits some historical or existing slope instability.
4-14 January 31, 2008