GEOLOGIC CONTRIBUTIONS TO DESIGN, TIDAL TURBINE PILOT PROJECT, SNOHOMISH COUNTY PUD, ADMIRALTY INLET, WASHINGTON

Snohomish County PUD No. 1 obtained a FERC permit in 2014 for pilot scale deployment of tidal current-powered turbine generators in Admiralty Inlet, west of Whidbey Island, WA. Strong tidal currents in the relatively narrow and shallow Admiralty Inlet, make the location attractive for possible tidal current energy generation and posed design challenges. Interpretation of geologic conditions supported design for the stability of the turbine structures and the connecting power cables. The proposed location of turbines west of Whidbey Island, and the 60-70 meter water depth, relatively high energy tidal currents, and coarse seafloor surface sediment conditions prevented use of routine sub-seafloor sampling equipment to collect geologic data. A gravel to cobble size sediment (pavement) on the seafloor is visible on ROV video. The pavement layer is likely a lag deposit formed by erosion of finer grained fractions of granular material by strong tidal currents. A preliminary geophysical survey did not provide suitable sub-seafloor information. Accordingly, low frequency seismic reflection geophysical data, bathymetric data, submarine video of seafloor conditions, mapped geologic conditions on adjacent Whidbey Island, and logs from adjacent upland borings were used to interpret geologic conditions in support of design. Interpretation of seismic reflection results showed that low reflectance/low density material consistent with glacial marine drift was present in a portion of the turbine site beneath the surface gravel-cobble pavement. These conditions at relatively shallow depths indicated a potential for settlement under loading. Below the interpreted glaciomarine unit, results show a geophysical unit consistent with sand, gravel, cobbles, and boulders approximately 30 to 40 m thick. Geophysical results identified units with similar conditions to geologic units present in Northern Puget Sound, and provided properties to assess geotechnical conditions to support design. Active tectonic considerations include the proximity to the South Whidbey Island Fault. After assessing design cost estimates, the PUD discontinued the project.