Sea Technology

AUG 2017

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30 st / August 2017 www.sea-technology.com We used the bottom-track feature of the Teledyne RDI ADCP to measure ice velocity, which enables conversion of the ice draft time series determined by the IPS, to a dis- tance series providing high-resolution, two-dimensional ice profiles. The frames in northern Cook Inlet were specifically de- signed for the high sediment concentrations, strong currents and moving debris that can be encountered there. During deployment, a tilt-pinger ensures that the frame is deployed upright. Transponders are also mounted on each frame to provide back-up range and bearing information during re- covery operations. An acoustic-release-activated pop-up float is the primary method for recovery. The pop-up canister is enclosed in an aluminum box on the frame that provides protection. For re- covery, a command is sent to the acoustic release. Because of the chance of burial, we chose releases with a push-off mechanism. In all cases, upon recovery, the releases oper- ated as planned, even when we suspect they were partially buried in sediment. Each frame was also deployed with a high-strength, abrasion-resistant ground line for backup re- covery. During the three most recent recovery trips, drag- ging was required only once because the pop-up float was packed with mud and could not rise to the surface. Taut-Line Moorings ADCPs were also deployed on taut-line moorings to measure near-bottom currents, as well as in areas where the threat of burial by sand waves was too risky to deploy bottom frames. The mooring consisted of a downward- looking ADCP housed in a streamlined float about 4 m off bottom. The floats are de- signed for high currents, and although they are depressed by the strong current, they tend to remain horizontal in the flow, which allows for the collection of high-quality current profile data. The downward-looking ADCPs provided near-bottom current velocity profiles, an important requirement for the pipeline design. The bottom- track feature also provided measurements of bedload transport. Current Velocity Profile Transects At the relatively high lati- tudes of Cook Inlet, the hori- zontal component of the Earth's magnetic field is weaker than at more southern latitudes and, consequently, the ADCP flux gate compass is generally less accurate. It is more easily affected by nearby ferrous material, even though the compass calibrations were carried out on site. The battery packs, particularly alkaline, generally have the most ef- fect on the ADCP compass, even after factory degaussing. ings were deployed. The boat's echosounder revealed sand wave features of about 1-m height and 2-m wave- length. The sounder image also captured the taut-line mooring. Bottom-track data from the downward-looking ADCP showed bedload sedi- ment velocities up to 0.8 m/ sec, compared to about 0.4 m/sec at the other northern Cook Inlet sites. Bottom Frames The bottom frame moorings were custom-built to house the instrumentation in a stable, easy-to-ship, corrosion- resistant aluminum design. Each frame is large enough to accommodate all the equipment required at a single site: ice-profiling sonar (IPS), ADCP current profiler, and the con- ductivity-temperature, turbidity (Tu) and dissolved oxygen (DO) loggers. (Top) Single-beam echo- sounder image showing sand waves in northern Cook Inlet, as well as the taut-line moor- ing housing a downward- looking ADCP. (Bottom) Bot- tom frame mooring being deployed off Nikiski April 2016.

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