SQ Summer 2017 - WaterColumn

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CSL Heron Squamish Operations
April-October 2017

Water Column Anomalies due to JD159C Flow

John E.Hughes Clarke, Anand Hiroji, Liam Cahill	Ian Church, Angela Gillis
Center for Coastal and Ocean Mapping University of New Hampshire, USA	Ocean Mapping Group University of New Brunswick, Canada

The JD159C event

On the first day of operations in June, the South Channel was investigated just a few minutes before the passage of a ~ 1.5 m/s flow. The second survey line fortuitously happened to take place as the flow was passing down the channel. As a result there is a remarkable view of the morphology of the flow itself.

The combined figure to the left illustrates the bathymetry and backscatter of the South Channel floor before, during and after the flow. The top panel clearly shows the expected upslope bedform migration. The bottom panel shows the change in the seabed backscatter signature, immediately before and after the flow. Note that the backscatter pattern does not conform to the bedform spacing, rather longer wavelength along channel patches are seen that clearly change with the passing of the flow.

The middle panel shows the identical backscatter imaging geometry during and after the flow. From that one can see the presence of the flow head and body itself.

The two red dots in the figure are the exact location of the ADCP, as identified in the water column image while the Heron was passing over the flow. As can be seen, the ADCP was within the main channel located just upstream of the crest of a cyclic step. After the flow passed, the cyclic step migrated enough upstream so that the ADCP was now looking into the trough. This can be seen from the ADCP data for that event:

The following set of figures below illustrate the change and activity in the multibeam water column scattering imagery - before/after the singular flow on JD 159 ( June 8th) 2017 that the Heron tracked. Note that this flow was actually the third of three on that day.

Bathymetry Changes
bathy before
backscatter while flowing	this is the image from the northern line that actually captured the flow while it is in the channel.
Along Track Volume Scattering Sections: The following figures represent vertical sections showing the acoustic volume scattering directly below the vessel as it steams along one of the three survey lines. The data are exactly co-registered along the X axis with the plots above. Note that there are three tracks: 125m to the north of the channel axis or 125 to the south of the channel (that way looking at the over bank flow), and directly along the axis (to look at the main channel flow).
line 20: to south seeing southern spillover
line 21: to north seeing northern spillover
line 22: along axis (steaming uphill) seeing head and gas release
line 23: along axis (steaming downhill) head died continuing gas release
line 24: along axis (steaming uphill) later gas release

Across Swath Water Column Imaging of flow dynamic and gas release

The figures above represent vertical sections along track. To better appreciate how the flow manifests across the channel, one can alternately examine registered "pie" displays to relate the enhanced scattering to the channel geometry. Two brief examples are shown below.


Line 21 - looking at the active flow showing enhanced scattering across the floor of the channel - either sheet like gas release or actually turbulence on the top of the flow. Also showing disturbance of the plankton layer up on the channel flanks indicating that a subset of the flow is leaking out over the sides.	Line 22 - looking at the wake and consequences of the flow Showing gas column release preferentially from the eroded flanks of the channel. Also showing bulbous displacements of the plankton layer suggesting that the flow is rising in the wake (as it drops sediment, it is slight brackish and thus buoyant). The line up near the surface is a multiple.

Positioning the ADCP relative to the bedforms

The ADCP is suspended from a surface buoy, which, while it is constrained by two anchors, is always moving depending on the tide and wind. In order to know precisely where it is relative to the bedforms, how can we locate the ADCP? For the case of the JD159C flow, we passed over it twice, during and just after the head passage. As it is a strong scattering target, and we know roughly where it is, we can pick it out and geo-reference it.


showing the whole lop-sided swath with the ADCP echo visible on the port side in the inner sector in the region after the minimum slant range	Zoom of the image to the left, showing the ADCP echo.

Showing ADCP location relative to cyclic step wave crest before and after the JD159C flow.	Showing the depth differences resulting from the upstream migration of the bedform (greyscale - black-white = -2m to +2m)

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