JdeF Canyon ADCP Transect

ADCP Test Transect
Juan de Fuca Canyon

v. 1.0 suggestions. John HC

As a suggestion, a nice test area to assess the relative performance of the new ES-150 against the OS-38 is the Juan de Fuca Canyon.

A canyon really only in name - it is a deep across-shelf trough that joins the open ocean to the mouth of the Strait of Juan de Fuca. It acts as a passageway to feed colder nutrient rich waters into the Strait and ultimately the Salish Sea region (Puget Sound and the Georgia Basin). Because there is a lot of vertical water column shear and longitudinal gradients in the currents, it should be an interesting place to compare the relative performance of these comparable ADCP systems.

showing the mouth of the Strait of Juan de Fuca and the link between it and the shelf break, formed by the Juan de Fuca "canyon".
The black line indicates the proposed transect. The white box shows the bounds of the more detailed figure below.
Superimposed is the NOAA chart showing the shipping lanes

The transect is 85.2 km (46 nm) and would take about 5.8 hours to cover running at 8 knots. Note that it starts in the outgoing shipping lane, but thereafter is outside the main shipping traffic. It is (presumably) an only slightly circuitous path on the way to the shelf break that the vessel will going anyway in order to reach deep water.

Proposed Coordinates are here.

A close up view is presented here (using GMRT bathymetry):

rotated map sheet showing the transect in detail (100m contour interval)

vertical depth profile along the proposed transect (note the sill at the SW end).

The idea is to run this transect just as part of a transit (in and/or out) during the pre-testing period. And perhaps on the way out for the first testing leg.

Originally my idea was to then focus on the sill at the mouth, and run back and forth along a 8 nm (1 hour) section for the duration of one M2 tidal period (so would cost 13 hours). It would have been nice to have the bottom mounted ADCP just in the landward side of the sill - but the logistics of dropping it there and needing to pick it up later, are not practical.

This is the site of a remarkable study :

Flow and Mixing in Juan de Fuca Canyon, Washington, Alford and MacCready, 2013 ,GRL
https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2013GL058967

In it, they show that there is a two level system. Above the depth of the surrounding shallower shelf (~< 140m) the watermasses are moving back and forth with tidal frequencies. Below that depth (> 150m) within the constraints of the canyon, there is net upstream (landward) flow of deeper (nutrient rich of course) water. Between the two watermasses, there is significant shear. The most interesting aspect of the study is the behaviour of the flow as one goes over the constraining sill at the shelf break. At this location they report a strong lee wave response with overturns (large scale turbulence).

The canyon axis is all > 200m, with local holes down to 380m and is thus an ideal area to test the performance of these lower frequency ADCPs. The WH300 will only look at the top 100m (at best), but the OS38 will see all of it (and much deeper if available). The ES-150 is a not yet a very well known system. But at 150 kHz it should be able to image the whole water column.

Suggested Data Collection:

OS38 - 10m(?) bins broad band - max range 600m
WH300 - 2m bins broad band - max range 100m
ES-150 - however KM suggest it be set up (perhaps alternating pings ,oblique and splitbeam nadir?) - max range 600m.
EM712 - logging water column +/-65 deg, forced to Medium Mode (all CW).
EM124 - logging water column +/- 65 deg forced to shallow mode (default).
EM2040 - sonar mode - forced to FM - 100m range. (un-synched)
ES-200 - logging 1ms CW
ES-38 - logging 1ms CW
MVP-300 dipping every 15 minutes to max depth (20m off bottom)?.

Quite how the synchronization should proceed, I'm not sure. All could be unsychronized?. Testing could be done in the transit down the Strait as all about the same depth. The EK-80's do NOT have to be on as they are not being tested. I suspect they would be synch'ed with the ES-150 anyway.

Other MBES WCD data samples

To get an idea about what we might find here, I've browsed the NCEI water column archives and found 4 MBES systems that transit through the area:

You'll see there is a noticeable scattering horizon separating the upper shelf waters from the isolated canyon waters.

Okeanos Explorer 2015 - EM302	Falkor 2013 - EM710

Tommy Thompson 2011 outbound - EM302	Tommy Thompson 2011 inbound - EM302

page made by John HC, June 2021