OMG Bute KnickPoint Changes 2014

KnickPoint Changes 2008-2010-2014
CSL Heron - Bute Inlet Operations
June 2014
Ocean Mapping Group, UNB

Overview:

Bute Inlet has been surveyed 3 times with multibeam. In early 2008, late 2010 and now early/mid 2014. The first pair of surveys cover a period over three freshets and the second pair also cover three freshets. Thus the apparent changes should be comparable if the process is pretty uniform.

Channel Floor "Knickpoints"

Along the channel axis, there are several locations of intensified scour that appear to represent the upstream head of an upstream migrating knickpoint. Based on the three surveys, it is apparent that these knickpoints are migrating progressively up channel. Over the 6 year period, they have typically migrated about 2-3km. Thus potentially they should shift about 300-500m in a single summer freshet window.

The three examples below show the best developed progressions.

Animation of 6 year progression of knickpoints.

300-350m talweg depth.	400-450m talweg depth	500-530m talweg depth
Stills of all 3 surveys. 10m contour interval

location map showing the position of the three illustrated knickpoint locations
and the white line shows the recommended talweg transit line.

Seeing if there is any knickpoint progression from 14th to 26th June:

For the period 8th to 13th June, the knickpoints (and the associated resolvable bedforms on the channel floor) did not appear to change in any resolvable way.
The Vector will, however, be visiting several weeks later. Given that several 100 metres of migration are expected over a season, there is a reasonable chance that these knickpoints may have migrated in the intervening period.

Thus it is requested that a dedicated line be run up the channel talweg axis.

Talweg Transit:

The second survey agenda is merely to run up the length of the channel using a narrow restricted swath to best identify the in-channel knickpoint features.

Learning from what the Heron achieved last week, the strongly recommended mode for best definition of the knickpoints, is to run the channel forcing the system into SHALLOW mode (dual swath) from the channel terminus in ~ 600m of water all the way up to the convergence point of the two prodeltas. And the swath should be restricted to a +/- 250m sector. The vessel should be run at a speed of about 7 knots or less and this way, maximum along track sounding density would be achieved.
Water column logging should be enabled (to a separate file) for all these operations.

Coordinates for talweg transit line: here this corresponds to the white line in the figure above.

Return transit ~ 250m offset to the west of the talweg:

On the way back out, the return transit should be deliberately offset to the side so that the talweg is now optimally viewed in backscatter. the coverage should be switch to MEDIUM mode and a +/- 65 degree sector with a swath width limit set wide to ~+/-600m. The bathymetric definition of the talweg is now poorer, but by avoiding the specular echo, the patchiness of the backscatter in the talweg can be best examined. If there has been any change associated with a recent flow, it may show up just in the backscatter.

Note for all Vector operations, hopefully the full POSPAC files will be logged to recalculate a best trajectory after the fact.

page generated by JEHC, , June20th 2014