Heron Fraser June 2022

a quiet traffic day on the mouth of the Fraser...

Imaging the Salt Wedge

Sands Head to Steveston,
Fraser River Mouth

CSL Heron, June 26, 27, 28, 29^th, 2022

page version
July 19th 2022

distal river plume front - off Gulf Islands
JD179's edge seen on JD180

John E. Hughes Clarke
Center for Coastal and Ocean Mapping
University of New Hampshire

Graham Christie
Ocean Mapping Group
University of New Brunswick

Mathew Wells
Dept. Physical and Environmental Sciences
University of Toronto

Phillip Hill
Pacific Geoscience Centre
Natural Resource Canada

CSL Heron professionally skippered by Jada Cavanaugh.

Research Objectives

This field program was undertaken as part of a collaboration between UNH, UofT, UNB and GSC-P. The main objectives were:

The PhD thesis of Indra Prasetyawan (UNH) on imaging estuarine turbulence
The NSERC-supported research of Prof. Mathew Wells (UofT) looking at suspended sediment behaviour across shearing haloclines.
The NRCan interest in the transfer of sediment across the salt wedge from the freshwater to the seawater below.
The ongoing performance evaluation of the EM712/2040C systems on board the CSL Heron.

Operational Window:

This June program builds on, and extends, the May 2022 program (preliminary report here) and midwinter December 2021 first investigation (preliminary report here). The aim is to capture the dynamics of the salt wedge and its associated suspended sediment load during peak freshet conditions. It was also a test of the combined capability of the two EM's the MVP and the ADCP, to see how well this combination of equipment could meet the needs of the various interested parties. The long term engineering aim (marine geomatics at UNH/UNB) is to learn how to best utilize this equipment (and develop software in support of) addressing the questions investigated.

11 longitudinal sections were collected over the 4 day period. They extended from Steveston harbour to ~2 km seaward of the Sand Heads lighthouse.

Location of the June 2022 CSL Heron Transects

The 4 day window in June was chosen as it was close to both the peak freshet conditions as well as the spring tide window. During the summer spring tides in this area, the lower low water (LLW) is close to midday. On the first day, we transited from IOS and picked up a section on the rising tide in the evening. On the second and third days, we undertook profiles on the falling, LLW and rising tide segments. And on the last day, we got a transect out, close to one of the HWs before transiting back to IOS.

Times of Acquisiton of ADCP transects,

The earlier surveys in May (~ JD143) were while the river discharge was just beginning to rise (~ 4,000 m3/s). These surveys (~JD178) were undertaken during full freshet conditions with a discharge in excess of 10,000 m3/s:

Fraser River Discharge - Spring/Summer 2022
Showing the two time windows of CSL Heron Operations
May (~ 4000 m3/s) and June (~10,000 m3/s)

The MVP was run continuously, corresponding to profiles about every 60 seconds. In total, 605 profiles were collected with the daily distribtuin being:

June 26th - 38
June 27th - 246
June 28th - 227
June 29th - 94

Longitudinal Variations over the Tidal Cycle

Combining the 4 days we can view the evolution of the salt wedge over the phases of the spring M2 tide:

cycling through 7 stages of the evolution of the salt wedge through a spring tide cycle.

top - EM712 imagery with salinity superimposed
middle - EM2040 imagery with suspended sediments superimposed
bottom - ADCP along channel velocities

Individual Sections:

For each of the 11 transects obtained - all the permutations are presented below with a brief description:

Epoch and tidal phase	EM712 view (70-100 kHz) ----- EM2040C view (250-350 kHz) ---- ADCP view (600 kHz)
JUNE 26th
JD 177 A	late on flood tide - salt wedge at bend, dropping into scour depression
JUNE 27th.
JD178 A	late of ebb tide - salt wedge being pushed back toward Sand Heads surface outflow at 5-6 knots! highest suspended sediment levels ever seen occur just upstream of nose of wedge.
JD178 B	just after LW - salt wedge arrested just below Sand Heads highest suspended sediment levels where freshwater outflow is able to shear directly against the seabed.
JD 178 C	just after LW - salt wedge now starting to move upstream, out of scour hole by Sand Heads
JD 178 D	mis flood tide - salt wedge halfway up reach and advancing
JD 178 E	late on rising tide - salt wedge advancing up toward the bend.
JUNE 28th
JD 179 A	late on falling tide - salt wedge being dragged back down the reach. surface outflow at ~ 5 knots.
across mouth section A performed now.
JD 179 B	just after low water - salt wedge sitting below Sand Heads in the scour hole highest suspended sediment levels are in the all fresh water region - but notably less than the previous day (?).
JD 179 C	mid flood tide - salt wedge finally advancing again.
across mouth section B performed now.
JD 179 D	late on rising tide - salt wedge moving back up the channel toward the bend.
JUNE 29th
JD 180 A	earliest phase of tide sampled - ~ 1 hour after HW - maximum salt wedge penetration - well past Steveston

Plume behaviour once decelerates off the lip

The main longitudinal sections shown above are mainly along the reach where there is lateral restriction due to the training dike. Once the plume expands, SW of Sand Heads, and passes off the edge of the shelf, the plume slows down and spreads out. Here it is now heavily impacted by the across river front tidal flows. Here is also where we might expect to see evidence of sediment moving from the river plume into the salt water below.

Each longitudinal section ran a slightly different radial path out off the delta lip past Sand Heads:

showing all the different sections run off the lip of the delta.
map is 3500m wide.

For each of these off-lip transects, a close up view of the surface plume character is presented below to illustrate how it varies as a function of phase of the tide:

data presented: Tidal Phase Map View ADCP on/off delta velocities EM2040 WCD with Salinity with OBS EM712 WCD	JD 177 A	JD 178 A	JD 178 B
JD 178 C	JD 178 D	JD 178 E	JD 179 A
JD 179 B	JD 179 C	JD 179 D	JD 180 A

Plume behaviour Across Delta Front

While easier to run, it became apparent that the longitudinal sections do not adequately address the lateral variability across the mouth of the channel. Given that there is such a pronounced interaction with the along-delta-front tidal streams, there needs to be an investigation to see what the the lateral variations show. Thus on the third day we added two cross-sections:

showing the morphology of the delta front just seawards of the Sand Head Light beacon.
The white dotted lines show the two sections run - black lines are 10m contours.
CHS Chart 349001 backdrop - depths in metres.

Two sections were undertaken - just-before and long-after the low water:

Across Mouth - JD 179 Section A just before LW	Across Mouth JD 179 Section B half way up the rising tide
EM2040 scattering with MVP observations superimposed clearly - just on the southern side, the surface plume suspended sediment load is highest. And it is the one region where we see extensive evidence of suspended sediment leaking into the lower layer. maybe it is just being swept off the adjacent tidal flats?	EM2040 scattering with MVP observations superimposed in both examples, you see the warm subsurface layer creeping in from the north around the Sand Head light.
EM712 scattering with MVP observations superimposed	EM712 scattering with MVP observations superimposed
ADCP observations	ADCP observations

Clearly lots to think about - a good basis to use to design a future program.

page developed by JEHC, July 5-20th 2022