OMG/UNB Salt Water Intrusion Dynamics

animation of 600 kHz volume backscatter imagery showing intrusion into the Kennebecasis

Salt Water Intrusion Dynamics
Grand Bay Sill
June 13^th and 19^th, 2003

John E. Hughes Clarke and Anya Duxfield
Ocean Mapping Group
Dept. Geodesy and Geomatics Engineering
University of New Brunswick
Canada

Marion Sutton
ISITV
Ocean Engineering
University of Toulon
France

As part of the ongoing investigations into the oceanography of the lower Saint John River Estuary, a series of tidal cycle experiments were undertaken in June 2004. The aim was to use an approach involving simultaneous multi-sensor oceangraphic sampling systems to try and watch the evoltion of salt-water intrusion dynamics over a sill.
Focusing in on the Grand Bay area of the lower Saint John River system, we see that there is a pronounced bathymetric restriction (fig. right) that prevents the deeper (> 10m) saline (~23-24ppt) waters in the gorge (that originate from the ocean after mixing over the Reversing Falls) from exchanging freely with the deeper waters of either the Westfield Channel and Long Reach (i.e. going up the main river system) or into the Kennebecasis (to refresh the trapped deep saline body that exists throughout that basin). Following on from preliminary ADCP and Acoustic volume scattering experiments in 2000 and ADCP and CTD experiments in 2001, it was recognised that there was a need to better understand the exchange of salt water across this sill.

As a result of multibeam operations over the the past 5 years (2000-2004) the major bathymetric extent of this region has been mapped (fig. left) with a spatial resolution of ~ 1m using 300 kHz multibeam sonars (a Simrad EM3000S). The bathymetric imagery, clearly indicate widepread erosional and depositional patterns on the sill top and adjacent channels, most probably related to the modern exchange of saline waters between the deeper basins.

Looking now in detail at the sill top (fig. right) we see that there are multiple potential pathways for water to flow between the Grand Bay channel and the deeper basin of the Kennebecasis. This area has been the focus of a variety of experiments in 2000 and 2001, but now with the availability of newer instrumentation (primarily the Brooke Ocean MVP-30) we have an opportunity to better examine the dynamics of that exchange.

During the spring tidal period in June 2003, when the river level had dropped to the usual summer levels, we know that there is active exchange of the deeper saline waters across the sill. On two occasionals (48 hour apart) the CSL Heron was deployed for a full 12.42 hour period to run a single transect back and forth (20 complete sections over the tidal cycle). there are two major challenges:

the first is to present the time sequence of the data effectively and
the second is to interpret it properly.

This web page illustrates the current approach to daa processing, management and display. Some interpretation is offered, but that is an underway research project at this time.

GORGE to WESTFIELD CHANNEL EXCHANGE - Up-River Intrusion

The adjacent animation illustrates a variety of simultaneous views of the exchange of salt and fresh water between the gorge, upstream of the Reversing Falls and the Westfield Channel, on the way up river to Long Reach. 20 images are presented representing 20 cross-sections of the ~7000m section.

For each section (indicated by phase of tide at Indiantown, 1500m away just above the Reversing Falls), all the salinity and temperature profiles are presented in a conventional plot together with a TS plot showing the mixing line between the two masses. Four images are presented:

combined TS plot (as T ands S are so strongly correlated, the colour is mapped into TS space to best highlight the location of the principle thermo/halo/pycno-cline).
ADCP derived horizontal component of the velocity field. The colours indicate direction (red-left, green-right, yellow-into, blue-outof screen). And the intensity of the colour indicates the magnitude of the current.
200 kHz acoustic volume backscatter - Which is seen to best indicate the fine scale location and structure of the thermo/halo/pycno-cline.
600 kHz acoustic volume backscatter - derived from the ADCP in 50cm bins, which differs in character significantly from the 200 kHz in that is appears to separate watermasses by mean backscatter within the watermass, rather than just scattering of the interfaces between the watermasses.

In this imagery one clearly sees the salt water intrusion develop in the gorge, and spread out upstream over the sill as a basal density flow, just (if perhaps not quite) making it into the deeper water of the Westfield channel at it maximum upriver extent by ~ half way through the ebb. From then on, the surface waters are flowing out and there is a slow leakage of the residual salt water layer at the base, back into the gorge (as seen better in the 2000 imagery, where an interfacial waterfall actually develops as it flows backwards).

SILL TOP CHANNEL to KENNEBECASIS EXCHANGE - Across-Sill Intrusion

The adjacent animation illustrates a variety of simultaneous views of the exchange of salt and fresh water between the main SE-NW through-channel in Grand Bay and the Kennebecasis Fjord to the NE. The section chosen, actually passes through the deepest lateral spill channel that lies just S of Kennebecasis Island. That channel is assumed to be the major conduit for most of the saline inflow into the Kennebecasis (although we also know that smaller sill top channels play a surprisingly significant role).

The display parameters are identical to the above case, and again 20 sections were acquired over a 12.42 hour cycle period. In this case we are looking for evidence of an interfacial hyperpycnal intrusion between the overlying freshwater and the underlying plug of relatively stagnant salt (23ppt) water.

As we can see, the halocline in the Kennebecasis is significantly perturbed (> 5m) as a result of the progression of the intrusion. The magnitude of the halocline perturbation decays ~ linearly to the NE as one goes into the fjord. This "internal tide/wave" of M2 period was actually previously noted by Page (BSc Hons thesis 1980's) from a series of point observations close to the line of this transect. What is new here, is evidence for the progression of that wave into the fjord.

Of particular interest is evidence for the hyperpycnal intrusion . We can see evidence of a mid water current maximum as well as a clear right ward (NE ward) propagating head running along the location of the primary acoustic scattering horizon (that represents closely the pycnocline). In the 600 kHz it almost seems that a shock wave actually propagates into the lower more stagnant waters. The nature of this remarkable volume scattering signature in not understood.

last modified by John E. Hughes Clarke, August 2004