OMG_Lameque_Bay

Lameque Bay Plume
Shippagan Bay Survey 2003

John E. Hughes Clarke, Anya Duxfield and Beth-Anne Martin
Ocean Mapping Group
University of New Brunswick

Whilst the CSL Heron Shippagan Bay survey was designed to cover the whole of the Shippagan Bay region, there was naturally a particular interest in the Lameque Bay area, especially in the vicinity of the fish plant effluent plume, just east of the Lameque harbour mouth. The following series of images, synthesize the acoustic findings in specifically that region. Note that these images by no means answer all the questions about the fate of the effluent, but they do clearly show a high-backscatter anomaly that radiates from the effluent outfall pipe suggesting that the extent of the seabed influenced by the plume can be delineated.

Extent of the backscatter anomaly in the vicinity of the Harbour breakwater.

If we examine this archived aerial photography (source: SNB orthophoto series, probably taken in 1996 or 1997?) we can see the harbour breakwater, the processing plant and the beach to the east of the breakwater. Most significantly one can actually see a bright plume in the water off that beach, emanating from a single point (the effluent outfall pipe mouth) which is drifting off to the east.

Looking at the identical area from an extract of CHS chart # 4913, one can see the regional bathymetry. The two main features to note are the dredged channel (maintained at a minimum depth of 5.5m) and the fact that the regional bay has average depths of only ~ 2.5 to 3 metres. One can see that the eastern harbour breakwater extends out beyond the 2m contour (actually dredged against the outer face of the wharf) but there is a gradual sloping beach front to the east of the breakwater where the effluent plume is located.

For the identical area we can look at the extent of multibeam bathymetric surveying (image to left, greyscale ranges from 7m (black) to 0m (white)). As the region is so shallow, there is only ~ 10-20% coverage achieved through the standard 80 m line spacing chosen for the regional survey. We went back in however, to get 100% coverage of the dredged channel, the harbour itself, and just outside the mouth of the harbour. The deepest point in the channel is just a little over 6m and the NE face of the outer harbour has also been dredge to in excess of 6m (the thin contour lines in the image are 2m contours).

The morphology of the bathymetric surface can be examined by making a synthetic sun-illuminated image. For this image (to the right) , the illumination is to the NW (top right). One can see that the main bay floor is almost entirely featureless. The only relief is seen in the dredged channel where a rough surface exists due to random motion of the dredge head.

Of more interest is the 300 kHz backscatter strength signal derived from the same multibeam data. From this (image to left) one sees that the highest backscatter (lightest colour) exists just outside the mouth of the harbour extending around to the east toward the effluent outfall. Most significantly the high backscatter anomaly does not extend into the outer or inner harbour suggesting that the source of this anomalous sediment is not from within the harbour itself.

Using the 200 kHz backscatter signal (image to right) from the keel-mounted sidescans we get a much better picture of the entire harbour approaches area. From this we can clearly see that the high backscatter anomaly (bright area) sweeps around the mouth of the harbour to the east and suggests a source right on the beach at the location of the outfall pipe. This strongly suggests a causal relationship between the source of this anomalous high backscatter sediment and the plume of material being ejected out the effluent pipe.
One can also see the outline of a shipwreck just subtidally on the beach immediately west of the outfall, outboard of the rock riprap edge of the carpark.

More regionaly we can see a Y shaped junction of some sort of old trench line (an old outfall pipe?) to the west of the dredged channel. And both to the east and west of the dredged channel are extensive areas of mussel aquaculture. One can see the individual parallel lines of the aquaculture site images by the sidescan as the vessel steamed along the outer limit of the mooring buoys.

The last image shown here (left) is more 200 kHz sidescan that was collected when the dredged channel and the inner harbour surveys were conducted. This confirms that the high backscatter sediment is not extending into the harbour itself (and thus is not originating from in there). As the inner harbour is extensively dredged, it suggests that the dredging itself is not the cause of the high backscatter.

The next question is how far away from the beach does this high backscatter sediment plume extend? For that we have to look at the same data but now extending out into the main part of Lameque Bay.

Extent of the backscatter anomaly along the Lameque Harbour Channel

Whilst it is easy to see the plume signature immediately to the east of the harbour mouth, it is less clear, where the distal limits of the plume are. The following series of images help delineate how far out into the bay this backscatter signature extends. These images examine the full length of the dredged channel, extending from harbour entrance by the break water to the limit of dredging. The extract from chart 4913 (left) defines the regional distribution of depths.

This next image shows the extent of multibeam bathymetric coverage in the bay (figure to right, colour range 0-7m below chart datum). The main survey lines are 80m apart but a special survey was conducted of the dredged channel and the inside of the harbour itself. On its own, little indication can be seen of the anomalous region. However, from the same dataset we can examine the distribution of 300 kHz backscatter strength in the area (next image). We can then compare the backscatter signature with the depth distribution.

The 300 kHz backscatter clearly shows that the northeastern half of the channel floor is high backscatter (bright), changing gradually to a lower backscatter than the surrounding sediments of the bay as one moves to the southwest. Interestingly the high backscatter data is restricted to the central axis of the channel but includes a series of circular high backcsatter anomalies on either side of the centre of the channel. If we look at the bathymetric model in more detail we see that the channel is deepest along its central axis , but that there are a series of small depressions on either side of the channel axis which matches the location of the small circular high backscatter anomalies.

It is thus assumed that whatever is causing the high backscatter anomaly (possibly excess organic material as discussed below) is preferentially settling in the deepest points of the channel.The fact that the positive backscatter anomaly is strongest to the NE and fades away to the SW suggests that the source of this anomalous sediment is coming from close to the harbour mouth. And as show in the close up set of images, the most likely source is not the harbour floor, but the beach immedately to the east of the harbour where the effluent source is.

The multibeam-derived 300kHz backscatter is better at delineating the sediment distribution on the floor of the dredged channel as it casts less shadows than the lower-aspect-ratio sidescans. Nevertheless the sidescan provides the only complete picture of the sediment distribution on the bay. As we can see (image to left) one can recognise tthe individual deposits of the dredge spoil at the SW end of the dredged channel . The ultimate limit of the dredging is seen as an abrupt change in the bottom backscatter. The SW end of the dredged channel is clearly filled with a very uniform sheet of low backscatteer sediment (see discussion below comparing this with the Shippagan Approaches dredged channel).

Combining the multibeam and the sidescan imaging ,we have a very clear picture of the lateral extent of this anomalous high backscatter seabed signature. We still don't know what it is, but one more clue can be obtained by looking at the simulataneous subbottom profiles.

Plume Signature, as seen from subbottom profiling

For exactly the same areas we can now look at the 3.5 kHz subbottom profiles (figure to right) across the Lameque Bay dredged channel. Remember the lines are spaced at 80m and run obliquely across the channel. A high surface reflection coefficient anomaly (extra dark surface echo) just on the dredged channel floor needs explaining. It looks similar to the buried gas reflectors seen elsewhere in the bay, except it is at the surface and only occurs on the dredged channel floor. There are a number of possible causes, including:

increased surface roughness (e.g. gravel rather than sand or mud)
increased impedance contrast (e.g. harder, more consolidated)
increased volume scattering (e.g.: gas bubbles within the otherwise soft smooth sediments)

From the regional 3.5 kHz subbottom profiles (see compilations) we have already established that there is a buried gas-rich horizon underlying much of the bay. Strong but diffuse reflectors exist normally about 2-5m below the modern seabed (zoom into figure above). These reflectors are probably a result of buried peat horizons, which contain organic matter that decomposes releasing CO2 and CH4 into the sediments. If the sediments above are low permeability, that gas is trapped within the sediment. Because, it is much lower impedance than the surrounding sediments (low density and sound speed) it shows up as a v. strong acoustic reflector. One possible explanation for the high reflectivity of the dredged channel floors might be that it is this gas rich horizon that has been exposed by the dredging..

3.5 kHz subbottom
across Shippagan dredged channel

In the vicinity of the dredged channel NW of Shippagan harbour (Pointe Brule), however, it is also clear from the subbottom profiling (image to left) that the dredging has extended down into this gas rich horizon. However, if one looks at the sidescan image of the same dredged channel (image to right), one can clearly see that the channel floor is distinctly low backscatter (as would be expected if it is actively being filled in by fine grained sediment deposited through current action).

Thus, by comparison to the analogous situation in the Shippagan dredged channel, exposure of the gas rich horizon would not be expected to produce a positive backscatter anomaly as is seen in the Lameque Bay dredged channel. In fact if we look at the sidescan image of the outer part of the Lameque channel, it is also low backscatter.

200 kHz sidescan
across Shippagan dredged channel

The adjacent image shows the correlation of the 3.5 kHz transects across the dredged channel showing that there is a pronounced high surface reflection coefficient on the channel floor where the high backscatter anomaly is seen in the EM3000 300 kHz backscatter images. As stated above, based on the results from the Shippagan dredged channel, this high reflectivity is not expected to be the result of exposing the buried gas-rich horizon. It is thus assumed that the anomaly is due to the presence of an unidentified deposit derived from fish plant effluent plume.

Based on comparison to the acoustic signature of rotting excess organic material seen off other fish plants (e.g. : Blacks Harbour fish plant outfall site and Letang Estuary salmon farm waste deposits) it is assumed that there is a layer of organic material that being slowly transported along the deepest axis of the dredged channel floor. This hypothesis needs to be tested by recovering sediment samples either from a grab or from divers.

back to Shippagan Survey Index

last modified Nov 14th by John E. Hughes Clarke