The Oceanographic Circulation of the Port of Saint John over tidal and seasonal timescales
Reenu Toodesh
Ocean Mapping Group

   


OVERVIEW:
As part of the sustainable management of the Port of Saint John there is a critical need to maintain sufficient under keel clearance for the various container and tour ship traffic in and out of the harbour. Because of the high and variable sedimentation, annual maintenance dredging is necessary to keep access to the port open and to provide sufficient depth at the berths for ships using the harbour.

 The Port of Saint John lies at the mouth of the Saint John River on the north side of the macrotidal Bay of Fundy. Because of this, the harbour sedimentation is influenced by two major sources of siltation. From the river, the sediment flux is strongly modulated by the seasonal variations in river discharge. From the ocean, there is significant resuspension of offshore marine sediments which occurs in response to pseudo random storm events and seasonal meteorology.

To better understand this complex interaction which occurs over tidal periods, high density oceanographic observations over a tidal cycle have been collected at four river level stages (winter minimum, spring freshet, summer minimum and fall freshet). Oceanographic profiles of temperature and salinity were collected using the towed survey sensor and current magnitude, direction and acoustic backscatter with the use of the Acoustic Doppler Current Profiler. Also, short wavelength variability in the halocline was observed using the 200kHz volume backscattering echosounder.


Figure 1:Map sshowing the location of the Saint John Harbour.

Figure 2:Satellite image showing the extent of the river plume.

The figures above show the location of the Saint John harbour with respect to the macrotidal Bay of Fundy. It can also be see from the satellite image, the river plume (shown by the turbid fresh water surface layer) from the Saint John river during periods of high river dischage. The turbidity of the water is related to the sedimentation from the river discharge upstream  of the harbour.


INSTRUMENTATION USED:

Moving Vessel Profiler, MVP30
The moving vessel profiler is an autonomous profiler used to collect logitudinal oceanographic sections along the axis of the Main Harbour and Courtenay Bay channel. Vertical dips are made while the vessel is underway collecting temperature and salinity (which derives the density and soundspeed) and also optical backscatter which derives the amount of suspended sediments  throughout the water column.

ADCP

The RDI 600 kHz workhorse ADCP was used to measure the magnitude of the velocity, direction and acoustic backscatter through the water column. The ADCP is mounted on a pole located on the port side of the vessel, with a draft of approximately 1.2m.  This model ADCP consists of four beams, each of width 20°. As the vessel moves, the ADCP emits pulses of sound which are reflected by scatterers in the water moving relative to the ADCP. While the vessel was operating at speed of 4m/s, the ping rate of the ADCP was approximately1Hz. The frequency of the pings controls the range and the resolution of the data obtained, which in this case would be 4m.


ECHOSOUNDER

A Knudsen 320 B/P 200 kHz echo sounder with a beam width of 6° and pulse duration of 0.1ms was used. Thus the echosounder had a pulse length of 0.15m and an effective range resolution of 0.075m. For the surveys the gain was set to its highest (100) whilst the power was set to its lowest (1). The system is equipped with an amplifier that increases the intensity of the return echo. Amplification of the signal (increasing the gain) was preferred rather than increasing the power because the weak backscatter targets in the water column are considered to be ambient noise which is independent of the power level.The echosounder was used to detect the short wave variability in the halocline. The range of the echosounder was set to measure depth of up to 50m. The horizontal resolution of data collected was 0.5 to 1.5m and the vertical resolution was 0.07m.
Ocean Mapping Group's Survey vessel showing location of sensors
 Ship Track along the Main Channel



This is an example of the results obtained from the specified instrumentation on board the Heron, using OMG's software for post processing. All sections are showns for a single transect along the main axis of the Main Harbour Channel.

For the plots the depth range is (0-40m). This data is very useful in analysing the seasonal and tidal variation of the salt water and fresh water mixing dynamics which is heavily influenced by the large tides and the variable freshwater discharge from the Saint John River and there the estuarine circulation of the Port of Saint John.


DATA
INITIAL RESULTS
Updated By Reenu Toodesh                                                                                                                                                                                                31/01/2011