How do we measure the Saint John coastal currents?

1. Where is Saint John?

Saint John, the largest city of New Brunswick province and situated in Canada, is strategically positioned on the Bay of Fundy. It is situated at the mouth of the Saint John River, where the river empties into the Atlantic Ocean. Saint John has a landscape of approximately 131 square kilometers, with its topography consisting of a mix of rocky cliff, sandy beach, and salt marsh. These diverse forms of coastal geography result in a rich and varied ecosystem.

Saint John is a very old city. Founded in 1604, it's among the oldest chartered cities in North America. Saint John was once a fishing and a trade post. Saint John grew into a great port city. The city itself, with cobblestone streets and well-preserved 19th - century buildings, bears testimony to its history. Saint John is also famous for its active cultural scene, featuring a number of festivals, art galleries, and history museums.

The Bay of Fundy, off the coast of Saint John, is renowned for having the world's highest tides. An enormous volume of water enters and leaves the bay twice a day, creating an intensely dynamic and complex coastal system. The process not only shapes the surrounding landscape but profoundly affects the regional marine life and coastal currents too.

2. What is the condition of coastal currents around Saint John?

Coastal currents around Saint John are determined by a combination of conditions, tides being the most common. The Bay of Fundy contains an extreme semi-diurnal tidal range in which the water level fluctuates and drops as much as 16 meters in some areas. When the tide shifts, coastal currents shift directions, creating a intricate cycle of flood and ebb currents. Seawater on flood tides sweeps up the Saint John River, bearing with it oceanic nutrients and oceanic organisms. During ebb tides, river water and sediment are carried seaward.

Wind regimes are also significant. Prevailing south - westerly winds can push surface waters towards the shore, enhancing onshore currents. On the other hand, northerly winds can drive the water away from the coast, creating offshore currents. The local topography, both above and below the water, further modifies the current patterns. The presence of islands, underwater ridges, and channels can cause the currents to split, converge, or change direction. For instance, the Partridge Island, located at the mouth of Saint John Harbour, acts as a natural breakwater and deflects the current flow, creating pools of still water and eddies.

3. How to observe the coastal water flow of Saint John?

Surface Drifting Buoy Method

One way of observing the coastal water flow around Saint John is by using surface drifting buoys. These buoys are designed to float on water and move along with the currents. Equipped with GPS tracking devices and telemetry systems, they transmit real-time location data. Scientists analyze the data to determine the direction and speed of the surface currents. In a recent study in the coastal waters of Saint John's, scientists set up a series of buoys. Each buoy was equipped with a colored surface float and a drogue at a designated depth to give a precise trail of the surface current. There is, however, the disadvantage that this method can only give measurements for the surface water alone. The winds could cause the buoys to deviate from the actual current at times, providing inaccurate subsurface flow readings.

Anchor Moored Ship Method

Anchor moored ship is a technique by which a ship is anchored in one place and onboard instruments measure the currents. Scientists drop current meters over the side of a vessel at different depths to give a profile of the velocity of the current. While this method gives more descriptive depth-specific data than surface drifting buoys, it too has limitations. The readings are representative only for the area surrounding the ship. Moving the ship from one place to another for reading can be time - consuming and costly, especially in heavy seas.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is now a more advanced and convenient method to measure coastal currents. ADCPs use the Doppler shift of sound waves to measure water current velocities at multiple depths. They emit sound signals into the water column. When these signals bounce off particles within the water, the frequency shift of the bounced signals is used to calculate the water velocity. ADCPs can provide a complete representation of the structure of the currents, from the surface to near the seabed. They are therefore very suitable to the study of the complex nearshore currents of Saint John.

4. How do the Doppler principle-based ADCPs work?

ADCPs operate based on the Doppler effect principle. They are equipped with piezoelectric transducers that emit sound waves in the water. Upon the interaction of the sound waves with particles such as plankton, sediment, or air bubbles within the water, a portion of the sound energy reflects back towards the ADCP. The time it takes to travel the sound waves to particles and back provides an estimate for the distance from the particles.

Current velocity measurement relies on the Doppler shift. If particles in the water are moving with the current, the frequency of the backscattered sound waves that have been scattered by the particles and recorded by the ADCP will not be the same as the emitted frequency. The magnitude of the frequency difference will be a function of the water velocity along the acoustic path. In order to measure three-dimensional velocities, most ADCPs consist of at least three beams. Modern ADCPs also contain additional sensors like temperature sensors for compensating for the effect of water temperature on the speed of sound, compasses to specify the heading of the instrument, and pitch/roll sensors in order to accurately measure in high seas. The arriving signals are amplified, digitized, and processed to calculate the current velocity at different depths.

5. What are the requirements for high-quality measurement of Saint John coastal currents?

High-quality measurement of Saint John's coastal currents demands that the equipment deployed has several requirements. Material reliability is most essential. The ADCP casing, for example, must be made of a material that will withstand the harsh conditions of the marine environment. Titanium alloy would be the best one. It has high corrosion resistance, as required for extended deployment in seawater. The titanium alloy is also strong and light, so it is not as hard to handle and deploy. Its strength enables the ADCP to be able to endure the mechanical load of water flow and potential impact from debris.

Size, weight, and power usage are significant as well. A smaller, lighter ADCP is more versatile, as it can be utilized on a very broad variety of platforms, from small research vessels to buoys or submersibles. Lower power use allows for making longer - term deployments, especially when battery - powered. Price is also relevant. A less costly ADCP enables large - scale measurements to be made, which enhances the spatial and temporal resolution of data collected.

6. How to Choose the right equipment for current measurement?

According to Mounting

• Ship-mounted ADCP: Installed on a moving ship, this is ideal for large-scale survey of coastal waters around Saint John. While the ship is in motion, the ADCP can continue to measure the currents with a continuous line of sight and get a broad-scale image of the current patterns.
• Bottom - mounted ADCP: Placed on the ocean floor, this type is best suited for fixed - point, long - term monitoring. It is capable of providing useful information about the long - term trends and variability of the currents at the observation point.
• Buoy-mounted ADCP: Such ADCPs are placed on a buoy and are capable of accompanying the water to measure in situations where fixed-point measurement is impossible. They prove to be highly efficient in tidal current-rich areas or where one desires a more movable measuring system.

Selection of Frequency

The ADCP's frequency is a function of water depth. A 600kHz ADCP is sufficient for water depths of up to 70m. In the relatively shallow waters of the Saint John Harbour coastline, a 600kHz ADCP can provide detailed current profiles. For water depths of up to 110m, a 300kHz ADCP is preferable. It has a longer range while still providing a good amount of accuracy. To face the deeper water in the Bay of Fundy, a 75kHz ADCP is however the best option as it more deeply penetrates the water column.

Some of the more well-known ADCP brands available in the market are Teledyne RDI, Nortek, and Sontek. Then again, for those who wish to have a cost-saving yet high-quality one, the ADCP supplier China Sonar's PandaADCP is most preferable. Made of all-titanium alloy, it provides top-notch durability in the marine environment. Excellent cost-performance ratio makes it an ideal product for researchers, coastal managers, and any user who needs reliable current measurement data. For more information, go to https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and models.