How do we quantify Portland's coastal currents?

1. Where is Portland?

Portland, situated on the southern coast of Maine in the United States, is a vibrant and historic seaport city. Portland hugs Casco Bay, a massive, semi-enclosed estuary in the Gulf of Maine. The fact that the city sits at the entrance of numerous waterways has made it a significant seafaring hub for centuries. Spanning a total area of approximately 36.6 square miles, of which 21.3 square miles are land and 15.3 square miles are water, Portland's physical makeup is a combination of rocky shores, numerous islands, and deep-water harbors.

Being in existence since the 17th century, Portland has experienced a rich history of development. It was once a fishing village but later became an important trading port. The city's architecture is a combination of colonial-era and modern buildings. The Old Port district, surrounded by cobblestone streets and old warehouses, is an indication of its maritime past. Casco Bay, which surrounds Portland, is a haven for marine life. The complex network of inlets, channels, and islands within the bay is home to a diverse set of species ranging from lobsters, fish, and seabirds. The bay's resources directly depend on shipping, tourism, and fishing.

2. How are the coastal currents around Portland?

Coastal currents around Portland are affected by a combination of various factors. Tidal forces are one of the primary determinants. The Gulf of Maine is semi-diurnal in tides, that is, two highs and two lows every day. While tides recede and come back again, they create major fluctuations in the direction and speed of coastal currents. While water flows up into Portland bays and estuaries at high tide, it comes back again to the sea at low tide.

Wind also has a determining role. Sustained south - westerly breezes tend to drive surface water along the coast to add to the shoreward current. Conversely, northern winds will drive the water away from the shore. The local topography, both on the surface and beneath the water, determines the currents. Islands, reefs, and underwater channels can deflect the currents to divide, combine, or reverse. For example, the numerous islands of Casco Bay interferes with the movement of the currents, causing areas of eddies and still water.

3. How to track the Portland coastal water flow?

Surface Drifting Buoy Method

One of the ways of monitoring the coastal water flow in and around Portland is through the use of surface drifting buoys. The buoys are designed to drift on the surface of the water with the currents. They are equipped with GPS tracking devices and telemetry systems and transmit real-time location data. Scientists analyze the data to find the rate and time of surface currents. Recent research on surface currents in waters off Portland involves scientists deploying a chain of floats. Each of these floats is equipped with an extremely reflective float on the water surface and also a drogue at a predefined depth to exactly follow the water surface current. However, with this method, only the upper layer of water is measured. Wind sometimes makes the buoys stray away from the real current, thereby producing a bogus reading of the subsurface movement.

Anchor Moored Ship Method

Anchor moored ship method involves tying up a ship at a chosen location and taking measurements of the currents with instrumentation on board. Scientists lower current meters over the ship's side at different elevations to acquire a profile of the current velocity. Though more accurate depth-specific information than from surface drifting buoys, this method is not without limitations. The reading is representative only of the area around the ship. Moving the ship to make successive measurements can be costly and time - consuming, especially in heavy seas.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is now a sophisticated and simple method of documenting coastal currents. ADCPs make use of the Doppler shift of sound waves to measure water current velocities at many depths. They emit pulses of sound into the water column. When the sound pulses reflect off water particles, the frequency shift in the reflected sound is used to calculate the water's velocity. ADCPs are able to give a detailed picture of the present structure, from the surface down to nearly the seabed. This makes them extremely suitable for examining the intricate coastal currents off Portland.

4. How do ADCPs based on the Doppler principle operate?

ADCPs operate according to the principle of the Doppler effect. They possess piezoelectric transducers which emit sound waves into the water. When the sound waves come into contact with particles such as plankton, sediment, or bubbles in the water, the sound energy gets scattered back towards the ADCP profiler. The sound wave travel time to the particles and back provides an estimate of the distance away the particles are.

The measurement principle of current velocity relies on the Doppler shift. If the water particles are carried by the current, the frequency at the ADCP of the backscattered sound waves will be other than the transmitted frequency. The magnitude of the frequency difference is proportional to the water velocity. In order to determine three-dimensional velocities, most ADCPs employ at least three beams. Modern ADCPs are also equipped with several sensors, including temperature sensors to compensate for the effect of water temperature on the speed of sound, compasses to determine the heading of the instrument, and pitch/roll sensors to take precise readings even during rough seas. The incoming signals are amplified, digitized, and processed to calculate the current velocity at different depths.

5. What does high-quality measurement of Portland coastal currents need?

High-quality measurement of Portland coastal currents needs the equipment deployed to meet a series of requirements. Material reliability is one of the most important requirements. The ADCP casing, for example, needs to be constructed from a material that can withstand the marine environment. A good illustration of such a material is titanium alloy. It is also extremely corrosion resistant, which is important for extended deployment in seawater. The titanium alloy is also strong and lightweight, therefore easier to deploy and maneuver. Its strength ensures that the ADCP will resist the mechanical load of water movement and potential collisions from debris.

Size, weight, and power needs are also essential. A light and compact ADCP flow meter is preferable, since it can be installed on various platforms, from small research ships to buoys or underwater unmanned systems. Lower power needs result in longer - term deployments, especially with the use of batteries. Cost is also important. A less expensive ADCP enables large - scale measurements to be made, increasing the spatial and temporal resolution of data gathered.

6. What equipment should be employed in measuring current?

By Mounting Type

• Ship-mounted ADCP: As the ship traverses from point to point on the water, this technique is appropriate for big - scale surveys of Portland coastal waters. If the ship is in motion, the ADCP can continuously record the current, providing a description of the flow patterns of current over a broad area.
• Bottom-mounted ADCP: Fixed on the seafloor, this is optimally utilized for long - term fixed - point observation. It could perhaps provide useful insight into the long - term trends and variability of the currents at a specific location.
• Buoy-mounted ADCP: The ADCPs are attached to a buoy and will ride with the water, giving measurements where fixed-point measurements would be inconvenient. They are particularly useful in areas of strong tidal currents or where a more portable platform of measurement is desired.

Selection of Frequency

The frequency of the ADCP is chosen depending on the depth of the water. A 600kHz ADCP would be suitable for water depths up to 70m. Where the water depth in the shallow coastal waters near Portland is often within this range, a 600kHz ADCP can provide detailed current profiles. A 300kHz ADCP would be better suited to water depths of up to 110m. It has greater range with a still relatively good accuracy. For use in the deeper water, for instance, out in the extremities of Casco Bay, a 75kHz ADCP is the best to use as it penetrates further into the water column.

There are quite a few popular ADCP brands available in the market, such as Teledyne RDI, Nortek, and Sontek. But for whoever is looking for a cost - effective but high - quality product, the ADCP manufacturer China Sonar's PandaADCP is an excellent option. Made of all-titanium alloy, it possesses greater durability in the marine environment. With a higher cost - performance ratio, it is ideally suited for use by researchers, coastal planners, and anyone who needs reliable current measurement data. To find out more, visit https://china-sonar.com/.

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