How do we measure Scarborough's coastal currents?

1. Where is Scarborough?

Scarborough, a town in Cumberland County, Maine, USA, sits on the southern coast of

he state, with the expansive sweep of the Gulf of Maine. The coastal gem encompasses some 26.2 square miles, of which 17.2 square miles is land and 9 square miles is water. Scarborough's unique geographical features are its extended sandy beaches, salt marshes, and network of tidal creeks, making it a refuge for aquatic and terrestrial wildlife alike.

The town is filled with history since the early 17th century. Scarborough evolved from a small fishing village to become a popular summer resort town. Its architecture, especially in the downtown area, bears witness to the town's long-standing heritage. The nearby Scarborough Marsh, one of the largest salt marshes in New England, is not only a vital part of the ecosystem but also helps to explain the town's natural beauty. The marsh serves as a significant breeding habitat for many birds, including herons, egrets, and ospreys. In addition, the marsh filters pollutants, protects the shoreline from erosion, and provides nursery grounds for numerous fish and crustacean species.

2. What is the status of the coastal currents in the area around Scarborough?

The coastal currents in the area around Scarborough are governed by a complex interplay of a number of factors. Tidal forces, which are caused by the gravitational pull of the Moon and the Sun, are one of the primary drivers. The Gulf of Maine experiences semi - diurnal tides, so there are two low tides and two high tides in a day. The tides, as they flow and ebb, also bring about profound variations in direction and speed in the coastal currents. During high tide, water moves into the estuaries and salt marshes and, during low tide, it returns to the sea.

Wind patterns also have a powerful impact on the coastal currents. Prevailing south-westerly winds drive surface waters towards the shore, reinforcing the onshore current. Northerly winds, however, can drive the water away from the shore. The local topography, both above and below water, also affects the patterns of current. The presence of sandbars, reefs, and submarine channels can split the currents, converge, or change their direction. For instance, the long sandbars on the Scarborough coast can act as natural barriers, diverting the direction of the currents and creating areas of calm water and eddies.

3. How to observe the coastal water flow of Scarborough?

Surface Drifting Buoy Method

One way of monitoring the movement of water along Scarborough's coast is through the use of surface drifting buoys. The buoys are placed on the water surface and drift along with the currents. Equipped with GPS tracking equipment and telemetry, they transmit real - time location information. Using this information, researchers compute the direction and speed of the surface currents. In a recent study in Scarborough's coastal waters, researchers employed an array of buoys. Each of the buoys was fitted with a brightly colored surface float and a drogue at a known depth so that it would closely follow the surface current. However, this method is limited to following only the surface layer of the water. Wind can sometimes cause the buoys to deviate from the actual current, hence giving incorrect readings of the subsurface flow.

Anchor Moored Ship Method

The anchor moored ship method involves anchoring a ship at a fixed location and measuring the currents by means of instruments on board. Scientists hang current meters over the side of the ship at different depths to obtain a profile of the current speed. While this method provides more depth - specific detail than is received from surface drifting buoys, it is not without its drawbacks. Measurements are only representative of the area in the immediate surroundings of the ship. It can be costly and time - consuming to relocate the ship to other areas to take measurements, especially in rough seas.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is a newer and more convenient method of measuring coastal currents. ADCPs use the Doppler shift of sound waves to measure water current velocities at different depths. ADCPs send sound signals into the water column. As the signals bounce off particles in the water, the change in frequency of the returning signals is used to calculate the velocity of the water. ADCPs can provide a full image of the current structure from the surface to close to the seabed. This makes them very well adapted to examining the complex coastal currents around Scarborough.

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

ADCPs operate based on the Doppler effect. ADCPs possess piezoelectric transducers that send sound waves into the water. When the sound waves encounter particles such as plankton, sediment, or bubbles in the water, a portion of the sound energy is scattered back to the ADCP profiler. The time taken for the sound waves to travel to the particles and back provides an estimate of the distance to the particles.

The key to measuring velocity is in the Doppler shift. Since the particles are moving with the water current, the frequency of the sound waves that are scattered back to the ADCP will be different from the frequency of the waves that were sent out. The size of this frequency shift is proportional to the velocity of the water along the acoustic path. To be able to measure three - dimensional velocities, a minimum of three beams is utilized by most ADCPs. Contemporary ADCPs also have a number of sensors on them, such as temperature sensors to compensate for the influence of water temperature on the velocity of sound, compasses to record the heading of the instrument, and pitch/roll sensors to maintain accurate measurements despite rough seas. Amplification, digitization, and processing of received signals are performed to compute the current velocity at varying depths.

5. What's needed for high-quality measurement of Scarborough coastal currents?

For measuring Scarborough's coastal currents with high quality, the measuring equipment used must meet some standards. Material reliability is the first. The case of the ADCP, for example, must be made from material that can withstand the corrosive seawater environment. Titanium alloy would be an excellent choice. It is extremely resistant to corrosion, which is essential for long-term deployment in seawater. Titanium alloy is also strong and light, making it easier to handle and deploy. Its strength will ensure that the ADCP will be able to resist water movement mechanical stress and potential impacts from any debris.

Size, weight, and power consumption also matter. A smaller and lighter ADCP flow meter is more versatile, as it can be installed on a variety of platforms, from small research vessels, buoys, or underwater unmanned vehicles. Low power consumption allows for longer - term deployments, especially when operating on battery power. Cost is another factor. A cheaper ADCP allows for large - scale measurements to be made, increasing the spatial and temporal resolution of data collected.

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

Types Based on Mounting

• Ship - mounted ADCP: Mounted on a moving ship, this is ideal for large - scale surveys of Scarborough's coastal waters. As the ship moves, the ADCP can record continuous measurements of the currents, providing a large - scale picture of the current patterns.
• Bottom - mounted ADCP: Placed on the ocean floor, this type is suitable for fixed - point, long - term monitoring. It can provide valuable information on the long - term variability and trends of the currents at a specific site.
• Buoy - mounted ADCP: These ADCPs are mounted on a buoy and can drift with the water, allowing measurements in areas where fixed - point measurements are not practical. They are employed particularly in areas with high tidal currents or where a more mobile measurement platform is desirable.

Frequency Selection

ADCP frequency selection is determined by water depth. A 600kHz ADCP is sufficient for water depths up to 70m. In the shallow coastal waters off Scarborough, where the water depth is often within this range, a 600kHz ADCP can provide detailed current profiles. For water depths up to 110m, a 300kHz ADCP is more appropriate. It has a greater range and still offers a reasonably good compromise of accuracy. For more open water, such as in the outer Gulf of Maine, the 75kHz ADCP would be the preferable option as it reaches further into 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.