How do we quantify Plymouth's coastal currents?

1. Where is Plymouth?

Plymouth, Plymouth County, Massachusetts, is a sanctified location in American geography and history. It is located on the state's southern coastline, along the broad Atlantic Ocean. Plymouth's coastal community is not only a geographical point, but also the basis of the nation's origin.

Rich in historical heritage, Plymouth is famous for the Pilgrims' landing in 1620 aboard the Mayflower. The Pilgrims who arrived here established one of the most successful of the English colonies in North America. Attic historical atmosphere is felt even today. The town square, with cobblestone streets and colonial architecture, is a living reminder of its past. Plymouth Rock, the mythical landing site of the Pilgrims, attracts thousands each year, who want to touch their own part of America's history.

The beach at Plymouth is a wealthy tapestry of nature. Sandy beaches such as Plymouth Beach curve along the coast, with ideal spots for sunbathing and for swimming on warm summer afternoons. These beaches are more than recreational sanctuaries; they are also crucial breeding grounds for numerous shore-dwelling organisms, including the piping plover. In addition to the beaches, rocky outcroppings are scattered along the coast and offer unique habitats for marine animals. Tidal flats, which are exposed at low tide, reveal a robust ecosystem of clams, crabs, and other intertidal animals.

The waters off Plymouth are of the dynamic Massachusetts Bay and the broader Atlantic Ocean. The bay, influenced by a complex interaction of tides, wind, and ocean currents, is a busy shipping route for commercial shipping, fishing vessels, and recreational boating. The proximity to the Atlantic Ocean exposes Plymouth to the full impact of the cycles of the ocean, and hence an understanding of the coastal currents here is of extreme importance both to marine trade and for protecting the environment.

2. What is the health of the coastal currents surrounding Plymouth?

The coastal currents surrounding Plymouth are shaped by a set of variables. Tidal forces, regulated by the moon's and sun's gravitational force, are a controlling force. The recurring increase and decrease in Massachusetts Bay tides result in water pouring onto Plymouth beach during high tide and running away from it during low tide. Tidal flow greatly impacts the direction and speed of the coastline currents. In narrow inlets and channels around Plymouth, for example, the Jones River Estuary, tidal flow may be extremely strong because the water is focused through narrow passages.

Wind patterns also have an important role. Local prevailing winds, usually from the southwest, may force the surface waters of the bay. Higher winds will produce bigger waves and stronger currents. An onshore wind will pile water up off the Plymouth coast, changing the local current patterns. An offshore wind will result in the removal of surface waters from the coast.

The bathymetry, or seafloor topography, of the area around Plymouth is also significant. Shallow banks, deep channels, and seafloor ridges are all able to control water flow. The shallow areas of the seafloor serve to slow the currents and scatter them. Deep channels will drive the water, speeding it up. The irregular shape of the Plymouth coastline, with its numerous bays, inlets, and headlands, also serves to make the current patterns more complicated. Headlands, for example, White Horse Beach Headland, can divide and accelerate the currents around them, while bays, for instance, Plymouth Bay, can trap and recycle water and create characteristic local current systems.

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

Surface Drift Buoy Method

One of the traditional methods of observing coastal water flow is through the use of surface drift buoys. These buoys are anchored on the surface of the water and carried away by the currents. Equipped with GPS trackers, they monitor their path over a period of time. Analyzing the path of such buoys, researchers gain information regarding the patterns of surface currents. However, this method has its own set of limitations. Since the buoys are exposed to both the currents and the wind-induced surface waves, it may be challenging to distinguish between the two. Thus, the data might not always represent the true underlying current flow, especially in the areas of strong winds. In addition, surface drift buoys are only measuring the surface of the water column and may not be indicating the complex vertical structure of the currents.

Anchor - Moored Ship Method

The anchor - moored ship technique uses an anchored vessel. Shipboard sensors, i.e., current meters, are lowered into the ocean at specific depths to feel the velocity and direction of the currents. This method allows data to be collected at several depths, providing a better representation of the vertical structure of the currents than the surface drift buoy method. The fixed ship position, on the other hand, limits it to sampling currents in a relatively small area. This may prove to be too localized to record the large-scale variability of the coastal current over a wide extent of space, especially in an unstable region like the Massachusetts Bay near Plymouth. Moreover, the presence of the ship can agitate the natural water flow and thus skew the measured data.

Acoustic Doppler Current Profiler (ADCP) Method

Over the past few years, the Acoustic Doppler Current Profiler (ADCP) has evolved more advanced and convenient measurement technology. ADCPs have been widely used in coastal surveys and oceanographic research. ADCPs can measure water speed at any depth for a great variety of vertical height. By transmitting sound waves out into the water and observing the Doppler shift of the backscattered sound waves off suspended material within the water, such as sediment or plankton, the ADCP can calculate the speed of the water. This provides a more detailed and precise image of the structure of the current within the water column and is therefore especially well-suited to studying the complex coastal currents within the Plymouth region. ADCPs can be employed in numerous configurations, e.g., on ships, on moorings, or even on autonomous underwater vehicles, to enable flexible measurements in different environments.

4. What is the working principle of ADCPs based on the Doppler principle?

ADCPs utilize the Doppler effect. They transmit acoustic pulses into the water at a specified frequency. When these signals strike moving particles in the water, such as suspended sediment or plankton, the frequency of the back-scattered signals changes. If the particles are approaching the ADCP, the back-scattered frequency is greater, and if they are moving away, the back-scattered frequency is less. The magnitude of this frequency change, referred to as the Doppler shift, is a function of the velocity of the particles and, by association, that of the water in which they are floating.

The ADCP typically has multiple transducers which can send out and receive signals with different angles. This enables the device to capture the three-dimensional velocity components of the water current (east-west, north-south, and vertical). By measuring the Doppler shift in several directions, the ADCP can calculate the water's velocity vectors at various depths. The data obtained is then processed to acquire a profile of the current velocity along a vertical section of the water column. This profile provides valuable information about the strength and direction of the currents at different depths, allowing scientists to better understand the complex flow patterns in the coastal waters off Plymouth.

5. What's required for high - quality measurement of Plymouth coastal currents?

In order to measure accurately the coastal currents of Plymouth, there are some properties that would be desirable for the measurement equipment. The material used in the equipment must be highly reliable. Because of the marine environment's severity, the equipment must resist corrosion from seawater, can withstand high pressure from water, especially in deep water, and must be capable of withstanding long-term usage.

The equipment must be compact in size. The more compact the size, the more easily it can be handled and transported to different coastal locations, including those with complex topography or limited access, such as narrow inlets or shallow bays off Plymouth. Lightweight equipment is also preferable as it reduces the effort of transport and handling, whether deployed from a ship or a small boat.

Low power consumption is vital. This ensures that the device can be used for long periods without the need for frequent battery replacement or the use of a large, heavy power supply, which may be impracticable in remote coastal areas or in long - term monitoring campaigns. Cost - effectiveness is also important. The use of a low - cost device enables the device to be deployed more extensively, enabling large - scale measurement of the coastal currents. This is of special importance in detailed studies which require data at different locations across the Massachusetts Bay off Plymouth.

For the case of ADCPs, housing is an essential component. One suitable material for ADCP housing is titanium alloy. Titanium alloy offers excellent corrosion resistance, which is essential in long - term use in corrosive seawater conditions. It is also lightweight, yet strong enough to protect the internal components of the ADCP from mechanical damage, e.g., waves or impacts by debris. Titanium alloy's resistance to wear and tear ensures that the ADCP will withstand the severity of the marine environment, and the measurements are reliable for a significant amount of time, which is critical to analyze the dynamic coastal currents around Plymouth accurately.

6. How to Choose the Right Equipment for Current Measurement?

Types of ADCPs Based on Use

The choice of ADCP will depend on the purpose for which it is to be used. If measurements have to be taken when the ship is in motion, a ship-mounted ADCP will be suitable. It can measure currents as the ship travels and provides real-time data along the ship track. This has the benefit for surveys that must cover large areas in a short time or investigating the manner in which currents change over large geographical scales.

A bottom - mounted (or moored) ADCP, or sit - on - the - bottom ADCP, is particularly good for long - term monitoring at a single site. It is put on the seafloor and left to take current data for an extended period of time. This can be used to resolve the long - term trends and variability of the currents in a region, say a particular bay or channel off Plymouth.

Buoy - based ADCPs, or floating ADCPs, are ideal for observing large - scale coastal or oceanic processes. They can be carried along by the currents, sending messages from different locations as they are swept along. This type of ADCP is well adapted to studying the broad circulation patterns inside the Massachusetts Bay and the exchange with the off - Plymouth coastal currents.

Frequency Selection

Frequency is also an important consideration for ADCP. Different frequencies are best suited for different water depths. For water depths of up to around 70m, an ADCP with a frequency of 600kHz would be a suitable choice. This kind of frequency has comparatively high - resolution measurements in shallow waters so that thorough study of the structure of the currents near the surface and at the surface of the water column is possible.

A 300kHz ADCP would be more appropriate at depths of about 110m. It has the ability to penetrate further in the water column but is highly accurate and should be used to measure at middle depths where moderate water depth can be found in areas like near Plymouth in parts of the Massachusetts Bay.

For very deep water, up to 1000m, a 75kHz ADCP is recommended. Lower frequency ADCPs have longer distances they can travel through the water, allowing for measurement at deeper depths. This is required for the study of the deep - water currents that may have an impact on the general coastal circulation off Plymouth.

Recommended Brands

Some of the well-known ADCP brands are Teledyne RDI, Nortek, and Sontek. But for budget buyers without sacrificing quality, the ADCP supplier China Sonar's PandaADCP is a good choice. More information can be found on its official website: https://china-sonar.com/.

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