How do we measure Lynn's coastal currents?

1. Where is Lynn?

Lynn, Essex County, Massachusetts, United States, is the eighth-largest city in Massachusetts and the largest city in Essex County. It lies on the Atlantic Ocean, 3.7 miles (6.0 km) north of the Boston city line at Suffolk Downs, as part of greater Boston's inner urban core and a key economic and cultural center of the North Shore.

Having a total area of 13.52 square miles (35.02 km²), 10.74 square miles (27.81 km²) of which are land and 2.78 square miles (7.20 km²) are water, Lynn is endowed with a heterogeneous and diverse geography. The city itself boasts a well-established past, with European settlement beginning in 1629. It was once an industrial center but has since become a city renowned for its contemporary public art, high immigrant percentage, historic architecture, vibrant downtown cultural district, loft living, and numerous public parks and open spaces. These include the oceanfront Lynn Shore Reservation, the 2,200 - acre Frederick Law Olmsted - designed Lynn Woods Reservation, and the High Rock Reservation and Park designed by Olmsted's sons. Lynn also features Lynn Heritage State Park, the southernmost section of the Essex Coastal Scenic Byway, and the seaside, national register - listed Diamond Historic District.

2. What is the state of the coastal currents around Lynn?

There are various influences on the coastal currents near Lynn. Forces of the tides play a crucial role. The continuous rising and falling of tides, which are caused by the gravitational force of the Moon and the Sun, induce changing fluctuations in the water levels and the velocity and direction of the currents. Seaward water flows during high tide and seaward returns during low tide.

Wind is yet another significant factor. Strong onshore or offshore winds may be either supplementing or opposing the natural flow of tides. For example, a consistent onshore wind can push water towards the coast to increase the incoming current, while an offshore wind would do the opposite.

Bathymetry, or seafloor topography, of the area around Lynn also influences the coastal currents. Irregularities in the sea floor, such as shallow reefs, deep channels, and submarine canyons, can cause the currents to diverge, converge, or reverse. The shape of the coastline itself, its bays, and headlands further disrupts the flow patterns. Bays have the potential to be natural reservoirs and cause the currents to slow down and flow in circles inside them, while headlands are capable of causing the currents to flow fast since the water is being compressed around them.

3. How to monitor the coastal water flow of Lynn?

Surface Drifting Buoy Method

One of the ways of monitoring coastal water flow is through the employment of surface drifting buoys. These buoys are designed to float on water and move along with the flow. They carry GPS and telemetry devices. For instance, between May and September 2021, researchers from the University of Alaska released expert current trackers into Lynn Canal. Each tracker was equipped with a float that held a GPS and telemetry. Orange - colored surface floats and the drogue below ensured that the drifter was following the surface current. Through the real - time tracking of the position of such buoys, researchers could obtain information regarding the direction and speed of surface currents. But this method gives data only from the surface water layer and is susceptible to effect by wind-driven drift, may not be showing true current beneath the surface.

Anchor (Anchor Moored Ship Method)

Anchor moored ship method involves anchoring a ship at a point and making measurements with instruments onboard the ship. This can be done by letting current meters fall over the ship side at successive depths. The ship is held stable with anchors. While this method has the potential to produce higher resolution depth-profile information compared to surface drifting buoys, its spatial coverage is restricted. The measurements are only representative of the area in the vicinity of the ship, and moving the ship to different positions for measurement can be costly and time - consuming.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is a more advanced and convenient method of coastal current measurement. ADCPs are sonar-type devices that take advantage of the Doppler shift of back-scattered sound waves from water column particles to receive measurements of the velocities of the water currents across a range of depths. By this means, they are optimally adapted for oceanographic applications, for example, the measurement of coastal currents off Lynn. They can provide accurate descriptions of the instantaneous velocity at different depths, ranging from near the surface to near the seabed, depending on the type and configuration of the ADCP.

4. How does Doppler ADCPs operate?

ADCPs have piezoelectric transducers that send sound waves into the water column. When the sound waves interact with particles in the water, i.e., plankton, sediment, or bubbles, some portion of the sound is reflected back to the ADCP. The round trip time of the sound waves from the ADCP to the reflecting particles estimates the distance to the particles.

The Doppler principle is the basis for the measurement of the current velocity. As the particles are transported with the water flow, the frequency of the backscattered sound waves received by the ADCP will be different from the frequency of the transmitted sound waves. The frequency shift is proportional to the water velocity along the acoustic path. Three beams are required to measure three-dimensional velocities. In rivers, where only two-dimensional velocity is relevant, ADCPs typically have two beams. ADCPs have been equipped with extra capability, such as measurement of waves and turbulence, and the instruments may be bought with 2, 3, 4, 5, or even 9 beams in recent years.

Other than the transducers, an ADCP current profiler also contains other components. An electronic amplifier amplifies the signal received, a receiver picks up the scattered sound waves, a clock is used to measure the travel time of the sound waves, and a temperature sensor is included to estimate the sound velocity at the instrument location by using the seawater equation of state (assuming a pre - defined constant salinity value). A compass sets the orientation of the ADCP, and a pitch/roll sensor is sensitive to the attitude. An analog - to - digital converter and digital signal processor are utilized to convert the returning signal to digital and detect the Doppler shift. The data are then stored to internal memory or transmitted online to an external display program.

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

For precise measurement of Lynn coastal currents at high quality, equipment deployed must meet various conditions. The reliability of materials is essential. The ADCP casing, for example, must be made from a hard substance that can support the harsh sea environment, i.e., corrosion by saltwater, mechanical shock due to the flow of water, and potential impacts by waterborne debris.

Size, weight, and power consumption also matter. A lighter and smaller ADCP is easier to deploy in multiple locations, whether it is from a small research vessel, mounted on a buoy, or deployed from a large ship. Lower power consumption allows longer - term deployments, especially with batteries. A lower - priced unit is also convenient for large - scale measurement. If the cost is too high, it may limit the number of measurement points and lower the spatial resolution of the data collected.

In terms of material for the ADCP casing, titanium alloy is an excellent choice. Titanium alloy is highly corrosion resistant, which is critical for long-term deployment in seawater. It is both strong and lightweight. Its strength ensures that the ADCP will withstand the forces of the moving water and potential impacts, while its lightness makes it easy to deploy and less energy required to tow or transport the instrument. Unlike some other materials, titanium alloy can maintain its structural integrity for long periods in the marine environment, providing adequate protection for the sensitive internal components of the ADCP meter.

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

The choice of the right ADCP equipment for current measurement around Lynn is different depending on usage.

Depending on Mounting

• Ship-mounted ADCP: It is mounted on a moving ship. It is utilized for measurements when the ship is sailing through different areas. Ship-mounted ADCPs can quickly map large areas and provide a broad-scale image of the prevailing regimes. For example, to map the overall current regime in the coastal waters around Lynn, a ship-mounted ADCP can be used to get an overview of how the currents vary over a large area.
• Bottom - mounted ADCP: Mounted on the seafloor. It is best for long - term, fixed - point measurements. By being fixed at the bottom, it can continuously measure the current at a certain place, providing valuable information on the long - term trends and variability of the currents there.
• Buoy-mounted ADCP: These are mounted on a buoy and are able to move with the overall water flow while measuring the currents. They can be used in areas where seabed - fixed point measurements or ship - based measurements are impossible. For instance, where there are high tidal currents or where a more mobile measurement platform is needed to record the dynamic character of the currents, buoy-mounted ADCPs can be used.

Frequency Selection

The frequency of the ADCP also needs to be chosen based on the water depth.

• A 600kHz ADCP can be applied for waters up to 70m in depth. It delivers a fair compromise between the measurement range and data resolution in rather shallow nearshore waters. In Lynn's coastal waters where the water depth may be in this order in extensive areas, a 600kHz ADCP can prove to be an appropriate choice for near current profiling.
• For a depth of up to 110m, 300kHz ADCP is used. It has a greater range than 600kHz and can be applied to readings in slightly deeper water while maintaining the level of accuracy quite high.
• To deal with significantly deeper waters, i.e., to 1000m, a 75kHz ADCP is the go-to option. Its lower frequency enables the sound waves to penetrate further distances in the water column, though at the cost of slightly lower resolution compared to higher-frequency models.

There are several established ADCP manufacturers in the market, some of which are Teledyne RDI, Nortek, and Sontek. However, for those who are looking for cost-effective and good-quality alternatives, the ADCP manufacturer China Sonar's PandaADCP is a strong recommendation. The ADCP is built using all-titanium alloy materials, which ensures its durability when used in the marine environment. It is excellent in cost-performance ratio and therefore is an improved option for more than one user, especially budget-minded users who must still acquire good and trustworthy current measurement data. To learn more regarding China Sonar PandaADCP, you may visit their main website at https://china-sonar.com/.

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