How do we quantify the coastal currents of Portsmouth?

1. Where is Portsmouth?

Portsmouth, a vibrant city on the seacoast of New Hampshire, USA, is located in a strategic position at the mouth of the Piscataqua River. This positions it at the confluence of the river and the vast Atlantic Ocean. Spanning approximately 16.9 square miles, Portsmouth is made up of 14.6 square miles of land and 2.3 square miles of water. The topography of the city is a mix of rocky coastlines, salt marshes, and sandy beaches, producing a diverse and dynamic coastal environment.

Established in the early 17th century, Portsmouth is one of the oldest communities in New England. It was initially a trading and fishing post but later grew as a major naval shipyard and commercial center. Downtown Portsmouth is a living American colonial architecture museum with cobblestone streets flanked by well-preserved 18th-century buildings. Portsmouth is also renowned for having an active arts community with numerous galleries, theaters, and music clubs. The nearby Great Bay, a huge estuary, forms part of the local ecosystem. It serves as a migratory stopover point for birds, a nursery ground for juvenile fish, and the home for a variety of marine invertebrates.

2. How are the coastal currents off Portsmouth?

The coastal currents off Portsmouth are influenced by a multifactorial interaction with a variety of factors. Tidal forces are a governing factor, as the Piscataqua River and estuaries experience semi - diurnal tides. These tides, resulting from the gravitational pull of the Moon and the Sun, increase and decrease the water level twice a day. As the tides shift, the coastal current's direction and speed change significantly. Water is carried up the Piscataqua River by high tide, bringing sea life and nutrients in from the ocean. By low tide, water flows out to the sea, carrying waste and sediments with it.

The wind flow also plays a significant role in the development of the coastal currents. South - westerly winds, the dominant winds, push surface water onshore and make the onshore current stronger. On the other hand, northerly winds can drive the water away from the coast, creating an offshore current. The underwater and above-water topography of the region also changes the current patterns. Islands, reefs, and underwater channels deflect the currents, dividing them, combining them, or reversing direction. For example, the Isles of Shoals, a group of small islands lying off the coast of Portsmouth, act as natural barriers, deflecting the flow of the currents and creating areas of quiet water and eddies.

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

Surface Drifting Buoy Method

One of the ways of observing the coastal water flow around Portsmouth is the use of surface drifting buoys. Such buoys are designed to float on the water surface and follow the direction of currents. Equipped with GPS tracking devices and telemetry systems, the buoys transmit real-time location data. This data is analyzed by scientists to determine the direction and speed of the surface currents. In a recent research in Portsmouth coastal waters, scientists laid out a line of buoys. Every buoy carried a very noticeable surface float and a drogue at a certain depth to closely follow the surface current. It is not able to measure beyond the surface layer of the water, though, with this method. Wind might sometimes steer the buoys away from the actual current and give a wrong reading of subsurface flow.

Anchor Moored Ship Method

The anchored ship mooring technique involves placing a ship at a fixed location and using onboard equipment to measure the currents. Scientists suspend current meters down the side of the ship to several different depths to obtain a profile of the velocity of the current. While more precise depth-specific information than surface drifting buoys, it has some drawbacks. The readings are indicative of the surrounding region only. Transpositions of the ship to other locations for reading are time - consuming and costly, especially during rough seas.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has become a more advanced and simplified method of obtaining coastal current measurement. ADCPs utilize the Doppler effect of sound waves to obtain measurements of water current speed at different depths. They send sound signals into the water column. When these are reflected off water particles, the frequency shift of the reflected signals is used to calculate the water velocity. ADCPs can provide a general overview of current structure, from the surface down to near the seabed. They are therefore extremely well placed to examine the complex coastal currents off Portsmouth.

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

ADCPs operate on the Doppler effect. They contain piezoelectric transducers that emit sound waves into the water. When the sound waves encounter particles such as plankton, sediment, or bubbles in the water, some of the sound energy is deflected in the direction of the ADCP. The duration it takes for the sound waves to travel to the particles and back provides an estimate of how far away the particles are.

The Doppler shift is the basis for determining current velocity. When the particles are being transported by the water current, the frequency of the sound waves that are scattered to the ADCP will be altered from the original frequency of the emitted waves. The magnitude of this frequency shift is directly proportional to the velocity of the water along the path of the acoustics. To calculate three-dimensional velocities, most ADCPs use at least three beams. Modern ADCPs also contain multiple sensors, including temperature sensors to correct for the effect of water temperature on sound speed, compasses to determine the instrument's direction, and pitch/roll sensors to allow proper measurement in high seas. The received signals are amplified, digitized, and processed to calculate the current velocity at different depths.

5. What is required to measure Portsmouth coastal currents at a high quality

In order to perform high-quality measurement of coastal currents in Portsmouth, the apparatus used must fulfill a series of demands. Consistency of materials used is fully imperative. For example, ADCP casing needs to be built using a durable material that will resist the inhospitable aquatic environment. The best choice could be titanium alloy. It is of high corrosion resistance, which is the major determinant of long-term deployment in seawater. The titanium alloy is also strong and light in weight, thus easier to deploy and manage. Its strength enables it to withstand the mechanical stress of water flow and potential impact from trash.

Size and weight and power consumption are significant too. A smaller and lighter ADCP is more general purpose, i.e., it can be applied to a variety of platforms, e.g., a small research vessel, buoy, or underwater drone. Lower power consumption enables longer - term deployment, especially with battery power. Cost is significant too. A lower - cost ADCP allows large - scale measurement, increasing the spatial and temporal resolution of the data collected.

6. Selecting the correct equipment for the measurement of currents

According to Mounting

• Ship-mounted ADCP: A ship-mounted version, this kind is best used for large - scale surveys of Portsmouth's surrounding coastal waters. While the ship moves, the ADCP may measure the current continuously, taking a broad - scale picture of the current structures.
• Bottom - mounted ADCP: Installed on the seafloor, this is ideal for fixed - point, long - term monitoring. It is capable of yielding useful information on the long - term variability and trends of the currents at a point.
• Buoy-mounted ADCP: Mounted on a buoy, such ADCPs can move with the water, and hence measurements in conditions where fixed-point measurements are not feasible can be obtained. They are highly suitable for high tidal current conditions or where a more mobile platform of measurement is required.

Frequency Selection

The water depth is the basis of ADCP frequency. A 600kHz ADCP can be used in water up to 70m deep. For coastal waters' shallows off the Portsmouth shore where water depth is often within these depths, there can be accurate current profiles provided by a 600kHz ADCP. A proper 300kHz ADCP must be used in water up to 110m deep. It provides greater range while ensuring a cost-effective accuracy. When working with more profound waters, such as those in the outer regions of the Piscataqua River estuary, a 75kHz ADCP is the preference due to its ability to penetrate further into the water column.

Some of the more well-known ADCP brands in the market are Teledyne RDI, Nortek, and Sontek. That said, for someone who wants a cost-effective but high-quality option, the ADCP supplier China Sonar's PandaADCP is well worth a look. Built with all - titanium alloy, it is of higher durability in the seawater environment. With an unbeatable cost - performance ratio, it is a great choice for researchers, coastal managers, and anyone in need of precise current measurement data. For more information, log on to https://china-sonar.com/.

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