How can we measure the coastal currents of Southampton

1. Where is Southampton?

Southampton, a major port city on the south coast of England in Hampshire, occupies a prime location on the Solent, a strait separating the mainland from the Isle of Wight. This strategic position has made it a significant center for maritime trade, passenger transport, and naval activities for centuries.

Southampton has a rich historical legacy. It has been a very important port since the Middle Ages, with evidence of settlements around it dating back to the Roman period. The architecture is historic as well as modern. Some of the medieval-era city walls remain standing along the present-day skyscrapers and commercial buildings. The people are very much attached to the sea. The city of Southampton hosts one of the busiest UK ports. It receives a considerable number of cargo ships, cruise liners, and ferries plying routes across Europe and beyond. Although traditional fishing has declined, it still maintains a modest fishing fleet operating from the city in local waters. The waters of the Solent and the English Channel that lie immediately to the south are an integral part of the local ecosystem. They provide home to a variety of marine life, including seals, different fish species such as mackerel, bass, flatfish, and innumerable seabirds. On the seabed, there is a complex morphology with lines of sandbank, rocky outcrops, and channels, each playing its part in shaping the movement of the water.

2. What is the state of the coastal currents off Southampton?

The coastal currents off Southampton are determined by many factors interacting in a complicated way. The most influencing ones are tidal forces. The Solent has semi-diurnal tides - there is a high and low tide twice each day. Sometimes this tidal range results in certain areas having particularly strong tidal streams, reaching as high as 3-4 knots, notably in narrow areas of water that separate the Isle of Wight from the mainland. This process at high tide pushes nutrients and sediment from the English Channel into the Solent. The converse happens on the low tide: the waters pull back out of the intertidal zones to expose them once again.   The other huge contributing factor to all this is:. Dominant winds from the southwest may force the surface waters landward and hence enhance tidal streams. The surface winds also cause waves, so there is likely to be considerable interaction between this oscillating component and the wind-driven current: this gives an extremely complicated pattern of flow in the area. Winds from northerly quarters can force water away from land. Convergences and divergences due to coastal geometry including inlets and the proximity of the Isle of Wight are certainly occurring. These contemporary patterns are additionally complicated by underwater features like sandbanks and rocky outcrops, which might act either as barriers or channels to the flow of water, producing areas of turbulence and calm. Besides, the inflow of freshwater from local rivers, such as the River Itchen and the River Test, can affect the local current patterns. The freshwater has different density compared to the seawater and thus, the density - driven currents form and interact with tidal and wind - induced currents.

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

Surface Drifting Buoy Method

Surface drifting buoys are a simple and efficient means to observe surface - level currents. These buoys are prepared with GPS or satellite - based tracking systems. These are carried by the surface currents once they were released into the water. They can track its movement over time, and from the buoy trajectory alone, they can estimate the speed and direction of the surface waters. This method is constrained to the upper few meters of the water column and may not be representative of deeper currents.

Moored Ship Method

In the moored ship method, a ship is moored in a certain position near the coast of Southampton. The instruments used to measure current would be deployed. These instruments can measure the flow of water at different depths, enabling a detailed vertical profile of the current velocity and direction. However, this is limited to one location only, and the presence of the ship may interfere with the natural water flow, which may affect the measurement results.

Acoustic Doppler Current Profiler (ADCP) Method

ADCPs have transformed the ways of measuring coastal currents around Southampton. They are capable of measuring the velocity profile of the whole water column from the surface to the seabed. They also work non-intrusively with sound waves, measuring current velocity at many different depths from an acoustic pulse through the air to measure a Doppler shift of the signals reflected back to it from particles in the water. This shows a rather well-developed outline of the structure of the currents, and makes ADCPs perfectly apt for examining in detail complex patterns of current flows in coasts, in general, the ones in Solent and English Channel off Southampton in particular.

4. How do ADCPs using the Doppler principle work?

The ADCPs work on the principle of the Doppler effect. They send high-frequency acoustic pulses into the water. These pulses, upon meeting small particles like sediment, plankton, or even bubbles in the water, reflect the acoustic signals back to the ADCP profiler. The signal that is reflected back will be at a different frequency than that which was transmitted if the particles are moving with the water current, a process known as the Doppler shift. This frequency shift is proportional to the relative velocity of the particles. The ADCP measures this Doppler shift at multiple depths within the water column, allowing it to calculate the water velocity at points within the column. The ADCP works by having many transducers which measure velocity components in the different directions to then deduce the three-dimensional water flow velocity vector.

5. What is required to measure good quality coastal currents of Southampton?

Reliability in Equipment Materials

Reliability of equipment materials plays a significant role in the measurement of good-quality coastal currents around Southampton. The casing of the ADCP flow meter would be required to be from a material that is capable of withstanding the harsh marine environment in the Solent and the English Channel. One of the best choices for that would be of titanium alloy. Its high strength should enable the material to withstand strong water currents that may strike it, collision with floating debris, and the corrosive nature of seawater. Because of its low elastic modulus, mechanical stress will be less likely to cause damage. Moreover, the good corrosion resistance enables ADCPs to operate in seawater for a long time with minimal degradation.

Compact Size, Light Weight, Low Power Consumption, and Low Cost

A compact, lightweight ADCP current profiler is easier to deploy and maintain either on a small research vessel, attached to a buoy, or placed on the seabed. It causes less disturbance of the natural flow of the water and gives results that are quite accurate. For long-term autonomous monitoring, power consumption is small if it is to depend upon a battery-powered system. Low cost is needed for an ADCP current meter for its wide-scale deployment that is essentially necessary for thorough understanding of the generally complex coastal current patterns around Southampton.

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

Based on Usage

• Shipborne ADCP: Suitable for real-time current data along a ship's route near Southampton, it can be used for oceanographic surveys, studying the general circulation in the area, and optimizing shipping routes.
• Bottom-mounted ADCP: Suitable for long-term fixed-point monitoring of the currents at the seabed, this proves valuable in studying long-term trends of the coastal currents, for example, on the impact that climate change might have on the local marine ecosystem.
• Buoy-mounted ADCP: Useful for monitoring the surface-level currents over a wider area as it drifts with the water. It helps understand the spatial variability of the surface currents and how the coastal waters interact with the open English Channel.

Based on Water Depth

• 600kHz ADCP: Suitable for water depths up to about 70m, yielding high-resolution measurements throughout the generally shallow waters off Southampton. - 300kHz ADCP: For water depths of approximately 110m, this can be helpful in some areas where there might be a little deeper water over the Solent or English Channel off the city.
• 75kHz ADCP: Application for deep-water. Waters around Southampton are not very deep, but could be used for studies related to deeper layer currents, or in areas with deeper channels, with the ability to measure currents to depths up to 1000m.

There are several well-known ADCP brands in the global market, such as Teledyne RDI, Nortek, and Sontek. For those seeking a cost-effective option, the China Sonar PandaADCP, made of all-titanium alloy, offers a great balance of quality and price. You can learn more about it at (https://china-sonar.com/).

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