How can we measure the coastal currents of Portsmouth?

1. Where is Portsmouth?

Portsmouth is a historic port city in the county of Hampshire, along England's southern coastline. It lies in the Portsea Island surrounded by waters of the Solent and the English Channel, which makes it an extremely significant center for naval operations, maritime trade, and tourism over centuries.

History goes back to Roman times, and it has played an important role in British naval history. It houses the Portsmouth Historic Dockyard, home to such iconic ships as HMS Victory, which served as the flagship of Admiral Nelson at the Battle of Trafalgar. Architecture in Portsmouth blends historical charm with modern functionality. These are well-preserved Georgian and Victorian buildings, particularly around the dockyard area, standing alongside more contemporary structures built to accommodate its ever-growing population and modern needs. The locals are deeply attached to the sea. Naval activities have become such an integral part of the city that the presence of the Royal Navy is immense. Yet there is also a small and dynamic fishing community, and the port is served by everything from fishing boats to large naval vessels and ferries. The adjacent waters of the Solent and the English Channel form a dynamic ecosystem, supporting a host of seal, dolphin, and fish species such as cod, herring, and mackerel. The topography of the seabed is very complex, with sandbanks, rocky outcrops, and subsea channels influencing the movement of water.

2. What is the situation of the coastal currents near Portsmouth?

Coastal currents near Portsmouth form under a very complex interplay of multiple contributing factors. The tidal forces form one of the dominant factors. The Solent experiences semi - diurnal tides, with two high tides and two low tides each day. However, the tidal range can be extensive, and, in some spots, the outcome of tidal streams can attain velocities of 3 - 4 knots, in particular in narrow channels between the land and either Portsea Island or the Isle of Wight. At high tide, a rush of water enters the Solent, heavy with nutrient, sediment, or other materials moved along by the tides within the English Channel. At low tide, the water recedes to expose its intertidal zones.

The second major factor is the wind. The prevailing south-westerly winds can push the surface waters towards the coast, enhancing the tidal currents. These winds also generate waves which interact with the currents to produce a complex pattern of flow. Northerly winds can drive the water away from the shore. The shape of the coastline, with its bays, inlets, and the shape of Portsea Island, means the currents converge and diverge. This is further complicated by underwater features such as sandbanks and rocky outcrops, which provide barriers or channels to the flow, causing turbulence and areas of calm. Added to that, the local river freshwater discharge, even though it is small with respect to the open sea, still produces a local effect on the currents, as well. Because of the different densities between fresh and saltwater, a density-driven current develops that together with the tidal and wind-induced currents interact.

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

Surface Drifting Buoy Method

The surface drifting buoys are one of the simple and efficient ways to observe surface-level currents. These buoys carry either GPS or satellite-based tracking systems. After deployment in water, they are carried by the surface currents. By tracing their movement over a certain period of time, the scientists can make an approximation of the speed and direction of the surface waters by analyzing the trajectory of the buoy. However, this method is restricted to the upper few meters of the water column and may not be representative of the deeper currents.

Moored Ship Method

In the moored ship method, a ship is anchored at a location near the coast of Portsmouth. Current-measuring instruments, like electromagnetic current meters, are deployed. These instruments can measure the flow of water at different depths, providing a detailed vertical profile of the current velocity and direction. However, this method is limited to a single location, and the presence of the ship may disturb the natural flow of water, which could affect the accuracy of the measurements.

Acoustic Doppler Current Profiler (ADCP) Method

ADCPs have transformed the measurement of coastal currents around Portsmouth. They are capable of measuring the velocity profile of the entire water column from surface to seabed. These ADCPs use sound and operate non-intrusively while measuring the flow of water. They work by transmitting acoustic pulses and then finding the Doppler shift of the reflected signals from suspended particles in the water, which can be used to estimate the current velocity at different depths. Thus, this is very well adapted for the current research into the complex coastal current patterns in the Solent and the English Channel off Portsmouth.

4. How do ADCPs applying the Doppler principle work?

The ADCPs apply the Doppler effect. They send out high-frequency acoustic pulses into the water. When these pulses hit small particles in the water, like sediment, plankton, or even bubbles, those particles reflect the acoustic signals back to the ADCP profiler. The particles moving with the water current will result in a shift in frequency of the reflected signal relative to the transmitted signal. The Doppler shift is directly proportional to the velocity of the particles relative to the ADCP. Measuring the Doppler shift at various depths of the water column allows the ADCP meter to pre-calculate the velocity of that depth. More than one number of transducers measures the ADCP's velocities in various directions, enabling determinations of three-dimensional velocity vector water flow.

5. What will be required in order to do a good quality measurement of currents around Portsmouth coasts?

Equipment Material Reliability

Given that it is to be used in open water in the Solent and the English Channel, the quality of the construction materials is key in ensuring that the instrument is reliable for measurement. The ADCP flow meter housing should be made from marine-quality materials that are resistant to seawater. Titanium alloy would be good. Its high strength resists high-impact water currents, and any collision that may occur by floating debris as well as from the corrosiveness of seawater. Its low elastic modulus is flexible, decreasing the possibility of mechanical damage resulting from mechanical tension. Besides that, its superior corrosion resistance properties ensure that even in seawater, the ADCP can also be deployed in it for longtime monitoring without the degradation of material properties. -

Compact Size, Low Weight, Low Power Consumption, and Low Price

A compact, lightweight ADCP current profiler is easier to install and operate, whether it is on a small research vessel, attached to a buoy, or placed on the seabed. It interacts less with natural water flow and will therefore result in more accurate measurements. Low power consumption allows for long-term autonomous monitoring; in the case of battery-powered systems, it's especially so. It will be capable of large-scale deployment due to its low cost, which would enable capturing the complex coastal current patterns around Portsmouth.

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

Based on Usage

• Shipborne ADCP: Will be used to derive real-time current data along a ship's route near the proximity of Portsmouth. It can be used for oceanographic surveys, general circulation in the area, and for optimizing shipping routes.
• Bottom-mounted ADCP: Suitable for long-term fixed-point monitoring of the currents on the seabed. This, in turn is useful for study of the trends in the current over long times, for instance, changes wrought by climate change on the regional marine ecosystem.
• Buoy-mounted ADCP: These are very useful for surface level current measurement with a large spread as the buoy drifts along with the water. This aids in the identification of the surface currents' spatial variability and also how the water bodies in the coastal areas interact with the open English Channel.

Based on Water Depth

• 600kHz ADCP: Suitable for water depths up to about 70m, high-resolution measurements may be derived in the generally shallow waters off Portsmouth.
• 300kHz ADCP: Suitable for water depths of approximately 110m, more usable for some areas of the Solent or English Channel close to the city that are deeper.
• 75kHz ADCP: For deep-water applications. While waters around Portsmouth are not usually too deep, it can be applied in studies dealing with the deeper-layer currents or for areas with deeper channels, capable of measuring currents at depths of up to 1000m.

There are some well-known brands of ADCP in the market, like Teledyne RDI, Nortek, and Sontek. If you want a cheaper one, China Sonar PandaADCP is a very good balance between quality and price, made of all-titanium alloy. You can learn more about it at [https://china-sonar.com/].

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