How can we measure the coastal currents of Dover

1. Where is Dover?

Dover is a historic port along the southeastern coast of England in the county of Kent. It occupies a strategic position on the Strait of Dover, separating the United Kingdom from continental Europe. This narrow waterway-actually the busiest shipping lane in the world-makes Dover an important focal point of international trade, ferry services, and maritime traffic.

The City of Dover can boast of two thousand years of history, filled with important events. The well-known Dover Castle stands on white cliffs, telling about its huge military-strategic importance for such a long time. It faced sieges and invasions and had its tunnels in use even during World War II. Naturally, the whole architecture of this town reflects this rich past-older buildings contrast with medieval ones, Georgian ones, and some Victorian-era constructions. The sea has linked the local community by a deep-rooted connection, both in a historical and cultural aspect. Fishing was once the most dominant industry, but even though it has declined, the town still supports a small but active fishing fleet. The port of Dover remains one of the busiest centers, moving a heavy flow of cross-channel ferries, cargo ships, and cruise vessels. The waters around the Strait of Dover are dynamic, with an area of high ecological importance. Marine life is in a range from seahorses and dolphins to different fish species. The seabed topography, formed of chalk cliffs, caves, and sandbanks, influences the water movements and the distribution of marine organisms.

2. What is the situation with the coastal currents near Dover?

In the coasts near Dover, the currents depend on the inter-relationship of a number of factors. Tidal forces exercise a dominant influence in this area. There are large-scale tidal variations in the Strait of Dover. In a typical day, the tides are semi-diurnal with two high tides and two low tides. Simultaneously, this intense tidal flow may exhibit 5 - 6 knots somewhere, mainly at the narrowest part of the strait. During high tide, water flows into the strait and goes back at low tide, developing an extraordinary flood and ebb.

The second most important element is the wind. Prevailing south - westerly winds are capable of pushing surface waters toward the shore and, hence, increase tidal currents. These winds can also produce huge waves interacting with the currents to form a very complex flow pattern. On the other hand, northerly winds have the opposite effect, driving the water away from the shore. Converging and diverging currents are also induced by the coastline shape with cliffs, bays, and headlands. Current patterns are further complicated by underwater sandbanks and rock formations. These features are barriers or conduits that result in the generation of turbulence areas and areas without turbulence. It is also highly influenced by fresh inflow into the North Sea and the English Channel, in which different types of water and salinity profiles may cause local density-driven currents interacting with tide- and wind-induced currents.

3. How to Observe the Coastal Water Flow of Dover?

Surface Drifting Buoy Method

Surface drifting buoys are a simple and efficient way to observe surface-level currents. These buoys have either GPS or satellite-based tracking systems. Once they enter the water, they are carried by the surface currents. By following the trajectory of the buoy, their speed and direction for the surface waters can be inferred with time. However, this method is only applicable to the upper few metres of the water column and probably does not represent the deeper currents.

Moored Ship Method

The moored ship method involves anchoring a ship over the bottom topography off the coast of Dover. Then, by using electromagnetic current meter as common metering instruments, they could be deployed for the measurements of flow at individual depths that offer a very fine vertical profile in the current speed and direction. However, measurements are only provided at a single location and often the ship would interfere with natural flow conditions. Thus, such interference can likely affect the measuring process.

Acoustic Doppler Current Profiler (ADCP) Method

These ADCPs have really transformed the manner in which the currents near Dover are measured. In principle, an ADCP is able to measure the velocity profile of the entire water column, from surface to seabed. Basically, ADCPs measure water flow non-intrusively by utilizing sound waves. They can estimate current velocity at different depths by transmitting acoustic pulses and measuring the Doppler shift of the reflected signals from suspended particles in the water. This provides a full-scale view of the existing structure and thus is highly suitable for studying the complicated coastal current patterns in the Strait of Dover.

4. How do ADCPs using the principle of the Doppler work?

The ADCPs work on the principle of the Doppler effect. They send high-frequency acoustic pulses into the water. When these pulses hit small particles in the water, such as sediment, plankton, or bubbles, these particles reflect the acoustic signals back to the ADCP. Assuming that the particles are moving with the water current, there will be a frequency shift of the reflected signal with respect to the transmitted signal because of the Doppler effect proportional to the velocity of the particle relative to the ADCP. By monitoring the Doppler shift at different water column depths, it can compute the velocity of the water at those depths. Multiple transducers on an ADCP are used to measure the components in different directions for the same velocity, which allows the determination of the three-dimensional velocity vector of the water flow.

5. What is required for high-quality measurement of Dover coastal currents?

Equipment Material Reliability

The reliability of equipment materials is of great importance to obtain a good measurement of coastal currents in this area of Dover. Regarding ADCP, casing should be in material resistant to the adverse environment of Strait Dover with its violent tidal stream and high salinity, also bearing in mind a possible influence from shipping. This would recommend an excellent titanium alloy. The high strength imparts resistance to strong water currents, possible collision with floating debris, and the salinity of seawater. Low elastic modulus imparts flexibility to the material, which minimizes the chances of damage due to mechanical stress. Besides that, its excellent corrosion resistance enables the ADCP to serve in seawater for long-term monitoring without severe degradation.

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

A compact, lightweight ADCP is easier to install and operate whether on a small research vessel, attached to a buoy, or placed on the seabed. Because of its lower size, it interacts less with natural water flow, thus offering more precise measurements. Low power consumption becomes a must during long-term autonomous monitoring, which relies on battery-powered systems. The low-cost ADCP enables the large-scale deployment, which is a necessary requirement for the comprehensive understanding of the complex coastal current patterns around Dover.

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

Based on Usage

• Shipborne ADCP: Suitable for real-time current data along the route of the ship across the Strait of Dover, oceanographic surveys, studies of the general circulation in the area, and optimization of shipping routes.
• Bottom-mounted ADCP: Enables long-term, fixed-point monitoring of the currents from the seabed. This allows the study of the long-term trends in the currents along the coast; for example, due to changes in the local marine ecosystem because of climate change.
• Buoy-mounted ADCP: Suitable for the monitoring of surface-level currents over a wide area while the buoy drifts with the water. It helps in understanding the spatial variability of the surface currents and how the coastal waters interact with the open English Channel and North Sea.

Based on Water Depth

• 600kHz ADCP: Suitable for water depths up to about 70m, giving high-resolution measurements in the relatively shallow waters near Dover.
• 300kHz ADCP: Suitable for water depths around 110m, which may be useful in areas with a little deeper sections in the strait.
• 75kHz ADCP: For deep-water applications. Although the Strait of Dover is not that deep in most parts, this can be used for studies related to the deeper layer currents or for areas with deeper channels, and are capable of measuring currents to 1000m.

There are some famous brands for ADCPs in the market, such as Teledyne RDI, Nortek, and Sontek. In recent years, China Sonar PandaADCP, made from all-titanium alloy, has a very good balance between price and quality. You can learn more about it at(https://china-sonar.com/).

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