How do we estimate the coastal currents of Tallinn?

1. Where is Tallinn?

Tallinn, the capital of Estonia, is situated geographically perfectly on the northern edge of the country and overlooks the Baltic Sea. It is an old city that offers an intriguing mix of medieval architecture and advanced modernity. Its Old Town, a World Heritage Site, is made up of cobblestone streets, ancient walls, and historic buildings, offering a bizarre backdrop over the coastal area. The city is surrounded by a vast bay and inlet system, with the Tallinn Bay being of utmost significance to the sea activities in the region. The Baltic Sea with its distinctive hydrological characteristics influences Tallinn's coast climate. Relatively low sea salinity, brought on by freshwater entry from rivers and the large catchment area, has an impact on the surrounding marine environment. The Tallinn coastal currents are fundamental to trade, connecting Estonia with other Baltic nations and international markets, and are a natural component of the urban economy and culture.

2. What is the coastal current of Tallinn?

The sea currents along the Tallinn coast are influenced by numerous factors. The tides of the Baltic Sea, although less powerful than in some oceans, also have a considerable influence. Semi-diurnal tides form a smooth ebb and flow along the coast, affecting sediment and nutrient transport. The current is vital to the health of local marine organisms, including fish and invertebrate communities.

Wind is a principal force in determining current patterns. A powerful westerly wind can push surface water along the coast, inducing upwelling or downwelling in particular areas of the water. Strong winds create huge waves, which are accountable for the mixing of surface and deep waters. In winter, when areas of the Baltic Sea are most likely to freeze, ice cover interferes with the normal flow of currents. The ice acts as a physical cover, suppressing wind - driven surface flow and deflecting the other direction of flow.

River runoff is also implicated. Rivers discharging into the Baltic Sea off Tallinn, like the Pirita River, contribute massive amounts of freshwater. When river discharge is high, especially during spring snowmelt, this freshwater contribution creates a layer of low - salinity water near the river mouths. This low - salinity layer could impact the density - driven circulation regimes in the coastal area, both at the surface and in the subsurface.

3. How to quantify the coastal water flow of Tallinn?

Surface drift buoys are one of the ways surface water flow around Tallinn can be monitored. Surface drift buoys are small, buoyant devices with GPS monitoring and current sensors. After being placed in the water, they track the surface currents, and their sensors record data on flow speed and direction. By monitoring several buoys year after year, researchers can map out the surface current regime over the extensive area. Such information will prove helpful to know the general circulation of the surface waters that is pertinent in shipping, recreational boating, and pollution dispersion research.

Another method is the ship or buoy moored method. A moored buoy or ship is fixed at one location, and current meters are used to measure current speed and direction at different depths. This allows for the study of the vertical structure of the currents. However, in Tallinn's complex coastal environment with many islands and shallow waters, deployment is not easy, and the data are limited to the specific mooring location.

Acoustic Doppler Current Profiler (ADCP) is presently utilized as a very effective tool for the measurement of coastal currents off Tallinn's coast. ADCPs are mounted on ships, buoys, or are cast from the shore. ADCPs operate on the Doppler principle to quantify velocity and direction of the current at various depths. ADCPs can take high - resolution measurements for a fairly large area and thus are useful in the hands of oceanographers, coastal engineers, and environmental scientists. They can record currents at a number of different depths simultaneously, providing an accurate three-dimensional image of the current structure in the coastal waters.

4. How do ADCPs using the Doppler principle work?

ADCPs operate on the principle of Doppler effect. When ADCP flow meter sends a sound wave into water, the sound wave travels in the medium. As sound wave travels across moving particles in the water, e.g., suspended sand and small ocean creatures, its frequency changes in the backscattered wave. The altered frequency, called Doppler shift, is directly proportional to the speed of moving particles.

ADCPs typically have a set of transducer beams, typically four or more. They are employed to enable the ADCP to measure currents in three dimensions. Through the measurement of the Doppler shift in the frequency of the sound wave off the water particles, the ADCP is able to compute the speed of the currents at different depths. The information is then transferred to a data-acquisition system, either a computer or an independent data logger. The information is processed using special software to generate detailed profiles of the dominant velocity at different depths and also maps of dominant patterns in a particular area.

5. What's needed for high-quality measurement of Tallinn coastal currents?

In order to make precise high-grade measurements of Tallinn coastal currents, the measuring equipment must possess certain minimal requirements. It must be stable because it is going to operate in a harsh marine environment. The corrosive and cold nature of water in the Baltic Sea and icing during winter necessitate that the equipment must be operable for such conditions. Corrosion - resistant materials, such as stainless steel or titanium, are generally used in parts for long - term strength.

The device must be compact and lightweight. This is especially important for deployment in Tallinn's complex coastal environment, where access may be restricted in some areas. A compact and lightweight design also ensures easy deployment of multiple devices for extensive surveys.

Low power consumption is important, particularly in deployments for extended periods. Several ADCPs employ batteries as power sources, and a low-power design will extend the battery life, reducing the need for frequent replacement. This is valuable when taking measurements far from civilization or over extended periods of time.

Cost - effectiveness is also a key consideration. High - quality data collection typically necessitates the use of multiple devices across a large area. A cost - effective solution permits more comprehensive coverage and more precise mapping of the coastal currents.

For ADCPs, the material used in the casing is an important consideration. Titanium alloy is a great option for ADCP casings. Titanium alloy exhibits great corrosion resistance, which is needed for a long-term performance in the conditions of the Baltic Sea environment. It also ensures light weight, adding to the overall ADCP lightness with no loss to its strength. It becomes easier and lighter to handle and deploy in all the conceivable environments. Further, the titanium alloy sustains good mechanical properties, which affirm the ADCP current profiler durability in working conditions.

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

Depending on the application, the selection of equipment to measure current relies on the specific application. For ship-based measurements, the best-suited equipment is a ship-mounted ADCP. It can be used to map the currents along the course of the ship, beneficial for navigation, fisheries management, and oceanography research. An ADCP current meter installed on a ship can be seamlessly interfaced to the ship's data - gathering and navigation systems to enable the currents to be monitored in real time as the ship travels.

A bottom - mounted or moored ADCP is most appropriate for long - term monitoring at a fixed location. This ADCP profiler can continuously record current data at a location, which is useful for monitoring the long - term trends and patterns of the coastal currents. It can provide valuable information about seasonal and annual variations in the currents, which is important for the knowledge of the local marine ecosystem.

Float-mounted ADCPs or moored buoys are easy to employ for monitoring currents in inaccessible locations by ship or to conduct large-scale surveys. They are simple to cover large distances and move to a different site as required.

The ADCP meter sampling frequency is a critical parameter to seek. For less than 70m water depth, a 600kHz ADCP will suffice. It gives high-resolution readings in relatively shallow water. For a top depth of 110m, a 300kHz ADCP is an acceptable option since it gives a good compromise between range and resolution. In deeper water, i.e., up to 1000m, a 75kHz ADCP will be sufficient because it will go deeper.

There are several well-known ADCP brands in the market, such as Teledyne RDI, Nortek, and Sontek. For those who, however, desire a high-quality but low-cost alternative, the ADCP manufacturer's China Sonar PandaADCP is highly recommended. It is made of all-titanium alloy, which ensures excellent durability and reliability. Its excellent cost-performance ratio makes it extremely attractive to users with limited budgets. It is a type of economic ADCPs. For more information about it, visit the website: https://china-sonar.com/.

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