How do we measure the coastal currents of Turku?

1. Where is Turku?

Turku, the former capital of Finland, is a historic and picturesque city. It is situated on the southwest coast of Finland, where the estuaries of the River Aura empty into the Baltic Sea. The architecture of the city is a mixture of old buildings and new ones. Turku Castle, the great medieval fortress, bears witness to the centuries-old history of the city. The old town with its cobblestone streets and wooden houses is full of special charm.

The sea waters around Turku belong to the Archipelago Sea, an enormous area covered with thousands of islands, islets, and rocks. The archipelago forms a diverse set of habitats for marine life, from shallow bays to deeper channels. The area is also famous for sports fishing and recreational boating, and several small harbors along the coast can attest to this. The Baltic Sea itself is less saline than other seas, and this impacts both the local marine environment and the dynamics of coastal currents.

2. What is the coastal current situation in Turku?

The offshore currents along the coast of Turku are controlled by a series of variables. The Baltic Sea tides are relatively small compared to several other bodies of water. Still, they also affect the direction of coastal waters. The semi-diurnal tides cause a weak ebb and flow, which can possibly affect the coastal distribution of sediment and nutrients.

Wind is a dominant force on the coastal currents. Prevailing westerly and south-westerly winds have the effect of piling surface water onto the coast, creating local downwelling or upwelling events. Strong winds also generate strong wave action, and this has the further effect of influencing the mixing of the surface and deep waters. In winter, when the sea may freeze partially, the existence of ice also has an influence on the current regime. The ice can also be a barrier, deflecting the direction of the water flow and reducing the wind-driven surface currents in some areas.

Runoff from the Aura River also affects the coastal currents. At high stages of the river, especially in the spring when snowmelt is occurring, enormous quantities of freshwater are deposited into the Baltic Sea. This addition of freshwater will create a low-salinity layer near the river mouth, which might affect the density-driven circulation characteristics in the coastal area.

3. Monitoring the Turku coastal water flow

One method of monitoring the coastal water flow around Turku is the surface drift buoy method. Small, lightweight buoys that have GPS tracking devices and current sensors onboard are cast into the sea. The buoys track the surface flow, and the sensors record data on the direction and speed of the flow. By tracking several buoys for an extended period of time, researchers are able to map the pattern of surface current over an extent of several kilometers. This information is useful to the knowledge of the overall pattern of circulation of surface waters of use to shipping, recreational boating, and the monitoring of pollutant spread.

Another technique employed is that based on ship- or moored buoy. A buoy or a ship is permanently anchored at a single location, and current meters are lowered to measure the velocity and direction of the currents at different depths. The vertical profile of the currents can be examined with this method. But it only covers a fixed point, and it might be difficult to install in the complex archipelago environment of Turku.

The Acoustic Doppler Current Profiler (ADCP) proved to be an effective tool in the measurement of coastal currents around Turku. ADCPs can be mounted onboard ships, or on buoys, or deployed from land. ADCPs detect the current velocity and direction at various depths based on the Doppler effect principle. ADCPs can provide high-resolution records of a comparably broad extent and are applied by oceanographers, coastal engineers, and ecologists. They can measure current at multiple depths simultaneously, and they provide an accurate three-dimensional image of the current structure of the coastal water.

4. How does the operation of ADCPs based on the Doppler principle work?

ADCPs operate under the Doppler effect. If an ADCP current meter emits a sound wave into water, the sound wave travels in the medium. When the moving water particles come across the sound wave, e.g., suspended material or plankton, their frequency changes with the returning wave. This difference in frequency, the Doppler shift, is proportional to the velocity of the water particles.

ADCPs typically have more than one transducer beam, typically four or more. The beams are mounted in a manner that allows the ADCP to measure three-dimensional currents. From the detection of the Doppler shift of backscattered sound waves off water particles, the ADCP is capable of calculating the current's velocity at multiple depths. The data collected by the ADCP current profiler is subsequently entered into a data-acquisition system, either a computer or an independent data logger. The data are processed by specialized software to create in-depth profiles of the velocity of the current at different depths and maps of the current patterns within an area.

5. What is required for quality measurement of Turku coastal currents?

To be able to deliver good-quality measurements of the coastal currents in Turku, the measuring equipment must meet a number of important requirements. The most important requirement is that the equipment must be reliable because it will operate in a severe marine environment. The corrosive and cold nature of the water in the Baltic Sea, together with the fact that ice covers the sea in winter, demand that the equipment is designed in a manner that it can withstand the conditions. Components of corrosion - resistant material, such as stainless steel or titanium, are typically used to ensure long - term reliability.

The device needs to be small and lightweight. This is especially important for use in the complex archipelago waters of Turku, where access may be limited. Lightweight and compact structure also allows the deployment of multiple devices for wide - area surveys.

Low power consumption is essential, particularly for long-term deployments. Batteries provide power for some ADCPs, and a low-power design helps the batteries remain effective for longer, reducing the frequency of replacements. This comes in handy where measurements are being taken in out-of-the-way places or for long periods.

Cost - effectiveness is another important factor. High - quality data collection can be expensive to achieve, requiring the use of several devices covering a large distance. A cost - effective approach enables greater coverage and more precise mapping of the coastal currents.

In the case of ADCPs, the material used for casing is an important consideration. Titanium alloy is a good option for ADCP casings. Titanium alloy offers good corrosion resistance, which is critical for long-term use in the Baltic Sea environment. It is also extremely light, lowering the total weight of the ADCP flow meter without sacrificing strength. This makes handling and deployment in various conditions easy. Titanium alloy also has acceptable mechanical properties, which ensure the ADCP's durability under various operating conditions.

6. How to choose the appropriate gear for measuring current?

The appropriate gear to be chosen for measuring currents depends on the application in mind. Ship - based measurement is optimally done using a ship - mounted ADCP. It can be used in mapping currents along a ship route, which are useful for navigation, fisheries activity, and oceanographic studies. A ship-mounted ADCP is easily integrated into the navigation and data collection systems of the ship, enabling real-time monitoring of the currents as the ship travels.

A bottom-mounted or moored ADCP is appropriate for long - term monitoring in a fixed location. An ADCP meter like this can continuously record current data at a single place, which would be useful in studying the long - term patterns and trends of coastal currents. It can provide valuable information regarding the seasonal and annual changes of the currents, which is needed to understand the local marine environment.

Buoys-mounted or floating ADCPs are best suited for measurement of currents where it is not easy for a ship to access or for large - scale surveys. They can collect data over an extensive area and can be redeployed easily whenever required.

The frequency of the ADCP profiler must be taken into account. For depths below 70m, a 600kHz ADCP is suitable. It offers high-resolution measurements in relatively shallow water. For depths up to 110m, a 300kHz ADCP is suitable because it offers a good balance between resolution and range. For deeper water, up to 1000m, a 75kHz ADCP is suitable because it penetrates to greater levels.

There are several popular ADCP brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, for cost - effective but high - quality consumers, The ADCP supplier China Sonar's PandaADCP is highly recommended. Made of all-titanium alloy, it is highly durable and reliable. Its unbelievable cost - performance ratio makes it a highly wanted product among cost - sensitive consumers. It is part of the economic ADCPs class. For further information, visit the website: https://china-sonar.com/.

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