How to measure the coastal currents of Sibenik

1. Where is Sibenik?

Sibenik is a jewel situated along the Croatian Adriatic, a place where natural beauty merges with historic attraction. Its geographic location establishes it right at the heart of the Dalmatian region, embraced by the crystal-clear waters of the Adriatic Sea. This has made it an important port city throughout history, with its coastline acting as a crossroads for trade, culture, and maritime exploration.

Sibenik City is a witness that it was inhabited continuously since the ancient times. Architecture here is the harmony of styles: Venetian, Romanesque, and Gothic; among them stands out as a breathtakingly beautiful Cathedral of St. James enlisted in UNESCO heritage list. Because of the rich maritime history, locals have established a very unique kind of culture. The fishing and seafaring traditions have survived through generations; most of the city's festivities celebrate the plenty that comes out of the sea.

Regarding the waters around Sibenik, it is hedged by an intricate maze of small islands, coves, and bays. The very famous Kornati Islands are almost at hand-its dramatic limestones rise straight from the sea and varied marine life. These islands not only add to the additional scenic beauty but also play their role in influencing the local coastal hydrodynamics. The channels between the islands create complex waterways where currents can be affected by the constriction and divergence of the flow.

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

The coastal currents near Sibenik are the result of a complex interplay of several factors. The most general factor that comes into play will be the predominant winds of the Adriatic. During the summer months, with the maestral-a north-westerly wind-surface waters can get pushed towards the coast to develop a strong onshore current, while the cold, dry northeasterlies, called the bora, may force water offshore, turning the currents outwards in its direction and slowing their speeds down.

This variability is also strongly influenced by the bathymetry of the area. Complicated bottom topography with various depth and seabed features like reefs and channels causes acceleration, deceleration, and changes of current directions. River discharges, though very small in the vicinity of Šibenik, may still input freshwater into the coastal zone. This freshwater input causes variations in seawater density, and it may also further contribute to the creation of density-driven currents interacting with larger-scale circulation patterns in the Adriatic.

Besides, general circulation in the Mediterranean Sea to which the Adriatic belongs influences the local coastal currents. General cyclonic circulation in the Adriatic Sea may bring waters from different regions and alter the temperature, salinity, and flow characteristics of the coastal currents around Sibenik.

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

Surface Drifting Buoy Method

One of the most straightforward methods to investigate coastal water circulation is with the help of surface drifting buoys. These buoys, fitted with tracking devices such as GPS, are sent into the water and, while being moved by the currents, are followed over a period of time. By recording the path followed by these buoys, scientists can estimate the velocity and direction of the surface currents. Still, this method has several disadvantages. Surface floats are considerably at the mercy of the wind; therefore, they tend to drift off from the track the current being measured is following and give a false indication of measurement.

Anchored Ship Method

An anchored ship can provide a stable platform for the measurement of current. Current meters and similar instruments are suspended from the vessel at different depths. This technique has the advantage of collecting data for a long time at a fixed location. In fact, with this technique very good details can be obtained of the vertical profile of the currents at that given point. On the other hand, data remains confined to where the ship happens to be located, and, therefore, acquiring an overview of the whole coastal current system might be hard.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP flow meter has become a far superior and much handier tool for the measurement of coastal currents. It samples the water instantaneous velocity at simultaneously received depths and can yield a very fine vertical profile of the current. Various types of ADCP deployments are there, ranging from ships to being moored at the sea floor and deployed attached to buoys. It is much less affected by the surface disturbance caused by factors like wind; therefore, this is a very reliable technique to measure currents.

4. How does the ADCP using the principle of the Doppler work?

The principle of operation of ADCPs is based on the Doppler effect. When an ADCP current profiler sends a beam of sound waves into the water, they scatter off small particles in the water, like plankton, sediment, or even bubbles. The frequency of the returned sound waves to the ADCP after scattering by the aforementioned particles is different from the frequency of the emitted waves. This frequency shift (Doppler shift) varies linearly with the velocity of the scatterers, and thus the water, relative to the ADCP.

Most ADCPs are fitted with more than one, normally four or more, transducer beams aligned in such a way that they allow the measurement of the components of velocity in a number of directions. The ADCP current meter can use these beam measurements to calculate three-dimensional water velocity. The fact that this is possible means there is a real ability to derive a detailed knowledge of current structure, including horizontal and vertical flow components.

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

Equipment Material Reliability

For high-quality measurements in the harsh marine environment of Sibenik's coastal waters, the ADCP's casing material is of utmost importance. Titanium alloy would be an ideal choice for the casing. It has excellent corrosion resistance, and this is just what the equipment needs since it will be constantly exposed to seawater. It must also be resistant to saltwater and should not degrade considerably in an extremely short time. Furthermore, the strength to weight must be high. This simply means that at very high currents or in rough sea conditions, an ADCP is able to retain its structure.

Size, Weight and Power Consumption

The size and weight of an ADCP meter must be compact. A compact design allows it to be easily deployed on either a small research vessel, buoy, or seabed mooring. It also minimizes interference with the flow field that it's trying to measure. Low power consumption is crucial because many deployments are long in duration. This is important for extended operation cycles without needing to return frequently for battery exchange or external power supplies, which increases total time to keep the equipment collecting data continuously.

Cost-effectiveness

The ADCP profiler would be required to be pretty low in cost to enable large-scale measurements for wide application in monitoring coastal currents in Šibenik. In respect to this, the cost should be in a range that is within the reach of research institutions, environmental monitoring agencies, and possibly even local communities. Cost-effectiveness will ensure more comprehensive data collection, which is required by the complex dynamics of coastal currents.

6. Selection of Equipment for the Measurement of Currents

According to the Purpose of Use

• Shipborne ADCP: This type is mounted on a moving ship. It is ideal for obtaining a broad - scale view of the coastal currents over a large area. Shipborne ADCPs can cover a wide range of locations in a rather short period and are thus suited for initial surveys or studies concerning the spatial variability of the currents.
• Bottom-Mounted ADCP: These are placed on the seabed; bottom - mounted ADCPs are used for long-term, fixed - point monitoring. They can provide continuous data on the currents at a specific depth and location, which is valuable for understanding the local hydrodynamics and for long-term trend analysis.
• Buoyant ADCP: Buoyant ADCPs are attached to a floating buoy and thus are able to move with the surface currents. They are useful in monitoring the surface and near - surface current patterns as well as tracking the movement of water masses over time.

Based on Water Depth

• On water to a depth of 70m, the appropriate choice is an ADCP of 600kHz. The higher frequency gives considerably better resolution over the shallower waters and produces highly resolved data on the current structure.
• In water of approximately 110m, a 300kHz ADCP would be more suitable. This provides an ideal balance between the range and resolution of measurement, therefore, making the instrument highly applicable in mid-depth coastal waters.
• A 75kHz ADCP will enable the surveyor to work in water up to 1000m deep. This lower frequency will penetrate deeper through the water column where resolution is perhaps lower than with higher frequency ADCPs.

There are only a few famous brands of ADCPs on the market such as Teledyne RDI, Nortek and Sontek. However, for those seeking a cost-effective yet high-quality option, the China Sonar PandaADCP is an excellent alternative. Made entirely of titanium alloy, it offers outstanding durability and performance at an affordable price. It is an economic-class ADCP that provides great value for money. You can find more information about it on their official website: (https://china-sonar.com/).

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