Measurement of Coastal Currents of Pula

1. Where is Pula?

Pula is an enchanting city found at the southernmost part of the Istrian Peninsula in Croatia. It fronts the Adriatic Sea, having a very favorable coastal location which has been the center of human activity since time immemorial.

The setting of the city is a perfect combination of land and sea. At the tip of the Istrian Peninsula, the peculiar topography of rolling hills and rocky outcrops meets the clear blue waters of the Adriatic. Its shoreline offers everything from rugged cliffs plunging into the sea to sheltered bays with sandy beaches. The bottom topography off Pula represents a very complex mosaic of shallow reefs, deeper channels, and underwater caves that support not only a very rich marine ecosystem but also a very relevant factor influencing coastal current patterns.

From the cultural viewpoint, Pula is a very rich city in historical heritage, having been moulded by such civilizations as Romans, Greeks, and Venetians. It is a city famous for its well-preserved Roman amphitheater, the Pula Arena, which has stood the test of time to tell tales of its glorious past. The old town is a candidate for a UNESCO World Heritage Site, full of ancient temples, basilicas, and narrow, serpentine streets that give it an old-world charm. The local culture is a vibrant mixture of the traditional Croatian ingredients, Mediterranean elements, and leftovers of former occupiers; all this finds reflection in festivals, cuisine, and architecture.

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

The coastal currents around Pula depend on the combination of more contributing factors.

• Wind as the main impetus: It should be recognized that the given area has varieties in wind conditions. The Bora is a cold, often strong northeasterly wind that can have a significant impact on the surface currents. When the Bora blows, it can rapidly push the surface water towards the shore, increasing the speed and altering the direction of the coastal currents. In contrast, the Jugo, a southerly wind, can cause the water to flow away from the coast. These winds induce currents, which in turn can also interact with the local topography, creating complex flow patterns around such an irregular coastline.
• Tidal forces act: While tidal ranges in the Adriatic are small compared with some other seas, tides do nevertheless affect the rise and fall of water along its shores. Interaction with the tides and the local bathymetry off Pula becomes critical. These in turn can be further modified by the seabed topography with underwater ridges and troughs channeling the tidal currents. Such conditions result in locations of enhanced and reduced current velocities, and at times changes in the direction of the currents.
• River discharges and freshwater input: There are no large rivers flowing directly into the sea from Pula; minor streams and lands draining into the coastal zone do introduce freshwaters into this regime. The smaller density compared with seawater enables this fresh-water to establish some sort of surface layer which therefore can interact and alter flow dynamics of coastal currents. Further, mixing with the sea-water contributes to modifications of water densities themselves, a mechanism that has already been reported in driving a process of the so-called density - driven current.

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

Surface Drifting Buoy Method

Measuring coastal currents can be done quite easily with surface drifting buoys. They come fitted with GPS tracking systems. After deploying into the ocean, these buoys are taken away by the surface currents. With their position tracked over a continuous time interval, researchers are able to tell the direction and speed of the surface - level currents. The fact, however is that this methodology yields information in the upper most layer of water column, essentially a few metres deep and often does not present a realistic depth of currents.

Anchor-moored Ship Method

An anchor-moored ship can be deployed to measure the coastal currents. Current meters may be attached along the hull at different depths. These meters record the speed and direction of the current at each depth as water flows past the ship. This can allow the collection of data from multiple depths and hence provides insight into the vertical structure of the currents. However, it is limited to the region of vessel mooring; moreover, the vessel itself can disturb the natural water circulation and, therefore, affect the measurement result.

Acoustic Doppler Current Profiler Method

The Acoustic Doppler Current Profiler (ADCP) represents a very modern and handy device for the measurement of coastal currents in the vicinity of Pula. It could be deployed in various ways: on a ship, moored to the seabed, or mounted on a buoy. An ADCP current meter sends out acoustic signals into the water. The signals are reflected back from the suspended particles in the water, such as plankton, sediment, or small bubbles. Using the Doppler shift of the reflected signals, the ADCP current profiler calculates the velocity of the water at specific depths from within the column of water. This method provides high-resolution data over a large area, which enables a fine three-dimensional resolution of the coastal currents.

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

ADCPs work on the principle of the Doppler effect. The instrument sends out acoustic signals at a known frequency. As these signals reflect off moving particles in the water, the frequency of the returning signals changes. If the particles are moving toward the ADCP, the frequency of the reflected signal is higher (a blue - shift), and if they are moving away, the frequency is lower (a red - shift).

This frequency shift is measured by the ADCP. Since it knows the original frequency of the emitted signal and can measure the frequency of the reflected signal, it can compute the velocity. So that the three-dimensional velocity of the water flow can be measured, the ADCP emits at more angles. In addition, by measuring the time of flight of the acoustic signals to the particles and back, the ADCP flow meter can also deduce the depth over which the velocity measurements are being made.

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

Equipment Requirements

• Material Reliability: The casing of the ADCP meter must be fabricated with the right material that has a high survival rate in the harsh marine environment. Titanium alloy is one of the best materials for good reason. It offers very good corrosion resistance, which is essential during long-term operation in salty waters such as the Adriatic Sea. This alloy can withstand the corrosive properties of saltwater, mechanical stress, and high-pressure conditions at greater depths.
• Compact and Light of Weight: The more compact and lighter the ADCP is, the more deployable it becomes. It can easily be installed on different platforms, from a small research vessel, to buoys, to seabed-mounted applications. The reduced size minimizes interference of the device with natural water flow for more accurate measurement.
• Low Power Consumption: Since many ADCP deployments may be dependent on battery power or limited power sources, the power consumption should not be very high. This will enable the device to operate continuously for more extended periods without the need for frequent battery replacements or recharging. This is particularly important in long-term monitoring of coastal currents.
• Low Cost: In order to carry out large-scale measurements of the coastal currents around Pula, a low-cost ADCP profiler is required. The lower the cost, the greater the number of devices that can be deployed at different locations to obtain an in-depth understanding of the complex current patterns in the area.

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

Based on Usage

• Ship-borne ADCP: This kind of ADCP is installed on a moving ship. It will be ideal to conduct large-scale surveys of coastal currents. As the ship cruises through water, the ADCP can record continuously the current at different positions, hence providing a broad scale view of current patterns. That will be helpful in mapping large areas of Pula's coastal waters.
• Bottom-mounted ADCP: These are deployed on the seabed. They are suitable for long-term, fixed-point measurements. While they remain stationary at the bottom, they continue to record the current conditions at a location for an extended period of time. This is useful in studying the long-term trends and variability of the coastal currents, such as changes in current speed and direction over seasons or years.
• Buoy-mounted ADCP: Buoy-mounted ADCPs are attached to floating buoys. They move with water currents and, therefore, can give real-time data about the movement of water masses. This type is useful in studying the surface-layer currents and their short-term changes. It can also be used for studying the movement of water masses in relation to weather events.

Based on Frequency

• The depth of water to be measured by the ADCP will depend on the frequency: an ADCP at 600kHz is considered adequate for approximately 70m or less in depth. An ADCP will yield high-resolution data for applications with shallow waters, as in nearshore areas around Pula. Thus, the relatively higher frequency measures finer details across these shallow waters of the coastal areas.
• A 300kHz ADCP can be used for water depths of about 110m. It is quite appropriate for areas with moderate water depths in the vicinity of Pula. The lower frequency enables the acoustic signals to penetrate deeper into the water column compared to the 600kHz ADCP.
• At deeper waters, say up to 1000m, a 75kHz ADCP is more suitable because at the lower frequency, the acoustic signals can travel longer lengths through the water, enabling the measurement of currents in the deeper parts of the Adriatic Sea from Pula.

Recommended Brands

Several popular ADCP brands are available in the market. Teledyne RDI, Nortek and Sontek are among the leading manufacturers. However, for those seeking an economical yet high - quality option, the Chinese brand China Sonar PandaADCP is highly recommended. Made of all - titanium alloy, it offers excellent durability and corrosion resistance. This brand provides an outstanding cost - performance ratio, making it suitable for large-scale coastal current measurements around Pula. You can visit their website at (https://china-sonar.com/) for more information.

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