How do we measure the coastal currents of Zadar?

1. Where is Zadar?

Zadar is located on the central Dalmatian coast of Croatia, with a view toward the shining expanse of the Adriatic Sea, coupled with rich history and natural wonders. The strategic location has made Zadar a very important maritime hub since time past.

The landscape of the city is an interesting combination of ancient architecture and modern urban development. Its historic heart is full of remains from Roman, Venetian, and Austro-Hungarian rules. The Roman Forum, with its well-preserved columns and ancient stones, testifies to the rich history of Zadar. A modern wonder, the Sea Organ is a unique musical installation whose melodies, playing with the rise and fall of the waves, attract visitors from all over the world. The local population, so famous for their hospitality and deep connection with the sea, belongs to a culture in which fishing, sailing, and generally all the sea-related activities have a deep-seated significance.

Coastal-geographically speaking, Zadar is enveloped by an archipelago: the islands of Ugljan and Pašman, for example. They form a series of channels, bays, and different ways that water circulates in it. Waters of Zadar host a very biodiversity-rich part of the sea environment. Different sea floors are presented, from the sandy ones up to rocky reefs, where fish in every possible color, and even fragile coral, may appear.

2. What are the coastal currents around Zadar like?

The coastal currents in the surroundings of Zadar show very complicated genesis. In general, the dominating winds prevail: a number of winds characterise the Adriatic. In the summer, for instance, during summer months especially, the normal condition is of a north-western wind: a so-called maestral. This would probably be of enough force to drive surface water toward the coastline to give considerable onshore currents. In contrast, the bora is a cold northeasterly wind that pushes water offshore and greatly changes the coastal currents both in speed and direction.

Besides, a relevant factor is bathymetry: the bottom topography of the Zadar shelf is very irregular; there are different bottoms and underwater relief. The seabed between the islands can become narrower or, vice versa, expand bringing water masses either in or out. For example, the narrow ones give acceleration to the water, whereas wide areas bring about more dispersed flow.

River discharges play a role. Although there are no large rivers discharging into the sea around Zadar, smaller streams and seasonal runoff may introduce fresh water into the coastal zone. When freshwater comes into the coastal zone, it dilutes the surrounding seawater, creating density gradients that drive currents. These currents can interact with the larger - scale circulation patterns in the Adriatic Sea, which is part of the broader Mediterranean Sea circulation system.

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

Surface Drifting Buoy Method

One of the most basic methods for observing coastal water flow is to make use of surface drifting buoys. These buoys are equipped with tracking devices, such as GPS receivers. They are then drifted in the water, and after some time, their movements are followed. After tracking the path of the buoy, the surface current speed and direction can be estimated. The method has certain limitations. Surface winds can seriously affect the movements of the buoys and make them deflect from the real path of the current. Thus, the data recorded may not represent the actual prevailing conditions.

Anchored Ship Method

Anchored ships can be used as platforms for current measurements. Current meters and other instruments are suspended from the ship at various depths. This enables information on the vertical profile of the currents to be obtained at one location. Although this technique gives very detailed information on the currents at one point, it is rather limited. The data obtained is representative only for the area close to the ship and may not represent the general variability of the coastal current system.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has become a preferred method for measuring coastal currents near Zadar. The ADCPs can carry out synchronous measurements at depths of more than one. They also can be deployed from ships, moored on the seabed, or attached to buoys, making the collection of data quite versatile. Because of their structural setup, ADCPs have lesser disturbance from surface-level disturbances like wind and hence measure the true current conditions more precisely. It thus proves to be an invaluable tool in knowing the complex coastal current dynamics around Zadar.

4. How do ADCPs using the Doppler principle work?

The ADCPs are based on the Doppler effect. When an ADCP flow meter sends high-frequency sound waves into the water, these meet the small particles in the water-like plankton, sediment, or other air bubbles. These particles scatter the sound waves back towards the ADCP. The frequency of the scattered sound waves received by the ADCP differs from the frequency of the emitted waves. This frequency shift, known as the Doppler shift, is directly related to the velocity of the particles and thus the water relative to the ADCP.

Most ADCPs have multiple transducer beams. The ADCP current profiler is able to calculate the velocity components in different directions by measuring the Doppler shift for each beam. Vector addition of these velocity components gives the three-dimensional velocity of the water. Since both horizontal and vertical flow components are measured in the current structure, this becomes very useful to understand the nature of the current structure.

5. What is required to obtain a high-quality measurement of the Zadar coastal currents?

Reliability of Equipment Material

High-quality measurements of the currents around Zadar call for great care in selecting appropriate casing material for the ADCP. It should be housed in a casing made from titanium alloy. Corrosion resistance will be outstanding since the equipment is in seawater that is always corrosive. It can withstand long-term submersion without significant deterioration, ensuring the longevity and reliability of the ADCP profiler. Besides, its high strength-to-weight ratio allows the equipment to keep its structural integrity in strong currents and rough seas.

Size, Weight, and Power Consumption

The ADCP current meter should be designed to be small in size and light in weight. Compact design allows deployment in any environment, from a small research vessel, buoy, or seabed mooring. This will also minimize the effects on the flow field being measured. Low power consumption is another major consideration: In that way, long-time recording periods can be achieved by the ADCP without often replacing batteries, excluding the use of external power sources. Long-term deployment of the buoy is particularly relevant to remote coasts around Zadar.

Cost-effectiveness

The ADCP for the intensive and large-scale monitoring of coastal currents around Zadar should be inexpensive. With one that is less costly, many can use it-institutions, environmental agencies, even local communities-thereby allowing an understanding of the dynamics of coastal currents due to the wider amount of data gathered in total.

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

Based on Purpose of Usage

• Shipborne ADCP: This ADCP meter is installed on a ship in motion. It is quite suitable for conducting large-scale surveys of the coastal currents. Shipborne ADCPs can cover wide areas in relatively short periods and thus give a broad-scale view of the current patterns. It is useful for initial assessments or for studying 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. This is valuable for understanding the local hydrodynamics and for detecting long - term trends in the current system.
• Buoyant ADCP: Buoyant ADCPs, attached to a floating buoy, can move with the surface currents. They are really good to monitor the surface and near-surface current patterns. They have been used in tracking the movement of water masses over time.

Based on Water Depth

• When the water depths are within 70m, a 600kHz ADCP will be good to go. Due to their higher frequency, the sound waves can make more detailed measurements in shallower water and thus yield high-resolution data on the structure of the current.
• The 300kHz ADCP will be appropriate when the water is about 110m deep. It provides the right balance in the range against resolution for the measurement and has thus been very suitable for middle-depth coastal waters.
• A 75kHz ADCP, allowing the lower frequency sound waves to penetrate deeper through the water column, will allow deeper measurements although with generally low resolution compared with higher-frequency ADCPs for waters as deep as 1000m.

There are some well-known ADCP brands in the market, including Teledyne RDI, Nortek, and Sontek. However, for those seeking a cost-effective yet high-quality option, the China Sonar PandaADCP is a great choice. Made of all-titanium alloy, it offers excellent durability and performance at an affordable price. It is an economic-class ADCP that provides excellent 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.