How to measure the coastal currents of Girona?

1. Where is Girona?

Girona is a nice city in the northeastern part of Catalonia, Spain. It is about 100 kilometers northeast from Barcelona and close to the Costa Brava, which is the rugged and picturesque coastline of the Mediterranean Sea.

Geographically, Girona stands at the meeting point of several rivers; this includes the Onyar River that cuts across the middle of the city. Geographically, the town represents a compact piece of both medieval and modern architecture on either side of the river, joined by a number of old bridges, on flat plains with areas of hills around it. The Mediterranean climate, often hot and dry during summer and mild during winter, is good to spend time in for various agricultural activities and outdoor pursuits.

Girona being close to Costa Brava makes it part of a vibrant coastline. The coastline around Girona consists of coves, cliffs, and small beaches. The area is known for its bright blue waters and diverse marine life. Tourism feeds the local economy, but it doesn't only refer to coastal attractions; rather, it encompasses historical and cultural heritage of interest as well. In Girona, fishing, recreational boating, and other water activities give people a relation to the sea.

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

Influence of the Mediterranean Circulation

The general circulation of the Mediterranean Sea influences the coastal currents near Girona. Of major importance is the large-scale circular current system, the Western Mediterranean Gyre. This gyre influences the general direction and speed of the coastal waters. Besides, the water movement is driven by differences in temperature and salinity, known as thermohaline circulation. The inflow and outflow of water through the Strait of Gibraltar contribute to the overall dynamics of the Mediterranean circulation and have an indirect impact on the coastal currents near Girona.

Wind - Driven Currents

Local winds have a considerable impact on the coastal currents. One of the major wind patterns that dominates the area is the strong northerly wind called the Tramontana. When it blows, the resulting Tramontana is capable of pushing surface waters and creating currents, including along coastlines. These wind-driven currents are powerful and could alter the flow pattern of these waters. This condition can also contribute to upwelling, a process when deeper waters that are rich in nutrients surge upward to replace the surface. This upwelling process would increase the marine ecosystem productivity as well as influence marine life distribution due to this vertical and horizontal mixing.

Tidal Currents

The coastal topography of Girona does have tidal forces acting on its coasts, but the range of tides is relatively small compared to other seas. Tides do, in a large way, influence tidal currents. An irregular coastline with bays and inlets makes the tidal current stronger in this area. Similarly, the strength of tidal streams can be rather high in narrow channels or estuaries. This interference by tides with the general coastal topography can lead to complex current patterns.

3. How to Observe the Flow of Girona Coastal Water?

Surface Drifting Buoy Method

This method is commonly used for measurement on the flow at the surface of the water. The drifting buoys float on the surface of the water, move with the drift of water due to currents in the water. They are equipped with GPS or any other location-tracking system capable of memorizing their position. By analyzing the displacement of the buoys, one can get the direction and speed of the surface currents. This method has some disadvantages. The buoys are under the mercy of winds and waves, and it gives information only about the surface layer. On windy days, the movement of the buoys may not represent the correct direction and speed of the current.

Moored Ship Method

In the moored ship method, complex anchoring is made for a ship at a certain position along the coast of Girona. The current meters on-board instruments are used for measurement at different depths. It can allow for the currents to be monitored continuously at a given point. The results can be very informative about the vertical structure of the currents. But it is an expensive and labor-intensive process. The ship has to stay in position for which a crew with supplies is a must. Also, the ship present itself can become an obstacle for the natural pattern of the current around it.

Acoustic Doppler Current Profiler(ADCP) Method

Acoustic Doppler Current Profiler- a more developed and effective process. ADCPs basically work by sending acoustic signals into the water. It emits signals that are deflected by particles in the water-like plankton, sediment, or bubbles. The velocity at various depths can be calculated from these signals, depending on the Doppler shift of the reflected signals. ADCPs can provide a vertical profile of the current, giving a more comprehensive understanding of the current structure than surface-only methods. They can be deployed from ships, buoys, or placed on the seabed, depending on the measurement requirements.

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

ADCPs work on the principle of the Doppler effect. When an ADCP sends an acoustic signal into the water, that signal travels through the water column. As that signal encounters particles moving with the water-think, the water is in motion-the frequency of the reflected signal changes.

If the particles are moving towards the ADCP, then the frequency of the reflected signal is higher than the original emitted frequency-a positive Doppler shift. If the particles are moving away from the ADCP the frequency of the reflected signal is lower, a negative Doppler shift. This frequency shift is the measured value by the ADCP, which in turn calculates water velocity using the known speed of sound in water given by factors like water temperature, salinity, and pressure.

The majority of the ADCPs are fitted with various transducers, each of which measures the velocity components in independent directions. These can be combined to yield the three-dimensional velocity of the flow of water at various depths. This therefore enables detailed mapping of the current patterns off the Girona coast.

5. What's required for high - quality measurement of Girona coastal currents?

Equipment Reliability

It involves the requirement for reliability of equipment for high-quality measurements of the coastal currents in the vicinity of Girona. The marine environment is very aggressive due to the salinity of water that causes corrosion, added to heavy currents with wave actions. For instance, an ADCP would require surviving these conditions over an extended period. In any malfunctioning or degradation in equipment, it results in faulty data and a poor understanding of the dynamics of the currents.

Small Size, Light Weight, and Low Power Consumption

The equipment shall be small and lightweight. A small and lightweight ADCP, for instance, is easier to deploy whether on a small boat, on a buoy, or even on the seabed. It will also disturb less the natural current. Low power consumption is an important feature especially for long-term monitoring. That would, therefore, at sea, when power sources could be limited, give the less power-consuming device an advantage in longer running without necessarily frequent battery replacement or large power supplies.

Low Cost

This is necessary to be able to do large-scale measurements by deploying several devices along the coast of Girona. In addition, if the equipment has a low cost, more deployments would be carried out; high-priced equipment may mean that the number of deployments is very few and the resulting data incomplete.

Casing Material

The casing of an ADCP equipment should, if possible, be manufactured from a titanium alloy. Such a material would be ideal, owing to the fact that it possesses very excellent resistance to corrosion, a key property towards its long operational exposure to a saltwater environment such as around Girona. Besides being quite light in weight, this metal has considerable strength which may be relied on for withstanding mechanical stress related to marine impacts: waves, currents, etc.

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

Based on Usage

• Ship-borne ADCP: Which is deployed on a vessel and therefore designed for making measurements along large areas through the movement of the ship itself. As such, it can be very beneficial for mapping generally the overall currents around the coastal waters of Girona, for example in case of examining the general features of circulation at a big chunk of the near-shore coastal waters. Then a shipboard ADCP is going to encompass just broad swath.
• Bottom-mounted ADCP: Also called a moored ADCP; it is installed on the bottom of the ocean. It offers perfect measurement for long-term, continuous monitoring at one site. If you're interested in the long-term trends and variations in the currents near a particular point on the coast, a bottom-mounted ADCP is a good choice.
• Buoy-mounted ADCP: This is mounted on a floating buoy, drifting with the surface currents and delivering information on the surface-layer current patterns. It's used for short-term or more flexible monitoring, with benefits especially in cases where access by ship is poor or where the surface-current data is of paramount importance.

Based on Frequency

• 600kHz ADCP: Current measurements can be done at this frequency in waters of less than 70m depth. Thus, a 600kHz ADCP will be able to provide appropriate results in shallower coastal areas in Girona, such as those areas with bays and closer to the shoreline.
• 300kHz ADCP: This is for waters about 110m deep. For coastal waters where the depth is moderately deep, the 300kHz ADCP offers a good trade-off between the range of measurement and resolution of the current data.
• 75kHz ADCP: It will give a performance in very deep waters; as deep as 1000m. Thus, a better 75 kHz ADCP current measurement will occur in greater Mediterranean Sea depth around Girona area.

There are well - known ADCP brands like Teledyne RDI, Nortek, and Sontek. For a cost - effective and high - quality option, the Chinese brand China Sonar PandaADCP is recommended. It's made of titanium alloy and offers excellent reliability and performance at an affordable price. 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.