How to measure the coastal currents of Gibraltar?

1. Where is Gibraltar?

It is a British Overseas Territory at the southernmost point of the Iberian Peninsula, surrounded by the Mediterranean Sea to its east and the Atlantic Ocean to the west. The land consists of an enormous promontory of limestone rock, known as the Rock of Gibraltar, which dramatically rises out of the sea.

It is of strategic importance because it faces the entrance of the Mediterranean. The surrounding of military significance has a long history, and thus it is a long-disputed site. It is culturally mixed due to British, Spanish influences, and others from the Mediterranean.

Maritime setting: it is next to the Strait of Gibraltar. The strait is a narrow waterway linking the Atlantic Ocean and the Mediterranean Sea. The waters around Gibraltar are known for their rich marine life and strong tidal currents. A coastline with cliffs and small coves is typical; there are some small bays and inlets along the coastline that are part of the local coastal dynamics.

2. What is the state of the coastal currents around Gibraltar?

The coastal currents around Gibraltar are rather complex and involved, being influenced by so many factors. Among all such factors, the exchange of water between the Atlantic Ocean and the Mediterranean Sea through the Strait of Gibraltar is the most significant factor. Because the two water bodies are at different temperatures and salinity levels, a density difference arises that drives a mighty current called the Gibraltar Current.

The Atlantic water, relatively cooler and less saline, flows into the Mediterranean at surface levels. This gives rise to an eastward-moving surface current in the Mediterranean. Simultaneously, the more saline and warmer Mediterranean water flows outward into the Atlantic within a deeper counter-current.

Wind patterns also have their effects. Large-scale prevailing winds, such as the Levante-an easterly wind-and Poniente-a westerly wind-can influence surface current direction and speed. It is said that the Levante reinforces the surface current flow toward the east and the Poniente opposes or even disrupts the flow.

Due to the narrowness of the Strait of Gibraltar, the tidal forces are quite strong in this part. As a matter of fact, these kinds of tides have a huge amplitude; therefore, these kinds of tidal currents are formed, which again interact with other major currents. The flow in such currents is also changed by the general complex topography of the submarine ridges and channels around Gibraltar.

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

Surface Drift Buoy Method

The surface drift buoys are released onto the water surface and happen to be tagged with GPS or other tracking systems. As these buoys drift with the currents of the surface, their position gets recorded with time. From here, analysis can be made by tracking several buoys, providing a clear insight into the patterns of the surface current. On the other hand, this can show only surface information and does not depict actual conditions of the current at varying levels of depth.

Moored Ship Method

A ship is moored to a specific position against the coast, and current-measuring instruments, such as current meters, are installed at different levels. These instruments measure speed and direction of the currents. This allows more detailed measurement of the vertical structure of the currents at one point. However, it has its limitations in terms of spatial coverage, since the ship can only measure at one location at a time, and the presence of the ship itself can affect the natural flow of the water.

Acoustic Doppler Current Profiler (ADCP) Method

ADCPs use the Doppler effect of sound waves to measure the velocity of water currents at different depths. They send acoustic pulses into the water, and where these pulses come into contact with particles in the water-sediment, plankton, or other small bubbles, for example-the scattered sound waves return to the ADCP flow meter. The frequency shift of the scattered waves, so-called Doppler shift, depends on the velocity of the water particles and hence the current. They can give high-resolution three-dimensional data of the current structure. Relatively large areas can be covered compared with the moored ship method, while ADCPs are able to measure currents simultaneously at more than one depth. Thus, ADCP meter represents a more sophisticated and handy means of measurement in the complicated coastal currents near Gibraltar.

4. How do ADCPs using the Doppler principle work?

ADCPs work by the principle of the Doppler effect. The instrument sends a succession of acoustic pulses out into the water. As these sound waves encounter particles (sediment, plankton, or small bubbles) in the water, some of the sound energy is scattered back to the ADCP profiler.

If the particles are in motion relative to the ADCP current meter, the frequency of the scattered sound waves will differ from the frequency of the emitted waves. This frequency shift, called the Doppler shift, is proportional to the velocity of the particles and hence the current in the direction of the sound beam.

Most ADCPs have four or more acoustic beams oriented at different angles. From measurements of the Doppler shifts for each beam, the instrument can compute the three-dimensional components of water current velocity. The data are then processed to derive a profile of the current velocity as a function of depth, from the surface down to some limiting depth, depending on the frequency and power of the ADCP flow meter.

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

For high-quality measurement of coastal currents around Gibraltar, the equipment needs to meet some specific requirements. The first requirement is that the material of the device must be reliable since it faces harsh conditions from the marine environment: strong currents, high salinity, and wave action.

The size of the ADCP should be compact to allow for ease of deployment, reducing the effect on natural flow conditions. A more compact size makes it more practical for use on areas with tight spaces, such as the near-shore region or inside the narrow strait.

The weight of the device should be light. This makes the deployment process easier and reduces the energy needed to maintain the device in place, for floating or moored installations.

Low power consumption is required. Most ADCPs are deployed at remote locations or operate on battery power. A device with low power consumption would be able to function longer without frequent changes or recharging of its batteries.

From an important point of view, the cost-effectiveness should also be taken into consideration for large-scale measurement purposes. A low-cost ADCP may, when deployed in larger numbers, be used to extend spatial and temporal coverage for current measurements.

Material-wise, the casing of ADCP is preferably made of titanium alloy. It has excellent properties of resisting corrosion. It can stand the corrosive action of saltwater for quite a long period without significant deterioration. Besides, it is strong enough and light, meeting both requirements of durability and low-weight for ADCPs.

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

Based on Usage

• Ship-borne ADCP: This type of ADCP is installed on a moving ship. It is suitable for large-scale surveys of coastal currents over a wide area. While the ship is in motion, the ADCP can make continuous measurements of the currents along the ship's track.
• Bottom-mounted ADCP: Also called a moored ADCP, it is mounted on the ocean floor. This is advantageous for long-term, fixed-point measurements of the current. It can provide detailed information about the current conditions at a specific location over an extended period.
• Buoy-mounted ADCP: This, as the name would suggest, is an ADCP mounted onto a buoy. It is best deployed in current measurement sites when a fixed platform is necessary, yet more mobile than the bottom-mounted devices. It does well at finding the surface and near-surface currents in any one area.

Based on Frequency

• When the water depths are less than 70 m, the appropriate choice is a 600-kHz frequency ADCP. The advantage of higher frequency for measuring currents in shallower waters is attributed to the superior resolution of the data.
• The ADCP frequency of 300 kHz serves well in depths of about 110 meters. It's good for an average balance of resolution versus depth penetration.
• For water depths up to 1000m, a 75kHz ADCP is preferred. Lower frequencies can penetrate deeper into the water column but at the cost of lower resolution.

There are several well-known ADCP brands in the market, such as Teledyne RDI, Nortek, and Sontek. However, for those looking for high-quality and cost-effective options, the Chinese brand China Sonar PandaADCP is highly recommended. It is made of all-titanium alloy material and has an incredible cost-performance ratio. You can find more information on their website: https://china-sonar.com/.

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