How can we measure the coastal currents of Marbella?

1. Where is Marbella?

Marbella is a luxurious and glamorous city located on the southern coast of Spain, in the autonomous community of Andalusia. It lies in the province of Málaga and is part of the renowned Costa del Sol (Coast of the Sun).

Geographically, the town of Marbella is privileged with a coastline facing the azure waters of the Mediterranean Sea. It borders several beaches, the major ones being the Puerto Banús Beach and Cabopino Beach. Besides their important appeal for sun-seekers, they play a significant role in local coastal dynamics.

Marbella has luxury resorts, high - class shopping districts, and lively nightlife. It also has a historical past, from ancient times to the present, which has once been a small fishing village that is now famous throughout the world. The old town of Marbella contrasts sharply with modern developments in recent decades, having cobblestone narrow streets, whitewashed houses, and old monuments.

From a more general point of view in regard to natural surroundings, Marbella borders the Sierra Blanca mountain range. The presence of mountains ensures a picturesque urban landscape of the city while simultaneously influencing the local climate. Sea and mountains together create a very unusual microclimate where winters are rather mild, while summer days are very warm and sunny.

2. What is the situation with the coastal currents close to Marbella?

Several elements interact in the general pattern of coastal currents around Marbella. First, there is the general Mediterranean Sea circulation, induced by the temperature and salinity differences, as well as wind. These prevailing winds around the area play an important role and include the Levante and Poniente. This can cause surface water to move in a westerly direction while, on the other hand, the Poniente can reverse the flow.

The relief of the seafloor along Marbella does also play a role in coastal circulation. Such seabed irregularities as submarine canyons and shoals may disturb or deflect the flow of water. For example, a current might decelerate or deflect over a shoal area; it may well speed up within a deeper channel.

Though relatively weak compared to other seas, tidal forces are manifested. The ebb and flow can be regarded as creating an "in-and-out" movement of the water, with interference from other factors in a general system such as a wind-driven current which then develops an interactive complicated water circulation pattern.

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

Surface Drift Buoy Method

The principle is rather straightforward, using a buoy floating on the water's surface and installing appropriate tracking devices like GPS on them. Carried by the currents on the surface, the position of the buoy would change with time; the GPS devices attached will, therefore, document it. For more general impressions on surface current patterns, parallel sets of buoy deployments will help. On the other hand, in this process, only surface data are estimated without showing proper information for different currents across the vertical level.

Moored Ship Method

In this technique, a ship remains anchored in one place and the instruments on it record the current velocity and direction at various depths with the help of current meters. This method gives more detail about the vertical structure of the currents, although the spatial coverage is limited since only one location can be measured by the ship at a time. The presence of the ship may itself disturb the natural flow of the water.

Acoustic Doppler Current Profiler (ADCP) Method

ADCPs work by emitting sound waves to determine the velocity of water currents at various depths. They send acoustic pulses through the water; when these pulses strike particles suspended in the water, part of the energy is scattered back. The frequency shift of the scattered sound waves is proportional to the velocity of the water particles and, by this, the water current. ADCPs can obtain high-resolution measurements in three dimensions of the current structure. Compared with the moored ship method, a relatively large area can be covered; moreover, currents can be measured simultaneously at several depths. That makes ADCP more contemporary and handier in the measurement of coastal currents at Marbella.

4. The operation of ADCPs adopting the principle of Doppler:

The ADCPs operate based on the principle known as the Doppler effect. An instrument emits a series of acoustic pulses in the water. The sound waves backscattered to the instrument after interaction with the particles in water-sediment, plankton, or small bubbles, some of the sound energy that comes back to the ADCP.

If the particles are in motion relative to the ADCP, then the frequency of the scattered sound waves will be different than that of the emitted waves. The frequency shift or Doppler shift is directly proportional to the velocity of the particles, and hence the water current, along the direction of the sound beam.

Typical ADCPs use several acoustic beams, oriented at some angle to each other. An ADCP measures the Doppler shift from each beam and then combines them to compute the three components of water current velocity. This velocity information is averaged along a vertical path extending from the surface downward, which may reach several hundred meters deep, depending on the ADCP power and frequency.

5. What are the conditions for high-quality measurement of the currents in the coastal area of Marbella?

The equipment for high-quality measurement of the coastal currents in Marbella has to fulfill a number of requirements. First of all, the material of the device should be reliable. It needs to withstand the harsh marine environment, including saltwater corrosion, strong currents, and wave action.

The ADCP shall be of the smallest size possible. A small size allows for easy deployment and minimizes its interference on the natural flow of water. It also makes the instrument more applicable in an area with limited space, such as nearshore regions.

The weight of the device should be as light as possible. It would ease the deployment process and reduce energy that keeps the device in place in floating or moored installations.

Low power consumption is also very important. Most ADCP deployments are in remote locations or on battery power. A device with low power consumption can operate for a longer period without frequent replacement or recharging of its batteries.

Cost-effectiveness is another important factor, especially for large-scale measurement applications. Relatively low-cost ADCPs can be deployed more widely to enhance the spatial and temporal coverage of the current measurements.

Material-wise, the housing of the ADCP is preferred to be of titanium alloy. The corrosion-resistant property of a titanium alloy is excellent, and it can resist the corrosive action of saltwater for a very long period with minimal deterioration. Moreover, it is solid and light; thus, both the requirements regarding strength and low weight of ADCPs are met.

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 is only suitable for large-scale surveys of the coastal currents for a wide area. An ADCP may continuously measure currents along the track which the ship will cover while in motion.
• Bottom-mounted ADCP: It is also called moored ADCP and is mounted on the sea bed. It can be advantageous for long-term fixed-point measurements of the current. In this, ample data can be recorded relating to the conditions of the current at a particular location over a long duration.
• Buoy-mounted ADCP: As the name would suggest, this ADCP is mounted on a buoy. It finds application in the measurement of currents where a fixed platform is required yet more mobile compared to a bottom-mounted device. This device can serve in monitoring the surface and near-surface currents of a particular area.

Based on Frequency

• An ADCP 600kHz should be used to work in a water depth below 70 m as higher frequency supports the investigation over smaller areas better.
• A suitable 300 kHz ADCP that provides approximately a good middle between resolution and depth of penetration in a certain depth range at 110m.
• For water depth up to 1000m, 75kHz ADCP is of course the preferred choice. Lower frequencies penetrate further into the water column but at the cost of lower resolution.

There are several well-known brands in the ADCP market in the market, such as Teledyne RDI, Nortek, and Sontek. But for those who like high quality but cost-effective options, I would highly recommend the Chinese brand China Sonar PandaADCP. It is a product of all-titanium alloy materials with an incredible cost-performance ratio. You can find their website: https://china-sonar.com/.

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