How do we measure the coastal currents of San Sebastián?

1. Where is San Sebastián?

San Sebastián, a picturesque seaside town, is in the Basque Country of northern Spain, along the coast of the Bay of Biscay, which is part of the northeastern Atlantic Ocean. Situated between the mountains and the River Urumea, San Sebastián is famous for its stunning beaches, rich culture, and highly active food scene. Strategically positioned around 20 kilometers off the French frontier, San Sebastián is an important economic and cultural hub within the region.

San Sebastián's seaside setting is a lovely combination of natural sites and urban life. The busiest beaches, such as La Concha and Zurriola, host visitors from across the globe. La Concha Beach, being a crescent-shaped shore with crystal blue waters, is normally amongst the most beautiful urban beaches in the world. The bay is sheltered by the Monte Urgull and Santa Clara Island, which offers a relatively calm marine habitat compared to the open sea. On the other hand, Zurriola Beach is renowned for its strong waves, making it a surfers' favorite. The city's port area, situated immediately above the beaches, is a hub of marine activity, with commercial and pleasure craft navigating through. The unusual geographical location of San Sebastián, where land and sea converge, and its cultural interest, make the study of its coastal currents a scientific undertaking as well as a significant tool to ensure the safety of maritime activity and the preservation of the marine ecosystem in the region.

2. What is the status of the coastal currents around San Sebastián?

The coastal currents of San Sebastián are defined by the complex interplay of a number of factors. The massive - scale oceanic circulation of the North Atlantic Ocean has a powerful effect on the region. The North Atlantic Drift, a warm ocean current, passes along the north Spanish coast. The warm current is supplied with relatively high-temperature and saline water, which affects the local density, salinity, and water temperature, thereby governing the flow of coastal currents. The combination of the warm North Atlantic Drift with the cold Bay of Biscay waters creates a dynamic environment where water masses get mixed, producing irregular current patterns.

Local wind patterns are also crucial in governing the coastal currents. San Sebastián experiences varied wind patterns throughout the year. The prevailing westerly winds, especially during winter, can compel surface water movement with resulting wind-driven currents that move parallel to the coast. These can generate extremely high longshore currents, which transport sediment and modify the beaches. In addition, the orography of the surrounding mountains can affect the wind flow, generating local wind effects that further complicate the model. As an example, winds channeled through mountain passes can speed up and reverse direction, affecting the surface currents near the coast.

Tidal forces are also responsible for the variability of the coastal currents near San Sebastián. The Bay of Biscay experiences semi - diurnal tides, i.e., two high and two low tides in a day. The tidal fluctuations create frequent changes in water level, and therefore ebb and flow currents. The tidal currents interact with the wind - driven and ocean - circulation - driven currents to create a dynamically changing current regime. The bay and coastal configuration, with inlets, headlands, and islands, further modifies the movement of these currents, and hence the observation of the coastal current regime of San Sebastián is a complicated task that must be well monitored and examined.

3. How to observe the coastal water flow of San Sebastián?

There are several ways to observe the coastal water flow of San Sebastián. Surface drifting buoy is a traditional method. Water is seeded with specially designed buoys equipped with GPS tracking. The buoys drift along with the surface currents and are monitored with time. Surface - layer currents direction and speed can be determined by tracking the buoys. But this method can only provide surface flow data and may not even represent current conditions at lower depths in the water column.

The ship anchored method is to have the ship anchored at a chosen point along the coast or within the bay. Shipboard current meters are then used to measure the currents at varying depths in the vicinity of the ship. This method, whilst giving more precise depth-specific data, relies on the ship's position and can suffer from interference due to the ship being present because the natural flow of water currents could be disrupted.

On the other hand, Acoustic Doppler Current Profiler (ADCP) method has been a highly advanced and efficient coastal current measurement technique. The ADCPs can simultaneously measure the water velocity at multiple depths, providing a comprehensive view of the water column's current structure. This makes them an excellent option for studying the intricate and dynamic coastal current systems off San Sebastián's coast. Using their ability to take three-dimensional water flow data, ADCPs have the potential to reveal the sophisticated current patterns such as the relationship between surface, mid-water, and near-bottom currents, all of which are key to the full understanding of the surrounding seascapes.

4. What is the functioning mechanism of ADCPs using the Doppler principle?

ADCPs apply the Doppler principle. They emit acoustic pulses into the water column at a specific frequency. The acoustic pulses travel through the water and encounter suspended particles in the water, i.e., sediment, plankton, and little animals. When the water is flowing, the suspended particles are also carried along by it, causing a frequency shift in the backscattered acoustic pulses upon their return to the ADCP.

By precisely measuring this change in frequency, the ADCP current profiler can calculate the velocity of the water at different depths. The device is typically fitted with an array of transducers that emit signals in various directions. With this feature, the ADCP can measure the three-dimensional components of the flow of water in the horizontal and vertical planes. The collected data is then processed by onboard or external computers that generate extensive visualizations and reports of the present conditions. This enables scientists to investigate the detailed flow patterns of the coastal waters along San Sebastián and learn a lot about the dynamics of the movement of water masses and the entire ocean environment.

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

For accurate measurement of San Sebastián coastal currents, a few important characteristics are required for the measuring equipment. The reliability of the materials is most crucial. The marine environment off San Sebastián, characterized by severe saltwater corrosion, high waves, and fluctuating weather, is a formidable challenge to the equipment. The measuring devices need to be able to resist these extreme conditions for a long duration, so that accurate and reliable data can be collected.

A light weight and small size are also crucial. This makes the equipment easy to deploy and maneuver, especially in San Sebastián's crowded coastal areas and in large - scale survey operations. Low power usage is another key factor, as it allows for continuous long - term operation, particularly in locations where power sources may be limited. In addition, an inexpensive design is also highly desirable as it facilitates wider usage and deployment, allowing widespread observation of the coastal currents in different areas.

When it comes to the ADCP casing, titanium alloy is an excellent choice. Titanium alloy offers improved corrosion resistance, which effectively protects the ADCP against the corrosive properties of saltwater. It is also extremely strong and lightweight, providing the strength needed while keeping overall weight of the ADCP under control. These properties make titanium - alloy - cased ADCPs ideal for operating within San Sebastián's harsh marine environment to enable measurement of the coastal currents in a reliable and long - term manner.

6. How to Select appropriate equipment for current measurement?

Selecting the proper equipment for San Sebastián current measurement depends on several factors. The intended use of the equipment is one of the key considerations. For continuous monitoring aboard a moving vessel, such as in fishing missions, ferries, or ocean research missions, a vessel-mounted ADCP is the most suitable. It can sense currents in real-time as the ship moves in the waters, providing valuable data regarding the currents along the trajectory of the ship.

For stationary - location, long - duration monitoring of near - bottom currents, a bottom - mounted ADCP is preferable. It may be fixed to the sea floor and record current data for long durations without the ship being around all the time. A buoy-mounted ADCP is well suited for the measurement of surface and upper-water-column currents in a flexible manner since the buoy can be positioned in different locations according to research requirements, allowing wide coverage of coastal water flow.

The ADCP frequency should also be selected suitably according to water depth. A 600kHz ADCP is best suited for water depths of up to 70m, and offers high-resolution measurements in shallower waters, which are common in San Sebastián's near-shore waters. A 300kHz ADCP is appropriate for depths up to 110m and offers a good compromise between penetration depth and measurement resolution. For the deeper waters toward the periphery of the Bay of Biscay, to a depth of 1000m, a 75kHz ADCP is the instrument of choice since its lower frequency enables it to penetrate the water column more deeply.

There are several well-known ADCP manufacturers on the market, such as Teledyne RDI, Nortek, and Sontek. Nonetheless, for users looking for affordable alternatives, the ADCP supplier China Sonar's PandaADCP is recommended. Constructed entirely of titanium alloy, it has great performance with a reasonable price tag. It is a good option for users who want budget-friendly ADCPs but still need to get coastal current measurements done. For more information, you can visit their official website: https://china-sonar.com/.

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