1. Where is Tangier?
Tangier, a city steeped in rich and storied history, is situated at the northernmost point of Morocco where the Atlantic Ocean converges with the Mediterranean Sea. This peculiar geographical location has made Tangier a vital crossroads for centuries in terms of trade, culture, and migration. It is located at the mouth of the Strait of Gibraltar, a strait that divides Europe from Africa and is an important sea route connecting the two continents.
The landscape of Tangier is an unfailing blend of urban and natural charm. The city boasts a beautiful coastline with a blend of sandy beaches and seashores. The beaches, such as Playa de Malabata, are explored by tourists from around the globe for their clear waters and soft sand, while the cliffs give cinematic views of the sea and the distant shores of Spain. In its heart, the medina, a labyrinth of narrow streets and colorful bazaars, is a UNESCO World Heritage site. Typically Moroccan architecture coexists alongside the traces of many other cultures which have passed through Tangier throughout history, i.e., Berber, Arab, and European.
The people of Tangier are as multicultural as is the history of the city. A melting pot of diverse cultures and ethnic groups, the population of the city is a vibrant and dynamic society. The city's economy is dependent on tourism, fishing, and overseas trade. Tangier Harbor is one of North Africa's busiest ports, exporting large amounts of cargo and a large ferry terminus for ferries traveling between Morocco and Europe. Surrounding waters are a lush marine environment, full of varied fish, marine mammals, and other sea animals. Where the Mediterranean and Atlantic converge, there exists a unique environment that is favorable to all sorts of marine life, from anchovies and sardines to whales and dolphins.
2. How are the coastal currents off Tangier?
The coastal currents of Tangier are governed by a combination of factors that create a complex and dynamic regime. The most powerful influence is the interaction between the Atlantic Ocean and the Mediterranean Sea through the Strait of Gibraltar. The Mediterranean Sea is saltier than the Atlantic because it has relatively low rainfall and high evaporation. As a result, water continuously flows from the Atlantic into the Mediterranean at the surface through the strait, and a denser, deeper movement of Mediterranean water exits into the Atlantic. This exchange of water mass makes an immense difference to the patterns of currents off Tangier.
Wind patterns also play a significant role in contributing to the coastal currents. The area is characterized by a broad array of wind systems, including the strong and often not-wisdom-inclined winds blowing through the Strait of Gibraltar, for example. The westerlies, in particular, can push surface - level currents along the coast, feeding into the intrinsic supply of water via the strait. The winds can provoke large - scale circulation tendencies in the coastal waters, altering direction and speed of the currents. Secondly, local wind patterns, in turn determined by the land topography and proximity to the mountains, may provoke short - term variations of the current direction and rate.
Tides are another significant factor influencing the coastal currents in the vicinity of Tangier. The tides within this region are semi - diurnal, and there are two high tides and two low tides daily. Tidal range could be fluctuating, especially in neap and spring tides. During high tide, the level of water rises, altering the flow of water over the strait and coast, thus altering the regimes of current. When the tide recedes in low tide, the direction and speed of the currents also change, which consequently have impacts on local fisheries, navigation, as well as the movement of sediment along the coastline.
3. How to measure the coastal water current of Tangier?
Surface Drift Buoy Method
The surface drift buoy method is an old method to measure coastal water current. As per this method, specially made buoys are released into the sea. These buoys are equipped with tracking devices, e.g., GPS receivers. When the buoys are drifted by the surface current, the GPS instruments record their positions at fixed time intervals. By tracking the movement of the buoys over a certain duration, scientists can estimate the speed and direction of the surface-level current. This is not an optimal technique, though. The buoys are affected by wind, causing them to drift from the actual path of the current. It only provides information about the surface current and tells us nothing about the currents' patterns at different depths.
Anchor Moored Ship Technique
The anchor moored ship technique involves anchoring a ship at a specified place in the seawaters near Tangier. From the ship, several current - measuring devices are dropped into the water. The measuring device used can be mechanical current meters, which calculate the velocity and direction of the current from the motion of a propeller - type device. By measurement at different depths, researchers can have a profile of the flow of water at that place. Even though the method allows accurate measurements at different depths, it is limited to the area surrounding the anchored ship. The boat may also disrupt regular current flow, which may give false readings.
Acoustic Doppler Current Profiler (ADCP) Method
Acoustic Doppler Current Profiler (ADCP) is now a more advanced and efficient method for measuring the coastal water currents. ADCPs use sound waves to quantify water current velocities at different depths. ADCPs transmit acoustic pulses into the water column, and the pulses bounce off suspended particles in the water, such as sediment or plankton. By determining the Doppler shift of the backscattered pulses, the ADCP can calculate the speed and direction of the water flow. This method can be employed to generate an entire profile of the current from the surface to close to the bottom of the body of water without touching the water. ADCPs are less affected by external influences like wind in comparison with surface drift buoys and are therefore a secure choice for accurate current measurement.
4. What is the principle behind ADCPs that operate on the Doppler principle?
ADCPs operate in accordance with the Doppler effect principle. Transducers inside the ADCP emit acoustic waves into the water. The sound waves travel through the water and bounce off particles that are in the custody of the current. As the particles approach the ADCP, the frequency of the sound waves reflected increases, and vice versa. The frequency shift, or the Doppler shift, is directly proportional to the speed of the water movement along the direction of the sound wave.
The majority of ADCPs measure the three - dimensional velocity of the water by utilizing multiple acoustic beams. Typically, four or more beams are arranged at different angles. From the observation of the Doppler shift for each beam, the ADCP can find the vertical and horizontal velocity components. The data from all the beams are utilized to determine the overall current direction and velocity at different depths in the water column. Modern ADCPs also feature additional sensors, such as temperature sensors to account for the temperature effect on sound velocity in water, and orientation sensors to ensure accurate readings even if the instrument is tilting or moving.
5. What does one need for high-quality measurement of Tangier coastal currents?
To achieve high-quality measurement of coastal currents along the coast of Tangier, there are some requirements that should be met. First, the measuring equipment has to be highly reliable in the harsh marine environment. The seawater along the coast of Tangier is subject to heavy currents, saltwater corrosion, and the influence of variable winds, so the materials used in its construction should be corrosion-resistant, robust, and mechanical stress-resistant.
The size of equipment is also important. Small-sized equipment is more versatile and can be employed in a great number of places, for example, in narrow and crowded waters around the Strait of Gibraltar and other areas with intricate coastal geometry. The smaller footprint reduces the likelihood of the equipment disturbing the usual current dynamics, resulting in more accurate measurement. Lightweight is desirable because it is easier to handle at deployment and recovery, especially in cases of limited access or from small boats.
Low power usage is a requirement due to the high quantity of current-measurement operations currently required to be performed for extended periods of time, often in areas with limited access to power sources. Low - power - consuming equipment can run for extended periods without requiring constant battery replacement or recharging. Cost - effectiveness is also important, particularly for large - scale measurement endeavors. Equipment that provides good performance at an affordable price enables more extensive data collection, allowing researchers to gain a better understanding of the coastal current patterns.
When it comes to ADCPs, the material of the casing is especially relevant. Titanium alloy is a great option for the casing of ADCPs deployed in the coastal waters of Tangier. Titanium alloy is highly corrosion-resistant, as required for its ability to withstand corrosive action caused by saltwater over extended periods. It is also extremely strong and light, providing the necessary protection for the internal components of the ADCP while still being easy to control and deploy. The titanium alloy also possesses good fatigue properties in that it can withstand repeated stress and strain without loss of structural integrity, thus imparting the ADCP with extended life and reliability within the dynamic coastal environment.
6. How to choose the right equipment for current measurement?
Choosing the right equipment for current measurement in Tangier is based on several factors, mainly the purpose of use and the nature of the measurement location.
ADCPs by Mounting
- Ship-mounted ADCP: It is mounted on a ship in motion. It is best suited for carrying out large-scale surveys of coastal currents. When the ship is cruising the coast of Tangier, like the Strait of Gibraltar, the ship-borne ADCP can collect data over an extensive region, providing a general description of the current streams. It is suitable for applications like marine navigation, large-scale oceanography studies, and monitoring environmental change in the area.
- Bottom - mounted ADCP: Bottom - mounted ADCPs are fixed on the seabed. They are designed to measure long - term patterns of currents at a single point. They have the capability of measuring continuous data over extended periods of time, which is useful for monitoring the seasonal and long - term variability of coastal currents and the impact of human activities on the marine habitat in specific locations near Tangier.
- Buoy - mounted ADCP: Buoy - mounted ADCPs are installed on floating buoys. They are most - suitable for measuring currents in areas where it is not convenient to install other forms of ADCPs, e.g., in open - water areas or in areas with strong and fluctuating currents. Buoy-mounted ADCPs can ride with the current, and this provides real-time data on the dynamic behavior of the current, which is convenient for applications like oil spill tracking, fishery, and short-term oceanography studies in Tangier waters.
Choosing the Right Frequency
The frequency of the ADCP is also a crucial factor in purchasing. Different frequencies are suitable for different water depths. A 600kHz ADCP is sufficient for water depths of up to about 70m. It gives relatively high-resolution measurements in shallow seas, and it is a suitable choice for near-shore regions, e.g., the beaches and the more shallow part of the Strait of Gibraltar. A 300kHz ADCP is suitable for water depths of about 110m and is a compromise between penetration depth and resolution. For water depth greater than 1000m, a 75kHz ADCP is ideal. ADCPs with lower frequency penetrate further into the water but are poor in spatial resolution compared to higher-frequency ADCPs.
There are only a couple of popular ADCP brands that have presence in the market, viz. Teledyne RDI, Nortek, and Sontek. However, for those budget-conscious, there is the ADCP manufacturer China Sonar's PandaADCP. It is made wholly of titanium alloy and performs pretty well without much expense. For price-sensitive individuals who want reliable ADCPs still, it makes a fine alternative. You may know more about them on their website: https://china-sonar.com/.
Here is a table with some well known ADCP instrument brands and models.
How do we quantify the coastal currents in Tangier?