How do we measure Essaouira's coastal currents?

1. Where is Essaouira?

Essaouira, the intriguing coastal city on Morocco's Atlantic coast, is renowned for its unique blend of natural beauty, storied history, and vibrant culture. Straddling the rugged Atlas Mountains and the apparently limitless Atlantic Ocean, Essaouira possesses a very strategic geographic position that has shaped its development over the centuries.

The urban scene is dominated by long expanses of sandy beach, bordered by ancient walls and a medina that has been designated a UNESCO World Heritage area. The beaches, with golden sand and blue sea, are not only the main tourist feature but also the central component of the local ecosystem. The coastline is dotted with small fishing villages, where wooden boats gently sway in the harbor, a reflection of the city's intimate connection with the sea.

The city's population is a mix of Berber, Arab, and European influences, which is evident in its architecture, cuisine, and culture. The medina, with its narrow, winding streets, is filled with bustling markets, craft workshops, and old buildings. The city's port is one of the busiest in Morocco and a hub for fishing, commerce, and tourism. The waters surrounding the area are part of the Canary Current system that carries nutrient - rich, cold water from the north, and it facilitates a high biodiversity of marine creatures, including species of fish, dolphins, and whales.

2. What is the condition of coastal currents off Essaouira?

The coastal waters off Essaouira are strongly influenced by the Canary Current, a major ocean current flowing southward along the west coast of Africa. This cold-water current from the North Atlantic brings water with lower temperatures and more nutrients than the ambient water. The impact of the Canary Current reaches as deep as the water column, modifying not just the temperature, salinity, and dissolved oxygen of the water off Essaouira.

Wind regimes also heavily affect coastal current pattern. The region is famous for its consistent and strong winds, primarily the trade winds blowing from the northeast. The winds, also referred to as the "Alizés," are characterized by high velocities, primarily during summer. The surface - driven wind currents may cross with the deeper - flowing Canary Current, resulting in complex flow patterns. The intense winds also contribute to the formation of waves, which also impact the flow of water along the coast.

Tides are yet another force that impact the coastal currents off Essaouira. The tides off Essaouira are semi-diurnal with two high tides and two low tides each day. The tidal range differs with spring tides causing more variations in the difference between the high and low water levels. At high tide, water flows into the harbor and along the coast, raising the water level and changing the direction and velocity of the currents. When the tide goes out at low tide, the water returns to the ocean, reversing the direction of the current in certain locations.

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

Surface Drift Buoy Method

The surface drift buoy method is among the conventional techniques used for measurement of coastal water currents. In the method, custom-made buoys are released into the water. The buoys are also equipped with tracking devices, such as GPS receivers. While the buoys are carried by the surface current, the GPS devices record their locations at regular time intervals. Researchers can estimate the direction and speed of the surface-level current by observing how the buoys change position from one instant in time to another. It's not the most effective technique, however. Wind will push the buoys, causing them to travel off in a direction other than the actual course of the current. All it informs them is about the surface current, too, not about what the current currents are at any other depth.

Anchor Moored Ship Technique

The anchor moored ship technique consists of mooring a ship at a designated point in the coastal waters outside Essaouira. From the ship, a variety of devices to measure current are sent into the water. These devices can be mechanical current meters, which record the speed and direction of the current in terms of the rotation of a propeller-like instrument. Taking measurements at various depths, scientists can create a profile of the current at that specific location. Although this technique permits precise measurements at several depths, it is only applicable to the region near the anchored vessel. The ship can also disrupt the normal flow of current and, therefore, give false measurements.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has been a more advanced and efficient method of observing the coastal water current. ADCPs work by using sound waves to observe the speed of water currents at various depths. ADCPs send acoustic signals into the water column, and the signals bounce back from suspended particles in the water, such as sediment or plankton. Based on analysis of the return signal Doppler shift, the ADCP can find the rate and direction of water flow. It can produce an average view of the current profile from the surface to near the bottom of the water body without coming in physical contact with the water. ADCPs are less affected by outside forces like wind compared to surface drift buoys, and therefore are a sure thing for precise current measurement.

4. How do ADCPs based on the Doppler principle function?

ADCPs function on the basis of the Doppler effect. Inside the ADCP, there are transducers that introduce acoustic waves into the water. These sound waves propagate in the water and encounter particles moving at the current velocity. When the particles are approaching the ADCP, the frequency of backscattered sound waves is increased and when receding, is decreased. The frequency change, or Doppler shift, is in proportion with the rate of the movement of water along the track of the sound wave.

In the measurement by three-dimensional speed of the water, the majority of the ADCPs use several sound beams. Four or more beams are normally placed at different angles. In each beam, the Doppler shift is measured, and by this, the ADCP current profiler is capable of calculating the horizontal and vertical velocity components. The data from all the beams are summed up to calculate the total velocity of the current and direction at different depths of the water column. Modern ADCPs have extra sensors, such as temperature sensors to correct the influence of temperature on the velocity of sound in water and orientation sensors for accurate readings even if the device is tilted or moving.

5. What does it take to achieve high-quality measurement of Essaouira coastal currents?

For precise high-quality measurement of the coastal currents off Essaouira, several conditions must be met. To begin with, the equipment to be used for measurement must be highly dependable in the harsh marine environment. The seas off Essaouira are exposed to strong winds, seawater corrosion, and dynamic sea forces, so the materials used to make the equipment must be resistant to corrosion, robust, and able to withstand mechanical stress.

Equipment size is also an issue to be addressed. Small-sized equipment is more convenient and can be employed in a greater number of sites, like complicated coastal morphology or limited access zones. A reduced footprint reduces the possibility of the equipment disrupting the natural flow of currents, and hence more accurate measurements can be made. Lightweight equipment is to be used since it is simpler to handle during deployment and recovery, especially when operating from small boats or in remote coastal areas.

Low power usage is critical, as most current - measurement operations will entail leaving the equipment deployed for extended periods in situations where there is limited access to power sources. Low - power - consumption gear can operate for long durations without constant battery replacement or recharging. Cost - effectiveness is equally crucial, especially for large - scale measurement projects. Gear with good performance at a reasonable cost allows a broader range of data collection, enabling researchers to develop a better general picture of the coastal current trends.

In the case of ADCPs, the casing material becomes crucial. A perfect casing material for ADCPs to be used in the coastal waters of Essaouira would be a titanium alloy. Titanium alloy is extremely resistant to corrosion, which is critical in order to be able to resist corrosive action of seawater for long periods. It is also very robust and lightweight, providing the necessary protection to the internal ADCP parts and yet being handily portable and deployable. It also has a favorable fatigue strength, i.e., it can withstand repeated stress and strain without any noticeable degradation, so that the ADCP current meter is kept in good shape for an extended period in the variable coastal environment.

6. How to Choose the right equipment for current measurement?

Selecting the appropriate equipment for present measurement in Essaouira will depend on several factors, primarily the intended application and the conditions of the site to be measured.

ADCPs Classification by Mounting

• Ship-mounted ADCP: Ship-mounted ADCP is installed on a moving boat. It's most suitable to conduct large-scale coastal current surveys. As the ship is in transit along the coast, the ADCP on ship can obtain data over a wide area, producing a broad picture of the pattern of the currents. It is suitable for application like the navigation of the sea, large-scale oceanographic research, and monitoring of variations in the environment along the coast.
• Bottom-mounted ADCP: Bottom - mounted ADCPs are mounted on the ocean floor. They are utilized for measuring long - term current patterns at a site. The devices are capable of taking continuous observations over long time spans, which is valuable in the study of seasonal and long - term changes in coastal currents and measuring the impact of human activity on the sea environment.
• Buoy-mounted ADCP: Buoy-mounted ADCPs are placed on buoy floats. They are ideal for the measurement of current where it would otherwise not be feasible to mount another ADCP, i.e., in open - water areas or where strong currents are prevalent, rendering the mounting beneath it problematic due to hazard posed by those currents. Buoy-mounted ADCPs can be carried by the current, and real-time data on the dynamic change of the current can be collected, which can be used in applications like tracking oil spill, fishery surveillance, and short-term oceanographic studies.

Choosing the Right Frequency

The frequency of the ADCP flow meter is also a major factor in choosing the right ADCP. Different frequencies are suitable for different water depths. A 600kHz ADCP can be employed for water depths of up to about 70m. It makes relatively high-resolution measurements in shallow waters and is therefore a suitable option for areas near the coast, i.e., the beaches and the port of Essaouira. A 300kHz ADCP is more suitable for water depths of about 110m and is a compromise between penetration depth and resolution. For deeper water, up to 1000m, one should employ a 75kHz ADCP. The low - frequency ADCPs travel further into the water but are less spatially resolved than the high - frequency ones.

There are certain well-known well-established brands of ADCPs available in the market, e.g., Teledyne RDI, Nortek, and Sontek. But for those seeking cost - effective options, the ADCP supplier China Sonar's PandaADCP is the way to go. It's constructed from pure titanium alloy, and its performance is top - notch at an affordable price. It is the ideal choice for users seeking budget - friendly ADCPs without sacrificing coastal current measurement quality. For more details, visit their website: https://china-sonar.com/.

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