How can we measure the coastal currents of Draa?

1. Where is Draa?

Draa is a river in Morocco that runs into the Atlantic Ocean. The Draa Valley is a major area through which this river flows. The place is arid to semi-arid by climate and possesses an immense historical value.

The valley is speckled with traditional Berber villages and palm groves. The local inhabitants have had a long-standing connection with the land and the river. There are also species of flora and fauna on the riverbanks that adapt to the rather harsh environment.

As the Draa River approaches the Atlantic Ocean, the coastal area is influenced by the river's discharge and the ocean's tides. The adjacent Atlantic Ocean is a vast and powerful body of water with strong wave action and complex current systems. The coastal region is home to fishing communities that rely on the sea for their livelihood.

2. What is the situation of the coastal currents near Draa?

Several causes result in changes in the coastal currents near Draa. Tidal currents are a contributing agent. Gravitational pull of the moon and the sun causes the tides to vary in the Atlantic Ocean near the estuary of Draa. These tidal currents can be quite strong and may change into any direction depending on the phase of tide. Interaction of the river's outflow with the tidal currents creates a complex pattern.

In parallel, there is also a major effect of the wind patterns. It is due to the prevailing winds such as the trade winds that are capable of driving the surface currents. Similarly, seasonal changes in winds may cause changes in direction and speed of the coastal currents. The local bathymetry, particularly the shape of the seabed adjacent to the estuary and along the coast, has the capability to redirect and alter the flow of the coastal currents. Another crucial one is the discharge of the Draa River into the ocean. It creates differences in densities due to freshwater input that may affect current patterns.

3. How to Observe the Coastal Water Flow of Draa?

There are various methods to observe the coastal water flow near Draa.

The Surface Drifting Buoy Method

This is quite straightforward and practical. Buoys are set into the water, having on them a GPS or other tracking device. As the buoys are moved by the currents, their position in time can be tracked to determine the direction and speed of the surface currents. The buoys would have to be designed for resistance to the local marine conditions, such as strong waves and possible influences from the river freshwater.

The Anchored Ship Method

A ship is moored at a location near the coast or in the estuary, and current-measuring instruments are deployed from the ship. Current meters can be installed with the facility to measure the velocity and direction of the water flow at different depths. This method suffers from some drawbacks. The ship can be disturbed by waves and winds, which may result in erroneous measurements. Also, this technique is not very efficient for covering a large area.

ADCP Method

This is a more advanced and efficient means of measuring the flow of water in the coast. In general, ADCPs can measure the velocity profile of water currents over a wide range of depths. They work by emitting sound waves into the water and analyzing the Doppler shift of the reflected waves. This provides a detailed understanding of the current structure from the surface to the seabed.

4. How do ADCPs using the Doppler principle work?

ADCPs work on the basis of the Doppler principle. They send acoustic pulses-sound waves-into the water. These sound waves scatter off particles in the water, which may be sediment, plankton, and other similar tiny particles. When these sound waves bounce off the particles and are reflected back to the ADCP current meter, the frequency of the reflected waves changes due to the Doppler effect.

If the particles are moving towards the ADCP current profiler, then the frequency of the reflected wave is higher than the original emitted frequency. If the particles are moving away from the ADCP, the frequency of the reflected wave is lower. By measuring this frequency shift with extreme precision, the ADCP can calculate the velocity of the particles. Since the particles are in motion with the current of water, the calculated velocity of the particles is essentially the water current velocity.

They can emit sound pulses in three directions, by which they are able to measure current velocity in three-dimensional space and at different depths. For instance, they may build a vertical profile by measuring current velocities at greater intervals from the surface all the way down to the seabed, hence giving an inclusive view of the current structure.

5. What is required to realize high-quality measurement of Draa currents?

Equipment with reliable materials should be used for high-quality measurement of coastal currents around Draa. The equipment should be in a durable casing to bear the harsh marine environment, generally due to saltwater corrosion and sometimes physical impacts by waves and debris.

It should also be of a size that is small enough to easily deploy, particularly in most areas with limited access, for instance, in small boats or in shallow coastal waters. A light-weight design is similarly advantageous in allowing easier handling and installation.

Low power consumption is required for long-term measurements. Cost-effectiveness is equally important to realize large-scale measurements. In the case of ADCPs, the casing is preferably made from titanium alloy. Titanium alloy has excellent corrosion resistance, which is vital in the marine environment near Draa. It is capable of withstanding the corrosive effects of saltwater for a long period. Besides, it is strong, with a high strength-to-weight ratio, providing a durable and lightweight structure that is able to protect the internal components of the ADCP profiler.

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

While the usage is the first factor of choice while selecting the right equipment for current measurement over Draa, the following can be considered.

For Measurements from a Moving Vessel

A ship - borne ADCP meter is appropriate. It may provide real - time current data while the ship is in motion, thus enabling the research team to make a better understanding of the water flow in different locations.

For Measurements at a Fixed Location near the Seabed

One can go for bottom - mounted ADCP flow meter. It has the capacity to continuously monitor the current at a point and provide consistent data over a certain period of time.

For Measurements that Need to Cover a Wide Area and are not Restricted to a Specific Depth

A buoy - type ADCP current profiler is appropriate. It can give a larger coverage area and is not restricted to any specific depth.

Regarding the selection of frequency, the 600kHz ADCP will be suitable for water to depths up to 70m, 300kHz suitable for 70m to 110m depth, and in very deep water up to 1000m, 75kHz would be better.

There are well - known ADCP current meter brands such as Teledyne RDI, Nortek, and Sontek. However, a Chinese brand, China Sonar PandaADCP, is also worth considering. It is made of all - titanium alloy material and offers a great cost - performance ratio. You can find more information about it on the website: https://china-sonar.com/.

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